Ground source heat pump, a sustainable source of energy. Llanerchaeron, Wales, UK. Used for heating Llanerchaeron House - a Welsh property of The National Trust. The National Trust have combined efficiency measures and sustainable heating technologies to reduce energy use. As well as cutting down on energy use it has also installed renewable sources of energy, including solar PV and hydro power. Ground source heat pumps, like the one shown here, draw heat from deep in the earth to transform into electricity or keep as heat.