Numeric model of a direct expansion solar assisted heat pump water heater operating with low GWP refrigerants (R1234YF,R290, R600A and R744) for replacement of R134A

Abstract

Heating water through of heat pump has a reduced consumption of electric energy duetotheuseoftheenergyavailableintheenvironment. Thisworkexploresthelackofstudies of direct expansion solar assisted heat pump (DX-SAHP) using refrigerants with low Global Warming Potential (GWP). The numeric model proposed uses a lumped parameter model for the heat exchangers and a semi-empirical model for the compressor. This model was validated fromanexperimentalsetuprunningwithR134a,equippedwithtwotypesofcondenser,comparing the experimentaland theoretical results ofCoefcient of Performance (COP). These results are available in the literature and were obtained under a solar radiation from 0 to 1100W/m2, an ambient temperature between 24.8 and 34.5oC, a nal water temperature around 45oC, and a wind velocity between 0 and 2.4 m/s. The mean difference between measured and calculated COP is 2%. The refrigerants with low GWP analyzed are R290, R600a, R744, and R1234yf. R290 has better COP for solar radiation between 50 W/m2 and 700 W/m2, as well as for environment temperature between 10oC and 35oC. On the other hand, for solar radiation less than 50 W/m2, the R134a has better COP than R290. Environmental analysis indicates that the indirect emissions are the most important effect, and then, the Total Equivalent Warming Impact (TEWI)resultsalmostfollowedtheCOPoutcome. ThelowestTEWIwasobtainedusingR290 and an economic analysis was performed usingthis refrigerant. An optimum collectorarea and condenser length were found in the simulations to reduce the payback time. The optimum size of the condenser increases with the augment of solar radiation, but the optimum collector size remains approximately constant with the increase of solar radiation. The payback time of a R290 DX-SAHP in Belo Horizonte is 3.9 years and is 13% higher than the payback time available in the literature for R134a DX-SAHP