Many energy sources exist today that can replace traditional energy supplies. Ground source heat pumps extract heat from the ground to heat a building, while reversing it can suck the heat out and cool the building. Biogas is produced by microbial digestion of manure and can be used for heat or electricity. Wind energy converts the moving energy of the wind and converts it into electricity or mechanical energy (they typically require about 15 km/h of wind speed). Solar energy can be used, but is dependent on the climate. Biodiesel is derived from plant or animal oil but requires careful consideration. Hydroelectric power is water-turning turbines to create energy (obviously the barn must be located near a source of water). Cogeneration uses a single energy source and outputs electricity and thermal energy in the form of hot water or steam. Microturbines can run on low or high-pressure natural gas, propane, diesel, and up to 7% H2S. Fuel cells are devices that convert hydrogen and oxygen into electricity and heat.