A novel PVT/PTC/ORC solar power system with PV totally immersed in transparent organic fluid

A novel hybrid PVT/parabolic trough concentrator (PTC)/organic Rankine cycle (ORC) solar power system integrated with underground heat exchanger has been proposed. The evaporator unit consists of a transparent flat PVT solar collector and a PTC connected in series. The first transparent solar collector has transparent covers and consists of solar cells totally immersed within a pressurized transparent organic fluid that allows the solar radiation to reach the solar cells, cools them effectively, and captures all thermal losses from the solar cells. The second concentrator is a conventional one with opaque black receiver used to reheat the transparent organic fluid to higher temperatures. Both solar collectors (the PVT and PTC) perform as the boiler and superheater for the ORC. The performance of the proposed system is investigated by a steady‐state mathematical model. The results show that, at design conditions, the efficiency of the PV modules stabilizes around 12%, absorber efficiency varies within 64% to 75%, and the ORC efficiency varies within 7% to 17%.     

空气集热器透明蜂窝透过率的二维理论推导

透明蜂窝的太阳透过率是衡量平板式太阳能集热器性能的主要参数.本文考虑入射光线所在平面垂直于集热器表面的情况,从而将光线在蜂窝内部的复杂的传播过程简化为在一个矩形单元内的二维多次反射.根据几何光学原理,推导出蜂窝的有效透过率的计算公式:τe=[(1-Atanθ+N)+(Atanθ-Nρe]τ2ρNe.根据公式,透明蜂窝的有效透过率为入射角θ、材料的透射率τ和反射率ρ、以及蜂窝单元的高宽比A的函数,与蜂窝的具体尺寸无关.该公式提供了计算透明蜂窝透过率的一个简单而有效的方法.     

A new trough solar concentrator and its performance analysis

The operation principle and design method of a new trough solar concentrator is presented in this paper. Some important design parameters about the concentrator are analyzed and optimized. Their magnitude ranges are given. Some characteristic parameters about the concentrator are compared with that of the conventional parabolic trough solar concentrator. The factors having influence on the performance of the unit are discussed. It is indicated through the analysis that the new trough solar concentrator can actualize reflection focusing for the sun light using multiple curved surface compound method. It also has the advantages of improving the work performance and environment of high-temperature solar absorber and enhancing the configuration intensity of the reflection surface.     

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.     

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.     

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.     

Theoretical and experimental study on the application of diffuse‐reflection concentrators in PV/T solar system

In order to improve the practicability of PV/T solar system, we proposed the theory and method on the application of diffuse‐reflection concentrator in the PV/T solar system and analyzed the concentration characteristics of this proposed application. In addition, we designed experimental prototype of PV/T solar system and conducted test and analysis of the thermal and electrical characteristics of the PV/T solar system with or without a concentrator, respectively. The results showed that for the PV/T solar system with diffuse‐reflection concentrator, the amount of incident irradiance was increased by an average of 26% during test period, and the 200‐L water in the system was heated to 58 °C, which was 12 °C higher than that of PV/T solar system without diffuse‐reflection concentrator; moreover, the max output power was increased by 11%. Therefore, it is a feasible way to improve the practicability of PT/V solar system by integrating a diffuse‐reflection concentrator. Copyright © 2016 John Wiley & Sons, Ltd.     

用于太阳能光电水泵的菲涅耳透镜聚光收集器

报道了具有第二级Ｖ形槽聚光器的玻璃直纹菲涅尔透镜与ＬＧＢＧ高效太阳电池组成的低倍聚光收集器的试验结果。宽度为３０ｍｍ、长度为１．５８ｍ的太阳电池组件，经聚光后峰值功率从７Ｗ增大到４４Ｗ，比常规太阳电池费用减少６０％，系统总费用降低４０％。     

Solar cell module colored with fluorescent plate

In order to color the solar cell module with a small decrease in energy conversion efficiency, a fluorescent plate was used as a protecting plate for the module. The effect of the coloration on the energy conversion efficiency was discussed on the basis of a simple model for the absorption and re-emission of light in the fluorescent plate and spectral reflection of the solar cell module. The measured energy conversion efficiency of the colored solar cell module was comparable to that of the non-colored module, when the fluorescent quantum efficiency was nearly equal to 1.0. The coloration of green yielded a 2.7% increase in energy conversion efficiency, and the colorations of orange, pink, and red, could be made with 0.5%, 1.5% and 5.5% decrease in energy conversion efficiency. The small increase in energy conversion efficiency was attributable to the fact that the reflectivity of the colored module was a little less than that of the non-colored module.     

