U.S. Dept. of Energy, EERE, Nov. 15, 2016, by Timothy Patrick Reinhardt.
http://energy.gov/eere/articles/heat-beneath-ground-working-advance-deep-direct-use-geothermal

"Beneath our feet lies a source of geothermal heat that can be tapped to provide heating and cooling to both residential and commercial buildings, manufacturing processes, greenhouses and aquaculture ponds. Deep direct-use (DDU) systems are an emerging technology area in the geothermal sector that draw on lower temperature geothermal resources. Deeper than geothermal heat pumps and other conventional direct-use systems, DDU is deployable at a similar temperature range—between 100°F and 300°F—but at a much larger scale. DDU maximizes system efficiencies and return on investment."