Solar-assisted heat pump

A solar-assisted heat pump (SAHP) is a system that combines a heat pump and thermal solar panels and/or PV solar panels in a single integrated system. Heat pumps require a low temperature heat source, which can be provided by solar energy. Typically, these two technologies are used separately (or only placing them in parallel) to produce warm air or hot water. In this system the solar thermal panel acts as the low temperature heat source, and the heat produced feeds the heat pump's evaporator. The goal of this system is to get high coefficient of performance (COP) and then produce energy in a more efficient and less expensive way. Air source heat pumps, which are preheated by solar air collectors, have an additional benefit of lower maintenance as the outside fan unit can be protected from the harsh winter environment.

Solar PV energy can power the heat pump electrically to enable electrification of heating buildings and greenhouses. These systems enable electrification of heating/cooling and are normally driven by economics and decarbonization goals. Such systems have been shown to be economic in the Middle East, North America, Asia and Europe.

It is possible to use any type of solar thermal system with air or liquid collectors, (sheet and tubes, roll-bond, heat pipe, thermal plates) or hybrid (mono/polycrystalline, thin film) in combination with the heat pump. Using hybrid panels, however, is most preferred because it allows covering a part of the electricity demand of the heat pump and reduces the power consumption and consequently the variable costs of the system.

Solar air collectors operate at maximum efficiency when heating ambient air and thus are ideal for supplying warm air to air source heat pumps. For solar liquid based systems the operating conditions' optimization of this system is the main challenge, because there are two opposing trends of the performance of the two sub-systems: by way of example, decreasing the evaporation temperature of the working fluid increases the thermal efficiency of the solar panel but decreases the performance of the heat pump, and consequently 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 a renewable source. For PV powered heat pump systems the goal is still to reduce grid-power, but there is an additional optimization to maximize self-sufficiency and self-consumption of PV and the energy imported/exported to the grid. Best practices have been developed to model PV-powered heat pumps that can be done with a range of open source software tools like TRNSYS, EnergyPlus and System Advisory Model (SAM).

Solar heated air source heat pumps are relatively simple to implement by connecting the outlet of the solar air collectors to the fan inlet of the heat pump. For liquid solar collectors, there are two possible configurations with heat pumps, 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:

Generally speaking the use of this integrated system is an efficient way to employ the heat produced by the thermal panels in winter period, something that normally would not be exploited because its temperature is too low.

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.

In comparison with a system with only thermal panels, it is possible to provide a greater part of the required winter heating using a non-fossil energy source.