Development and optimization of a concentrating photovoltaic/thermal cogeneration system

Abstract

The scope of the doctoral dissertation is the theoretical and experimental evaluation of a linear Concentrating Photovoltaic Thermal (CPVT) system. The system comprises three distinct constituents, namely the optical device to concentrate solar irradiation, the solar-cell module and the active cooling system. Concentrated solar irradiation is incident on the active surface of the solar-cell module at the backside of which, a suitable cooling device (heat sink) is thermally bonded to extract excess heat. The most significant parameters that affect the operation of a CPVT system are the intensity of the solar irradiation and the temperature of the solar cells.At a first stage, several heat-sink layouts were evaluated using three-dimensional, computational fluid dynamics and heat transfer models in terms of, among others, thermal resistance, temperature uniformity of the heated surface and cooling fluid pressure drop. The configurations taken into consideration can be divided into two general groups based on the cross-section of the ducts employed for the flow of the cooling fluid (water): configurations with orthogonal and circular ducts, respectively.The most promising heat-sink designs were subsequently optimized using a novel, muti-objective optimization procedure, which allows the determination of the optimal heat-sink geometrical parameters through the use of a genetic algorithm. The optimal devices were manufactured and experimentally evaluated. The performance evaluation of the manufactured heat sinks comprised the measurement of the induced fluid pressure drop and the substrate temperature for constant heat-flux heating and negligible heat losses.In reference to the development of the integrated CPVT system, the main focus initially was on the procurement of appropriate materials and the manufacturing of the custom-made components required for the development of the prototype system. The optical quality of the parabolic trough was experimentally evaluated with the use of an appropriate measuring device that bears a grid of sensors (photodiodes), so that the irradiation distribution on the receiver surface and the achieved concentration ratio can be determined. Besides, the main factors that have a significant effect on the trough optical quality were designated using a ray-tracing software.An experimental rig was developed for the evaluation of the integrated CPVT system. The experimental evaluation led to the determination of the solar-cell modules voltage-current curves and temperature factor, as well as the electric and thermal efficiency of the system. In addition, the main thermal losses mechanisms were identified and the CPVT system thermal performance was compared to that of flat-plate thermal collectors, commercially available in the Greek market.Finally, a computational code was developed in FORTRAN for the dynamic simulation of the CPVT system long-term performance. A dynamic theoretical model was formulated that allows the prediction of the system energetic and exergetic efficiency, as well as its output in a daily, monthly or annual basis. The characteristics of the system sub-components that have the most significant impact on its exergetic performance are highlighted.