Abstract
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 a site energy basis (as used by the US DOE for rating water heater EF), and a primary energy-basis energy factor (PEF) is also defined and included.
Typical ranges of values for the six parameters are given. For gas HPWHs, using typical ranges for component performance, EF will be 59 – 80% of the heat pump cycle thermal COP (for example, a COP of 1.60 may result in an EF of 0.94 – 1.28). Most of the reduction in COP is due to burner efficiency and tank heat losses. Gas-fired HPWHs are theoretically be capable of an EF of up to 1.7 (PEF of 1.6); while an EF of 1.1 – 1.3 (PEF of 1.0 – 1.1) is expected from an early market entry.
