Thermoelectric (TE) modules utilize available temperature differences to generate electricity by the Seebeck effect. The current study investigates the merits of employing thermoelectrics to harvest additional electric energy instead of just cooling concentrating photovoltaic (CPV) modules by heat sinks (heat extractors). One of the attractive options to convert solar energy into electricity efficiently is to laminate TE modules between CPV modules and heat extractors to form a CPV-TE/thermal (CPV-TE/T) hybrid system. In order to perform an accurate estimation of the additional electrical energy harvested, a coupled-field model is developed to calculate the electrical performance of TE devices, which incorporates a rigorous interfacial energy balance including the Seebeck effect, the Peltier effect, and Joule heating, and results in better predictions of the conversion capability. Moreover, a 3D multiphysics computational model for the HCPV-TE/T water collector system consisting of a solar concentrator, 10 serially connected GaAs/Ge photovoltaic (PV) cells, 300 couples of bismuth telluride TE modules, and a cooling channel with heat-recovery capability, is implemented by using the commercial FE-tool Comsol Multiphysics V R . A conjugate heat transfer model is used, assuming laminar flow through the cooling channel. The performance and efficiencies of the hybrid system are analyzed. As compared with the traditional photovoltaic/thermal (PV/T) system, a comparable thermal efficiency and a higher 8% increase of the electrical efficiency can be observed through the PV-TE hybrid system. Additionally, with the identical convective surface area and cooling flow rate in both configurations, the PV-TE/T hybrid system yields higher PV cell temperatures but more uniform temperature distributions across the cell array, which thus eliminates the current matching problem; however, the higher cell temperatures lower the PV module's fatigue life, which has become one of the biggest challenges in the PV-TE hybrid system.
In this paper, a multiphysics, finite element computational model for a hybrid concentrating photovoltaic/thermal (CPV/T) water collector is developed. The collector consists of a solar concentrator, 18 single junction germanium cells connected in series, and a water channel cooling system with heat-recovery capability. The electrical characteristics of the entire module are obtained from an equivalent electrical model for a single solar cell. A detailed thermal and electrical model is developed to calculate the thermal and electrical characteristics of the collector at different water flow rates. These characteristics include the system temperature distribution, outlet water temperature and the thermal and electrical efficiencies. The model is used to study the effect of flow rate on the efficiencies. It is found that both efficiencies improve as the flow rate increases up to a point (0.03 m/s), and after that point remain at relatively constant levels. However, as the flow rate increases the outlet water temperature decreases, reducing the quality of the extracted thermal energy. In addition to the thermal and electrical modeling, finite element analysis is used to estimate the fatigue life of the module based on the different temperature profiles obtained from the thermal model at flow rates of 0.01 m/s and 0.03 m/s. Results show that for the higher flow rate, the outlet water temperature decreases, but the fatigue life improves. Based on the fatigue life model predictions, it is shown that the thickness of die attach layer must be increased for high outlet temperature applications of the hybrid CPV/T collector.
Excess temperatures in concentrating photovoltaic (PV) modules can lead to electrical efficiency loss and irreversible structural damage. Therefore, designing an appropriate cooling system is necessary to increase the lifetime and performance of concentrating PV modules. The basic design considerations for cooling systems include low and uniform cell temperature, minimal pumping power, high PV electrical efficiency and system reliability. In this paper, a 3D multiphysics computational model for a hybrid concentrating photovoltaic/thermal (HCPV/T) water collector is developed and implemented using the commercial FEA software COMSOL TM . The collector consists of a solar concentrator, 40 silicon cells connected in series, and a tree-shaped channel cooling system with heat-recovery capability. Laminar flow and conjugate heat transfer through the tree-shaped branching channel cooling networks is investigated. The temperature profile along the cells is determined for different cooling strategies. Comparisons are made of the thermal and electrical operating conditions, such as the silicon cell temperature, electrical efficiency, and total pressure drop in the collector incorporating a tree-shaped channel network with a collector having a straight parallel channel cooling array. For the same total convective surface area and pressure drop (15Pa) in both configurations, the tree-shaped channel cooling networks yield a 10 o C lower maximum cell temperature and a more uniform temperature distribution between the cells. In addition, the temperature distribution obtained in the collector with the tree-shaped channel cooling system reduces the 'current matching problem' between the cells along the flow direction and reduces the thermal stresses significantly, thus increasing the reliability of the system. KEY WORDS: HCPV/T system, Tree-shaped channel network, Thermal management, System efficiency, Cell temperature, FEA 978-1-4244-9532-0/12/$31.00 ©2012 IEEE
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