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 thermopower 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.
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.
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 put in 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 the technical point of view, allows to obtain several advantages:
- 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;
- presence of an evaporating fluid allows to obtain 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);
- using hybrid solar panel, in addition to the advantage described in the previous point, the electrical efficiency of the panel increases (for similar considerations).
Generally speaking the use of this integrated system is an efficient way to employ the heat produced by the thermal panels in winter period, which wouldn't be normally exploited because of its too low temperature.
In comparison with only heat pump utilization, it is possible to reduce the amount of electrical energy consumed by the machine during the weather evolution from winter season to the spring, and then finally only use thermal solar panels to produce all the heat demand required (only in case of indirect-expansion machine), thus saving on variable costs.
Compared to geothermal heat pumps, the main advantage is that the installation of a piping field in the soil is not required, which results in a lower cost of investment (in the investment cost of a geothermal heat pump system about 50% is given by drilling costs) and in more flexibility of machine installation, even in areas in which there is poor space availability. Furthermore, there are no risks related to possible thermal soil impoverishment. A disadvantage, similarly to as occurs for air heat pumps, solar-assisted heat pump performances are affected by atmospheric conditions.
In general, a heat pump can evaporate at temperatures below the ambient temperature. In a solar-assisted heat pump this generates a temperature distribution of the thermal panels below that temperature. In this condition thermal losses of the panels towards the environment become additional available energy to the heat pump. In this case it is possible that the thermal efficiency of solar panels is more than 100%.
The simple configuration of solar-assisted heat pump as only solar panels as heat source for the evaporator. It can also exist a configuration with an additional heat source. The goal is to have further advantages in energy saving but, in the other hand, the management and optimization of the system become more complex.
