Performance analysis of water cooled concentrated photovoltaic (CPV) system

The concentrated photovoltaic (CPV) system focuses solar radiation on the solar cells. CPV systems need to track the sun for keeping the reflected radiation focussed on the solar cell. A CPV module and its active water-cooling system are developed at the School of Energy and Environment, Southeast University, China and its performance has been reported here. This developed system has been used for testing the PV module's performance for different parameters such as operating temperature, power output, and efficiency. The experimental results show that the operating temperature of the CPV module under water cooling is reduced under 60 °C and therefore the efficiency of the CPV has increased and produced the more electric power output. The effect of water flow rate has been analyzed for the CPV efficiency and output.     

Operating characteristics of multijunction solar cells

Multijunction solar cells produced by Spectrolab are the most efficient solar cells in the world, with a record efficiency of over 40%. Cell designs have been modified for high performance in concentrator photovoltaic (CPV) systems with the potential for low-cost, high-volume manufacturing. High-performance CPV cells have been designed, tested, and entered into production for field testing in CPV systems. Performance under variable concentrations and temperatures has been characterized and compared to semiconductor theory. The cell response has been applied to a spectral irradiance model to predict field performance at reference locations. Cell qualification has been completed for the current-generation C1MJ design.     

聚光型太阳能光伏光热系统研究进展

聚光型太阳能光伏光热系统(CPV/T)在传统光伏发电系统的基础上增加了聚光系统和光热系统,在通过聚光系统提高光伏效率的同时将系统中多余的热量加以利用,以达到太阳能最大化利用的目的。本文介绍了CPV/T系统的工作原理及其能效影响因素,以直接影响系统太阳能综合利用效率的聚光器技术、光伏电池技术和光伏冷却技术作为分析对象,结合近几年国内外最新研究成果比较了不同类型聚光器、光伏电池以及冷却方式的优劣,列举了常见的光伏余热利用方式。分析认为:CPV/T系统虽然具有更高的太阳能利用率,但应加大对系统尤其是聚光器经济性的分析;考虑在系统中应用叠层光伏电池缓解聚光器带来的系统体积过大问题;新电池开发过程中应更注意光伏电池的温度系数以减少冷却系统的压力,冷却技术在强化散热的同时也应注意热量的收集方法及其与利用途径的有效结合。     

Investigation of functionally graded metal foam thermal management system for solar cell

Concentrated photovoltaic cell (CPV) is a solar energy harvesting device that converts solar energy into electrical energy. However, the performance and efficiency of the CPV are heavily dependent on the temperature. Besides, nonuniformity of temperature distribution on the CPV will lead to thermal aging and affects the cycle life. Hence, an effective cooling system is required to remove excess heat generated to ensure that the CPV operates at optimum operating temperature with minimum variation of temperature. Metal foam is a new class of material that possesses huge potential for thermal management. In this study, a functionally graded metal foam is proposed for the CPV thermal management system. Computational thermal fluid dynamic analysis is conducted to investigate the effect of porosity and pore density on the flow field and thermal performance of the aluminum foam heat sink. The investigation results revealed that 10 PPI functionally graded aluminum foam heat sink with two stages of porosity gradient 0.794 and 0.682 produced the lowest pressure drop and highest thermal performance. Temperature difference of 3.9°C was achieved for a solar cell with total heat generation of 900 W under water mass flow rate of 20 gs⁻¹.     

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.     

Performance Optimization of Water-Cooled Concentrated Photovoltaic System

In this paper, a three-dimensional computational fluid dynamics model is developed to predict the thermal and electrical performance of a water-cooled concentrated photovoltaic (CPV) system. Based on the good agreement between the numerical results and experimental data from literature, an attempt was made to improve this system performance. Indeed, as the developed model is able to predict the thermal behavior of the different system components, many hot spots were detected in the cell module. In order to avoid this disadvantage while promoting solar cell cooling, the number of water cooling pipes of the CPV module was first increased and then a rectangular channel was employed. Numerical simulation results indicate the potential of the different modified systems for reducing these hot spots and the CPV module temperature, thus providing increased electrical and thermal efficiencies. The optimum design, which presents a solar cell temperature of 315.15 K and respectively a thermal and combined (thermal plus electrical) efficiency of 74.2% and 83.5%, is also evaluated.     

