Concentration characteristics of a two stage solar concentrator: Effect of primary mirror surface errors

The concentration characteristics of a two-stage linear solar concentrator employing a perfectly tracked parabolic trough as the primary and a seasonally adjusted compound parabolic concentrator as the secondary stage with a flat horizontal absorber are studied. The Monte Carlo ray trace technique is used for this purpose. The effect of randomly distributed primary mirror surface errors on the concentration characteristics of the two stage concentrator is also investigated.     

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.     

Losses in a three-dimensional compound parabolic concentrator as a second stage of a solar concentrator

In this article, the losses due to reflection properties in a three-dimensional compound parabolic concentrator (3-D C.P.C.) are calculated and the effect of these losses on the concentration is analyzed. The 3-D C.P.C. is used as a second stage in two configurations of a two-stage concentrator: (a) a parabolic dish as a first stage, and (b) a Cassegrainian as a first stage. The nonplanar rays play an important role in 3-D C.P.C. losses. Thus, a ray-tracing procedure is needed to evaluate the losses. In this study, a rigorous three-dimensional ray tracing program was specially developed. The reflection losses and their effect on the concentration were determined from the reflection distribution derived by the developed ray-tracing program. As a result, the reflection losses were approximated by a simple empirical linear model useful in practical ranges of the 3-D C.P.C. acceptance angles and reflectivities. This model facilitates design and system optimization by analytical methods without resorting to a ray-tracing procedure. The approach presented in this article may be used to compare the performance of the two-system configurations, taking into account the 3-D C.P.C. losses.     

Comparison of solar concentrators

Even though most variations of solar concentrators have been studied or built at some time or other, an important class of concentrators has been overlooked until very recently. These novel concentrators have been called ideal because of their optical properties, and an example, the compound parabolic concentrator, is being tested at Argonne National Laboratory. Ideal concentrators differ radically from conventional instruments such as focussing parabolas. They act as radiation funnel and do not have a focus. For a given acceptance angle their concentration surpasses that of other solar concentrators by a factor of two to four, but a rather large reflector area is required. The number of reflections varies with angle of incidence, with an average value around one in most cases of interest. In order to help provide a rational basis for deciding which concentrator type is best suited for a particular application, we have compared a variety of solar concentrators in terms of their most important general characteristics, namely concentration, acceptance angle, sensitivity to mirror errors, size of reflector area and average number of reflections.The connection between concentration, acceptance angle and operating temperature of a solar collector is analysed in simple intuitive terms, leading to a straightforward recipe for designing collectors with maximal concentration (no radiation emitted by the absorber must be allowed to leave the concentrator outside its acceptance angle). We propose some new concentrators, including the use of compound parabolic concentrators as second stage concentrators for conventional parabolic or Fresnel mirrors. Such a combination approaches the performance of an ideal concentrator without demanding a large reflector; it may offer significant advantages for high temperature solar systems.     

Solar thermal engine systems using two stage concentrators

The overall efficiency of a system including a solar paraboloidal concentrator augmented by a new type of nonimaging second stage concentrator (shaped like the bell of a trumpet) and coupled to a heat engine is investigated. Typically, a relative increase of 10 per cent in system efficiency was found due to the second stage.     

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.     

A fresnel-winston tandem concentrator system

Some geometrical concentration characteristics of a solar concentrator system employing a Fresnel reflector concentrator and a compound parabolic concentrator in tandem are discussed.     

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.     

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.     

Practical design considerations for CPC solar collectors

Several practical problems are addressed which arise in the design of solar collectors with compound parabolic concentrators (CPC's). They deal with the selection of a receiver type, the optimum method for introducing a gap between receiver and reflector to minimize optical and thermal losses, and the effect of a glass envelope around the receiver. This paper also deals with the effect of mirror errors and receiver misalignment, and the effect of the temperature difference between fluid and absorber plate. The merits of a CPC as a second stage concentrator are analyzed.     

Heat transfer in a conical cavity calorimeter for measuring thermal power of a point focus concentrator

A theoretical study of the heat transfer process that takes place in a special calorimeter of conical cavity named CAVICAL is presented. This instrument is used to measure the thermal power of a point focus solar concentrator system named DEFRAC, developed at the Center for Energy Research of the National University of Mexico. The DEFRAC concentrator has a power of 1.3 kWth and a very fine optical system. The calorimeter has a cavity opening of 8.24 cm². A detailed heat transfer study was done using FLUENT code. The heat transfer processes taken into account for the analysis were the radiative energy absorbed by the inner cavity wall, the energy transfer from the outer cavity wall to the air by natural convection, the energy transferred by conduction through the inner metallic wall of the calorimeter, and by forced convection to the fluid in the cooling system. The calorimetric information gathered allowed determining the thermal power that the concentrator is able to capture. Temperature and velocity fields have been calculated for each of the thermal fluids considered inside of the calorimeter. The analysis gave thermal losses and measured the thermal efficiency of the device. The information generated is useful to further optimize the design of the calorimeter.     

