Geothermal energy is obtained from thermal energy generated and stored in the Earth. The most common form of geothermal energy systems in Scotland provide heating through a ground source heat pump. These devices transfer energy from the thermal reservoir of the earth to the surface via shallow pipe works, utilising a heat exchanger. Ground source heat pumps generally achieve a Coefficient of performance of between 3–4, meaning for each unit of energy in, 3-4 units of useful heat energy is outputted. The carbon intensity of this energy is dependent on the carbon intensity of the electricity powering the pump.