Cost-Effectiveness of a Photovoltaic-Powered Heat Pump Water Heating System vs. Solar Thermal Water Heating

The cost-effectiveness of a photovoltaic (PV) powered heat pump water heater (HPWH) system is compared to that of a traditional solar thermal water heating system. HPWH evaporators are most often located inside the conditioned building space, resulting in a year-round cooling effect in the building. This effect is beneficial during the cooling season but detrimental during the heating season. The significance of this cooling effect was evaluated as part of the life cycle cost (LCC) analysis of the PV-powered HPWH system. Four different locations were considered: Boulder, CO; Miami, FL; Chicago, IL; and Seattle, WA. For the solar thermal analysis, both electric resistance and gas-fired auxiliary water heating scenarios were considered. Life cycle costs for the PV-HPWH system were calculated for the case of a PV system dedicated to providing electricity for the HPWH, and for the case of a previously planned residential PV system being increased in size to accommodate the HPWH. This latter case uses a lower, incremental cost of increasing the size of the PV system. The most notable results of the analysis are summarized below: • In general, the solar thermal system is more cost effective than the PV-HPWH system, even using the incremental cost of increasing the size of a planned PV system. • In locations where there are incentives that apply to PV but not solar thermal systems, as in much of Colorado, the PV-HPWH system will be more cost-effective than solar thermal. • The cooling effect of the HPWH evaporator is a net benefit in Miami, FL, but a net penalty in the other three locations. • The PV-HPWH system becomes more cost-effective than solar thermal with gas auxiliary in Miami when the price of natural gas is increased from $1 to $1.50 per therm. • Increasing the price of gas in the other locations does not make the PV-HPWH system compete against solar thermal because the cooling effect penalty also increases with the price of natural gas.