Design and Development of a Lens-walled Compound Parabolic Concentrator-A Review

Compound parabolic concentrator(CPC) is a representative among solar concentrators, one of whose disadvantage is that the concentration ratio limits the half acceptance angle. Based on this, researchers put forward a novel structure, named the lens-walled CPC. This paper reviews the design and development of lens-walled CPC. The structure of the symmetric and asymmetric lens-walled CPC and the improved ones are presented, and their indoor and outdoor performances are also illustrated. The lens-walled CPC has a larger half acceptance angle and a more uniform flux distribution that is suitable for PV application. Furthermore, the life-cycle assessment for building integrated with PV is performed and it shows that the energy payback time of such integrated system has a significant advantage. In addition, future research areas are also indicated that may provide more functions and more stable performance. The design methods and developmental directions given in this study would provide many references in solar optical research and solar concentrator optimization.     

Three-dimensional simulating of concentrator photovoltaic modules using ray trace and equivalent circuit simulators

A 3-dimensional (3D) simulation for a concentrator photovoltaic (CPV) module using a triple-junction solar cell was developed. By connecting ray-trace simulation for an optics model and 3D equivalent circuit simulation for a triple-junction solar cell, the operating characteristics of a CPV module were calculated. By using the 3D simulation, we considered the influence of tracking error on the CPV module. It was found that maximum output power was not correlated with optical efficiency of the optics system and was strongly dependent on fill factor. We can use this total 3D simulation for the evaluation and optimization of CPV modules.     

An experimental study on thermal management of concentrated photovoltaic cell using loop heat pipe and heat sink

One of the most critical innovations in the solar energy conversion is the use of concentrators for generating power from a smaller area of the cell. The thermal management has an exceptional role in the concentrated photovoltaic (CPV) cell, without which the operating temperature will increase owing to the thermal degradation. In the present study, a prototype of low CPV with single‐cell configuration using a Fresnel lens and a manual tracker with geometrical concentration ratio of up to 25 Suns is made. The performance of the CPV with passive cooling arrangements, such as heat sink and loop heat pipes (LHPs), is analyzed under real‐time outdoor conditions. The results obtained infer that the LHP‐based cooling system has brought down the average temperature rise above ambient to 37.8°C from 54.16°C and 72.6°C in the heat sink and bare CPV systems, respectively. Also, the LHP managed to reject the heat to the surrounding with an average thermal resistance of 1.005°C/W, which is the least when compared with the heat sink. Apart from the instabilities caused by the interference of clouds, the CPV with the LHP cooling system could generate 10% more power output than the one with a heat sink.     

An optical analysis of a static 3-D solar concentrator

Concentrating technology is long established in the field of solar thermal applications. However, there is still scope for improvement due to innovation in design, materials and manufacturing methods. The optical efficiency of a solar concentrator depends largely on the geometry of the concentrator profile. This paper evaluates the optical performance of a static 3-D Elliptical Hyperboloid Concentrator (EHC) using ray tracing software. Ray tracing has been used extensively to calculate the optical efficiency of the static 3-D EHC. Performance parameters such as effective concentration ratio, optical efficiency and geometric concentration ratio are also evaluated for different aspect ratios of the elliptical profile. Optimization of the concentrator profile and geometry is also carried out to improve the overall performance; this parametric study includes the concentrator height, solar incidence angle and aspect ratio of the ellipse. The overall performance of the concentrator was assessed based on the acceptance angle, effective concentration ratio and optical efficiency. Finally, the flux distribution on the receiver area for different concentrator heights is also presented.     

