This page describes selected literature available on PVT[edit | edit source]
Dispatch strategy and model for hybrid photovoltaic and trigeneration power systems[edit | edit source]
Abstract: In this work Author(s) present a study of the optoelectronic properties of nanocrystalline GaN (nc-GaN) and amorphous GaON (a‐GaON) grown by ion-assisted deposition. The two classes of film show very distinct photoconductive responses; the nc-GaN has a fast small response while the a‐GaON films have a much larger response which is persistent. To describe the observed intensity, wavelength, and temperature dependence of the photoconductivity in each class of film, Author(s) build a model which takes into account the role of a large density of localized states in the gap. The photoconductivity measurements are supplemented by thermally stimulated conductivity, measurement of the absorption coefficient, and determination of the Fermi level. Using the model to aid author(s) interpretation of this data set, Author(s) are able to characterize the density of states in the gap for the two materials.
- First generation PV + CHP hybrids where the PV simply decreased some small fraction of the run time of the CHP, have been eclipsed by 2nd generation systems, where the PV is expanded and is completely backed up by CHP, usually with a diesel generator.
- Additional complexity of a PV-CCHP system over a PV-CHP system creates system dynamics that require numerical simulation in order to optimize system design.
- Electricity is generated by both the PV array and the CHP unit. In order to allow for more flexibility of matching thermal loads to electric loads, conversion and storage equipment for both electric and thermal loads are also incorporated.
- Advantages of this design include better supply–demand correlation, maximized CHP fuel efficiency, and minimized CHP maintenance costs.
- In the design and optimization of such a system, it is important that the load profile is representative of the annual onsumption and not subject to extreme anomalies that may lead to oversizing the system capabilities.
- This paper overcame the limitations of available modeling techniques for hybrid systems that incapable of accounting for cooling loads by demonstrating a new simulation algorithm and dispatch strategy for the modeling of hybrid PV-CCHP systems.
Abstract: A significant amount of research and development work on the photovoltaic/thermal (PVT) technology has been done since the 1970s. Many innovative systems and products have been put forward and their quality evaluated by academics and professionals. A range of theoretical models has been introduced and their appropriateness validated by experimental data. Important design parameters are identified. Collaborations have been underway amongst institutions or countries, helping to sort out the suitable products and systems with the best marketing potential. This article gives a review of the trend of development of the technology, in particular the advancements in recent years and the future work required.
- This article gives a review of the trend of development of the technology, starting from the early groundwork and placing more emphasis on the developments after year 2000.
- A common PV module converts 4–17% of the incoming solar radiation into electricity, depending on the type of solar cells in use and the working conditions. In other words, more than 50% of the incident solar energy is converted as heat.
- There can be two undesirable consequences: (i) a drop in cell efficiency (typically 0.4% per °C rise for c-Si cells) and (ii) a permanent structural damage of the module if the thermal stress remains for prolonged period.
- Applications: Improved longwave absorption,High temperature applications,Autonomous applications,Commercial applications,The market potential.
Energy performance of water hybrid PV/T collectors applied to combisystems of Direct Solar Floor type [edit | edit source]
Abstract: The integration of photovoltaic (PV) modules in buildings allows one to consider a multifunctional frame and then to reduce the cost by substitution of components. In order to limit the rise of the cell operating temperature, a photovoltaics/thermal (PV/T) collector combines a solar water heating collector and PV cells. The recovered heat energy can be used for heating systems and domestic hot water. A combination with a Direct Solar Floor is studied. Its low operating temperature level is appropriate for the operating conditions of the mono- or poly-crystalline photovoltaic modules which are selected in that study. However, for a system including a glass covered collector and localised in Mâcon area in France, we show that the annual photovoltaic cell efficiency is 6.8% which represents a decrease of 28% in comparison with a conventional non-integrated PV module of 9.4% annual efficiency. This is obviously due to a temperature increase related to the cover. On the other hand, we show that without a glass cover, the efficiency is 10% which is 6% better than a standard module due to the cooling effect. Moreover, in the case of a glazed PV/T collector with a conventional control system for Direct Solar Floor, the maximum temperature reached at the level of the PV modules is higher than 100 °C. This is due to the oversize of the collectors during the summer when the heating needs are null, i.e. without a heated swimming pool for example. This temperature level does not allow the use of EVA resin (ethylene vinyl acetate) in PV modules due to strong risks of degradation. The current solution consists of using amorphous cells or, if we do not enhance the thermal production, uncovered PV/T collector. Further research led to water hybrid PV/T solar collectors as a one-piece component, both reliable and efficient, and including the thermal absorber, the heat exchanger and the photovoltaic functions.
- The mono-crystalline (m-C) and poly-crystalline (PC) cells see their electric production decreasing when the temperature of the cell increases because of their negative temperature coefficient (approximately −0.5%/K).
- The thermal efficiency of a PV/T collector is lower than for a traditional system because part of solar energy is converted into electricity, the optical factor is weaker and the global coefficient of the thermal losses is higher. On one hand, the presence of a glazed cover at the top of the collector increases the thermal efficiency. On the other hand, it deteriorates the electric efficiency by increasing both optical losses and temperature of PV cells. This decrease can be compensated by the installation of reflectors.
- The hot water produced by the hybrid collector is used for domestic hot-water production and the Direct Solar Floor. The advantage of the DSF is that an additional solar tank is not used since collected solar energy is directly stored in the floor.
- The dynamic behavior of the collector takes into account the heat capacity of each node. It is important because of the strong variations of flow rate related to control. The model of the wall (or roof) is defined with a model of type 3R4C.
- The model, based on the electric diagram, is equivalent to a photovoltaic cell including a generator of current (photocurrent), a diode and a series resistance (Fig. 3). The photovoltaic module includes Nms modules in series and Nmp modules in parallel.
Abstract: A new reversible photoelectronic effect is reported for amorphous Si produced by glow discharge of SiH •. Long exposure to light decreases:'o.'both the photoconductivity and the dark conductivity, the latter by nearly four orders of magnitude. Annealing above 150·C reverses the process. A model involving optically induced changes in gap states is proposed. The results have strong implications for both the physical nature of the material and for its applications in thin-film solar cells. as well as the reproducibility of measurements on discharge-produced Si.
- Both the photo-conductivity and the dark conductivity decrease with light exposure .
- Dark conductivity as a function of temperature through this process the conductivity increases with increasing temperature following the expression a= ao exp(Ea/kT).
- The hydrogen content, moreover,is stable at annealing temperatures below the deposition temperature.
- The change in dark conductivity does not necessarily limit the performance because the bulk series resistance in an operating cell is determined by the photo-conductivity
DEVELOPMENT IN UNDERSTANDING AND CONTROLLING THE STAEBLER-WRONSKI EFFECT IN a-Si:H [edit | edit source]
Abstract: Hydrogenated amorphous silicon (a-Si:H) exhibits a metastable light induced degradation of its optoelectronic properties that is called the Staebler-Wronski effect, after its discoverers. This degradation effect is associated with the relatively high diffusion coefficient of hydrogen and the changes in local bonding coordination promoted by hydrogen. Reviewed are the fundamental aspects of the interplay between hydrogen and electronic energy states that form the basis of competing microscopic models for explaining the degradation effect. These models are tested against the latest experimental observations, and material and preparation parameters that reduce the Staebler-Wronski effect are discussed.
- Dangling bond defects acting as recombination centers, decreased the photocarrier lifetime and, owing to their location near midgap shift EF, thereby decreased the dark conductivity.
- Extended illumination appreciably degrading the optoelectronic properties of this material. This lightinduced degradation is called the Staebler-Wronski effect (SWE). Even though these defects anneal away between 150 and 200±C, the SWE seriously limits the use of a-Si:H in solar photovoltaic applications.
- In crystalline Si, atomic motions that could produce coordination defects occur only near 1000±C. The low-anneal temperature of the light-induced coordination defects (dangling bonds) in a-Si:H is related to the diffusion of bonded hydrogen. Above TE » 200±C, hydrogen diffusion enables the equilibration of solid-state chemical reactions, which determine the concentration of coordination defects of silicon as well as of dopant atom.
Intrinsic and light induced gap states in a-Si:H materials and solar cells—effects of microstructure [edit | edit source]
Abstract: The effects of microstructure on the gap states of hydrogen diluted and undiluted hydrogenated amorphous silicon (a-Si:H) thin film materials and their solar cells have been investigated. In characterizing the films the commonly used methodology of relating just the magnitudes of photocurrents and subgap absorption, a(E), was expanded to take into account states other than those due to dangling bond defects. The electron mobility-lifetime products were characterized as a function of carrier generation rates and analysis was carried out of the entire a(E) spectra and their evolution with light induced degradation. Two distinctly different defect states at 1.0 and 1.2 eV from the conduction band and their contributions to carrier recombination were identified and their respective evolution under 1 sun illumination characterized. Direct correlations were obtained between the recombination in thin films with that of corresponding solar cells. The effects of the difference in microstructure on the changes in these two gap states in films and solar cells were also identified. It is found that improved stability of protocrystalline Si:H can in part be attributed to the reduction of the 1.2 eV defects. It is also shown that ignoring the presence of multiple defects leads to erroneous conclusions being drawn about the stability of a-Si:H and SWE.
- Because as yet the nature and densities of the states are not known it is not possible to correlate them directly to those in the corresponding cells, but it is possible to relate them through their role as carrier recombination centers.
- The distinct difference between the times taken to reach the 1 sun DSS with the cells having the protocrystalline and the undiluted a-Si:H i-layers can be attributed to a difference in the relative densities.
- The same kinetics are observed for the cells and films where in the undiluted cell, there is an ‘overshoot’ in the recovery so-called ‘fast’ and ‘slow’ defect states.
Abstract:Light-induced degradation of hydrogenated amorphous silicon (a-Si:H) solar cells has been modeled using computer simulations. In the computer model, the creation of light-induced defects as a function of position in the solar cell was calculated using the recombination profile. In this way, a new defect profile in the solar cell was obtained and the performance was calculated again. The results of computer simulations were compared to experimental results obtained on a-Si:H solar cell with different intrinsic layer thickness. These experimental solar cells were degraded under both open- and short-circuit conditions, because the recombination profile in the solar cells could then be altered significantly. A reasonable match was obtained between the experimental and simulation results if only the mid-gap defect density was increased. To our knowledge, it is the first time that light-induced degradation of the performance and the quantum efficiency of a thickness series of a-Si:H solar cells has been modeled at once using computer simulations.
- When two mobile hydrogen atoms ‘collide’, a metastable complex containing two Si–H bonds is formed together with two dangling-bond defects.
- Recombination on weak bonds initiates the defect creation.
- Computer simulation concluded that the dark-conductivity degradation was mainly determined by the shift in peak position of the positively charged/neutral defect-state peak of the DDOS and the photoconductivity by the total defect density.
- Light soaking mainly leads to a change in the defect density of a-Si:H and the creation of defects is initiated by recombination events in the intrinsic layer of the device.
- Light-soak the cells under both open- and short-circuit conditions with the objective to alter the recombination profile substantially in the cells.
Light-induced defect states in hydrogenated amorphous silicon centered around 1.0 and 1.2 eV from the conduction band edge [edit | edit source]
Abstract:To take into account the presence of multiple light-induced defect states in hydrogenated amorphous silicon (a-Si:H) the evolution of the entire spectra of photoconductive subgap absorption, a(hn ),has been analyzed. Using this approach two distinctly different light-induced defect states centered around 1.0 and 1.2 eV from the conduction band edge are clearly identified. Results are presented on their evolution and respective effects on carrier recombination that clearly point to the importance of these states in evaluating the stability of different a-Si:H solar cell materials, as well as elucidating the origin of the Staebler–Wronski effect.
- LI degradationis associated with the creation of dangling bonds.
- a(hn)is interpreted solely in terms of D0 defect states.However, results have also been reported that point to the introduction of other defect states and that a(hn ) cannot be interpreted in such a simple manner.
- Two distinctly different light induced defect states centered around 1.0 and 1.2 eV from the conduction band ~CB! edge are clearly identified and their evolution found to be consistent with the corresponding changes in mt
- It is shown that in the AS, the mt product of the R510 material is about five times higher than in the R50 material, as is generally expected for better quality materials.
- At 25 °C, the protocrystalline material attains a DSS in approximately 100 h, whereas in the R50 material, the commonly reported kinetics with t21/3 extends for 400 h with no approach to DSS. When the temperature of degradation is raised to 75 °C, there is virtually no change in the kinetics of the 20 Å/s material, whereas the R510 reaches a DSS with a mt values that is two times higher than at 25 °C.