Influence of the light interaction on the optical behaviour of a-Si:H solar cells

Analytical expressions are proposed for light absorption in the active layer of an α-Si:H solar cell (SC), for non-coherent light interaction in this layer, as well as forcoherent interaction, considering inclined incidence of light. The contribution of the scattered light to the integral collection efficiency (ICE) of SC with indium-tin-oxide conductive transparent oxide contact (CTOC), or with SnO₂ CTOC is estimated. Computations are implemented about the collection efficiency of SC for various cases of light interaction in the layers of SC. This allows different appearances of the collection efficiency of SC in practice to be explained,based on the nature of light interaction in CTOC and the active layer, which is related to their surface roughness. It is possible to calculate some optical characteristics of the transparent contact or the active layer using the collection efficiency, when the light interaction is coherent in the respective layer. The influence of an anti-reflection coating and two-layer CTOC on the collection efficiency of SC is also investigated, as well as the thickness dependence of ICE.     

Design and analysis of a CPV/T solar receiver with volumetric absorption combined spectral splitter

Spectral beam splitting is a promising technology to achieve the maximum electrical and thermal outputs from concentrating photovoltaic/thermal (CPV/T) systems simultaneously. In this article, a novel CPV/T receiver is proposed by incorporating a fluid based filter together with a solid absorptive filter. The geometry of the receiver is developed for a designed linear flat mirror concentrator. According to the optical transmittance of both fluid based filters and solid absorptive filters, as well as their corresponding merit functions, four fluid filters and two solid filters are determined to be the candidates of the combined filter for the silicon concentrator solar cell. Then, a complete solar radiation propagation process from concentrator to the designed CPV/T receiver is simulated using ray tracing software-LightTools. The results show that the surface illumination uniformity of the PV module filtered by each combined filter under the linear flat mirror concentrator is higher than 96%. Using 5 g/L CoSO₄ solution and HB650 as the combined filter, 33.67% of the concentrated light can be directed to the PV module with the remainder collected by the filter as thermal energy and the silicon CPV cells can convert 27.06% of this energy into electrical power. This contributes to the fact that 92.43% of the light striking the PV module is within 650-1100 nm, which is the spectral response range of the cell can work efficiently. The total efficiency of 49.88% can be achieved with such a filter and the electrical efficiency is 9.1% with respect to the total incident power on the receiver.     

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.     

Distribution of absorbed heat in luminescent solar concentrator lightguides and effect on temperatures of mounted photovoltaic cells

Polymer plates containing fluorescent dyes with photovoltaic cells attached to one or more edges have the potential to be used for solar light collection in the built environment. The heating of the polymer plates and the resulting effects this may have on the operating temperatures of the attached photovoltaic cells and the integrity of the cells are studied. The results indicate that the luminescent solar concentrator under a solar simulator will allow the edge attached photovoltaic cells to operate up to 10 °C cooler compared to cells experiencing direct solar exposure because of the avoidance of infrared light. We also demonstrate the importance of correct adhesive to avoid rupture of the attached cell.     

Influence of radiation attenuation coefficient of light transparent coating and radiation reflection coefficient of radiation absorbing surface of heat exchange panel on radiative heat exchange coefficient in the “light transparent coating/radiation absorbing heat exchange panel” system of flat solar collectors

An analytic expression is proposed for determining the coefficient of radiative heat exchange between the inner surface of the heat exchange panel as a function of the radiation attenuation coefficient of the light transparent coating and the radiation reflection coefficient of the surface of the radiation absorbing surface of the heat exchange panel of flat solar collectors.     

Computer simulation of collection efficiency of a-Si : H tandem solar cells interconnected by transparent conductive oxide

In tandem solar cells, the interconnection between the cells is one of the factors limiting the performance of the photovoltaic device. For high performance, the interconnection must have good ohmic contact as well as good utilization of solar radiation to the bottom cells. Thin film of transparent conductive oxide with high conductivity and optical transparency is useful for this interconnection. In this paper, we made computer simulation of reflectance, transmittance and absorbance as a function of the wavelength of incident photons in single cell as well as two junction tandem solar cells. The collection efficiency was calculated for different thickness of transparent conductive oxide (TCO) TiO₂ interposed as interconnection between the two cells.     