Thermal management of concentrator photovoltaic systems using nano-enhanced phase change materials-based heat sink

Temperature regulation of concentrator photovoltaic systems is essential in reducing operating temperatures with higher system performance. A new nano-enhanced phase change material, with multi-cavity heat sinks, integrated with a concentrator photovoltaic (CPV) system is developed. The multi-cavity heat sink includes a single-, triple-, and quintuple-cavity configuration in both parallel and series pattern filled with n-octadecane PCM and graphene nanoparticle additives with 2% and 5 wt%. Numerical simulations are performed using the developed two-dimensional model for photovoltaic layers integrated with the nano-enhanced phase change material-based heat sink. The predicted results are compared with the available numerical results and measurements. Results indicate that increasing the number of parallel cavities, along with weight fraction of nanoparticles, significantly improves the thermal conductivity, and consequently attains better performance for the CPV system. Using a parallel quintuple-cavity configuration, with 5 wt% NPCM, achieves maximum reduction in the solar cell mean temperature along with the best temperature uniformity compared to other configurations. At a concentration ratio of 20, the thermal efficiency is 65%, the electrical efficiency is about 10%, and the output electrical power of the system is 235 W per m width of the cell. On the contrary, using a series pattern of the heat sink has an unfavorable effect on the mean solar cell temperature, as well as on electrical efficiency and thermal performance of the CPV system. The obtained result can assist in identifying the best possible design of the heat sink in addition to the most appropriate selection of PCM and nanoparticle additives.     

Uniqueness verification of direct solar spectral index for estimating outdoor performance of concentrator photovoltaic systems

To analyze the impact of a direct spectral distribution of the solar spectrum on the outdoor performance of concentrator photovoltaic (CPV) systems, an index for the direct spectral distribution is needed. Average photon energy (APE), the average energy of a photon in the direct solar spectrum, is one of these indexes. In this contribution, the uniqueness of APE to the direct solar spectral distribution is statistically analyzed to assure that an APE value uniquely yields the shape of a direct solar radiation spectrum. The results have exhibited the uniqueness of the direct normal solar spectrum with each APE value, in which the standard deviations are quite small. Short-circuit current density of the InGaP/InGaAs/Ge triple-junction solar cell in the CPV system is additionally calculated using the direct spectral irradiance with different APE values. It is revealed that APE is a useful index to describe the direct spectral distribution to evaluate the outdoor performance of the CPV systems.     

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.     

Exploitation of solar energy in Cyprus

This paper reviews the application of solar energy technology in Cyprus and presents an energy analysis with emphasis on the contribution of solar energy to the energy consumption in the island. The almost full reliance of Cyprus on imported oil to meet its energy demand, together with the abundance of solar radiation and a good technological base, created favourable conditions for the exploitation and development of solar energy in the island. Cyprys began manufacturing solar water heaters in the early 1960s and today it produces more than 30,000 m² of solar collectors yearly. It is estimated that more than 130,000 solar water heaters are in operation providing the equivalent of 9% of the total electricity consumption in the country; this corresponds to, approximately, 4% of the national energy consumption. However, the use of solar energy for space heating and cooling provides a further challenge, because it does not appear to be economic under the climatic conditions and system design practices currently prevailing in Cyprus. The paper provides a statistical analysis of the energy demand and identifies areas of further growth for solar energy technology.     

Assessment of the dye-sensitized solar cell

The field of solar electricity, or photovoltaics (PV), is rich in that there are many materials and concepts for converting sunlight into electricity. The technologies accepted as conventional are those well along in the process of commercialization. The dye-sensitized solar cell, developed in the 1990s, is a nonconventional solar electric technology that has attracted much attention, perhaps a result of its record cell efficiency above 10%. This paper reviews the technology, discusses new research results and approaches presented at a recent symposium of many of the world's important dye solar cell researchers, and presents an assessment of the dye-sensitized solar cell in a comparison with current conventional solar electric technologies. It concludes the dye solar cell has potential for becoming a cost-effective means for producing electricity, capable of competing with available solar electric technologies and, eventually, with today's conventional power technologies. But it is a relatively new technology and faces many hurdles on the path to commercialization. Because of its potential, this assessment recommends further funding for research and development (R&D) of the dye-sensitized solar cell technology on the basis of the promising technical characteristics of the technology, a strong US and worldwide research base, positive industry interest, and today's relatively small funding allocation for its R&D.     

