Thermodynamic analysis and optimization of densely-packed receiver assembly components in high-concentration CPVT solar collectors

Abstract Concentrated photovoltaic thermal (CPVT) solar collectors are one of the most promising solar concepts due to their compactness, multi-output nature, and high exergy efficiencies. However, accurate design models and clear simulation algorithms on the component-level are critical for the proper system-level engineering and evaluation of CPVT collectors. In this study, detailed design models and simulation algorithms of three state-of-the-art components commonly incorporated into the densely-packed receiver assemblies of high-concentration CPVT solar collectors are presented. These components, namely multi-junction photovoltaic cells, segmented thermoelectric generators with interconnectors, and finned minichannel heat extractors, could be integrated to form CPVT receiver assemblies in a number of different configurations. Thermodynamic component-level analyses that avoid oversimplified as well as computationally-expensive modeling approaches and provide clear and robust simulation algorithms with reasonable accuracy are separately developed for the three addressed components. Performance variations of InGaP/InGaAs/Ge cells with respect to cell temperature and flux concentration ratio are identified using a two-diode equivalent circuit model and relations for the irradiance-dependent temperature coefficients are provided. The effects of heat source/sink temperature and thermal impedance, load resistance, thermal and electrical contact resistance, and geometrical parameters on the performance of segmented thermoelectric generators are identified using a 1D thermoelectric model. The thermal and hydraulic performance of minichannel heat extractors when designed under fixed mass flowrate or fixed HTF velocity operation modes, as the number of minichannels is varied and using pure and nanoparticles-suspended HTFs, are studied to find their optimum geometries using a 1D total effective thermal resistance model. The obtained results provide valuable insight into the critical factors to be taken into account in the engineering of the addressed CPVT receiver assembly components. The separately-series equivalent thermal resistance network technique is employed in the thermal analyses in order to treat two-dimensional steady-state heat transfer in composite structures with different thermal conductivities as one-dimensional without a loss of accuracy. Finally, using the developed design models and simulation algorithms, constrained non-linear multi-variable geometric optimization of the assembly components has been carried out to obtain minimum pumping power, maximum extractor heat transfer coefficient, and maximum thermoelectric power output. Results show the existence of optimum geometrical design vectors, given a set of operation conditions, ensuring that the system-level performance of a CPVT employing the optimized components is maximized.