Static concentrator photovoltaic module with prism array

To obtain cost-effective photovoltaic modules, we have developed static prism-array concentrator modules consisting of prism concentrators about 4 mm thick assembled unidirectionally under a 3.2-mm-thick cover glass. Calculating the optical collection efficiency for the annual solar irradiation in Tokyo, we found that the theoretical efficiency of the modules is 94.4% when the geometrical concentration ratio is 1.88 and that it is 89.1% when that ratio is 2.66, respectively. Fabricating prism-array-concentrator modules with a geometrical concentration ratio of 2.66, we obtained a maximum optical collection efficiency of 82% with a flat reflector and 81.7% with a V-grooved reflector.     

Performance improvement of a static concentrator module with an asymmetric v-groove backsheet structure

A light-trapping type concentrator module with a new asymmetric V-groove structure at the rear surface is proposed to improve the performance of static concentrator module. Fundamental optical properties of various asymmetric V-grooves are calculated using a ray-tracing method. Based on these results, yearly integrated irradiance ratios of the concentrator module to a conventional flat-plate module are calculated using meteorological data. By the use of Ag as a reflection material, yearly integrated irradiance ratio of concentrator module with an asymmetric V-groove is 1.34, and the occupation area of Si cells in a module can be reduced to 74% compared with a conventional flat-plate module.     

Development of bifacial PV cells for new applications of flat-plate modules

Bifacial cells have been developed for use in flat-plate static-concentrator modules and bifacial photovoltaic modules. All but one of the fabrication processes are conventional mass production processes, making it possible to produce these cells at low cost. By using 12.5×12.5 cm SOG wafers, bifacial cells with about 15% front- and 10.5% rear-illumination efficiencies have already been developed, and cells achieving higher efficiencies for both surfaces are being developed now. By applying these cells, a bifacial-cell-type flat-plate static-concentrator module showed 82% optical-collection efficiency for normal-incident light with a 2.0 concentration ratio. We have also applied bifacial cells for non-concentrating bifacial modules and developed a new application that reduces the limitations on PV module installation.     

Simulation and fabrication of flat-plate concentrator modules

A flat-plate concentrator (FPC) module has been proposed and developed for cost reduction of PV modules. The FPC has V-grooved rear reflectors on spaces between adjacent cells. To find appropriate design parameters, yearly optical performance is simulated by using meteorological data and introducing a performance index. As a result of the simulation, it is shown that the FPC module can reduce occupation area of solar cells to 75% compared with that of a conventional module, although its module area increases to 1.16 times. Several prototypes are fabricated and measured under a solar simulator. The measured results are somewhat lower than the simulated ones, but generally show the validity of the simulation.     

A comprehensive study of dense-array concentrator photovoltaic system using non-imaging planar concentrator

A special modeling method using Simulink has been developed to analyze the electrical performance of dense-array concentrator photovoltaic (CPV) system. To optimize the performance of CPV system, we have adopted computational modeling method to design the best configuration of dense-array layout specially tailored for flux distribution profile of solar concentrator. It is an expeditious, efficient and cost effective approach to optimize any dense-array configuration for any solar concentrator. A prototype of non-imaging planar concentrator (NIPC) was chosen in this study for verifying the effectiveness of this method. Mismatch effects in dense array solar cells caused by non-uniform irradiance as well as sun-tracking error normally happens at the peripheral of the array. It is a crucial drawback that affects the electrical performance of CPV systems because maximum output power of the array is considerably reduced when a current–voltage (I–V) curve has many mismatch steps and thus leads to lower fill factor (FF) and conversion efficiency. The modeling method is validated by assembling, installing and field testing on an optimized configuration of solar cells with the NIPC prototype to achieve a conversion efficiency of 34.18%. The measured results are in close agreement with simulated results with a less than 3% deviation in maximum output power.     

A concentrator module of spherical Si solar cell

Spherical Si solar cell, which is made up of Si spheres with a diameter of approximately 1.0 mm, is expected to be a promising candidate for low consumption of Si feedstock and simple process technology. This paper describes the formation process and the structure of a concentrator module in detail. The concentrator lens was formed by casting with ultraviolet light hardening resin. The concentration ratio was 4.4 times and the pitch between the spheres was 2.0 mm. By this module design, it was possible to realize a consumption of the Si feedstock of about 3.0 g/W. Conversion efficiencies of 11.3% from single-sphere cell, 8.5% from a 23-spheres module and 5.2% from a 105-spheres module under AM1.5, 100 mW/cm² illumination were achieved.     

