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.
- A. Nosrat and J. M. Pearce, "Dispatch strategy and model for hybrid photovoltaic and trigeneration power systems," Applied Energy, vol. 88, no. 9, pp. 3270-3276, Sep. 2011.
- C. T.T., "A review on photovoltaic/thermal hybrid solar technology," Applied Energy, vol. 87, no. 2, pp. 365-379, Feb. 2010.
- "Energy performance of water hybrid PV/T collectors applied to combisystems of Direct Solar Floor type", Volume 81, Issue 11, November 2007, Pages 1426–1438
- "Reversible conductivity changes in discharge-produced amorphous Silicon ", Appl. Phys.Lett. 31,292 (1977);doi;10.1063/1.89674
- "DEVELOPMENT IN UNDERSTANDING AND CONTROLLING THE STAEBLER-WRONSKI EFFECT IN a-Si:H ", Energy Conversion Devices, Inc., Troy, Michigan 48084
- "Intrinsic and light induced gap states in a-Si:H materials and solar cells—effects of microstructure ", Thin Solid Films 451 –452 (2004) 470–475
- "Modeling of light-induced degradation of amorphous silicon solar cells ", Volume 92, Issue 1, January 2008, Pages 50–60
- "Light-induced defect states in hydrogenated amorphous silicon centered around 1.0 and 1.2 eV from the conduction band edge", Appl. Phys. Lett. 83, 3725 (2003); doi: 10.1063/1.1624637
- "PERFORMANCE TEST OF AMORPHOUS SILICON MODULES IN DIFFERENT CLIMATES – YEAR THREE: HIGHER MINIMUM OPERATING TEMPERATURES LEAD TO HIGHER PERFORMANCE LEVELS"
- "The potential of solar industrial process heat applications", Volume 76, Issue 4, December 2003, Pages 337–361