On the Potential of Using Heat From Solar Thermal Collectors for Heat Pump Evaporators
Abstract
Solar thermal collectors can be used in combination with heat pumps to cover the heat demand for space heating and domestic hot water preparation. Different concepts exist for the combination of these two components into a system concept. Some of these concepts offer the ability to switch from using heat from the solar collectors directly to serve the demand to using heat from the solar collectors indirectly as a heat source for the evaporator of the heat pump. In the same system concept, the heat pump may be able to switch from using heat from the solar thermal collector to using an alternative low temperature heat source, such as ambient air. In this paper a general mathematical relationship is derived for determining whether using heat from solar collectors for the evaporator of the heat pump instead of using it directly is beneficial for the energetic performance of these systems. It is shown that there is a limit for the solar irradiation on the collector field above which using collector heat for the evaporator of the heat pump instead of using it directly is not advantageous. This irradiation limit depends on the characteristic performance curves of the solar collector and of the heat pump, as well as on the operating conditions, above all on the temperature levels of the heat sink and the different heat sources. Based on dynamic annual simulations, examples are shown for the maximum amount of heat that can be delivered from the collector to the heat pump evaporator at times where this mode of operation is of advantage for the performance of a solar and air source heat pump heating system. Both the mathematical relationship and the examples show that using solar collector heat for the evaporator of a heat pump is more beneficial for systems that operate with a large temperature difference between the non-solar heat source and the heat demand.