Geothermal energy systems are one of the most sustainable power options, and are the only renewable energy source that does not come originally from the sun. There are two types of geothermal systems- heat pumps and turbine generators. The distinction is that heat pump systems are useful if only thermal regulation is desired, while turbine generators are a better choice for fulfilling electricity needs. Because geothermal systems use the temperature difference between the earth’s surface and subterranean levels as the energy source, there is no need for a centralized generator. This makes geothermal a very scalable power option, meaning it’s a good choice for even a single residence.

Geothermal heat pumps can be likened to the more “passive” geothermal option, just as thermal solar is more “passive” than photovoltaics. The basic principle behind using geothermal energy for thermally conditioning a building is that the subterranean environment maintains a more consistent temperature than the earth’s surface. This makes it a good resource for heating in the winter, and cooling in the summer. Some areas may also have pools of extremely hot subterranean water, which can make the heating component even more powerful. The working principle of a geothermal heat pump is that a fluid, often water, is circulated between the residential space and the earth below, using thermal radiation to transfer heat energy to where it is desired to be moved.

Areas that do have accessible reservoirs of very hot subterranean water, or even steam vents, are good candidates for the second type of geothermal energy system, a turbine generator. This is the more “active” form of geothermal generation, and it can be likened to the use of photovoltaic panel arrays in solar energy generation. The principle behind this type of system is much like a traditional turbine generator. Fluid (water) that comes in contact with the heat source, in this case the earth’s internal heat, will become steam and will rise. This rising steam causes the rotor of a turbine generator to turn, this turns the shaft, and that turning generates electrical current which is transported away using conductive wires. Once generated, this can be carried longer distances than the thermal energy of heat pump geothermal systems, and can be used for end uses other than heating, such as powering appliances. Once the steam rises out from the hot area, it will cool, and sink back down as water, beginning the process again. This circular use of water is just another reason why geothermal generation is such a sustainable choice.

While geothermal energy systems are well suited to single residences, their use is not limited to small applications. There are geothermal power plants at the scale of typical energy utilities, and even at the scale of entire countries. The country of Iceland is currently generating 25% of its energy from geothermal power plants.¹ These energy systems can function very differently depending on the scale they are designed to serve. What remains constant, however, is that this is an incredibly valuable and sustainable way to harness available energy for human use.