I don't understand the animus in replies to your post. It's as if people want to deny that people like myself heat our homes in the winter by blowing air over hot water in a heat-exchanger. We can and many do heat that water during the day with the magic of the sun's rays or by burning wood (the only true renewable we have). Humans have been using thermal property of rock (not concrete, concrete doesn't match rock's efficiency) and water to heat ourselves and our homes for hundreds of years, and will likely continue to do so for hundreds more.