- Any interpretation of a(hn ) in terms of the density and energy distribution of multiple defect states is complicated by the nature of photoconductive subgap absorption, which is determined by N(E),
PERFORMANCE TEST OF AMORPHOUS SILICON MODULES IN DIFFERENT CLIMATES – YEAR THREE: HIGHER MINIMUM OPERATING TEMPERATURES LEAD TO HIGHER PERFORMANCE LEVELS [edit | edit source]
Abstract:This paper presents third year results of a round robin exposure experiment designed to assess the performance of thin-film amorphous silicon (a-Si) solar modules operating in different climatic conditions. Three identical sets of commercially available a-Si PV modules from five different manufacturers were simultaneously deployed outdoors in three sites with distinct climates (Arizona – USA, Colorado – USA and Florianopolis – Brazil). Every year all PV module sets were sent to the National Renewable Energy Laboratory (NREL) for Standard Testing Conditions measurements under a SPIRE simulator. The fouryear experiment aims to determine the light-induced degradation and stabilization characteristics of a-Si regarding specific history of exposure, and to monitor and compare degradation rates in different climates. We present results from the first three years of measurements, showing that while most of the manufacturers underrate their products by 20 to 25% to account for the light-induced degradation, outdoor exposure temperature seems to be what will ultimately determine the stabilized performance level of a-Si.
- After the first year of outdoor exposure, modules deployed at the site with the highest minimum operating temperature experienced the highest stabilized output level
- Further reporting on the first and second years of this four-year experiment, indicated that there is barely any lifetime memory of the stabilized state in a-Si
- In the first year of deployment outdoors, PV modules exposed at the coldest site underwent the largest amount of degradation, and outdoor exposure at the warmest site led to the smallest amount of degradation
- In the second year, Set A went from the coldest site to the warmest site, which led to some recovery in output performance upon further light soaking, indicating that these modules do not seem to show a lifetime memory of the stabilized state.
- In the third year, Set A was deployed outdoors at the year round- climate-intermediate site,and reached a new stabilized state, at a higher performance level than after the second year.
Abstract:The temperature requirements of solar industrial process heat applications range from 60 °C to 260 °C. The characteristics of medium to medium-high temperature solar collectors are given and an overview of efficiency and cost of existing technologies is presented. Five collector types have been considered in this study varying from the simple stationary flat-plate to movable parabolic trough ones. Based on TRNSYS simulations, an estimation of the system efficiency of solar process heat plants operating in the Mediterranean climate are given for the different collector technologies. The annual energy gains of such systems are from 550 to 1100 kWh/m2 a. The resulting energy costs obtained for solar heat are from 0.015 to 0.028 C£/kWh depending on the collector type applied. The viabilities of the systems depend on their initial cost and the fuel price. None of these costs however is stable but change continuously depending on international market trends and oil production rates. The costs will turn out to be more favourable when the solar collectors become cheaper and subsidisation of fuel is removed. Therefore the optimisation procedure suggested in this paper should be followed in order to select the most appropriate system in each case.
- The industrial heat-demand constitutes about 15% of the overall demand of final energy requirements in the southern European countries
- Investment costs should be comparatively low, even if the costs for the collector are higher.One way to cause economically easy terms is to design systems without heat storage.
- Solar thermal energy can be used for low-pressure steam generation at 100–110*C and for refrigeration of the wort, which can be accomplished with absorption cooling.
- In a solar process heat system, interfacing of the collectors with conventional energy supplies must be done in a way compatible with the process. The easiest way to accomplish this is by using heat storage, which can also allow the system to work in periods of low irradiation and/or night time.
- In the case of water preheating, higher efficiencies are obtained due to the low input-temperature to the solar system: thus low-technology collectors can work effectively and the required load supply temperature has no or little effect on the performance of the solar system.
- The optimum collector area, storage tank volume, life-cycle savings and solar contribution of the various collector types and demand temperatures considered are investigated.
- There are two major types of collectors that can be applied for industrial process-heat, non-tracking (stationary) collectors and one-axis sun-tracking parabolic trough collectors.
[edit | edit source]
Abstract:The influence of the operation temperature on the output properties of solar cells with hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon germanium (a-SiGe:H) photovoltaic layers was investigated. The output power after longtime operation of an a-Si:H single junction, an a-Si:H/a-Si:H tandem, and an a-Si:H/a-SiGe:H tandem solar cell was calculated based on the experimental results of two types of temperature dependence for both conversion efficiency and light-induced degradation. It was found that the a-Si:H/a-SiGe:H tandem solar cell maintained a higher output power than the others even after longtime operation during which a temperature range of 25°C to 80°C. These results confirm the advantages of the a-Si:H/a-SiGe:H tandem solar cell for practical use, especially in high-temperature regions.
- The variation in the conversion efficiency of an a-Si:H solar cell during light soaking is determined by the balance of light-induced deterioration and thermal-induced recovery.
- SiGe:H tandem solar cell maintains a higher output power than both the a-Si:H single and a-Si:H/a-Si:H tandem solar cells when the operation temperature was maintained from 25*C to 80*C
- The influence ofthe operation temperature on the output properties ofa-Si: Hrelated solar cells was investigated by measuring the operation temperature dependence for both conversion efficiency and light-induced degradation.
ROOM TEMPERATURE ANNEALING OF FAST STATES FROM 1 SUN ILLUMINATION IN PROTOCRYSTALLINE SI:H MATERIALS AND SOLAR CELLS [edit | edit source]
Abstract:In order to obtain more insight into the nature of the recovery in the light induced changes at room temperature in hydrogenated amorphous silicon (a-Si:H) solar cells the relaxation of the photocurrents in- the light induced changes in protocrystalline a-Si:H thin films were investigated. Immediately upon the removal of I sun illumination recoveries in the photocurrents are found like those present in the currents in the dark current-voltage characteristics in corresponding p-i-n solar cells. The striking similarity between the results on thin films and the corresponding dark foiward bias current-voltage characteristics of solar cells suggest that the recoveries obtained with low generation rates (5~1O~~cm"sin~ 't)h e films are a measure of annealing kinetics of the defect states around midgap in the bulk of the films. The mtes of recoveries decrease with higher camer generation rates and the length of the light induced degradation. Results are presented which indicate that the history of creation and annealing of light induced defect states is important in determining subsequent creation and annealing kinetics.
- An overshoot (or 'hump") in annealing under 1 sun illumination at 50'C after soaking at 50 suns for 2 hours. This hump was more evident in undiluted - samples
- 'Fast' states that are created faster also anneal out faster, while the 'slow' states created after long light soaking times are more difficult to anneal out.
- Although recovery at 25'C after high illumination intensity in cells and films was observed the reported results indicated that it was very slow. It is important to note that in studies on both the solar cell.
- Differences in results of room temperature annealing have been observed in this study between different films deposited under identical conditions and also co-deposited films.
CORRELATION OF LIGHT-INDUCED CHANGES IN a-Si:H FILMS WITH CHARACTERISTICS OF CORRESPONDING SOLAR CELLS [edit | edit source]
Abstract:For the first time direct correlations are obtained between the light induced changes under 1 sun illumination in the properties of a-Si:H and those in the characteristics of p-i-n cells incorporating identically-prepared i-layers. These correlations were obtained after account was taken of the effects that the location of the electron and hole quasi-Fermi levels have on the carrier recombination that occurs through the different gap states. The changes in midgap state density, as measured on the films and reflected in the subgap absorption at 1.2 eV, are directly correlated with changes in the dark I-V characteristics under low forward bias. In this case small quasi-Fermi level splitting is present so the recombination of the injected carriers is determined by the midgap states in the bulk of the i-layer. In addition, the changes in the electron mobility–lifetime products as measured on the films are correlated with changes in the fill factor measured on cells under the same conditions as long as large quasi-Fermi level splitting is present and recombination occurs through states spanning a wide region of the gap, such as occurs under 1 sun illumination. The results explain (i) the failure of numerous attempts to correlate the degradation of solar cells reliably with the creation of dangling bond defects and (ii) the inadequacy of the large number of modeling results that assume such a correlation.
- Attempts by Wyrsch et al.  at correlating the light induced changes in effective mobility-lifetime products, representing the transport of both holes and electrons in films with the changes in cells, had somewhat better success but still left many unanswered questions.
- The rate of defect creation in cells is found to be approximately proportional to the square of the intensity (I) rather than to the predicted
- The degradation of solar cell efficiencies, primarily due to changes in fill factors (FF), could be directly related to the increase in the D0 density.
- Comparison of defects states that are created under 1 sun illumination are the same in thin films as in the identically-prepared i-layers of the corresponding solar cells.
TEMPERATURE COEFFICIENTS FOR PV MODULES AND ARRAYS:MEASUREMENT METHODS, DIFFICULTIES, AND RESULTS [edit | edit source]
Abstract:The term “temperature coefficient” has been applied to several different photovoltaic performance parameters,including voltage, current, and power. The procedures for measuring the coefficient(s) for modules and arrays are not yet standardized, and systematic influences are common in the test methods used to measure them. There are also misconceptions regarding their application. Yet, temperature coefficients, however obtained, play an important role in PV system design and sizing, where often the worst case operating condition dictates the array size.This paper describes effective methods for determining temperature coefficients for cells, modules, and arrays;identifies sources of systematic errors in measurements; gives typical measured values for modules; and provides guidance for their application in system engineering.
- Four temperature coefficients for Isc, Imp, Voc, and Vmp, are necessary and sufficient to accurately model electrical performance for a wide range of operating conditions.
- Coefficients are typically calculated from previously measured module coefficients by accounting for the series/parallel configuration of modules in the array.
Abstract: An aperture-area conversion efficiency of 20.0% (intrinsic efficiency: 21.0%) has been achieved for a 1.0 cm2 CZ n-type single crystalline silicon (c-Si) solar cell, by using the “HIT (heterojunction with intrinsic thin-layer)” structure on both sides of the cell. This is the world's highest value for a c-Si solar cell in which the junction is fabricated at a low temperature of below 200°C. In this paper, the junction fabrication technologies and features of the HIT structure are reviewed. The stability under light and thermal exposure, and the temperature dependence on performance of a high-efficiency HIT solar cell are also reported.
- Important techniques a) Back Surface Field (BSF) b) Light trapping by surface texturization c) Surface passivation d) Contact passivization
- Excellent surface passivation and a p-n junction at quite a IOW temperature, which makes it possible to achieve high efficiency using solargrade CZ materials.
- P-type a-Si layer should be as thin as possible if a good junction property is ensured.
- Although the junction is fabricated at a temperature below 150%, no degradation was observed. Thus, it can be concluded that the HIT solar cell offers good stability in the practical applications.
Abstract: This paper presents a review of the available literature on PV/T collectors. The review is presented in a thematic way, in order to enable an easier comparison of the findings obtained by various researchers, especially on parameters affecting PV/T performance (electrical and thermal). The review covers the description of flat plate and concentrating, water and air PV/T collector types, analytical and numerical models, simulation and experimental work and qualitative evaluation of thermal/electrical output. The parameters affecting PV/T performance, such as covered versus uncovered PV/T collectors, optimum mass flow rate, absorber plate parameters (i.e. tube spacing, tube diameter, fin thickness), absorber to fluid thermal conductance and configuration design types are extensively discussed. Based on an exergy analysis, it was reported that the coverless PV/T collector produces the largest available total (electrical + thermal) exergy. From the literature review, it is clear that PV/T collectors are very promising devices and further work should be carried out aiming at improving their efficiency and reducing their cost, making them more competitive and thus aid towards global expansion and utilization of this environmentally friendly renewable energy device.
- The purpose of the absorber plate is twofold. Firstly, to cool the PV module and thus improve its electrical performance and secondly to collect the thermal energy produced,which would have otherwise been lost as heat to the environment.
- Literature on liquid and air PV/T collectors which covers the work of the last 25 years
- A PV/T collector basically combines the functions of a flat plate solar (thermal) collector and those of a photovoltaic panel.
- The generic conclusion they reached was that PV/T efficiency is dependent on flow rate.
- The thermal performance of a coverless PV/T collector is reduced especially at high temperatures due to heat losses from the top. However, the coverless PV/T collectors have a better electrical performance.
Industrial application of PV/T solar energy systems [edit | edit source]
Abstract:Hybrid photovoltaic/thermal (PV/T) systems consist of PV modules and heat extraction units mounted together. These systems can simultaneously provide electrical and thermal energy, thus achieving a higher energy conversion rate of the absorbed solar radiation than plain photovoltaics. Industries show high demand of energy for both heat and electricity and the hybrid PV/T systems could be used in order to meet this requirement. In this paper the application aspects in the industry of PV/T systems with water heat extraction is presented. The systems are analyzed with TRNSYS program for three locations Nicosia, Athens and Madison that are located at different latitudes. The system comprises 300 m2 of hybrid PV/T collectors producing both electricity and thermal energy and a 10 m3 water storage tank. The work includes the study of an industrial process heat system operated at two load supply temperatures of 60*C and 80*C. The results show that the electrical production of the system, employing polycrystalline solar cells, is more than the amorphous ones but the solar thermal contribution is slightly lower. A non-hybrid PV system produces about 25% more electrical energy but the present system covers also, depending on the location, a large percentage of the thermal energy requirement of the industry considered. The economic viability of the systems is proven, as positive life cycle savings are obtained in the case of hybrid systems and the savings are increased for higher load temperature applications. Additionally, although amorphous silicon panels are much less efficient than the polycrystalline ones, better economic figures are obtained due to their lower initial cost, i.e., they have better cost/benefit ratio.
- Two basic types of PV/T systems are considered depending on the heat extraction fluid used, the water type and the air type PV/T systems.