太阳能会聚器的优化设计

讨论了用光学设计软件CODE V对太阳能会聚器进行优化设计的问题。在描述了太阳能会聚器的主要设计参数后，我们分析了抛物型会聚器的特性。利用光学设计软件，设计出一类新的高次非球面会聚器。实验结果表明，此类会聚器可实行间断跟踪太阳，且接收面光能量分布均匀，提高了系统转换效率，还具有节约能量、降低成本等优点。     

Concentrator performance within a centrally fuel-rich primary air burner: Influence of multiple levels

Within a gas/particle two-phase testing facility, multi-level concentrators were investigated by obtaining gas/particle two-phase characteristics of each concentrator within a centrally fuel-rich swirl burner. Measurements of velocities, particle volume flux profiles and relative particle number concentrations were obtained and analyzed to assess the performance of these concentrators. For concentrators with fewer levels, peak values of the axial particle volume flux were observed to move away from the central axis of the primary air duct. The rich/lean air ratios near the exit of the last ring were much larger for the single-level concentrator than for the 2- and 3-level concentrators. Conversely, the concentration ratio is always smaller for the single-level concentrator than for the others. In addition, the performance ratio R_CR near the exit is always larger than 2, indicating that the 2- and 3-level concentrators can achieve a stable flame. The resistance coefficient increased with fewer rings making up the concentrator.     

A novel multiple curved surfaces compound concentrator

A novel multiple curved surfaces compound concentrator is developed in this paper. It is composed of a parabolic and a flat contour. This new concentrator has a focus at the backside which is extremely useful and convenient for some applications. The reflected rays here are transmitted forward instead of backward as in the conventional parabolic concentrators. The design method of the concentrator is introduced. Some of important parameters are discussed and the value rang of them is determined. Simple comparisons between proposed concentrators and traditional paraboloid and CPC are made. Light rays tracing are carried out in the concentrator.     

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.     

具有二次聚光器的新型大开口槽式太阳集热器设计与光学性能分析

针对大开口和更高运行温度的槽式太阳能热发电系统,提出一种可实现高聚光比、低辐射热损及能流密度均匀的新型槽式太阳集热器,即在集热管内放置外壁具有太阳选择吸收膜层和内壁具有反射膜层二次聚光器的大开口槽式太阳集热器。建立圆弧为微元段的自适应设计新方法,提出3种典型的二次聚光器面型,利用蒙特卡洛光线追迹方法仿真新型集热器的能流密度分布特性,验证该光学仿真方法,分析影响集热器光学性能的各种因素。结果表明,该集热器可显著提升集热效率。     

Nontracking solar concentrators with louvered heliostats: A calculation algorithm

An algorithm for calculating the solar radiation flux on the receiving surface of a nontracking parabolic trough concentrator with a louvered heliostat is presented. Cylindrical geometry of the concentrator and shape and arrangement of mirror lamellae of the heliostat make it possible to consider sunlight passage across the mirror system of the module as a whole band, which significantly increases the computational efficiency of the algorithm proposed.     

Thermodynamic optimisation of the integrated design of a small‐scale solar thermal Brayton cycle

The Brayton cycle's heat source does not need to be from combustion but can be extracted from solar energy. When a black cavity receiver is mounted at the focus of a parabolic dish concentrator, the reflected light is absorbed and converted into a heat source. The second law of thermodynamics and entropy generation minimisation are applied to optimise the geometries of the recuperator and receiver. The irreversibilities in the recuperative solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In a small‐scale open and direct solar thermal Brayton cycle with a micro‐turbine operating at its highest compressor efficiency, the geometries of a cavity receiver and counterflow‐plated recuperator can be optimised in such a way that the system produces maximum net power output. A modified cavity receiver is used in the analysis, and parabolic dish concentrator diameters of 6 to 18 m are considered. Two cavity construction methods are compared. Results show that the maximum thermal efficiency of the system is a function of the solar concentrator diameter and choice of micro‐turbine. The optimum receiver tube diameter is relatively large when compared with the receiver size. The optimum recuperator channel aspect ratio for the highest maximum net power output of a micro‐turbine is a linear function of the system mass flow rate for a constant recuperator height. For a system operating at a relatively small mass flow rate, with a specific concentrator size, the optimum recuperator length is small. For the systems with the highest maximum net power output, the irreversibilities are spread throughout the system in such a way that the internal irreversibility rate is almost three times the external irreversibility rate. Copyright © 2011 John Wiley & Sons, Ltd.     

Thermal performance of solar concentrator/cavity receiver systems

To utilize solar energy at a high temperature, a parabolic dish/cavity receiver configuration is often used. The energy loss mechanisms of such a system are analyzed. System efficiency is defined as the power absorbed by the working fluid circulating in the cavity divided by the solar power falling on the concentrator aperture. Power profiles produced in cavities of varying geometry with concentrators of varying rim angle are also discussed. It is found that varying concentrator rim angle and cavity geometry can greatly affect the cavity power profile without a large effect on system efficiency. Cavity isothermality often requires a nonlinear power profile , particularly in a thermochemical system. The methodology described can be used to optimize concentrator/cavity design variables.     

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.