Experimental investigation on a heat pump water heater using R744/R290 mixture for domestic hot water

The natural eco-friendly mixture of R744/R290 was investigated to replace R22 in a fully instrumented water-water heat pump test rig. The test results show that the R744/R290 mixture with optimum concentration of 12%/88% (Mopt) is the suitable working medium for heat pump water heaters because of its higher COP and heating capacity than those of R22 under variable conditions as well as the nominal working condition. The heat pump system with Mopt can supply higher temperature hot water and promote compressor lifetime in substitution of R22. The test results also reveal that the heating COPs of heat pump system with Mopt are 4.98-11.00% higher than those of R22 at the heat sink outlet temperature ranging from 45 °C to 65 °C, and the heat sink outlet temperature has obvious effect on the system efficiency but little on the heating capacity.