A commonality among all renewable energy heat sources is that they perform best when coupled to low-temperature hydronic distribution systems.
Operating water temperature differences of 5 to 10º F can have a major effect on the COP (coefficient of performance) of geothermal water-to-water heat pumps as well as air-to-water heat pumps. Operating a flat-plate solar collector at an inlet fluid temperature of 110º rather than 170º can double the amount of solar energy it collects under clear skies on a cold by clear winter day.