- In case of PV thermal efficiency is reduced for higher operating temperatures due to the increased thermal losses from the PV module front surface.
- The PV/T systems can be used in several industrial applications, but the most suitable should be the applications that need heat in medium (60–80*C) and mainly in low (<50*C) temperatures, as in these cases both the electrical and the thermal efficiency of the PV/T system can be kept at an acceptable level.
- The cost of the conventional energy source replaced for the electrical and thermal needs was taken 0.1 €/kW h for the electricity and 0.06 €/kW h for the oil, considering energy conversion efficiencies for the final energy use of 100% and 85%, respectively.
Hybrid photovoltaic and thermal solar-collector designed for natural circulation of water[edit | edit source]
Abstract:The electricity conversion-efficiency of a solar cell for commercial application is about 6–15%. More than 85% of the incoming solar energy is either reflected or absorbed as heat energy. Consequently, the working temperature of the solar cells increases considerably after prolonged operations and the cell’s efficiency drops significantly. The hybrid photovoltaic and thermal (PVT) collector technology using water as the coolant has been seen as a solution for improving the energy performance. Through good thermal-contact between the thermal absorber and the PV module, both the electrical efficiency and the thermal efficiency can be raised. Fin performance of the heat exchanger is one crucial factor in achieving a high overall energy yield. In this paper, the design developments of the PVT collectors are briefly reviewed. Author(s) observation is that very few studies have been done on the PVT system adopting a flat-box absorber design. Accordingly, an aluminum-alloy flat-box type hybrid solar collector functioned as a thermosyphon system was constructed. While the system efficiencies did vary with the operating conditions, the test results indicated that the daily thermal efficiency could reach around 40% when the initial water-temperature in the system is the same as the daily mean ambient temperature.
- Effect of Glazing on PV/T module has been analyzed.
- Reducing the size of the collector device, achieving better overall system-efficiency, and sharing effectively the balance of-system costs.
- Collector-fin efficiency and tube-bonding quality have been identified as the crucial design factors, which often bring limitations to the overall efficiency achievable.
- The electrical efficiency was 9%, with the characteristic daily efficiency of the system as 38%.
- The a-value of a hybrid solar-collector is expected to be lower than the conventional solar thermal collector,because on one hand,the PV module above the thermal absorber surface reduces the solar energy collected by the absorber and on the other hand there is an increased thermal resistance between the irradiated surface and the water streams in the flow channels.
- Uniform temperature distribution across the width of the absorber.
Abstract: Renewable energy (RE) resources have enormous potential and can meet the present world energy demand by using the locally available RE resources. One of the most promising RE technologies is photovoltaic (PV) technology. This paper presents a review of the available literature covering the various types of up and coming PV modules based on generation of solar cell and their applications in terms of electrical as well thermal outputs. The review covers detailed description and thermal model of PV and hybrid photovoltaic thermal (HPVT) systems, using water and air as the working fluid. Numerical model analysis and qualitative evaluation of thermal and electrical output in terms of an overall thermal energy and exergy has been carried out. Based on the thorough review, it is clear that PVT modules are very promising devices and there exists a lot of scope to further improve their performances particularly if integrated to roof top. Appropriate recommendations are made which will aid PVT systems to improve their overall thermal and electrical efficiency and reducing their cost, making them more competitive in the present market.
- The paper describe different types & techniques (Crystalline,Thin film,III–V single and multi-junction ) of PV modules.
- It focuses of the application i.e: PV in agriculture, Medical refrigeration, PV in street lights, PV in buildings.
- It also describes a figure of merit for Photovoltaic thermal (PVT) systems.
In an effort to reduce the cost of conventional fin and tube photovoltaic thermal (PV/T) collectors a novel mathematical analysis was developed which determines the optimum absorber plate configuration having the least material content and thus cost, whilst maintaining high collection efficiency. The analysis was based on the “low-flow” concept whose advantages include: improved system performance, smaller pump (less expensive with lower power consumption), smaller diameter tubes requiring lower thickness and thus cost of insulation, less construction power and time for the optimum absorber configuration. From the optimization methodology developed it was found that very thin fins (typically 50 μm) and small tubes (of 1.65 mm inside diameter for the risers, in the header and riser arrangement and 4.83 mm for the serpentine arrangement), with a tube spacing of 62 mm and 64 mm (both corresponding to 97% fin efficiency) and a mass of 1.185 kg/m2 and 2.140 kg/m2, respectively, can be used. This optimum serpentine absorber plate contains 40.50% less material content and mass, as compared to the serpentine prototype proposed by others. In one such design a mass of 3.596 kg/m2 was used (with 10 mm diameter tubes, 95 mm tube spacing and 200 μm thick absorber). To predict the performance of the determined optimum configurations, a steady-state model (using the EES code) was developed. To validate the steady-state model two prototypes, one in Header and Riser and the other in Serpentine configuration, were built and tested. It was found from the experiments that there is a good agreement between the computational and the experimental results. Moreover, it was found that optimum PV/T configurations do indeed have thermal and electrical performance comparable to non-optimum ones of greater mass and cost.
- This paper applies this strategy, by optimizing the absorber plate material content of a conventional fin and tube PV/T collector, leading to a substantial reduction in material content and thus cost, whilst maintaining high collection efficiency (electrical and thermal).
- The purpose of this PV/T absorber plate is twofold. Firstly, to cool the PV module and thus improve its electrical performance and secondly to collect the thermal energy produced, which would have otherwise been lost as heat to the environment.
- From absorber plate design consideration, it is seen that by keeping the PV/T collector efficiency factor F′ constant, the useful collected heat Qu (or thermal efficiency) of the PV/T collector is also held constant.
- By using low collector flow rates (of about 2–4 g per square meter per second) would yield thermally stratified tanks and thus the calculated performance of solar hot water systems can be improved by as much as 38% as compared to a fully mixed tank and a high flow rate.
- Result shows that the thermal performance also reduces by about 7% with electricity production of the serpentine PV/T prototype. Moreover, the serpentine prototype thermal performance was found to be higher than the corresponding one of the header and riser prototype by about 4% in both with and without electricity production.
- A novel mathematical analysis was developed which finds the optimum absorber plate configuration having the least material content and thus cost, whilst maintaining high collection efficiency.
A combined optimisation concet for the design and operation strategy of hybrid-PV energy systems[edit | edit source]
Abstract: This paper presents a method to jointly determine the sizing and operation control of hybrid-PV systems. Hybrid energy systems use different energy sources such as solar and wind energy and diesel gensets. They are an economical option in areas remote from the grid. In this context the correct and cost-effective system sizing as well as efficient system operation are important. The problem becomes complicated through uncertain renewable energy supplies and load demand, non-linear characteristics of some components, and the fact that optimum operation strategies and optimum sizing of hybrid system components are interdependent. The outlined approach finds an optimum operation strategy for a hybrid system by carrying out a search through possible options for the system operation control. The search is conducted over some time period using estimated weather and demand data and long-term system component characteristics. The costing of the operating strategies is evaluated and component sizes are changed by the designed algorithm according to optimum search rules. As a result an optimum system configuration is chosen by the algorithm together with an optimum operation strategy for a given site and application requirement.
- With this method the interdependency of hybrid operation strategies and system sizing can be incorporated. Operation strategies are selected by searching through possible settings for the system operation control, considering the non-linear characteristics of some components. The operation control and sizing selection method is based on genetic optimization techniques.
Analytical expression for electrical efficiency of PV/T hybrid air collector[edit | edit source]
Abstract: The overall electrical efficiency of the photovoltaic (PV) module can be increased by reducing the temperature of the PV module by withdrawing the thermal energy associated with the PV module. In this communication an attempt has been made to develop analytical expression for electrical efficiency of PV module with and without flow as a function of climatic and design parameters. The four different configurations of PV modules are considered for the present study which are defined as; case A (Glass to glass PV module with duct), case B (Glass to glass PV module without duct), case C (Glass to tedlar PV module with duct), case D (Glass to tedlar PV module without duct). Further, experiments were carried out for all configurations under composite climate of New Delhi.
- It is found that the glass to glass PV modules with duct gives higher electrical efficiency as well as the higher outlet air temperature amongst the all four cases. The annual effect on electrical efficiency of glass to glass type PV module with and without duct is also evaluated. The annual average efficiency of glass to glass type PV module with and without duct is 10.41% and 9.75%, respectively.
Expanding photovoltaic penetration with residential distributed generation from hybrid solar photovoltaic and combined heat and power systems[edit | edit source]
Abstract: The recent development of small scale combined heat and power (CHP) systems has provided the opportunity for in-house power backup of residential-scale photovoltaic (PV) arrays. This paper investigates the potential of deploying a distributed network of PV + CHP hybrid systems in order to increase the PV penetration level in the U.S. The temporal distribution of solar flux, electrical and heating requirements for representative U.S. single family residences were analyzed and the results clearly show that hybridizing CHP with PV can enable additional PV deployment above what is possible with a conventional centralized electric generation system. The technical evolution of such PV + CHP hybrid systems was developed from the present (near market) technology through four generations, which enable high utilization rates of both PV-generated electricity and CHP-generated heat. A method to determine the maximum percent of PV-generated electricity on the grid without energy storage was derived and applied to an example area. The results show that a PV + CHP hybrid system not only has the potential to radically reduce energy waste in the status quo electrical and heating systems, but it also enables the share of solar PV to be expanded by about a factor of five.
- Here it shows that a PV + CHP hybrid system not only has the potential to radically reduce energy waste to 16% from the status quo of 65% for thermal electrical generation, but it also enables the share of solar PV to be expanded without the use of large amounts of storage technology.
Design of Novel Compound Fresnel Lens for High-Performance Photovoltaic Concentrator[edit | edit source]
Abstract: Author(s) present a new design of compound Fresnel-R concentrator which is composed of two lenses: a primary lens (Fresnel lens) that works by total internal reflection at outer sawteeth but refraction at inner sawteeth, and a ringed secondary lens that works by refraction. In contrast to previous Fresnel lens concentrators, this design increases the acceptance angle, improves the irradiance uniformity on the solar cell, and reduces the aspect ratio significantly. Meanwhile several sawteeth of the primary Fresnel lens can correspond to a same ring of secondary lens, which will efficiently lower the complexity of designing and manufacturing. Moreover, in order to reduce the influence of manufacturing tolerances and to increase the optical efficiency further, the central part of the bottom of the secondary lens which directly adhered to the solar cell is designed as a cone-shaped prism to collect the sunlight that does not reach the solar cell. Finally, Author(s) provide simulations and analyses of the design method an optical efficiency more than 80% and an aspect ratio smaller than 0.5 can be achieved.
- An effective way to reduce the cost is to cut down the amount of the semiconductor material by means of combination with concentrating optics.The Fresnel lens has been used as a concentrator in photovoltaic field.
- One of the purposes of our work is to design nonimaging Fresnel lenses used in concentrating photovoltaic systems (CPVs) with a high concentration factor but its aspect ratio maintains a relatively small value.
- Design of primary Fresnel lens and design of secondary lens of CPV system. The solar cell is adhered at the bottom of the secondary lens directly, making it simple to seal against moisture and prevent misalignment.
- The primary Fresnel lens has six TIR sawteeth and two refractive sawteeth.The secondary lens has three aspherical rings. The first three TIR sawteeth of primary Fresnel lens correspond to the first ring of secondary lens and the secondary three TIR sawteeth correspond to the secondary ring.
- Full internal reflection is the working principal
- Fresnel concentrator,the aspect ratio can be less than 0.5 and the optical efficiency of the optical system can be obtained more than 80%
Parametric analysis of a coupled photovoltaic/thermal concentrating solar collector for electricity generation[edit | edit source]
Abstract: The analysis of the combined efficiencies in a coupled photovoltaic (PV)/thermal concentrating solar collector are presented based on a coupled electrical/thermal model. The calculations take into account the drop in efficiency that accompanies the operation of PV cells at elevated temperatures along with a detailed analysis of the thermal system including losses. An iterative numerical scheme is described that involves a coupled electrothermal simulation of the solar energy conversion process. In the proposed configuration losses in the PV cell due to reduced efficiencies at elevated temperatures and the incident solar energy below the PV bandgap are both harnessed as heat. This thermal energy is then used to drive a thermodynamic power cycle. The simulations show that it is possible to optimize the overall efficiency of the system by variation in key factors such as the solar concentration factor, the band gap of the PV material, and the system thermal design configuration, leading to a maximum combined efficiency of ∼ 32.3% for solar concentrations between 10–50 and a band-gap around 1.5–2.0 eV.
- This work extends the concept of a hybrid PV/thermal system at high concentration ratios and temperatures by creating a coupled electrothermal model of the entire system.
- Electrical model of a PV cell inclusion of concentrated solar irradiance and coupling to a detailed heat transfer model.
- The overall energy balance relies on the PV efficiency requiring a coupled iterative approach with the PV modeling equations of thermal & electrical parts.
- Performance analysis depending of band-gap, mass-flow rate, concentration ration.
- Following values for the empirical parameters in this calculation following the discussion in K=0.05, m=1.02, n=0.98, and A=1.
