Numerical And Experimental Study On A High Concentration Photovoltaic/Thermal System

As one of the substitutes of traditional fossil energy, solar energy is clean, renewable and widely used. Photovoltaic technology, one of the important utilization patterns of solar energy, is worldwidely concerned. Concentrating photovoltaic technology uses the concentrator to improve surface energy density of PV module and reduce the necessary area of expensive photovoltaic cells. Therefore, it is an effective way to improve system efficiency and reduce system costs.Concerning the problem of uneven distribution of energy flux density in concentrating photovoltaic system and overtemperature of PV module, aiming to improve solar energy utilization efficiency and reduce system costs, this paper proposes a high concentration photovoltaic/thermal system, meanwhile designs and studies two high concentration photovoltaic/thermal systems (HCPV/T) of reflection and refraction with forced cooling device, by combining solar photovoltaic technology and photovoltaic/thermal technology. The systems collect waste heat besides electrical output, which improves photovoltaic efficiency and gains additional heat to improve overall efficiency of the system greatly. For the two HCPV/T systems, this paper establishes corresponding dynamic mathematical models and experimental test rigs, conducts intensive study on output characteristic of the photovoltaic cells, uniformity of energy flux density distribution and coupling relationship of photovoltaic and thermal features of the system under different concentration ratio, mass flow rate and other environment parameters by combining numerical simulation and experimental study.The main contents of this paper include the following aspects:Firstly, this paper establishes optical model of parabolic high-concentration concentrator, theoretically designs the focal length and area of the concentrator, according to optical theories such as concentration and solar image. To solve the problem of uniformity, this paper proposes multi-disc focal concentrator of high concentration ratio using a modular approach, and proposes the concentrating mode of replacing the continuous surface with plane mirror array. According to corresponding theoretical analysis, this paper builds a parabolic concentration system and a plane mirror array concentration system, and experimentally studied the energy flux density distribution. The result shows that the plane mirror concentration system fundamentally solves the problem of uniformity, and the system components are easily installed, low-cost, stable and of good wind resistance, while its concentration ratio is adjustable and the system can be applied to concentrating photovoltaic system of various concentration ratio.Secondly, combining high concentration ratio concentrator with plane mirror array and CPV/T module, this paper establishes a dynamic mathematical model and studies the design of the cells, the substrate and the heat exchanger under high energy flux. Moreover, this paper theoretically simulates temperature distribution of PV substrate, the heat exchanger and the cooling water along the flow direction, and calculates system thermal efficiency, electrical efficiency and overall efficiency. Additionally, this paper designs and studies a dual-axis tracking system according to photosensitive tracking and mechanical tracking program.Thirdly, this paper builds a HCPV/T system of plane mirror array, tests and calculates key temperature parameters, thermal efficiency, electrical efficiency, and overall efficiency of the system, while deeply analyses the influence of different concentration ratios, mass flow rates and environment parameters on the thermal and electrical efficiency of the system. Experimental results show that the HCPV/T system has good thermal and electrical performance, with electrical efficiency of22%, thermal efficiency of47%, and overall efficiency of about70%. Also, this paper experimentally and theoretically studies temperature coefficient of CPV/T module’s electrical output.Fourthly, this paper proposes a Fresnel HCPV/T system of water cooling device, and theoretically studies the system, analyses design and optical performance of the Fresnel prism. Dynamic model of the Fresnel cooling system is established, including system thermal model and electrical model. This paper also establishes the single diode five parameter model under standard conditions, discusses the solving method of five parameters, and gives the performance change equation at different temperatures and different irradiation conditions, while providing relatively accurate initial value method, and giving the program process of the thermal and electrical model of the system.Fifthly, this paper introduces traditional high concentration system with Fresnel array and related cooling fins, and proposes Fresnel HCPV/T system of water cooling module. Water cooling module of the Fresnel HCPV/T system is produced and replaces traditional cooling fins to form Fresnel water cooling module, and a test rig based on it is built. The experiment compares the performance of the Fresnel HCPV/T system with water cooling device and HCPV module based on air-cooled aluminum fins. Transient thermal efficiency of the Fresnel HCPV/T system can reach49.3%, electrical efficiency is26.5%, and overall efficiency of the system can be up to75.8%. The average thermal efficiency of the HCPV/T module can reach46.8%throughout the experiment. To intensively study thermal efficiency, electrical efficiency, and overall efficiency of the HCPV/T system, the experimental data and the simulation results are compared, while temperature coefficient of electrical efficiency of the system, performance under different inlet water temperature are studied.