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.     

Stability of nickel catalyst supported by mesoporous alumina for hydrogen iodide decomposition and hybrid decomposer development in sulfur–iodine hydrogen production cycle

We propose a hybrid HI decomposer, which combines a high-temperature catalytic decomposition reactor with a dual bed temperature-swing decomposer. For the high temperature step, we screened and identified a catalyst that is stable above 700 °C by preparing nickel catalysts supported on mesoporous alumina and evaluating their activity toward the high-temperature catalytic decomposition reaction. The catalysts achieved HI decomposition yields up to 23% at 650 °C and maintained over 20% yield after 100 h of operation. For the temperature-swing process, we investigated the adsorption, desorption, and regeneration efficiency, and the optimal regeneration temperature of nickel catalysts supported on silica/alumina adsorbents. The optimal regeneration temperature was 400 °C. Based on these results, we propose hybrid HI decomposers in two configurations: 1) residual HI decomposer and 2) HI concentrator. The residual HI decomposer improves the overall conversion efficiency to levels above the thermodynamic limit, while the HI concentrator increases the concentration of HI by simple temperature control, even at below-azeotropic HI concentrations.     

Design and fabrication of low concentrating second generation PRIDE concentrator

Prototype first generation Photovoltaic Facades of Reduced Costs Incorporating Devices with Optically Concentrating Elements (PRIDE) technology incorporating 3 and 9 mm wide single crystal silicon solar cells showed excellent power output compared to a similar non-concentrating system when it was characterized both indoors using a flash and continuous solar simulator. However, durability and instability of the dielectric material occurred in long-term characterisation when the concentrator was made by using casting technology. For large scale manufacturing process, durability, and to reduce the weight of the concentrator, second generation PRIDE design incorporated 6 mm wide “Saturn” solar cells at the absorber of dielectric concentrators. Injection moulding was used to manufacture 3 kWp of such PV concentrator module for building façade integration in Europe. Special design techniques and cost implications are implemented in this paper. A randomly selected PV concentrator was characterised at outdoors from twenty-four (≈3 kWp) 2nd-G PRIDE manufactured concentrators. The initial PV concentrators achieved a power ratio of 2.01 when compared to a similar non-concentrating system. The solar to electrical conversion efficiency achieved for the PV panel was 10.2% when characterised outdoors. In large scale manufacturing process, cost reduction of 40% is achievable using this concentrator manufacturing technology.     

Multi-junction III–V solar cells: current status and future potential

Our recent R&D activities of III–V compound multi-junction (MJ) solar cells are presented. Conversion efficiency of InGaP/InGaAs/Ge has been improved up to 31–32% (AM1.5) as a result of technologies development such as double hetero-wide band-gap tunnel junction, InGaP–Ge hetero-face structure bottom cell, and precise lattice-matching of InGaAs middle cell to Ge substrate by adding indium into the conventional GaAs layer. For concentrator applications, grid structure has been designed in order to reduce the energy loss due to series resistance, and world-record efficiency InGaP/InGaAs/Ge 3-junction concentrator solar cell with an efficiency of 37.4% (AM1.5G, 200-suns) has been fabricated. In addition, we have also demonstrated high-efficiency and large-area (7000 cm²) concentrator InGaP/InGaAs/Ge 3-junction solar cell modules of an outdoor efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing high thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd generation solar cells in addition to 1st generation crystalline Si solar cells and 2nd generation thin-film solar cells. We are now developing low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications.     

