Energy Factor Analysis for Gas Heat Pump Water Heaters (ST-16-C057)

Gas heat pump water heaters (HPWHs) can improve water heating efficiency with zero GWP and zero ODP working fluids. The energy factor (EF) of a gas HPWH is sensitive to several factors. In this work, expressions are derived for EF of gas HPWHs, as a function of heat pump cycle COP, tank heat losses, burner efficiency, electrical draw, and effectiveness of supplemental heat exchangers. The expressions are used to investigate the sensitivity of EF to each parameter. EF is evaluated on two bases: site energy (as used by DOE for rating water heater efficiency in the US), and source energy.

For gas HPWHs, it is found that using typical component efficiencies, EF will be 75-90% of the heat pump cycle COP. The contribution of each parameter to the difference between EF and cycle thermal COP is as follows: burner efficiency accounts for 50-80% of difference, parasitic electrical draws for 10 – 30%. Independent of COP, the presence of a condensing heat exchanger can make a 5-10% difference in EF, and tank losses reduce EF by 6 – 10%, depending on the insulation level.