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.     

An investigation of design concepts and control strategies of a double-stage expansion solar organic Rankine cycle

A small-scale, low-temperature solar organic Rankine cycle (ORC) is investigated, focusing on some key aspects of its design and operation, in order for the system to track the optimised point under variable weather conditions. Two major configurations can be identified for small-scale ORCs using two expansion stages, for improving the performance at variable loads. The first concerns the use of a cascade ORC and the second one of a single circuit with two in-series expanders. The first goal of this study is to theoretically investigate the performance of these two alternative configurations under variable heat inputs from evacuated tube solar collectors. The configuration finally selected is the ORC with two in-series expanders, showing higher efficiency for the entire range of incident solar radiations. This configuration is then further investigated, focusing on its operation and control, such as the by-passing of the first expander, in order to optimise its operation.     

Performance of a concentrating photovoltaic/thermal solar collector

The performance of a parabolic trough photovoltaic/thermal collector with a geometric concentration ratio of 37× is described. Measured results under typical operating conditions show thermal efficiency around 58% and electrical efficiency around 11%, therefore a combined efficiency of 69%. The impact of non-uniform illumination on the solar cells is investigated using purpose built equipment that moves a calibrated solar cell along the line of the receiver and measures short circuit current. The measured illumination flux profile along the length shows significant variation, despite the mirror shape error being less than 1 mm for most of the mirror area. The impact of the illumination non-uniformities due to the shape error, receiver support post shading and gaps between the mirrors is shown to have a significant effect on the overall electrical performance. The flux profile transverse to the receiver length is also investigated. Peak flux intensities are shown to be around 100 suns. The impact on efficiency due to open circuit voltage reduction is discussed.     

Mitigating the non-uniform illumination in low concentrating CPCs using structured reflectors

One problem in concentrating photovoltaic systems without active cooling is the formation of hot spots on the solar module cells. These hot spots are a result of uneven concentration of radiation within the solar module cells. The overall effect of concentrated heating is the reduction in the fill-factor of the solar module cell and the subsequent decrease in the overall efficiency of the system. In this paper, we investigate one alternative of improving the performance of a low concentrating photovoltaic system using semi-diffuse rolled reflective elements. Our results indicate that rolling marks on the reflector aligned parallel to the plane of the solar module cell improve the performance of the photovoltaic system.     

A review for the applications of solar chimneys in buildings

As a simple and practical bioclimatic design methodology, solar chimneys are receiving considerable attention for reducing heat gain and inducing natural cooling or heating in both commercial and residential buildings because of their potential benefits in terms of operational cost, energy requirement and carbon dioxide emission. In practical civil buildings, solar chimneys can be installed on the walls and roofs. For the purpose of improving natural ventilation performance and achieving better indoor thermal comfort, solar chimneys are always applied in the form of integrated configurations. Solar chimneys can also be used to combine with natural cooling systems so as to enhance the cooling effect inside buildings. Besides, active solar systems may be utilized to enhance the ventilation performance of solar chimneys. In this paper, the main configurations and the integrated renewable energy systems based on solar chimneys were summarized. Then the suggestions were given. Generally, solar chimney technology has been regarded as an effective and economical design method in low carbon buildings. As for the integrated energy systems based upon solar chimneys, it is still necessary to carry out more experimental investigations to acquire objective data for the system design. Besides, it is suggested to further study the optimization and control strategy of such integrated systems in different climates.