Enhanced performance of solar cells with anti-reflection layer fabricated by nano-imprint lithography

Because of increase in demand for clean energy sources, photovoltaic device is becoming more important as a new power plant. To replace fossil fuels with photovoltaics, generating electricity with solar cells should meet cost effectiveness and high efficiency. An anti-reflection technique is one of the effective methods to achieve high efficiency.To reduce the reflection at the surface of concentrated photovoltaic device, a nanometer scale dot-pattern array was formed on the surface of GaInP/Ga(In)As/Ge solar cells by nano-imprint lithography. Since this nano-pattern is smaller than the wavelength of visible light, the effective refractive index near the surface changes gradually. Thus, the reflection of the light at the surface with moth-eye structure can be effectively reduced for overall spectral region. As a result, a solar cell with the moth-eye pattern as an anti-reflection layer showed lower reflectance and enhanced total conversion efficiency, compared to a solar cell without a moth-eye patterned layer.The patterned solar cell was characterized using UV-vis spectrophotometer, atomic force microscope (AFM) and scanning electron microscope (SEM), and its total conversion efficiency was measured by solar simulator.     

Electrical and thermal performance of silicon concentrator solar cells immersed in dielectric liquids

Direct liquid-immersion cooling of concentrator solar cells was proposed as a solution for receiver thermal management of concentrating photovoltaic (CPV) and hybrid concentrating photovoltaic thermal (CPV-T) systems. De-ionized (DI) water, isopropyl alcohol (IPA), ethyl acetate, and dimethyl silicon oil were selected as potential immersion liquids based on optical transmittance measurement results. Improvements to the electrical performance of silicon CPV cells were observed under a range of concentrations in the candidate dielectric liquids, arising from improved light collection and reduced cell surface recombination losses from surface adsorption of polar molecules. Three-dimensional numerical simulations with the four candidate liquids as the working fluids, exploring the thermal performance of a silicon CPV cell array in a liquid immersion prototype receiver, have been performed. Simulation results show that the direct-immersion cooling approach can maintain low and uniform cell temperature in the designed liquid immersion receiver. The fluid inlet velocity and flow mode, along with the fluid thermal properties, all have a significant influence on the cell array temperature.     

Mathematical modeling of a box-type solar cooker employing an asymmetric compound parabolic concentrator

A novel design of solar cooker is introduced. The cooker is of box-type equipped with an asymmetric compound parabolic concentrator (CPC) as booster-reflector. It consists of an insulated box equipped with a vertical double glazing cover on a side, and a vertical absorber plate laid out just behind the transparent cover. The booster-reflector is fixed on the glazed side of the box. The absorber plate and the glazing form a vertical channel, open at the top and bottom, and enclosed at the sides. The two openings allow the inside air circulation. A mathematical model of the heat transfer processes involved with this solar cooker, containing a cooking pot loaded with water and deposited on the box floor; was developed and the effects of various parameters, such as solar radiation, load of water and clouds on the dynamic behavior of the cooker are studied.     

Improved solutions for temperature and thermal power delivery profiles in linear solar collectors

For the conversion of absorbed sunlight into useful thermal power, we demonstrate that the profiles of absorber temperature, fluid temperature and thermal power delivery along linear solar collectors can be solved in closed form even when the collector heat-loss coefficient is far from constant over the collector operating range. This analytic solution eliminates the errors inherent in earlier approximate solutions, and makes the dependence of collector performance on component properties transparent. An example for a realistic solar concentrator illustrates the improvement in prediction accuracy.     

An alternative solar pumping approach by a light guide assembly elliptical-cylindrical cavity

To improve solar laser output performance, what we believe to be a novel light guide assembly is proposed to pump a Nd:YAG laser rod within a two-dimensional elliptical-cylindrical (2D-EL-CYL) cavity with intervening optics. Based on refractive and total internal reflection principles, the concentrated solar radiation from a primary parabolic mirror is coupled through the square input face of a fused silica light guide assembly to its rectangular output end, located along the first focal line of the 2D-EL-CYL cavity. Optimized pumping conditions are found through ZEMAX™non-sequential ray-tracing software. By comparing with the laser performance of a two-dimensional dielectric compound parabolic concentrator (2D-DCPC) cavity, significant improvements in absorption distribution, TEM₀₀ laser power and tracking error dependent output power stability are obtained by the proposed pumping approach.