Procedure for thermal performance evaluation of steam generating point-focus solar concentrators

Solar energy can be used for substitution of the depleting fossil fuels in thermal applications and electricity generation through thermal route. For medium and high temperature applications, solar concentrators are required. Proper sizing and selection of concentrator for any thermal application calls for characterization of the concentrator at the required operating temperature. There are few procedures reported in literature for testing and evaluating solar concentrator performance which are based on sensible heating of the working fluid. One of the limitations of these procedures is requirement of precise operating conditions during testing. A test procedure for characterization of point-focus steam generating solar concentrators based on latent heating at different operating temperatures is proposed. The proposed procedure uses the phase change characteristic of water at constant temperature to measure the thermal performance. This procedure can be used to estimate thermal efficiency of solar concentrator at different operating temperatures above 100 °C. This procedure was used to estimate the efficiency of a point-focus solar concentrator having 25 m² aperture area at 161 °C (equivalent to 5.4 bar (g)). The efficiency was estimated as 47 ± 3.5%. The test procedure can be used for field evaluation of existing systems also with minimum amount of instrumentation and controls.     

Super high-efficiency multi-junction and concentrator solar cells

III–V compound multi-junction (MJ) (tandem) solar cells have the potential for achieving high conversion efficiencies of over 50% and are promising for space and terrestrial applications.We have proposed AlInP–InGaP double hetero (DH) structure top cell, wide-band gap InGaP DH structure tunnel junction for sub cell interconnection, and lattice-matched InGaAs middle cell. In 2004, we have successfully fabricated world-record efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cells with an efficiency of 37.4% at 200-suns AM1.5 as a result of widening top cell band gap, current matching of sub cells, precise lattice matching of sub cell materials, proposal of InGaP–Ge heteroface bottom cell, and introduction of DH-structure tunnel junction. In addition, we have realized high-efficiency concentrator InGaP/InGaAs/Ge 3-junction solar cell modules (with area of 7000 cm²) with an out-door efficiency of 27% as a result of developing high-efficiency InGaP/InGaAs/Ge 3-junction cells, low optical loss Fresnel lens and homogenizers, and designing low thermal conductivity modules.Future prospects are also presented. We have proposed concentrator III–V compound MJ solar cells as the 3rd-generation solar cells in addition to 1st-generation crystalline Si solar cells and 2nd-generation thin-film solar cells. We are now challenging to develop low-cost and high output power concentrator MJ solar cell modules with an output power of 400 W/m² for terrestrial applications and high-efficiency, light-weight and low-cost MJ solar cells for space applications.     

Two-stage concentrator permitting concentration factors up to 300x with one-axis tracking

Economic operation of high-efficiency concentrator solar cells requires solar concentration ratios which up to now can only be achieved with two-axis tracking. In this paper we present a two-stage concentrator approaching concentration ratios up to 300X while being tracked around only one polar axis. Its principle is as follows: a parabolic trough focuses the direct solar radiation onto a line parallel to the polar tracking axis. The half rim angle of this first concentrating stage is chosen to be equal to the sun's maximum declination of 23.5°. The second stage consists of a row of dielectric, nonimaging 3-D-concentrators, which couple the concentrated light directly into square solar cells. In contrast to linear secondaries the 3-D-secondaries make use of the limited divergence of ± 23.5° in the NS-direction to achieve additional concentration. The performance of the system depends sensitively on how well the angular acceptance characteristic of the second stage matches with the square-shaped angular irradiance distribution in the focal line of the parabolic trough. A new concentrator profile has been found that exhibits an almost ideal square acceptance characteristic with a very sharp cut-off. A prototype two-stage concentrator has been constructed with a total geometrical concentration of 214X. In outdoor measurements a total optical efficiency of 77.5% was obtained.     

