Experimental assessment of a CO2 direct-expansion solar-assisted heat pump operating with capillary tubes and air-solar heat source

This paper presents an experimental study on a direct-expansion solar-assisted heat pump (DX-SAHP) operating with capillary tubes in a transcritical carbon dioxide cycle. An air-solar dual-source evaporator is also proposed as an alternative to mitigate the performance drop in low solar radiation conditions. The experiments followed a factorial design, considering capillary tubes (number 1 and number 2) and operating schemes (Low Solar Radiation, Low Solar Radiation with fan, and High Solar Radiation) as factors. The results showed that altering from Low Solar Radiation (7 W/m2) to High Solar Radiation (969 W/m2) caused an augment on the heating capacity (58%), on coefficient of performance (from 2.19 to 3.12; 42%), on the refrigerant mass flow rate (35%), and on the degree of superheat (from 4.4 K to 30.6 K). Changing from Low Solar Radiation to Low Solar Radiation with fan, a 17% improvement on the heating capacity was found, with no variation on coefficient of performance. Besides the evaporating pressure, the gas cooler pressure was also sensitive to solar radiation variations. It is estimated that for lower levels of solar radiation, up to 570 W/m2, a single capillary tube could be a reasonable choice for a DX-SAHP. This work aims to be the first to experimentally investigate the use of capillary tubes in a DX-SAHP under solar radiation variations greater than 960 W/m2.