Many hydronic-based renewable energy systems include thermal storage tanks. When the heat input is from solar thermal collectors or biomass boilers, it is possible for the water temperature in those tanks to reach temperatures of 180-200º F. Thermal storage tanks supplied by electric boilers operating on off-peak electricity also can reach these high temperatures by the end of the charging cycle.
Although few would argue a 500 gal. tank filled with 190º water doesn’t contain a lot of energy, the usefulness of that energy depends on the supply temperature required by the heat emitters to meet the building’s heating load. The lower this temperature, the greater the temperature drop the tank can undergo and thus the more useful energy it can deliver.