We have spoken of heat pumps and of an ancient form of geothermal energy that can heat and light homes. Now a fairly recent innovation has wedding the two and made possible a source of home heat and cooling that can be used virtually everywhere in the world. At a price. Geothermal heat pumps (GHPs) work like a conventional heat pump that extracts warmth from even very cold air, uses it to warm interior space and then functions in reverse in hot weather. In the case of a geothermal pump, however,...