Water is much better by mass, and your heat pump will only work well at temperatures where it's a liquid. With a 30 degree delta in a system that uses 1MWh/mon

Water is much better by mass, and your heat pump will only work well at temperatures where it's a liquid. With a 30 degree delta in a system that uses 1MWh/month you could store 2 months of heat/cold in a 5mx5mx2m tank.