Packaging III–V tandem solar cells for practical terrestrial applications achievable to 27% of module efficiency by conventional machine assemble technology

A new concentrator receiver containing a 7 mm×7 mm 3J concentrator solar cell with a 37.4% peak efficiency was developed. The receiver design includes a homogenizer, heat-handling (epoxy lamination) technologies and a low-resistance soldered connection and can be applied to various concentrator optics, including dish systems. The outdoor efficiency with a combination of a plastic Fresnel lens, made by low-cost injection molding, reached 27% on a hot summer day under 35.0 °C ambient temperature without additional cooling. With this newly developed receiver, mechanical engineers will be able to design their own concentrator module suitable for their environment, using their mechanical knowledge and local industrial resources. A 400X and 7056 cm² concentrator module was fabricated with 36 concentrator receivers connected in series and the same number of newly developed dome-shaped, non-imaging Fresnel lenses. The power rating was 200 Wp. The peak outdoor efficiency on a clear sky day was 26.8±1.5%. The integrated efficiency over the course of the day was 25.3±1.4%. This is the highest module efficiency that has been achieved using a practical module size and electrical rating.     

Performance comparison of wedge-shaped and planar luminescent solar concentrators

A wedge-shaped luminescent solar concentrator (LSC) with the capability of high collection efficiency and flux gains is described. Monte Carlo simulations of direct insolation conditions for a mid-latitude location are used to form a comparison of a realistically sized wedge LSC and conventional LSC with the same area footprint. The results show that when the sun is high in the sky, such as during early summer, the planar LSC outperforms the wedge LSC in terms of efficiently concentrating light. Under these conditions, the average wedge LSC concentrator efficiency is 3.5%, while the planar LSC achieves an efficiency of 6.3%. However, when the sun stays low in the sky, such as during early winter, the wedge LSC concentrates light with a maximum efficiency of 32.8%; more than four times greater than that of the planar LSC at 7.6%. Moreover, on a seasonal basis, the wedge LSC is estimated to produce more electrical energy per square meter of PV cells than a planar LSC or conventional solar panel placed parallel to the horizon.     

Performance evaluation of v-trough solar concentrator for water desalination applications

The working principle and thermal performance of a new v-trough solar concentrator are presented in this paper. Compared with the common parabolic trough solar concentrators, the new concentrator has two parabolic troughs which form a V-shape with the focal line at the bottom of the troughs. This is beneficial for the installation and insulation of the receiver, and the shadow on the reflective surface is avoided. The new v-trough collector does not require high precision tracking devices and reflective material. And therefore the cost of the system could be significantly reduced. Various experimental tests were carried out both outdoor and indoor using different types of receiver tubes. The results show that the collector system can have thermal efficiency up to 38% at 100 °C operating temperature. System modelling was used to predict the rate of fresh water produced by four different solar collector systems which include both static and one-axis solar tracking technologies. Comparison of the solar collectors at different temperature ranges for humidification/dehumidification desalination process using specific air flow rate were considered. At each temperature range, suitable solar collectors were compared in the aspect of fresh water production and area of solar collector required. Results showed that the new v-trough solar collector is the most promising technology for small to medium scale solar powered water desalination.     

An Si concentrator cell by single photolithography process

A mid-range concentrator cell by low-cost process is investigated and a new design is proposed. We tried to develop a single lithography CZ-Si cell process. The current conversion efficiency (not optimized) is 16% under X20 concentration without AR coating. The present moderate efficiency is possibly due to low bulk lifetime (approx. 5 μs after process). Cell efficiency and open-circuit voltage (V_oc) are expected to improve by using a better quality Si material and developing a low-stress process to maintain crystal quality. The equivalent circuit analysis reveals degradation of diode characteristics under concentration, which implies decline of fill-factor (FF) by use of low-quality concentrator optics.     

Development and performance analysis of a two‐axis solar tracker for concentrated photovoltaics

This study presents a two‐axis solar tracking system equipped with a small concentrator module for electricity generation through a multijunction solar cell. The system can accurately track the sun without the need of calibration for an extended period and operate as a stand‐alone system. High‐precision solar tracking was achieved by a combination of open‐loop and closed‐loop controls. A camera tracking sensor was introduced as a feedback device in closed‐loop control. Two different types of solar concentrator modules were designed and fabricated. Their concentration ratios were analyzed against solar tracking errors by means of ray tracing software. One is made up of a paraboloidal primary concentrator and a paraboloidal secondary reflector, whereas the other has a paraboloidal primary concentrator and a hyperboloidal secondary reflector. Both modules showed an almost identical concentration ratio of 610 provided that the solar tracker is pointing perfectly at the sun. However, their performance differs considerably when tracking error is present. The maximum power output was obtained near solar noon with multijunction cells, whose average solar conversion efficiency was 21%, much higher than that of conventional photovoltaic systems. Copyright © 2015 John Wiley & Sons, Ltd.     

