Geothermal energy can be generated from heat beneath the earth’s surface or from heat in the atmosphere and oceans. Geothermal energy can be captured from naturally occurring underground steam to produce electricity. Another way in this category is by heating and cooling achieved by temperature differential between outside air and the ground or groundwater (heat pump). In other words, if there is a differential in temperature, the difference can either heat or cool a gas like a fridge or air conditioner.
This heat pump technology is usually combined with hot water heating or cooling and can be large or small. Large-scale applications are more for electrical power generation.