Analysis of Potential Conversion Efficiency of a Solar Hybrid System With High-Temperature Stage[edit | edit source]
Abstract: The analysis is given of hybrid system of solar energy conversion having a stage operating at high temperature. The system contains a radiation concentrator, a photovoltaic solar cell, and a thermal generator, which could be thermoelectric one or a heat engine. Two options are discussed, one (a) with concentration of the whole solar radiation on the PV cell working at high temperature and coupled to the high-temperature stage, and another (b) with a special PV cell construction, which allows the use of the part of solar spectrum not absorbed in the semiconductor material of the cell ("thermal energy") to drive the high-temperature stage while the cell is working at ambient temperature. The possibilities of using different semiconductor materials are analyzed. It is shown that the demands to the cell material are different in the two cases examined: in system (a) with high temperature of cell operation, the materials providing minimum temperature dependence of the conversion efficiency are necessary, for another system (b) the materials with the larger band gap are profitable. The efficiency of thermal generator is assumed to be proportional to that of the Carnot engine. The optical and thermal energy losses are taken into account, including the losses by convection and radiation in the high-temperature stage. The radiation losses impose restrictions upon the working temperature of the thermal generator in the system (b), thus defining the highest possible concentration ratio. The calculations made show that the hybrid system proposed could be both efficient and practical, promising the total conversion efficiency around 25–30 % for system (a), and 30–40 % for system (b).
- It is a two-stage hybrid converter including solar cell with optical or thermal concentrator combined with a heat-toelectric/ mechanic energy converter, which is denoted as thermalgenerator (TG).
- The general theoretical analysis of the system which is actually a coupled thermal and photovoltaic converter was performed where the thermal converter was assumed to be the Carnot engine, and the photovoltaic converter having the maximum ideal efficiency not depending on temperature which, of course, is not realistic.
- It can be said that around 80% of concentrated solar radiation energy will be transformed to heat within a cell, and may be used for a heat-to-electric energy conversion by the second stage of a hybrid system—a TG.
- The system is proportional to that of the Carnot engine, and the coefficient K<1 shows how close the TG efficiency to the ideal one is.
- Efficiency estimation the values of K=0.5 or 0.6 for the Carnot cycle can be used. On the other hand, the second stage can be a real heat engine which has the efficiency close to that of Carnot cycle, with parameter K of the same order. The mechanical energy produced by the heat engine could be converted into electricity with effectivity of 90–95%, so the overall efficiency can be characterized by that of the Carnot cycle.
Comparative Study on Hybrid PV/T Heat Pump Systems Using Different PV Panels[edit | edit source]
Abstract: Many studies have found that the photovoltaic (PV) cell temperature plays an important impact on the solar-to-electricity conversion efficiency. Different cooling liquids like air and water have been introduced to pass across the PVs to reduce the cell temperature, and thus increase the electrical efficiency. In this paper, the refrigerant R134a is used as the cooling liquid and the PV/thermal (PV/T) panel is coupled with a heat pump system acting as the evaporator, which is expected to achieve a better cooling effect and energy performance due to its low boiling temperature. Two different kinds of PV/T panels, glass vacuum tube (GVT) type and flat plate (FP) type, are proposed for the study on the energy performance comparison. The results show that the GVT PV/T panel has an average thermal efficiency of 0.775 and an average electrical efficiency of 0.089 (based on the reference efficiency of 0.12), which is 73.4% and 1.1% higher than that of the FP PV/T panel respectively, with the solar radiation varying from 200 W/m2 to 1000 W/m2. The GVT PV/T heat pump system has an average COP of 5.6, 9.8% higher the FP PV/T heat pump system. The GVT PV/T heat pump system has a better energy performance than the FP PV/T heat pump system.
- The performance comparison of the GVT and FP PV/T heat pump systems is carried out in this study. Result follows.
- The thermal efficiency increases by 0.02 for the GVT PV/T panel and 0.03 for the FP PV/T panel with the increase in solar radiation by 100 W/m2. The GVT PV/T panel has an average thermal efficiency of 0.775, 73.4% higher than that of the FP PV/T panel of 0.447.
- The electrical efficiency decreases by 0.004 for the GVT PV/T panel and 0.005 for the FP PV/T panel with the increase in solar radiation by 100 W/m2. The GVT PV/T panel has an average electrical efficiency of 0.089, slightly higher than that of the FP PV/T panel of 0.088.
Abstract: Hybrid photovoltaic/thermal (PV/T) solar systems can simultaneously provide electricity and heat, achieving a higher conversion rate of the absorbed solar radiation than standard PV modules. When properly designed, PV/T systems can extract heat from PV modules, heating water or air to reduce the operating temperature of the PV modules and keep the electrical efficiency at a sufficient level. In this paper, Author(s) present TRNSYS simulation results for hybrid PV/T solar systems for domestic hot water applications both passive (thermosyphonic) and active. Prototype models made from polycrystalline silicon (pc-Si) and amorphous silicon (a-Si) PV module types combined with water heat extraction units were tested with respect to their electrical and thermal efficiencies, and their performance characteristics were evaluated. The TRNSYS simulation results are based on these PV/T systems and were performed for three locations at different latitudes, Nicosia (35°), Athens (38°) and Madison (43°). In this study, Author(s) considered a domestic thermosyphonic system and a larger active system suitable for a block of flats or for small office buildings. The results show that a considerable amount of thermal and electrical energy is produced by the PV/T systems, and the economic viability of the systems is improved. Thus, the PVs have better chances of success especially when both electricity and hot water is required as in domestic applications.
- It shows that the electrical production of the system employing polycrystalline solar cells is more than that employing the amorphous ones, but the solar thermal contribution is slightly lower. A non-hybrid PV system produces about 38% more electrical energy, but the present system covers also, depending on the location, a large percentage of the hot water needs of the buildings considered. The derived TRNSYS results give an account of the energy and cost benefits of the studied PV/T systems with thermosyphon and forced water flow.
Abstract: Approximately 90 per cent of the world's power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30–40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1, the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here Author(s) report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20–300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.
- Efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature
- Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.
- The main advantage of using Si nanowires for thermoelectric applications lies in the large difference in mean free path lengths between electrons and phonons at room temperature
- It is possible to achieve ZT= 0.6 at room temperature in rough Si nanowires of 50nm diameter that were processed by a wafer-scale manufacturing technique
- Achieving broadband impedance of phonon transport, we have demonstrated that the EE Si nanowire system is capable of approaching the limits of minimum lattice thermal conductivity in Si.
Comparative Study on Exergy Efficiency of Solar Photovoltaic/Thermal (PV/T) System[edit | edit source]
Abstract: A brief comparative study is presented regarding different exergy efficiency correlations of solar photovoltaic/thermal (PV/T) system that have been proposed in the literature. Performance evaluation results of a certain PV/T hybrid system are found to be inconsistent with each other when using different correlations of exergy efficiency. It is mainly due to that no consensus has been reached in the calculation of thermal exergy, solar radiation exergy and other causal factors. Also, the issues that need to be further investigated are briefly discussed. This study will be beneficial to the design and performance assessment of PV/T hybrid system.
- Through the electricity and heat co-generation, the overall output is higher for a given PV/T collector than the outputs of two separated PV and solar thermal systems placed side-by-side.
- Unlike PV system, PV/T system uses the thermal energy available on the PV panel, the thermal energy gain can be utilized as useful energy and hence, the desirable exergy of PV/T system becomes the sum of the electrical exergy and thermal exergy
- Mathematical model of exergy analysis.
- Exergy has been analyzed for both natural & force cooling system
- At low solar radiation intensity, they show that the exergy efficiency of PV/T system equals the electrical efficiency of the reference conditions.
- Exergy annalysis by considering the temperature difference between the cell and ambient on the premise that there is no heat loss gives higher value
Performance evaluation of low concentrating photovoltaic/thermal systems: A case study from Sweden[edit | edit source]
Abstract: Some of the main bottlenecks for the development and commercialization of photovoltaic/thermal hybrids are the lack of an internationally recognized standard testing procedure as well as a method to compare different hybrids with each other and with conventional alternatives. A complete methodology to characterize, simulate and evaluate concentrating photovoltaic/thermal hybrids has been proposed and exemplified in a particular case study. By using the suggested testing method, the hybrid parameters were experimentally determined. These were used in a validated simulation model that estimates the hybrid outputs in different geographic locations. Furthermore, the method includes a comparison of the hybrid performance with conventional collectors and photovoltaic modules working side-by-side. The measurements show that the hybrid electrical efficiency is 6.4% while the optical efficiency is 0.45 and the U-value 1.9 W/m2 °C. These values are poor when compared with the parameters of standard PV modules and flat plate collectors. Also, the beam irradiation incident on a north–south axis tracking surface is 20–40% lower than the global irradiation incident on a fixed surface at optimal tilt. There is margin of improvement for the studied hybrid but this combination makes it difficult for concentrating hybrids to compete with conventional PV modules and flat plate collectors.
- Propose a testing method to characterize concentrating photovoltaic/thermal hybrids.
- Suggest a series of simulations and performance analysis for different latitudes based on the results from the testing method.
- Compare the hybrid performance with conventional PV modules and solar collectors.
- Electrical & thermal performance of of PV/T hybrid model
- Two hybrid areas were defined: total glazed area and active glazed area
Abstract: PV modules show temperature increase during their operation due to the absorption of solar radiation, as most of it is converted into heat and not into electricity. Hybrid Photovoltaic/Thermal (PV/T) solar systems combine a simultaneous conversion of solar radiation in electricity and heat. These devices consist of PV modules and heat extraction units mounted together, by which a circulating fluid of lower temperature than that of PV modules is heated by cooling them. An extensive study on water and air cooled PV/T solar systems has been conducted at the University of Patras, where hybrid prototypes have been experimentally studied. The water cooled PV/T systems consist of metallic heat exchanger placed at PV module rear surface, by which water circulating through pipes is heated. The methodology of Life Cycle Assessment (LCA) has been used to do an energetic and environmental assessment of the heat recovery system. The goal of this study, carried out at the University of Rome “ La Sapienza”, was to verify the benefits of heat recovery, implemented by a specific software for LCA, SimaPro 5.0. In this work Author(s) present the design, performance and aspects of improved PV/T systems based on the LCA results, giving guidelines for their application.
- Natural or forced air circulation is a simple and low-cost method to remove heat from PV modules, but it is less effective if the ambient air temperature is over 20*C
- PV/T systems with and without glazing cover are presented, suggesting also the concept of using a stationary diffuse reflector, instead of the specular reflector, to increase the total energy output.
- Description of PV/T module with reflection & its application
- To increase the system operating temperature, an additional glazing cover is necessary (like that of the usual solar thermal collectors), but it has as result the decrease of the PV module electrical output from the additional absorption of the solar radiation
[A dynamic model of hybrid photovoltaic/thermal panel][edit | edit source]
Abstract: In this paper, a dynamic simulation model of a photovoltaic and water heating system (PV/T) is developed. The model consists of a set of mathematical equations governing the main components of the system; namely: transparent cover, solar cell, absorber plate, operating fluid and storage tank. The model is based on the analysis of the energy balance which includes the photo electric conversion and the thermal conduction, convection and radiation. The model gathers all components equations so as to reflect the electrical and thermal behaviour of the PV/T system. It delivers the state equation of the system function of the climatic parameters and the fluid flow rate. The investigation of the effect of water mass flow rate through the collector on PV/T outputs have been carried out.
- A hybrid system, the PV/T collector can simultaneously produce thermal and electric energy.
- Utilization of a copolymer for the total design of the solar collector has numerous advantages as reducing the weight, facilitating the manufacturing, and reducing the cost.
- The hybrid photovoltaic thermal system is basically constructed by pasting photovoltaic solar cells directly over the absorber plate of the solar collector in a conventional forced circulation type solar water heater.
- The energy and fluid flow equations are developed on the basis of the four nodes. All sub-parts in each node are considered lumping together in proportion to give the average properties of the representing major component.
- This paper shows details mathematical model for hybrid photovoltaic/thermal panel and corresponding theoretical output.
Abstract: This paper presents a general energetic and entropic analysis of ideal photovoltaic and solar-thermal converters. Its purpose is to determine the efficiency limit when both types of converters operate together (hybrid converters). It has been found that, while in practical cases hybrid converters may give very high efficiency (61.7% vs. 40.0% of the solar thermal at 500 K and 40.7% of the photovoltaic at 300 K), in the limiting case of a system formed by an infinite number of band gaps, the efficiency of hybrid converters, 86.8%, is strictly equivalent to photovoltaic or solar thermal converters. Conversely, hybrid systems operating with one gap give efficiency of 86.7%, very close to the previous one and higher than the top efficiency achievable with a single temperature solar thermal, 85.4%.
- Theoretical analysis of maximum efficiency of a PV cell
- In a hybrid system, the escaping radiation will be a matter-coupled radiation at a temperature above the ambient and in some cases will be characterized by a zero chemical potential at some wavelengths and by a non-zero chemical potential at others.
- Low pass hybrid converter, the receiver only absorbs photons with energy above the bandgap of the solar cell, leaving the rest of the photons available for further use, for example, in a second solar cell located beneath.