High Efficiency Large Area Back Contact Concentrator Solar Cells with a Multilevel Interconnection

Very high efficiencies have been demonstrated under concentration with silicon solar cells having interdigitated contacts on the backside. However, only laboratory cells of small dimension have reached very high efficiencies. The need for developing a multilevel metallization technology for back contact concentrator solar cells of large area is demonstrated. The particular features required for such a multilevel interconnection are studied and a process using anodic oxidation of aluminum is presented. Back contact silicon solar cells of 0.64 cm² have been processed in this technology resulting in 26.2% efficiencies at 10W/cm² (100 suns AM1.5, 25.5 ^°C). the highest efficiency reported to date for a solar cell of this area. The one-sun efficiency of this cell is 21.7% (AMI.5, 25.2^°C). We propose also a new design for the metallization of back contact cells which allows an increase in the size of the cell without increasing the series resistance.     

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.     

Physical models for predicting the performance of Si/Si, AlGaAs/GaAs, and Si/SiGe solar cells

Analytical and physical models for homojunction and heterojunction solar cells are developed, and the performances of solar cells made by the Si/Si homojunction and made by the increasingly important and popular AlGaAs/GaAs and Si/SiGe heterojunctions compared. The models developed, which include relevant device physics such as the effective surface recombination velocity at the high-low junction and band discontinuities associated with heterojunctions, correctly explain the solar cell characteristics experimentally observed. Our calculations suggest that the highest efficiencies attainable for AlGaAs/GaAs, Si/Si, and Si/SiGe cells, with optimized doping concentrations but without surface passivation and geometry optimization, are 21.25%, 17.8% and 13.5%, respectively, under 1 AM1.5 sun condition. For concentrator cell applications, the efficiencies improve to about 24.5%, 22.2%, and 22.0% for AlGaAs/GaAs, Si/Si, and Si/SiGe cells, respectively, under 100 AM1.5 suns. While the AlGaAs/GaAs cell possesses the highest efficiency among the three cells, the Si/Si and Si/SiGe cells can achieve a satisfactory conversion efficiency at high sun concentration (22% at 100 suns), making them attractive for concentrator cell applications because their processing is the same as or is compatible with existing silicon technology. Model predictions for two Si/Si and one AlGaAs/GaAs cells compare favorably with data reported in the literature.     

Radiation resistant AlGaAs/GaAs concentrator solar cells with internal Bragg reflector

The problem of increasing efficiency, reliability and radiation resistance of solar cells based on AlGaAs/GaAs heterostructures can be solved by using an internal Bragg reflector. The Bragg reflector as a back surface reflector and as a back surface potential barrier which allows to conserve the high photosensitivity in the long- and middle-wavelength parts of the spectrum after electron and proton irradiation. The effect of base doping and base thickness on the radiation resistance of AlGaAs/GaAs solar cells with the internal Bragg reflector has been investigated. Concentrator solar cells efficiency and related parameters before and after 3 MeV electron irradiation at the fluence up to 3×10¹⁵ cm⁻² are represented. A base doping level of 1×10¹⁵ cm⁻³ and base thickness in the range 1.1–1.6 μm give an EOL AM0 efficiency of 15.8% (BOL–22%) at 30 Suns concentration after exposure to 1×10¹⁵ cm⁻² electron fluence. This EOL efficiency is among the highest reported for GaAs single-junction concentrator cells under AM0 conditions. Making the base doping level lower and the base thinner allows retaining a j_EOL/j_BOL ratio of 0.96 upon exposure up to 3×10¹⁵e/cm² 3 MeV electron fluence. These results are additionally supported by the modeling calculations of the relative damage coefficient.     

