Parametric study and sensitivity analysis of a PV/microchannel direct-expansion CO2 heat pump

The work numerically investigates the effect of operating and ambient conditions on the performance of a new PV-heat pump configuration: a PV/microchannel evaporator, direct-expansion heat pump (DXHP) which uses CO2 in a transcritical cycle. A steady-state model of the heat pump is used to conduct a parametric study and sensitivity analysis. The variables considered are the evaporation temperature, CO2 and water mass flow rates, insolation, and ambient temperature. The parametric study shows that the system can achieve a COP of 5.16 with an outlet water temperature of 45 degrees C or an outlet water temperature of 95 degrees C at a COP of 3.34. At the baseline operating conditions, the maximum increase in PV efficiency due to the evaporator is similar to 1.20%-points. The PV efficiency increases with a reduction in evaporation temperature. It is insensitive to CO2 and water mass flow rates. The sensitivity analysis shows that the COP has a strong dependency on the CO2 and water mass flow rates. The water outlet temperature has a strong dependency on the water mass flow rate followed by the insolation, ambient temperature, and evaporation temperature. The results of the study can help guide how to operate the system. For example, for the average ambient conditions of Fargo, ND, an evaporation temperature between 0 and 2 degrees C, a CO2 mass flow rate of 0.017-0.024 kg.s(-1), and water mass flow rate of 0.020-0.026 kg.s(-1) should be used to maintain a relatively high COP (e.g. 3.8-4.8) while producing hot water over 55 degrees C.