- In case of Opaque hybrid converter, occurs when the photons below the semiconductor bandgap are also absorbed in a perfect sub-band absorber located beneath the cell. This absorber is at the cell temperature and the heat produced is converted reversibly into work in the thermal engine.
Band-Gap Tuned Direct Absorption for a Hybrid Concentrating Solar Photovoltaic/Thermal System[edit | edit source]
Abstract: Two methods often proposed for harnessing renewable energy, photovoltaics and solar thermal, both utilize the power of the sun. Each of these systems independently presents unique engineering challenges but when coupled together the challenge intensifies due to competing operating requirements. Recent research has demonstrated these hybrid systems for low-temperature applications but there exists limited studies at higher concentration ratios and thus higher temperatures. What these studies have shown is that keeping the photovoltaic (PV) cell temperature low keeps the overall system efficiency relatively high but results in low efficiencies from the thermal system. This study presents a unique design strategy for a hybrid PV/thermal system that only has mild thermal coupling which can lead to enhanced efficiency. By creating a fluid filter that absorbs energy directly in the fluid below the band-gap and a PV cell with an active cooling strategy combined efficiencies greater than 38% can be achieved.
- Thermally decouple the photovoltaic (PV) cell from the heat transfer fluid (HTF)
- The approach to harvesting solar energy is through the use of photovoltaics which directly converts solar energy, specifically solar energy above the band-gap of the solar cell, into electricity.
- In PV systems incoming solar flux below the band-gap and various loss mechanisms for energy above the bandgap result in heat generation, increasing the temperature of the cell and decreasing the cell efficiency.
- Concentrating systems utilize a form of focusing optics, often a Fresnel lens or mirrors, to focus incoming sunlight onto the cell. These systems generate much higher temperatures in the system resulting in excess thermal energy but less electrical energy.
- Two main approaches exist for this method. The first, utilize optical techniques that actually split the incoming irradiance into separate beams of different spectrums which are then directed either to the PV cell or the thermal absorber. The second, approach utilizes an absorption-based filter that is highly absorptive in the thermal regions of interest while transmitting as much light as possible.
- The thermal system and the PV system are only lightly coupled thermally since they are not in direct contact as is typically done in conventional hybrid collectors.
- Result shows that using a coupled thermo-electric model demonstrated that it is possible to increase the efficiency of a hybrid system through utilizing the HTF to remove excess heat as well as by small increases in the fluid absorptance which provided a decrease in PV cell temperature while having limited impact on the thermal efficiency.
Abstract: Hybrid conversion of solar radiation implies simultaneous solar radiation conversion into thermal and electrical energy in the PV/Thermal collector. In order to get more thermal and electrical energy, flat solar radiation reflectors have been mounted on PV/T collector. To obtain higher solar radiation intensity on PV/T collector, the position of reflectors has been changed and the optimal position of reflectors has been determined by both experimental measurements and numerical calculation so as to obtain maximal concentration of solar radiation intensity. The calculated values have been found to be in good agreement with the measured ones, both yielding the optimal position of the flat reflector to be the lowest (5°) in December and the highest (38°) in June. In this paper, the thermal and electrical efficiency of PV/T collector without reflectors and with reflectors in optimal position have been calculated. Using these results, the total efficiency and energy-saving efficiency of PV/T collector have been determined. The energy-saving efficiency for PV/T collectors without reflectors is 60.1%, which is above the conventional solar thermal collector, whereas the energy-saving efficiency for PV/T collector with reflectors in optimal position is 46.7%, which is almost equal to the values for the conventional solar thermal collector. Though the energy-saving efficiency of PV/T collector decreases slightly with the solar radiation intensity concentration factor, i.e. the thermal and electrical efficiency of PV/T collector with reflectors are lower than those of PV/T collector without reflectors, the total thermal and electrical energy generated by PV/T collector with reflectors in optimal position are significantly higher than total thermal and electrical energy generated by PV/T collector without reflectors.
- Optimal design & inclination angle of reflector for PV/T module
- Comparisons were made between the performances of the two types of combined photovoltaic thermalcollectors, and the results showed that the double-pass photovoltaic thermal collector has superior performance in relation to the single-pass PV/T collector.
- They considered four different configurations of two types of PV modules and carried out experiments for all configurations under composite climate of New Delhi. Agrawal and Tiwari analyzed energy and exergy of building integrated photovoltaic thermal (BIPVT) systems under cold climatic condition in India. This system was used as the roof top of a building to generate higher electrical energy per unit area and to produce necessary thermal energy required for space heating.
- An increase in the power output and in hot water can be obtained. Garg et al. studied the effect of plane booster reflectors on the performance of a solar air heater with solar cells suitable for a solar dryer.
- In order to obtain the highest solar radiation intensity on PV/T collector, position of the bottom and upper reflectors have been changed and optimal positions of reflectors have been determined.
- optimal positions for the upper and bottom reflector are at the angles of 0 degree and 36 degree, respectively. In this study, measurements have been done in the period February-October 2008 for different days.
Design, fabrication and performance evaluation of a hybrid photovoltaic thermal (PVT) double slope active solar still[edit | edit source]
Abstract: A modified photovoltaic thermal (PVT) double slope active solar still was designed and fabricated for remote locations. The system has been installed at the campus of KIET, Ghaziabad (India) and its performance has been experimentally evaluated under field conditions in natural and forced circulation mode (series and parallel). Photovoltaic operated DC water pump has been used between solar still and photovoltaic (PV) integrated flat plate collector to re-circulate the water through the collectors and transfer it to the solar still. The production rate has been accelerated to 1.4 times than the single slope hybrid (PVT) active solar still and obtained highest (7.54 kg/day) for the parallel configuration in forced mode in the month of October, 2010. The daily average energy efficiency of the solar still is obtained as 17.4%. Comparative results have been predicted on annual basis with the single slope hybrid (PVT) active solar still accounting 250, 275 and 300 clear days in a year. Author(s) have found that energy payback time is significantly reduced by almost 30% in present design with less capital investment.
- In this paper, a new simple design of hybrid photovoltaic thermal (PVT) double slope active solar still has been fabricated and its performance is evaluated in field conditions.
- Potable water can be produced at reasonable cost by solar stills.
- The concept behind the hybrid is that a solar cell converts solar radiation to electrical energy with peak efficiency in the range of 9–12%, depending on specific solarcell type and thermal energy dissipated for air or water heating
- The productivity of the solar still can be improved by increasing the temperature of water in the solar still as one of the parameters.The main objective of the work is, to enhance the productivity of the double slope solar still to provide distilled water for isolated communities, facing electricity problems and good quality of water for commercial use.
Abstract: Author(s) present test results on hybrid solar systems, consisting of photovoltaic modules and thermal collectors (hybrid PV/T systems). The solar radiation increases the temperature of PV modules, resulting in a drop of their electrical efficiency. By proper circulation of a fluid with low inlet temperature, heat is extracted from the PV modules keeping the electrical efficiency at satisfactory values. The extracted thermal energy can be used in several ways, increasing the total energy output of the system. Hybrid PV/T systems can be applied mainly in buildings for the production of electricity and heat and are suitable for PV applications under high values of solar radiation and ambient temperature. Hybrid PV/T experimental models based on commercial PV modules of typical size are described and outdoor test results of the systems are presented and discussed. The results showed that PV cooling can increase the electrical efficiency of PV modules, increasing the total efficiency of the systems. Improvement of the system performance can be achieved by the use of an additional glazing to increase thermal output, a booster diffuse reflector to increase electrical and thermal output, or both, giving flexibility in system design.
- The electrical and thermal output of hybrid PV/T systems can be increased by using concen-trators of solar radiation of low concentrating ratio as proposed by Al-Baali.
- We include design considerations and experimental results from constructed and tested outdoors hybrid PV/T systems.
- In PV building installation at locations with high solar input and high ambient tempera-tures, liquid PV cooling can be onsidered as the most efficient mode for water preheating all year, most efficient mode for water preheating all year.
Abstract: The major purpose of the present study is to understand the performance of an integrated photovoltaic and thermal solar system (IPVTS) as compared to a conventional solar water heater and to demonstrate the idea of an IPVTS design. A commercial polycrystalline PV module is used for making a PV/T collector. The PV/T collector is used to build an IPVTS. The test results show that the solar PV/T collector made from a corrugated polycarbonate panel can obtain a good thermal efficiency. The present study introduces the concept of primary-energy saving efficiency for the evaluation of a PV/T system. The primary-energy saving efficiency of the present IPVTS exceeds 0.60. This is higher than for a pure solar hot water heater or a pure PV system. The characteristic daily efficiency ηs* reaches 0.38 which is about 76% of the value for a conventional solar hot water heater using glazed collectors (ηs*=0.50). The performance of a PV/T collector can be improved if the heat-collecting plate, the PV cells and the glass cover are directly packed together to form a glazed collector. The manufacturing cost of the PV/T collector and the system cost of the IPVTS can also be reduced. The present study shows that the idea of IPVTS is economically feasible too.
- Electric energy is a high-grade form of energy since it is converted from thermal energy.
- The heat-collecting plate adheres directly to the back of the commercial PV module.
- Increasing hot water temperature in order to meet some application requirements would in turn cause the power generation efficiency of solar PV to decrease.
Abstract: This work represents the second step of the development of a new concept of photovoltaic/thermal (PV/T) collector. This type of collector combines preheating of the air and the production of hot water in addition to the classical electrical function of the solar cells. The alternate positioning of the thermal solar collector section and the PV section permits the production of water at higher mean temperatures than most of existing hybrid collectors. These higher temperatures will allow the coupling of components such as solar cooling devices during the summer and obviously a direct domestic hot water (DHW) system without the need for additional auxiliary heating systems. In this paper, a simplified steady-state two-dimensional mathematical model of a PV/T bi-fluid (air and water) collector with a metal absorber is developed. Then, a parametric study (numerically and experimentally) is undertaken to determine the effect of various factors such as the water mass flow rate on the solar collector thermal performances. Finally, the results from an experimental test bench and the first simulation results obtained on full scale experiments are compared.
- The experiments provide the collection of weather data from a weather station and experimental values for various operating temperatures
- The advantage of this configuration is that the hot water production function is an option which can be added to the basic solar PV/T air collector, depending on the energy needs of the building.
- There is an air gap between the absorber and an insulation layer. It is in the rib which is originally used for the mechanical rigidity of the sheet steels that the hot water production section is positioned. This rib includes an insulation layer of polystyrene covered by a thin reflective layer as well as a water circulation pipe.
- A parametric study permitted the trends in the variation of the temperature of cells and the fluids as a function of water and air mass flow rates and the collector length to be determined.
Optimizing design of household scale hybrid solar photovoltaic + combined heat and power systems for Ontario[edit | edit source]
Abstract: This paper investigates the feasibility of implementing a hybrid solar photovoltaic (PV) + combined heat and power (CHP) and battery bank system for a residential application to generate reliable base load power to the grid in Ontario. Deploying PV on a large-scale has a penetration level threshold due to the inherent power supply intermittency associated with the solar resource. By creating a hybrid PV+CHP system there is potential of increasing the PV penetration level. One year of one second resolution pyranometer data is analyzed for Kingston Ontario to determine the total amount of PV energy generation potential, the rate of change of PV power generation due to intermittent cloud cover, and the daily CHP run time required to supply reliable base load power to the grid using this hybrid system. This analysis found that the vast majority of solar energy fluctuations are small in magnitude and the worst case energy fluctuation can be accommodated by relatively inexpensive and simple storage with conventional lead-acid batteries. For systems where the PV power rating is identical to the CHP unit, the CHP unit must run for more than twenty hours a day for the system to meet the base load requirement during the winter months. This provides a fortunate supply of heat, which can be used for the needed home heating. This paper provides analysis for a preliminary base line system.
- When the cost of PV electricity is equivalent to conventional grid electricity the PV penetration level is set only by technical limitations.
- This tilt angle also eliminates the complication of snow shadowing which can have a significant impact on yearly system performance.
- The frequency of varying magnitudes of solar energy change on a per second basis (dE/dt) was found by extracting histogram.
- The vast majority of the change in solar energy is small in magnitude and is associated with the natural daily cycle of solar energy and noise in the measurement system.
Abstract: Over the last 30 years, a large amount of research on PV-Thermal (PVT) collectors has been carried out. An overview of this research is presented, both in terms of an historic overview of research projects and in the form of a thematic overview, addressing the different research issues for PVT.
- The idea of an air collector that can run without access to the grid, with the additional benefit of having an irradiance-controlled mass flow.However research institutes and commercial companies have extended this idea to PVT-air collectors with PV over the entire absorber. However, this method also has some drawbacks. The thermal resistance between the PV laminate and the absorber may become too large for good thermal performance especially when air enclosure in the glue layer is significant, and the additional glueing step is not optimal for commercial manufacturing. Furthermore, the white tedlar rear that is generally used for c-Si modules, has relatively large reflection losses.
- Ventilation of BIPV: Whereas the initial question was how to cool the PV, this research naturally lead to the question how much heat was produced and how it could be applied.
- In the case of a solar thermal collector, a good efficiency requires a good solar absorption and a good heat transfer. Furthermore, the higher the required temperature level, the higher the required amount of insulation.