Pilot production of concentrator silicon solar cells: Approaching industrialization

Using a simple process, high-efficiency silicon concentrator solar cells have proved to achieve up to 21% efficiency at 100×. The purpose of this work is to prove the feasibility of their industrialisation by setting up a pilot line and manufacturing a significant number of cells for a 100× concentrator system. The process has been successfully verified by modifying the antireflection coating, the annealing process and the back contact. This yielded an average efficiency of 18.5% at 100× with 70% of cells having an efficiency >18% and costs ranging from 0.31 to 0.41 €/W. A fast learning curve is shown which suggests optimistic results indeed for further industrialisation.     

Characterisation of a 300× photovoltaic concentrator system with one-axis tracking

A solar concentrator with one-axis tracking is being developed at our institute. This concentrator system achieves a high geometrical concentration ratio of 300 using a parabolic trough mirror and a three-dimensional second stage consisting of compound parabolic concentrators. The design of the system as well as the characterisation of the second stage is described in this paper.     

Enhanced heat dissipation of V-trough PV modules for better performance

A concentrator photovoltaic (PV) module, in which solar cells are integrated in V-troughs, is designed for better heat dissipation. All channels in the V-trough channels are made using thin single Al metal sheet to achieve better heat dissipation from the cells under concentration. Six PV module strips each containing single row of 6 mono-crystalline Si cells are fabricated and mounted in 6 V-trough channels to get concentrator V-trough PV module of 36 cells with maximum power point under standard test condition (STC) of 44.5 W. The V-trough walls are used for light concentration as well as heat dissipation from the cells which provides 4 times higher heat dissipation area than the case when V-trough walls are not used for cooling. The cell temperature in the V-trough module remains nearly same as that in a flat plate PV module, despite light concentration. The controlled temperature and increased current density in concentrator V-trough cells results in higher V_oc of the module.     

Excellent thermal stability of remote plasma-enhanced chemical vapour deposited silicon nitride films for the rear of screen-printed bifacial silicon solar cells

In this work the thermal stability of the electronic surface passivation of remote plasma-enhanced chemical vapour deposited (RPECVD) silicon nitride (SiN) films is investigated with the aim to establish a cost-effective screen-printing and firing-through-the-SiN process for bifacial silicon (Si) solar cells. As a key result, RPECVD SiN films provide an excellently thermally stable surface passivation quality if they feature a refractive index in the range between 2.0 and 2.2. After a short anneal above 850°C the surface recombination velocity on 1.5 Ωcm p-type float-zone (FZ) Si remains at a very low level of about 20 cm/s. First bifacial silicon solar cells with screen-printed rear contacts on 1.5 Ωcm p-type FZ Si yield a very promising rear efficiency of 13.4%.     

Influences of spectrum change to 3-junction concentrator cells

PV output of multi-junction cells is strongly influenced by spectrum change. The influence of Sun height was quantitatively analyzed, considering seasonally and daily changes of spectrum. The new model also considered the presence of clouds. The influence of daily random fluctuation of spectrum was shown averaged out in the integration of yearly PV output fluctuation. It was suggested that the overall mismatch loss by the change of Sun height and Sun orbital would be <4% for III–V multi-junction cells for concentration application, when the bandgap of each junction was well balanced. The sensitivity of spectrum fluctuation was shown enlarged with the discrepancy of current matching conditions.     

Extended description of tunnel junctions for distributed modeling of concentrator multi-junction solar cells

One of the key components of highly efficient multi-junction concentrator solar cells is the tunnel junction interconnection. In this paper, an improved 3D distributed model is presented that considers real operation regimes in a tunnel junction. This advanced model is able to accurately simulate the operation of the solar cell at high concentrations at which the photogenerated current surpasses the peak current of the tunnel junction. Simulations of dual-junction solar cells were carried out with the improved model to illustrate its capabilities and the results have been correlated with experimental data reported in the literature. These simulations show that, under certain circumstances, the solar cell's short circuit current may be slightly higher than the tunnel junction peak current without showing the characteristic dip in the J-V curve. This behavior is caused by the lateral current spreading toward dark regions, which occurs through the anode/p-barrier of the tunnel junction.     

Assessment of a low-cost gold-free metallization for III–V high concentrator solar cells

A gold-free metallization is proposed to be used as the grid contact in III–V concentrator solar cells. This metallization is based on the Cu/Ge system which has been reported to attain very low specific contact resistances on n-GaAs. In this letter, we show that metal layers with low resistivity (13 μΩ cm) can be obtained if the copper content in the alloy is around 28% in weight for a wide range of annealing temperatures (400–450 °C). Finally, this metallization has been used to manufacture single-junction GaAs high concentrator solar cells. Efficiencies of 26.2% at 1000 suns have been reached.     

