If you don't want fossil fuels (even natural gas), then it'll likely be a ground source heat pump. You just drill a number of deep wells -- each 100' of well g

If you don't want fossil fuels (even natural gas), then it'll likely be a ground source heat pump. You just drill a number of deep wells -- each 100' of well gives you about 1 ton of heating/cooling (1 ton is 12,000 BTU/hrs or ~3.5KWh). For a decent sized building you'd be looking at probably a dozen or two 1-2K ft deep wells. The nice thing is, you don't need much room on site, you can even drill them into the basement after the foundation is done before the steel goes up. With a split system, the heat-exchangers can be in the basement and the refrigerant (R410A) run to the air handlers located elsewhere. Of course, the more you insulate the building, the less heating and cooling you need.