- The reduction in thermal efficiency is due to 4 effects: 1. the absorption factor of the PV-surface is lower than the absorption factor of a conventional collector surface due to reflections at the various layers in the PVlaminate; 2. the PV-surface is not spectrally selective, resulting in large thermal radiation losses; 3. the heat resistance between the absorbing surface and the heat transfer medium is increased due to additional layers of material. This implies a relatively hot surface of the PVT-panel, leading to additional heat losses and a small decrease in electrical performance and 4. the energy that is converted to electrical output is lost for the thermal output. However, as this effect is intended, it will not be discussed further.
- Five aspects have been found in the literature on the absorbance of PVT-collectors: 1. reducing reflection at the additional top cover (in case of a glazed module); 2. reducing reflection at the PVT-absorber top surface; 3. reducing reflection at the PV top grid; 4. increasing absorption in PV and rear contact and 5. increasing absorption in the opaque surface below the PV.
Performance analysis of photovoltaic-thermal collector by explicit dynamic model[edit | edit source]
Abstract: Although the performance of hybrid photovoltaic-thermal (PV/T) collector had been studied both experimentally and numerically for some years, the thermal models developed in previous studies were mostly steady-state models for predicting the annual yields. The operation of a PV/T collector is inherently dynamic. A steady-state model is not suitable for predicting working temperatures of the PV module and the heat-removal fluid during periods of fluctuating irradiance or intermittent fluid flow. Based on the control-volume finite-difference approach, an explicit dynamic model was developed for a single-glazed flat-plate water-heating PV/T collector. A transport delay fluid flow model was incorporated. The proposed model is suitable for dynamic system simulation applications. It allows detailed analysis of the transient energy flow across various collector components and captures the instantaneous energy outputs.
- The operation of a PV/T collector is inherently dynamic. The excitations like solar irradiance and wind are transient in nature.
- There will be no heat flow across this plane at any time under proper operation. The edges and bottom surface of the panel are inserted with thermal insulation. For a compact and thin panel design, the losses of the absorbed solar energy.
- Temperature gradient is treated separately with that in the transverse direction (Y -direction). In this way, the energy exchange across various components can then be handled by considering their mean temperatures.
Abstract: Long term evaluation of an asymmetric CPC PV-thermal hybrid built for high latitudes, MaReCo (MaximumReflectorCollector), is performed in Lund, lat 55.7°, and this paper discusses output estimates and characteristics of the system. The output estimates are calculated using the MINSUN simulation program. To get the input for MINSUN, measurements were performed on two MaReCo prototypes. These measurements show that the front reflector collects most of the irradiation in the summer, and the back reflector in the spring and fall. Two different reflector materials were used, anodized aluminium and aluminium laminated steel. The steel based reflector was selected for its rigidness. The output estimates show no difference in yearly output between the two reflector materials, both back reflectors deliver 168 kW h/(m2 cell area) of electricity compared to 136 kW h/m2 cell area for cells without reflectors. The cells facing the front reflector deliver 205 kW h/(m2 cell area) of electricity. The estimated output of thermal energy was 145 kW h/(m2 glazed area) at 50 °C. The estimates show that the optimal placement of the photovoltaic cells is facing the front reflector, but having cells on both sides is in most cases the best option.
- The electricity generation is impeded by high temperatures, and cooling the cells actively with water is one efficient way to increase the yield.
- Two different MaReCo prototypes were characterized, MaReCo1 and MaReCo2.
- The cells were laminated onto an aluminium profile that was eloxidized to a dark colour to improve its heat absorbing properties
- In the mornings and evenings,the part of the absorber closest to the gables will be shaded.
- The reflectance of the steel based reflector is slightly lower in the wavelength interval where the solar cells operate.
- By visual inspection, there was a considerably larger number of imperfections in the aluminium reflector troughs, which shows the difference in rigidness between the steel reflector and the aluminium reflector.
Abstract: The utilization of solar energy can be made by photovoltaic (PV) cells to generate electric power directly and solar thermal (T) panels can be applied to generate heat power. When the utilization of the solar energy is necessary to generate electric power, the option of using T panels in combination with some heat / electric power conversion technology can be a viable solution. The power generated by utilizing the solar energy absorbed by a given area of solar panel can be increased if the two technologies, PV and T cells, are combined in such a way that the resulting unit will be capable of co-generation of heat and electric power. In the present paper combined Photovoltaic / Thermal panels are suggested to generate heat power to produce hot water, while the photovoltaic part is used to obtain electric power mainly for covering the electric power consumption of the system, to supply the electronic control units and to operate pump drives etc. Ac and dc supplies are provided by converters for covering self-consumption and possibly the need of some household appliances. The development and design of the system is made by extensive use of modeling and simulation techniques. In the paper a part of the simulation studies, carried out to determine the energy balance in the electric energy conversion section of the system and the control structure, assuming stand-alone operation is presented.
- The mechanical energy provided by the turbine is converted into electric energy and fed to the utility mains or a group of loads via an AC to AC converter.
- As both the PV cells and the batteries are expensive components. The energy balance in the long run is basically determined by the energy supplied by the PV cells and the energy consumed by the load. However, losses of the energy conversion and the charge controller have to be taken into account.
- The operation of the charge controller is based on a voltage regulation loop that keeps the battery voltage at the level determined by a battery voltage reference signal in case of surplus charge.
- The operation of the MPPT controller in this study is based on the principle of determining the derivative c = dP/dV of the solar cell.
Abstract: The present commercial photovoltaic solar cells (PV) converts solar energy into electricity with a relatively low efficiency, less than 20%. More than 80% of the absorbed solar energy is dumped to the surroundings again after photovoltaic conversion. Hybrid PV/T systems consist of PV modules coupled with the heat extraction devices. The PV/T collectors generate electric power and heat simultaneously. Stabilizing temperature of photovoltaic modules at low level is higly desirable to obtain efficiency increase. The total efficiency of 60–80% can be achieved with the whole PV/T system provided that the T system is operated near ambient temperature. The value of the low-T heat energy is typically much smaller than the value of the PV electricity. The PV/T systems can exist in many designs, but the most common models are with the use of water or air as a working fuid. Efficiency is the most valuable parameter for the economic analysis. It has substantial meaning in the case of installations with great nominal power, as air-cooled Building Integrated Photovoltaic Systems (BIPV). In this paper the performance analysis of a hybrid PV/T system is presented: an energetic analysis as well as an exergetic analysis. Exergy is always destroyed when a process involves a temperature change. This destruction is proportional to the entropy increase of the system together with its surroundings—the destroyed exergy has been called anergy. Exergy analysis identifies the location, the magnitude, and the sources of thermodynamic inefficiences in a system. This information, which cannot be provided by other means (e.g., an energy analysis), is very useful for the improvement and cost-effictiveness of the system. Calculations were carried out for the tested water-cooled ASE-100-DGL-SM Solarwatt module.
- Applications of solar energy can be broadly classified into two categories: thermal systems (T) that convert solar energy into thermal energy and photovoltaic systems (PV) that convert solar energy directly into electrical energy.
- To reduce the module temperature an air-cooling or water-cooling of a flat plate collector is used in a hybrid PV/T system
- In this configuration, the air at first enters the flow channel formed by the glass cover and the upper metallic collector, and then under it. In consequence, this flow arrangement effects greater heat removal from the top absorber plate and reduces the heat loss from the collector.
- It is impossible to obtain the maximum electric and thermal efficiency simultaneously—a kind of tradeoff is necessary. The most important parameters are the flow rate and the inlet temperature, since the cell temperature depends strongly on them
- Exergy analysis is used in the field of industrial ecology as a tool to both decrease the amount of exergy required for a process and use available exergy more efficiently.
Abstract: It is well known that efficiency of photovoltaic solar cells decreases with an increase of temperature, and cooling is necessary at high illumination conditions such as concentrated sunlight, or cosmic or tropical conditions. The purpose of present study was to investigate the opposite option: to make a cell work at relatively high temperature (around 100–200 °C) and use the excessive heat in a hybrid system of some kind to increase the total efficiency of solar energy utilization. Author(s) studied the temperature dependence of the solar cell parameters both theoretically and experimentally, for the basic cells with p–n junction and the Schottky barrier, taking account of the different carrier transport mechanisms and recombination parameters of the cell material. The possibility of usage of the concentrated sunlight was also taken into account. The experiments conducted in the temperature interval of 25–170 °C and the calculated data show a real possibility of construction of a two-stage solar-to-electric energy converter with high-temperature second stage, having the overall conversion efficiency of 30–40%.
- A PV solar cell operated at high temperature could be coupled with a heat engine which hot side temperature is determined by the PV cell, making a two-stage hybrid conversion system.
- One-dimensional theoretical models of p–n junction and Schottky barrier valid for ideal diode approximation is considered.
- Rapid short-circuit current increase with S while open-circuit voltage increases slowly under the same conditions. The efficiency η features almost linear increase; it reaches the value of 22.3% for 100-times concentrated sunlight, while the modest concentration S=10 already gives 20.3% efficiency yield.
- The efficiency of the cells with metal–semiconductor (MS) junction depends mainly on the height of potential barrier φb formed between the parts of the device.
- Current transport through MS interface was considered to fit thermal emission approximation for the case of crystalline silicon characterized with comparatively high carrier mobility values. Total device current was determined from the continuity equations under minority carrier drift current transport approximation.
- It follows from our theoretical consideration that the initial (room temperature) efficiency of both devices is practically the same; Schottky diodes are less sensitive to the deep level impurities than the p–n junction diode and have a smaller temperature dependence of the efficiency at equal conditions; the concentration of radiation leads to the decrease of the temperature dependence of cell’s efficiency.
Rural electrification with photovoltaic hybrid plants—state of the art and future trends[edit | edit source]
Abstract: This paper presents an economic approach for rural electrification with photovoltaics world-wide. It concentrates on the most suitable technologies for supplying single or multiple consumers via stand-alone systems and examines the perspectives for local grid formation. After considering the promising applications in rural areas, advantageous system configurations that can cope with the various requirements of decentralized electrification stand out clearly. Apart from the already well-established photovoltaic (PV) application in supplying power to minute isolated consumers far away from the grid, such as solar homes, stand-alone PV hybrid configurations for power needs up to a few 10 kW are now presenting a promising application.
The results of a comprehensive cost analysis comparing the most applicable supply configurations are presented in this paper in order to determine cost-effective solutions. The design options and the technical performance features of the various PV systems applied are discussed, thus covering important applications and different power ranges. Conventional systems of today, as well as advanced systems offering the potential for covering all fields of application for decentralized power generation in the future are highlighted. The recent developments presented concern the system design techniques, modern energy management systems as well as suitable monitoring, supervising and controlling for decentralized PV integration on a large scale.
- The paper concludes with a reference list of institutes working on systems technology/power conditioning and control of PV hybrid plants.
- PV-only and PV±diesel hybrid systems are at the beginning of the development cycle and considerable cost reductions are to be expected in the future.
- In order to cope with the power fluctuations and the resulting dynamic plant performance, DC busses have to be applied for coupling PV generators or other renewable energy converters to batteries where AC busses are used for coupling consumer loads. In spite of the achievable reliability, this design oers specialized hybrid plant solutions and does not allow decentralized systems configurations with utility-grid-compatible components.
- By adding an auxiliary generator system this unit can be regarded as a supply guarantee, whereas the PV generator and the battery can be designed to a very much smaller scale, as was done with the previous one.
- Obviously the electricity price will increase if the electric consumption becomes smaller than the design value.The plant is also able to provide more energy annually than it is designed for, but in this case with a reduced power supply guarantee. In contrast, the PV hybrid system appears not to be sensitive to power demand variations around its designed size, which is caused by the collaboration between the PV system and the gen-set.
Abstract: Solar energy is one of renewable energy sources which have potential for future energy applications. New technology developments in solar energy utilization are expected to result in the improvement of the photovoltaic performance with lower production cost. This will increase the demand and viability for commercial applications. The current popular technology converts solar energy into electricity and heat separately. The photovoltaic-thermal (PV/T) hybrid system is designed to generate thermal and electrical energy simultaneously. It is well known that using a hybrid system can eliminate the need for external source of electrical energy. This paper presents research and development activity being carried out at Solar Energy Research Institute (SERI), Universiti Kebangsaan Malaysia in order to realise the technology.
- PV modules can only provide electrical energy. By changing the preserve PV module with some specific modification, we can thus produce electrical energy and thermal energy with the new design of this PV/T.
- Based on the design, the efficiency of the electricity is reduced more than 50% when the PV plate is covered by the glass plate as in commercial module.
- The principle of water based PV/T is similar to the air based collector where cold water is used as a medium to absorb heat from the sun which is later can be used for low heat temperature processes.
- The open circuit voltage (Voc) is similar for different solar irradiance but the short circuit current (Isc) increase when solar irradiance increases. Increasing the flow rate will increase the heat transfer coefficient between the channel walls and the working fluid, resulting in a lower mean photovoltaic cells temperature. This will increase the electrical efficiencies of the collector.
