Use of R-744 in a solar assisted heat pump for residential heating applications

Heat pumps are fundamental to decarbonize the building heating sector. To reduce the emissions of greenhouse gases into the atmosphere, a prototype heat pump working with carbon dioxide (R-744) as refrigerant is presented. However, R-744 heat pumps have to operate a trans-critical cycle to produce hot water and this penalizes their performance compared to systems operating subcritical cycles.

The present prototype adopts several innovative solutions: a double heat source at the evaporator and the electricity production by means of photovoltaic-thermal modules. Indeed, the present heat pump water heater for residential building applications can work with the solar radiation or the air as low temperature heat sources. In the solar mode, photovoltaic-thermal collectors are used to evaporate the R-744, whereas in the air mode a conventional finned coil is used.
The photovoltaic-thermal collector has a double effect: it allows to evaporate the R-744 that flows in the tubes and to cool down the photovoltaic cells improving the photovoltaic conversion efficiency.

In this study, the heat pump is described in detail and a comparison between the various operative modes is presented. The results show that at partial loads and under high irradiance, the heat pump working with the PV-T evaporators displays an evaporation temperature close to the air temperature and the performance coefficient can increase up to 30 % compared to the air mode. In addition, the measurements allow to validate a mathematical model of the heat pump, which can be used to predict the performance when varying the ambient conditions or the photovoltaic-thermal evaporator area. The iso-COP maps obtained from the model can be used to develop a control algorithm of the heat pump to decide a switching strategy between the air-source and the solar source but also to determine the minimum evaporator area in the design of the system.