Combined photovoltaic solar thermal systems (PVT) literature review/The influence of operation temperature on the output properties of amorphous silicon-related solar cells

The influence of operation temperature on the output properties of amorphous silicon-related solar cells[1][1][1][1][1][1][1][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^[2][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[3][3][3][3][3][3][3][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. [3] 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^[4][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.

High-efficiency a-Si/c-Si heterojunction solar cell[5][5][5][5][5][5][5][edit | edit source]

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.

Photovoltaic thermal (PV/T) collectors: A review[6][6][6][6][6][6][6][edit | edit source]

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^[7][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[8][8][8][8][8][8][8][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.

Photovoltaic modules and their applications: A review on thermal modelling[9][9][9][9][9][9][9][edit | edit source]

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.

Optimization of the photovoltaic thermal (PV/T) collector absorber[10][10][10][10][10][10][10][edit | edit source]

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.