Optimization of heat transfer systems and use of the environmental energy potential: application to compact heat exchangers and heat pumps

Abstract

In this thesis, the optimization of forced convection heat sinks and groundwater- -source heat pumps is addressed with the purpose of improving energy efficiency. Parallel ducts heat sinks are considered under constrained (fixed) pressure drop, pumping power and heat transfer rate. The intersection-of-asymptotes method is employed together with numerical simulations and relationships for determining optimum hydraulic diameter are put forward. An optimal design emerges under fixed heat transfer rate, which matches that found through the joint minimization of pressure drop and pumping power. With regard to heat pumps optimization, the relation between coefficient-of-performance and air-to-ground exergy potential is established, showing that energy saving as compared to air-to-air systems depends on the square root of that potential. The exergy potential in the ´Evora region is estimated, and exergy analysis of groundwater-source systems helps identifying distinct conditions of operation: maximum/null net exergy output and best trade-off between environmental exergy utilization and power input; RESUMO:Esta tese apresenta a optimiza¸c˜ao de dissipadores de calor e de bombas de calor geot´ermicas para melhoramento da eficiˆencia energ´etica. S˜ao estudados dissipadores de passagens paralelas sujeitos a constrangimentos de queda de press˜ao, potˆencia de bombeamento e taxa de transferˆencia de calor. Utiliza-se o m´etodo da intersec¸c˜ao- -das-assimptotas juntamente com simula¸c˜oes num´ericas, e apresentam-se express˜oes do diˆametro hidr´aulico ´optimo. Um design ideal emerge no caso de transferˆencia de calor fixa, o qual se aproxima da minimiza¸c˜ao conjunta de queda de press˜ao e potˆencia de bombeamento. Relativamente `as bombas de calor, estabelece-se uma rela¸c˜ao entre coeficiente-de-performance e potencial de exergia ar-solo, e mostra-se que a redu¸c˜ao de consumo energ´etico comparado com sistemas ar-ar depende da raiz quadrada desse potencial. O potencial de exergia em ´Evora ´e avaliado, e identificamse v´arias condi¸c˜oes de funcionamento usando an´alise exerg´etica: m´axima/nula sa´ıda l´ıquida de exergia e compromisso ´optimo entre utiliza¸c˜ao de exergia ambiental e potˆencia fornecida.