Abstract: In this communication, an attempt has been made to develop a thermal model of an integrated photovoltaic and thermal solar (IPVTS) system developed by previous researchers. Based on energy balance of each component of IPVTS system, an analytical expression for the temperature of PV module and the water have been derived. Numerical computations have been carried out for climatic data and design parameters of an experimental IPVTS system. The simulations predict a daily thermal efficiency of around 58%, which is very close to the experimental value (61.3%) obtained by Huang et al.
- In this paper, a thermal model of an IPVTS system as proposed validated by their experimental results. The design parameters and climatic data of IPVTS system have been used for numerical computations.
- In the thermal section: i)heat capacity of PV/T system,solar cell material,tedlar and insulation have been neglected. ii)One-dimensional (1D) heat conduction has been considered for the present study. iii)The transmissivity of EVA is approximately 100%.iv)A mean temperature is assumed across each layer. v)Water flow between the tedlar and the insulation material is uniform for forced convection.vi)The system is in quasi-steady state.
- Similarly heat transfer between solar cell & tedlar, tedlar & insulator , and also energy balance for the storage tank were modeled.
- Result shows that there is fair agreement between experimental value of cell temperature, Tc(exp) and the theoretical value, Tc(th). The correlation coefficient and root mean square deviation are found to be 0.98% and 7.22% respectively.
- Hourly variations of theoretical and experimental water temperatures in the storage tank shows that theoretical value is higher than the experimental value, the correlation coefficient and root mean square percent deviation are 0.99% and 5.87% respectively.
- The effect of mass flow rate on the hourly variation of water temperature shows that the flow rate has only a small effect on the hourly variation of water temperature over the range of flow rates considered. Hence one can conclude that the optimum mass flow rate lies between 0.005 and 0.075 kg/s.
- The water temperature increases with increasing length as expected. It is also important to note that there is only marginal increase in water temperature for lengths greater than 4 m. Hence the optimum length of the PV module is 4 m for the present set of design and climatic parameters.
- Similarly result indicates that the optimum value of water mass is about 60 kg for present set of design and climatic parameters, depending upon requirement of water temperature.
Performance analysis of a hybrid photovoltaic/thermal (PV/T) collector with integrated CPC troughs[edit | edit source]
Abstract: In the present investigation a theoretical analysis has been presented for the modelling of thermal and electrical processes of a hybrid PV/T air heating collector coupled with a compound parabolic concentrator (CPC). In this design, several CPC troughs are combined in a single PV/T collector panel. The absorber of the hybrid PV/T collector under investigation consists of an array of solar cells for generation of electricity, while collector fluid circulating past the absorber provides useful thermal energy as in a conventional flat plate collector. In the analysis, it is assumed that solar cell efficiency can be represented by a linear decreasing function of its temperature. Energy balance equations have been developed for the various components of the system. Based on the developed analysis, both thermal and electrical performance of the system as a function of system design parameters are presented and discussed. Results have been presented to compare the performance of hybrid PV/T collector coupled with and without CPC.
- The major applications of solar energy include solar collectors and solar photovoltaic systems. Solar collectors are designed to generate thermal energy; however, photovoltaic cell produces electricity directly from solar energy.
- Based on detailed heat transfer analysis the energy balance equations have been developed for each component of the system. Thermal and electrical performance of the system as a function of system design parameters are presented and discussed.
- After a certain point the system with CPC performs better. It implies that the integration of a CPC with PV/T system is appropriate for the application in the higher temperature range. For both the configurations, with and without CPC, the system performs better in the case of selective absorber.
- It has been observed that both the thermal and electric output decrease with an increase in duct depth for configurations with and without CPC.
- It is seen that the system performance increases with an increase in collector length; however, the percentage increase in performance output decrease for the larger values of collector length. This suggests that an optimum value of collector length can be obtained for fixed values of system and design parameters.
- It has been seen that increasing mass flow rate increases the thermal and electrical output for both the configurations. Higher mass flow rate results in a lower temperature of the absorber plate.It is also quite evident that the performance output of the system with CPC is quite higher than without CPC.
- The effect of packing fraction of solar cells on the performance of the system indicates that increasing the area covered by solar cells increases electrical output of the system quite rapidly. However, the thermal output remains more or less same. The system coupled with CPC shows better performance in terms of both the thermal and electrical output.
Abstract: In order to solve the freezing problem associated with the traditional photovoltaic/thermal (PV/T) system, a novel heat pipe-type PV/T (HP-PV/T) system was designed and constructed in the present study. Outdoor tests were carried out from May to July, 2010. The performance of the system was also studied. The results showed that the average photothermal efficiency was 41.30% and the photoelectric efficiency was 9.42%. The average first law efficiency of the system was 48.52%, and the second law efficiency of 6.87%.
- PV cooling can increase the electrical efficiency of PV modules, increasing the total efficiency of the systems.
- Integration of a PV/T flat-plate collector and heat pipe were designed and constructed in this study.
- The photothermal efficiency had also a trend of increasing first and then decreasing, it was because the transmittance of glass increased with the solar altitude at the initial stage, so there was an efficiency increase at first; but the heat loss between the system and the ambient temperature also increased when the water temperature rose, thus decreasing efficiency.
- At the beginning water temperature was lower than the ambient temperature, so the exergy had a negative value, which was cold quantity exergy.
- Total efficiencies of the testing day show a trend of increasing at first and then decreasing gradually, as in the second law efficiency, the photoelectric exergy played a decisive role, so the second law efficiency showed a trend similar to that of the photoelectric efficiency.
- As in the second law efficiency, which is 6.87%, because the photoelectric exergy played a decisive role, the overall second law efficiency had a trend similar to that of photoelectric efficiency.
Annual analysis of heat pipe PV/T systems for domestic hot water and electricity production[edit | edit source]
Abstract: Heat-pipe photovoltaic/thermal (HP-PV/T) systems can simultaneously provide electrical and thermal energy. Compared with traditional water-type photovoltaic/thermal systems, HP-PV/T systems can be used in cold regions without being frozen with the aid of a carefully selected heat-pipe working fluid. The current research presents a detailed simulation model of the HP-PV/T system. Using this model, the annual electrical and thermal behavior of the HP-PV/T system used in three typical climate areas of China, namely, Hong Kong, Lhasa, and Beijing, are predicted and analyzed. Two HP-PV/T systems, with and without auxiliary heating equipment, are studied annually under four different kinds of hot-water load per unit collecting area (64.5, 77.4, 90.3, and 103.2 kg/m2).
- The HP-PV/T collector can be used in cold regions without freezing, and corrosion an be reduced as well.
- A low-iron tempered glass plate is used as the upper glaze for the collector, permitting sunlight passage but preventing thermal loss and the entry of dust particles and rain. A thermal insulation layer is placed behind the aluminum plate to prevent thermal loss.
- The water pump circulates the water between the water-storage tank and the collectors, such that the heat energy of the collectors is removed from the storage tank by the circulating water.
- Heat conduction along longitudinal direction of Al layer & the heat capacity the adhesive layer was neglected.
- HP-PV/T system with auxiliary heating equipment: when the daily solar energy is insufficient to cover the daily hot water load (in our simulation, a water temperature higher than 45 *C can be considered available), auxiliary energy is required to cover the hot water load.
- HP-PV/T system without auxiliary heating equipment: when a daily solar energy is insufficient to heat the water to reach the available temperature of 45 *C, the water is sequentially heated in the following day until the available temperature is reached.
- The system with small water storage capacity obtains less thermal energy than that with large water storage capacity.
Abstract: In this paper, a new approach is proposed to evaluate the effect of colors of light on the photovoltaic cell performance. Based on the energy and exergy analyses of a photovoltaic thermal (PV/T) system and by using the photonic theory, the energy available on the PV/T surface and the exergy of the PV/T system have been evaluated. A case study is conducted to experimentally validate the model by using solar radiation data for four different months, namely January, April, August and October for New Delhi, India. The results show that the present day PV technology is influenced by the red color of light. In other words, the energy available on the PV surface lies between the wavelengths of orange and red colors whereas the exergy of the system lies between yellow and green colors of light.
- A new approach to evaluate the exergetic performance of a solar cell based on photonic theory and compared it with the exergetic analysis using the fundamentals of second law efficiency and found some interesting results that the new photonic theory proposed is in good agreement with the exergy analysis based on thermodynamics.
- The energy of the measured incident solar radiation and the experimental exergy of the PV/T system are in accordance with the predicted energy and exergy levels of the different wavelengths of the visible spectrum by using photonic approach.
- Both, photonic and exergy-based methods give energy and exergy of the PV/T system close to each other. Hence, either can be used to evaluate the performance of photovoltaic systems.
- The energy of the solar radiation received on the PV surface has a fair agreement with the energy levels of red and orange colors of the visible spectrum. The specific color of the spectrum (that is, red) may influence the performance of the system as the present day PV technology works between the wavelengths of red color and/or infrared light.
- The exergy from the PV/T system has a fair agreement with the exergy level of yellow color in general and for higher thermal output it shifts towards the higher exergy levels that is, green (in August) and blue (in October) colors.
Thermal-photovoltaic solar hybrid system for efficient solar energy conversion Band-Gap Tuned Direct Absorption for a Hybrid Concentrating Solar Photovoltaic/Thermal System[edit | edit source]
Abstract: A hybrid solar system with high temperature stage is described. The system contains a radiation concentrator, a photovoltaic solar cell and a heat engine or thermoelectric generator. Two options are discussed, one with a special PV cell construction, which uses the heat energy from the part of solar spectrum not absorbed in the semiconductor material of the cell; the other with concentration of the whole solar radiation on the PV cell working at high temperature and coupled to the high temperature stage. The possibilities of using semiconductor materials with different band gap values are analyzed, as well as of the different thermoelectric materials. The calculations made show that the proposed hybrid system could be practical and efficient.
- Theoretical limits of photovoltaic conversion efficiency for a multi-junction cell predicts an efficiency of about 90%, but in practice not even half of that value has been obtained. On the other hand,the solar energy converters using a high temperature stage have promised very high efficiency but not proved to be practical yet.
- A non-traditional approach is developed based on the utilization of the ‘‘thermal part of the solar spectrum’’. This could be done in two ways: one, by separating of the long wave length part of the spectrum (not absorbed in a semiconductor material of the cell) with its subsequent concentration and further conversion using a heat engine or a thermoelectric generator, and the other, by operating the cell at elevated temperatures, and use a heat engine of some kind to utilize the excess heat.
- For the first case total efficiency is the sum of thermal & electrical efficiency. in the case of a semiconductor with Eg = 1.75 eV, approximately 50% of solar radiation corresponds to the condition hm > Eg, and is suitable for photovoltaic conversion, and the other 50%, with hm < Eg, could be used as thermal energy.
- For the second case the cell is subjected to concentrated sunlight, which usually enhances its efficiency; the thermal flux through the cell is directed into the HTS by direct thermal contact, thus the working temperature of the cell is equal to the Th parameter of the HTS. Although the lifetime of the PV cell in question needs special study.
- A solar radiation concentration of approximately 50 times which is sufficient to achieve the cell temperature higher than 450 K. efficiency is 14.3% for single junction (SJ) cell, and 17.8% for multi-junction (mj) cell. Thus, practically 80% of solar radiation will be transformed into heat within the cell, and may be used for a heat-to-electric/mechanic energy conversion by the second stage of the hybrid system—a HTS
- The efficiency of TEG can be near to 90%. It works as a Carnot cycle.
- For higher TEG efficiency (Z = figure of merit) proper thermoelectric materials should be chosen. It is necessary to point out that Z for semiconductors depends on temperature, and the different kinds of semiconductor materials should be selected for different operating temperatures.
Hydrogenated Amorphous Silicon PV an an Absorber Coating for Photovoltaic Thermal Systems [edit | edit source]
Abstract: Driven by the limitations of solar-optimized roof space and International Energy Association (IEA) Task 35, there is a renewed interest in photovoltaic solar thermal (PVT) hybrid systems. Current PVT systems focus on cooling the solar photovoltaic (PV) cells to improve the electrical performance. This however, causes the thermal component (T) to underperform. An exergetic study was completed comparing a PVT, PV + T and a PV only system in Detroit, Denver and Phoenix. It was found that the PVT system outperformed the PV + T system by 72% for each location and by 8, 8.6 and 9.9% for Detroit, Denver and Phoenix when compared to the PV only system. To further improve the PVT system, using hydrogenated amorphous silicon (a-Si:H) PV as the absorber layer of the solar thermal device was explored. The temperature coefficient and annealing properties of a-Si:H allow the thermal component to run more efficiently, while enabling the a-Si:H i-layers to be thicker resulting in more electricity production. It was found that running i-layer thicker cells (630nm and 840nm) stabilized at higher efficiencies at 90°C (potential PVT operating temperatures) than the thinner cell (420nm) by 2% and 0.5% respectively. In addition, spike annealing, which is a new concept of stagnating a PVT system to allow for the a-Si:H PV to anneal and return it to its original efficiencies was also investigated. It was found that over the lifetime of the system with the spike annealing occurring once a day 10.6% more electricity was produced than a system without stagnation.
