An abandoned farm in rural Spain, a mile from the nearest tiny village, is the unlikely site of cutting-edge energy technology. The new farmhouse under construction on the property will eventually be powered partly by the owners’ poop. The mini waste-to-energy system takes organic waste—straight from toilets, from kitchen scraps, and from horse manure on the farm—and converts it into energy that can provide hot water, heat, and gas for cooking. The same technology is…