\A solar-assisted heat pump (SAHP) is a machine that represents the integration of a heat pump and thermal solar panels in a single integrated system. Typically these two technologies are used separately (or only placing them in parallel) to produce hot water. In this system the solar thermal panel performs the function of the low temperature heat source and the heat produced is used to feed the heat pump’s evaporator. The goal of this system is to get high COP and then produce energy in a more efficient and less expensive way.
It is possible to use any type of solar thermal panel (sheet and tubes, roll-bond, heat pipe, thermal plates) or hybrid (mono/polycrystalline, thin film) in combination with the heat pump. The use of a hybrid panel is preferable because it allows covering a part of the electricity demand of the heat pump and reduce the power consumption and consequently the variable costs of the system.
Optimization
The operating conditions’ optimization of this system is the main problem, because there are two opposing trends of the performance of the two sub-systems: by way of example, a decreasing of the evaporation temperature of the working fluid generates an increasing of the thermal efficiency of the solar panel but a decreasing in the performance of the heat pump, with a decreasing in the COP. The target for the optimization is normally the minimization of the electrical consumption of the heat pump, or primary energy required by an auxiliary boiler which supplies the load not covered by renewable source.
Configurations
There are two possible configurations of this system, which are distinguished by the presence or not of an intermediate fluid that transports the heat from the panel to the heat pump. Machines called indirect- expansion mainly use water as a heat transfer fluid, mixed with an antifreeze fluid (usually glycol) to avoid ice formation phenomena during winter period. The machines called direct-expansion place the refrigerant fluid directly inside the hydraulic circuit of the thermal panel, where the phase transition takes place. This second configuration, even though it is more complex from a technical point of view, has several advantages:
(1)a better transfer of the heat produced by the thermal panel to the working fluid which involves a greater thermal efficiency of the evaporator, linked to the absence of an intermediate fluid;
(2)presence of an evaporating fluid allows a uniform temperature distribution in the thermal panel with a consequent increase in the thermal efficiency (in normal operating conditions of the solar panel, the local thermal efficiency decreases from inlet to outlet of the fluid because the fluid temperature increases);
(3)using hybrid solar panel, in addition to the advantage described in the previous point, the electrical efficiency of the panel increases (for similar considerations).
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Post time: Sep-28-2022