Non-axisymmetric reflectors concentrating radiation from an asymmetric heliostat field onto a circular absorber

In solar tower plants, where a rotationally symmetric field of heliostats surrounds the tower, an axisymmetric secondary concentrator such as a compound parabolic concentrator (CPC) or a tailored concentrator or a cone is the obvious choice. For locations at higher latitudes, however, the reflecting area of the heliostats may be used more efficiently if the field of heliostats is located opposite to the sun as seen from the tower. Then the field is asymmetric with regard to the tower. In the case of an asymmetric field, an axisymmetric concentrator necessarily has a concentration significantly lower than the upper limit. Furthermore, the area on the ground from which a tilted axisymmetric concentrator accepts radiation is an ellipse, including also heliostats very distant to the tower producing a large image of the sun. For these reasons we investigate asymmetric secondaries. From the shape of the edge ray reflectors constructed for rays in the central south–north plane we conclude that a skew cone reflector might be appropriate for the field, and optimize its free parameters by means of ray tracing. Asymmetric concentrators may increase the concentration by up to 25% at the same efficiency compared to optimized axisymmetric CPC or cone reflectors.     

Optical design of a flat-facet solar concentrator

We present a procedure to design a facet concentrator for laboratory-scale research on medium-temperature thermal processes. The facet concentrator approximates a parabolic surface with a number of flat square facets supported by a parabolic frame and having two edges perpendicular to the concentrator axis. The optimum size and position of each facet are found by employing Monte Carlo ray tracing analysis to achieve desired flux characteristics in the focal plane. A 164-facet concentrator with realistic specularly-reflecting surface and facet positioning accuracy will deliver up to 8.15 kW of radiative power over a 15 cm radius disk located in the focal plane, with average concentration ratio exceeding 100.     

Two-stage tilting solar concentrators

In this paper a novel type of two-stage non-focussing concentrator is described which makes economic photovoltaic and thermal solar conversion possible at current prices in many locations. The new concentrators use exceptionally little mirror (1.3–1.8 times full aperture), yet achieve concentrations up to 12 × without diurnal tracking. Optical efficiency is high, and no moving parts are required except for a simple pivot and securing device.     

Evaluation of a large dish‐type concentrator solar lighting system for underground car park

To utilize solar energy more efficiently and reduce lighting power consumption in underground public spaces such as car park, a large dish‐type concentrator solar lighting system is put forward along with its evaluation, which is a unique design to apply a laminated layer of beam split thin‐film coating and thin‐film solar cells onto the dish reflector. The collected sunlight is split into 2 parts, one being reflected into a fiber optical bundle and transmitted for daylighting, while the rest being absorbed by solar cells for electricity generation as the other way to replenish daylighting. A set of 4 solar lighting systems using 3.28‐m diameter dish are designed to meet the lighting requirement in a 1771‐m² underground car park. A mathematical model is adopted to calculate the output power and conversion efficiency of solar cells distributed on the parabolic dish surface. The indoor illuminance distribution is given by lighting simulation. The results indicate that the average daylight illuminance in the car park can vary between 62.7 and 284 lx on February 25, 2016 and between 62.7 and 353 lx on August 17, 2016 for 2 chosen days, respectively. For the presented design, the electricity produced by solar cells is just enough to power light‐emitting diodes for lighting meeting a criterion at night. Considering about 19% conversion efficiency of solar cells and the efficacy of 129.5 lm/W of light‐emitting diodes, the hybrid solar lighting system can have about 40% utilization ratio of solar energy, so it can be concluded that a sufficient lighting provision can be provided by the proposed large dish‐type concentrator solar lighting system for applications in underground car park.