Assessment of Metal-Hydride Energy Storage Coupled with Heat Pumps and Solar PV for Residential Cooling and Heating
metal hydrides, energy storage, HVAC, load-shifting, payback period
The extension of time-of-use (TOU) rates to residential buildings has incentivized the use of thermal energy storage (TES) to shift heating and cooling loads away from on-peak hours. At the same time, the growing use of solar photovoltaics (PV) for on-site energy generation with variable buy back rates also encourages the use of TES to store energy generated at hours of peak solar radiation. Metal hydrides are reversible chemical compounds that form when certain metals react with hydrogen. Since these reactions depend on both temperature and pressure, metal hydrides can be used for thermochemical energy storage across a range of temperatures. Because of this, metal hydrides can be used as storage for both heating and cooling, enabling year-round utilization. In this paper, we present and evaluate a system that combines metal hydride energy storage with heat pumps and on-site PV. The system is designed to reduce operating costs by discharging the energy storage system during on-peak hours when electricity is expensive and charging the system during off-peak periods when electricity is less expensive and there is less incentive to sell excess PV electricity generation back to the grid. The TOU rates considered are designed for a future scenario with high solar penetration, where greater solar generation in the afternoon leads to on-peak periods in the mornings and evenings rather than the afternoon. The payback period of the energy storage system is estimated using energy savings relative to a baseline of a solar home without an energy storage system along with estimates of additional installed costs for the storage system. Payback period calculations for a range of rates show that rates with on-peak demand charges result in larger cost savings than rates with high on-peak rates, but even for the most favorable rates, the payback period is too long for the system to be economically viable.