Abstract: A theoretical analysis of novel PV/T solar collector is presented in this paper. The collector is made of vacuum tube-PV sandwich and the heat extraction from PV panel by the water passing through u-shape cooper tube of the collector results in the reduction of the PV cells’ working temperature. This also improves the electrical and thermal efficiencies of the PV cells. Based on energy balance of each parts of the vacuum-tube-PV sandwich, mathematical models are developed to evaluate the energy performance of the PV collector and analyze its affecting factors. The simulation results indicate that the thermal efficiency increases slightly while the electrical efficiency decreases slightly with the increasing radiation. Both the thermal and electrical efficiencies increase by 1.4% and 0.23% respectively with every 10 kg/h increase in water mass flow, and decrease by 3.8% and 0.6% respectively with every 10 ℃ increase in inlet water temperature.
Abstract: In this paper, Author(s) undertake a study to investigate the performance of hybrid photovoltaic thermal air collector systems through energy and exergy efficiencies and improvement potential factors and compare them for practical purposes. This will help identify the irreversibilities (exergy destructions) for performance improvement purposes. A case study is presented to highlight the importance of the efficiency modelings and compare them using some actual data. It is also aimed to find if there is room for improvement. It is found that the energy efficiency varies between 33 and 45% where as the variation in the exergy efficiency is from 11 to 16%, respectively. There is obviously a large scope for improvement in the existing system as about 11–16% of the exergy from the solar radiation is used.
Abstract: The work presented in this article aims to investigate a PV/T hybrid solar window on a system level. A PV/T hybrid is an absorber on which solar cells have been laminated. The solar window is a PV/T hybrid collector with tiltable insulated reflectors integrated into a window. It simultaneously replaces thermal collectors, PV-modules and sunshade. The building integration lowers the total price of the construction since the collector utilizes the frame and the glazing in the window. When it is placed in the window a complex interaction takes place. On the positive side is the reduction of the thermal losses due to the insulated reflectors. On the negative side is the blocking of solar radiation that would otherwise heat the building passively. This limits the performance of the solar window since a photon can only be used once. To investigate the sum of such complex interaction a system analysis has to be performed. In this paper results are presented from such a system analysis showing both benefits and problems with the product. The building system with individual solar energy components, i.e. solar collector and PV modules, of the same size as the solar window, uses 1100 kW h less auxiliary energy than the system with a solar window. However, the solar window system uses 600 kW h less auxiliary energy than a system with no solar collector.
Abstract: Flat plate photovoltaic/thermal (PV/T) solar collector produces both thermal energy and electricity simultaneously. This paper presents the state-of-the-art on flat plate PV/T collector classification, design and performance evaluation of water, air and combination of water and/or air based. This review also covers the future development of flat plate PV/T solar collector on building integrated photovoltaic (BIPV) and building integrated photovoltaic/thermal (BIPVT) applications. Different designs feature and performance of flat plate PV/T solar collectors have been compared and discussed. Future research and development (R&D) works have been elaborated. The tube and sheet design is the simplest and easiest to be manufactured, even though, the efficiency is 2% lower compared to other types of collectors such as, channel, free flow and two-absorber. It is clear from the review that for both air and water based PV/T solar collectors, the important key factors that influenced the efficiency of the system are the area where the collector covered, the number of passes and the gap between the absorber collector and solar cells. From the literature review, it is obvious that the flat plate PV/T solar collector is an alternative promising system for low-energy applications in residential, industrial and commercial buildings. Other possible areas for the future works of BIPVT are also mentioned.
Abstract: A novel heat-pipe photovoltaic/thermal system was designed and constructed by the authors. This system can simultaneously supply electrical and thermal energy. In addition, when compared with the traditional water-type photovoltaic/thermal system, this system can be used in cold regions without freezing. A dynamic model was developed to predict the performances of the heat-pipe photovoltaic/thermal system. Experiments were also conducted to validate results obtained for the simulation. A comparison between simulation values and experimental results demonstrated that the model was able to yield satisfactory predictions. Results indicated that the daily thermal and electrical efficiencies of the heat-pipe photovoltaic/thermal system were 41.9% and 9.4%, respectively, while the average heat and electrical gains were 276.9 and 62.3 W/m2, respectively. In addition, second-law efficiency, based on the second law of thermodynamics, is provided to analyze the total efficiency of the heat-pipe photovoltaic/thermal system, and the average total second-law efficiency of the system is 6.8%.
Abstract: In order to get more power and heat from PV/T system, it is necessary to cool the PV cell and decrease its temperature. This is not an easy task especially in hot and humid climate areas. There is a lack of an effective cooling strategy of PV/T panels. The liquid based photovoltaic thermal collector systems are practically more desirable and effective than air based systems. Temperature fluctuation in liquid based PV/T is much less than the air based PV/T collectors which subjected to variation in solar radiation levels. In this study a review of the available literature on PV/T collector systems which utilize water and refrigerant (working fluid) as heat removal medium for different applications has been conducted. Future direction of water-cooled and refrigerant hybrid photovoltaic thermal systems was presented. This study revealed that the direct expansion solar-assisted heat pump system achieved better cooling effect of the PV/T collector.
[A Review on Suitable Standards for Hybrid Photovoltaic∕Thermal Systems][edit | edit source]
Abstract: This paper will present an evaluation of the available standards and their considerations when using active‐cooled CPV systems, along with an initial assessment of the most appropriate tests, including additional test requirements, for hybrid Photovoltaic‐Thermal (PV‐T) systems in order to guarantee their long‐time electrical and thermal performance.
Performance evaluation of a solar photovoltaic thermal air collector using energy and exergy analysis[edit | edit source]
Abstract: In this article, a comparative study is carried out between two equations for the exergy efficiency of photovoltaic thermal (PV/T) air collectors; the first equation is based on net output exergy and the second equation is in terms of exergy losses. The exergy efficiency equation parametrically is dependent on thermal and electrical parameters of PV/T air collector; therefore, improved thermal and electrical models are used to calculate them. Developing an exergy balance for PV/T air collector system, the various exergy components in PV/T system are introduced and two equations for the exergy efficiency of PV/T air collector are derived. A computer simulation program is also developed which is based on the used improved thermal and electrical models. In order to validate the simulation results, a typical PV/T air collector has been built and some experiments have been carried out on it. The results of numerical simulation are in good agreement with the experimental results. Finally, parametric studies have been carried out and the effect of design and climatic parameters on two exergy efficiency equations has been investigated. It is observed that the improved exergy efficiency obtained in this paper is in good agreement with the one given by the previous literature and it is better because it shows the portion of each of exergy losses in the exergy efficiency equation, directly
Abstract: The supply of distant electric devices that cannot be connected to the public electricity grid for reasons of cost, waiting time or due to the need of local flexibility has been a major problem. To date, the power supply of such stand-alone systems has been based mainly on battery-buffered fossil-fueled motor-generators. Apart from the consumption of limited fossil fuel reserves, the disadvantages of these systems include the creation of noise and exhaust gases, the constant need to obtain fuel and, most important, the high amount of maintenance and repairs. For these reasons, and due to the progress in regenerative energy conversion made in the last decade, battery-buffered PV power systems are used more and more often. Their advantages are high reliability and low cost of repairs. However, far away from the equator, where solar radiation is very low during the winter, large PV generators are needed to guarantee sufficient reliability. Therefore, system costs are high. Another disadvantage is that the battery lifetime in PV power systems is significantly reduced compared to its lifetime in fossil fueled systems. To avoid these disadvantages, the PV generator can be combined with fossil fueled power generators. In the medium power range, from 10 W up to several hundred W, thermoelectric generators appear to be particularly qualified because of their reliability and lifetime. In this paper, a (so called) “photovoltaic hybrid system” is compared to a purely PV power system on the basis of model calculations starting with the solar radiation situation on the Earth's surface.
Abstract: This paper reports on the operational experience acquired with a photovoltaic (PV) hybrid system installed as a line extension alternative at a residence located in northern New York State. The system includes an 850 W PV array, 25 kWh worth of battery storage, and a 4 kW propane generator. The paper features a detailed analysis of the energy flows through the system and quantifies all losses caused by battery storage round-trip, rectifier and inverter conversions, and non-optimum operation of the generator and of the PV array. The paper also analyzes the evolution of end-use electricity consumption since the installation of the PV hybrid system.
Review of R&D progress and practical application of the solar photovoltaic/thermal (PV/T) technologies[edit | edit source]
Abstract: In this paper, the global market potential of solar thermal, photovoltaic (PV) and combined photovoltaic/thermal (PV/T) technologies in current time and near future was discussed. The concept of the PV/T and the theory behind the PV/T operation were briefly introduced, and standards for evaluating technical, economic and environmental performance of the PV/T systems were addressed. A comprehensive literature review into R&D works and practical application of the PV/T technology was illustrated and the review results were critically analysed in terms of PV/T type and research methodology used. The major features, current status, research focuses and existing difficulties/barriers related to the various types of PV/T were identified. The research methods, including theoretical analyses and computer simulation, experimental and combined experimental/theoretical investigation, demonstration and feasibility study, as well as economic and environmental analyses, applied into the PV/T technology were individually discussed, and the achievement and problems remaining in each research method category were described. Finally, opportunities for further work to carry on PV/T study were identified. The review research indicated that air/water-based PV/T systems are the commonly used technologies but their thermal removal effectiveness is lower. Refrigerant/heat-pipe-based PV/Ts, although still in research/laboratory stage, could achieve much higher solar conversion efficiencies over the air/water-based systems. However, these systems were found a few technical challenges in practice which require further resolutions. The review research suggested that further works could be undertaken to (1) develop new feasible, economic and energy efficient PV/T systems; (2) optimise the structural/geometrical configurations of the existing PV/T systems; (3) study long term dynamic performance of the PV/T systems; (4) demonstrate the PV/T systems in real buildings and conduct the feasibility study; and (5) carry on advanced economic and environmental analyses. This review research helps finding the questions remaining in PV/T technology, identify new research topics/directions to further improve the performance of the PV/T, remove the barriers in PV/T practical application, establish the standards/regulations related to PV/T design and installation, and promote its market penetration throughout the world.
Modeling and optimization of hybrid solar thermoelectric systems with thermosyphons[edit | edit source]
Abstract: Author(s) present the modeling and optimization of a new hybrid solar thermoelectric (HSTE) system which uses a thermosyphon to passively transfer heat to a bottoming cycle for various applications. A parabolic trough mirror concentrates solar energy onto a selective surface coated thermoelectric to produce electrical power. Meanwhile, a thermosyphon adjacent to the back side of the thermoelectric maintains the temperature of the cold junction and carries the remaining thermal energy to a bottoming cycle. Bismuth telluride, lead telluride, and silicon germanium thermoelectrics were studied with copper–water, stainless steel–mercury, and nickel–liquid potassium thermosyphon-working fluid combinations. An energy-based model of the HSTE system with a thermal resistance network was developed to determine overall performance. In addition, the HSTE system efficiency was investigated for temperatures of 300–1200 K, solar concentrations of 1–100 suns, and different thermosyphon and thermoelectric materials with a geometry resembling an evacuated tube solar collector. Optimizations of the HSTE show ideal system efficiencies as high as 52.6% can be achieved at solar concentrations of 100 suns and bottoming cycle temperatures of 776 K. For solar concentrations less than 4 suns, systems with thermosyphon wall thermal conductivities as low as 1.2 W/mK have comparable efficiencies to that of high conductivity material thermosyphons, i.e. copper, which suggests that lower cost materials including glass can be used. This work provides guidelines for the design, as well as the optimization and selection of thermoelectric and thermosyphon components for future high performance HSTE systems.
Abstract:The light-induced recovery in efficiency of amorphous silicon (a-Si) solar cells has been studied. The recovery of solar cells degraded by a concentrated light-soaking was accelerated under 1 sun illumination as compared with that in the dark. A similar phenomenon has been observed under current injection. The kinetics of light-induced annealing has been discussed on the basis of a series of the experiments.
Development and characterization of high-efficiency Ga0.5In0.5P/GaAs/Ge dual- and triple-junction solar cells[edit | edit source]
Abstract: This paper describes recent progress in the characterization, analysis, and development of high-efficiency, radiation-resistant Ga0.5In0.5P/GaAs/Ge dual-junction (DJ) and triple-junction (TJ) solar cells. DJ cells have rapidly transitioned from the laboratory to full-scale (325 kW/year) production at Spectrolab. Performance data for over 470000 large-area (26.94 cm2 ), thin (140 μm) DJ solar cells grown on low-cost, high-strength Ge substrates are shown. Advances in next-generation triple-junction Ga0.5In0.5P/GaAs/Ge cells with an active Ge component cell are discussed, giving efficiencies up to 26.7% (21.65-cm2 area), AM0, at 28°C. Final-to-initial power ratios P/P0 of 0.83 were measured for these n-on-p DJ and TJ cells after irradiation with 1015 1-MeV electrons/cm2 . Time-resolved photoluminescence measurements are applied to double heterostructures grown with semiconductor layers and interfaces relevant to these multijunction solar cells, to characterize surface and bulk recombination and guide further device improvements. Dual- and triple-junction Ga0.5In0.5P/GaAs/Ge cells are compared to competing space photovoltaic technologies, and found to offer 60-75% more end-of-life power than high-efficiency Si cells at a nominal array temperature of 60°C
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