Numerical Modeling of a Photovoltaic/Microchannel Direct-Expansion Evaporator for a CO2 Heat Pump

Abstract

Abstract A numerical model of a PV/microchannel direct-expansion evaporator for a CO2 heat pump is developed and validated with experimental data from the literature. The effects of degree of superheating, CO2 mass flux, and evaporation temperature on the amount of heat absorbed, pressure drop in the microchannel evaporator, PV temperature, and electrical efficiency are analyzed. The analysis shows that increasing the degree of superheating decreases the amount of heat absorbed, has minimal effect on the PV temperature (for superheating <15 °C), but reduces the pressure drop. The variation of CO2 mass flux has a minimal effect on the amount of heat absorbed and the PV temperature, but the pressure drop increases with increasing CO2 mass flux. Increasing the evaporation temperature decreases the amount of heat absorbed, reduces the pressure drop, and increases the PV temperature. For average ambient conditions for Fargo, North Dakota, a 5–10 °C of superheating at the evaporator outlet, an evaporator temperature between −5 and +5 °C, and a CO2 mass flux of 330–550 kg · m−2 · s−1 balance maximizing the heat absorption while minimizing the pressure drop. To maximize the PV efficiency, lower evaporation temperatures should be used. At an evaporation temperature of 0 °C and an insolation level of 1000 W · m−2, the CO2 microchannel evaporator causes a 23 °C reduction in PV panel temperature which corresponds to a 1.44% absolute increase in PV efficiency.