Experimental study of a CO2 direct-expansion solar-assisted heat pump operating with an adiabatic coiled capillary tube

Heat pumps are equipment used to promote space heating, to supply hot water or used for other heating purposes. The number of these systems should raise 16 times by 2050, as a contribution to meet the Paris agreement objectives. Despite presenting a relatively high efficiency, e.g. compared to electric resistance heaters, heat pumps still need to be developed. Direct-expansion solar-assisted heat pumps (DX-SAHP) are one alternative to improve the performance of air source heat pumps (ASHP). In this work, the behavior of a CO2 DX-SAHP is investigated under different operational conditions. However, the adjustable area expansion device (e.g. electronic expansion valve – EEV), generally used, was replaced by a capillary tube as an alternative to reduce the manufacturing costs. Initially, an algebraic solution to design an adiabatic coiled capillary tube operating in transcritical CO2 cycle was developed. Three different friction factors and three different k factors, related to the specific volume, were analyzed, creating a total of 9 possible combinations. After designing and assembling the tubes onto the workbench, tests were performed following a factorial design, with 2 different capillary tubes and 3 different operating conditions, namely: high solar radiation (HSR), low solar radiation (LSR), and low solar radiation with fan (LSR+fan). The results indicated an average augment of 57.9% on the heating capacity, 42.3% on COP, and 35% on the refrigerant mass flow rate, when changing from LSR (6 W/m²) to HSR (969 W/m²). The superheat degree went from 4.4 to 30.6°C and the evaporating pressure changed from 40.0 to 51.2 bar. However, the gas cooler pressure also showed a significant increase, from 83.5 to 87.9 bar. To limit the raise of the superheat degree and to keep the compressor integrity, minor modifications are recommended. Altering from LSR to LSR+fan, an improvement of 17.2% on heating capacity was found, with no penalty to COP, showing that the addition of the fan is advantageous in low solar radiation conditions. Apart from the factorial design, additional runs were carried out to increase the number of experimental points to validate the capillary tube algebraic solution. Overall, the C-M&N friction factor demonstrated to be the best for the proposed solution. The percentual of mass flow rate points predicted within 10% and 15% error bands were 95% and 100%, respectively, for the best combination.