Achievement of 27% efficient and 200 Wp concentrator module and the technological roadmap toward realization of more than 31% efficient modules

A 400× and 7056 cm² concentrator module was fabricated from 36 concentrator receivers, connected in series and with the same number of newly developed dome-shape Fresnel lenses. The averaged outdoor efficiency on a clear sky day was 26.8±1.5% (25C STC). This is the highest module efficiency achieved to date using a module of practical size and electrical rating. The heat was dissipated by the module wall and no heat sinks nor external cooling were used. A glass homogenizer was introduced to give uniform illumination to the square cell, and afforded a reasonable assembly tolerance, without the need for optical alignment.     

Comparison of 1D and 3D analysis of the front contact influence on GaAs concentrator solar cell performance

An extensive analysis of the front contact influence on concentrator GaAs solar cell performance has been carried out. The fill factor, open circuit voltage and efficiency have been calculated by varying the front contact specific resistance and the metal sheet resistance for 500X, 1000X and 2000X. An optimum front grid design has also been developed. The simulations have been carried out using a 3D model based on distributed circuit units, and by a classic lumped model, showing the need to use distributed models to achieve an accurate concentrator solar cell modeling as well as a precise front grid design.     

Solar distillation of water using a V-trough solar concentrator with a wick-type solar still

A V-trough solar concentrator has been combined with an inclined flat-plate wick-type solar still. Outdoor testing was carried out with and without the solar concentrator on clear days in summer and winter. The equipment was used to investigate the enhancement of the outdoor performance of the wick-type solar still by the solar concentrator.It has been concluded that use of the solar concentrator with the inclined wick-type solar still can lead to a greater fractional increase in still efficiency and productivity on clear days in winter than on clear days in summer.     

Development and experimental investigation of a compound parabolic concentrator

The numerical simulation and experimental validation of a compound parabolic concentrator (CPC) are presented. The solar device had an aperture area of 1.33 m², a real concentration ratio of 3.5, an acceptance half angle of 15°, and a carbon steel (or aluminum) tubular receiver with an outer diameter of 0.0603 m and coated with a commercial selective surface. Experimental tests were performed using water as working fluid at solar noon; the inlet temperatures used varied from 30 °C to 70 °C and the mass flow rates from 0.05 kg/s to 0.25 kg/s. A comparison of the experimental results with the numerical model developed was carried out. The results of the thermal efficiency, outlet temperature, and pressure drop were compared and found to be in close agreement with the experimental data. Therefore, the model is a reliable tool for the design and optimization of compound parabolic concentrators. Because the numerical model is based on the application of physical laws, it is possible to extrapolate its use with confidence to other fluids, mixtures, and operating conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

Design of grided Cu(In,Ga)Se2 thin-film PV modules

Results from modeling designs of Cu(In,Ga)Se₂ thin-film PV modules show that grided modules, at standard test conditions as well as at low-concentrated light, exhibit significantly improved performance when compared with conventional designs. It is further discussed that a grided design is advantageous from a synthesis and manufacturing point of view, since it provides higher front contact process tolerance and throughput as well as improved degrees of freedom of the module geometry.     

Optimum absorber temperature of a once-reflecting full conical concentrator of a low temperature differential Stirling engine

This paper provides a theoretical investigation on the optimum absorber temperature of a once-reflecting full conical concentrator for maximizing overall efficiency of a solar-powered low temperature differential Stirling engine. A mathematical model for the overall efficiency of the solar-powered Stirling engine is developed. The optimum absorber temperature for maximum overall efficiency for both limiting conditions of maximum possible engine efficiency and maximum possible engine power output is determined. The results indicated that the optimum absorber temperatures calculated from these two limiting cases are not significantly different. For a given concentrated solar intensity, the maximum overall efficiency characterized by the condition of maximum possible engine power output is very close to that of the real engine of 55% Carnot efficiency, approximately.     

Reliability assessment of silicone coated silicon concentrator solar cells by accelerated aging tests for immersing in de-ionized water

Direct de-ionized (DI) water immersion cooling has been verified to be an effective method for managing the operating temperature of silicon solar cells under concentration. However, the stable electrical performance was difficult to be achieved. The following investigation on mechanism indicated that galvanic corrosion occurred on cells. In this study, silicone coating was proposed to apply for the silicon concentrator (CPV) solar cells to eliminate or minimize the degradation when operated in DI water for a long time. The reliability of the selected silicone coatings and the silicone coated silicon CPV cells was assessed through designed accelerated aging tests, which include damp heat test, thermal cycling test and DI water immersion test. The selected silicone coatings exhibited excellent optical transparency. The tests results showed that no issues related with the selected silicone coatings’ reliability were observed. Some variations in the electrical performance of the silicone coated cells were detected, but the results gave confidence of the reliable performance of coated cells since they represented less than the total degradation allowed for the module under these tests. The simulation results indicated that temperature of the coated cell in DI water can be still maintained lower than that of conventionally encapsulated cell.