Almost everyone understands solar energy, wind power and hydroelectricity intuitively. Solar energy comes from the sun’s warming rays. Wind power is produced by the natural movement of air in the environment. Hydroelectricity is generated by the flow of water. However, there is a fourth source of renewable energy that has a huge amount of potential but is not as well understood by the general public: geothermal energy. Geothermal energy is the heat emitted by the Earth, from the core to the crust. The extraction of geothermal energy makes use of the difference in temperature between the earth’s surface and warmer regions below ground.
Core to Crust
The earth has a super-hot core that consists primarily of molten metal. Above that is the mantle, which is hot and flexible enough to shift over long periods of time. The surface of the Earth is the outer edge of the crust, which is solid and is divided into tectonic plates that move very slowly in response to changes in the mantle. The crust is as little as five kilometres thick under the oceans and up to 70 kilometres thick under the continents. In the upper crust, the temperature underground increases by as much as 30 °C per kilometre of depth. According to the Union of Concerned Scientists, the outer ten kilometres of the earth’s crust contains 50,000 times more potential energy than the world’s gas and oil reserves.
The Availability of Geothermal Energy
Hot spots near cracks in the tectonic plates, volcanos and other geologically active areas have the greatest potential for providing heat, but geothermal energy can be a useful source of renewable power almost anywhere. Because of the steep temperature gradient below ground, the difference between the temperature at the Earth’s surface and the temperature just four metres below can lower heating and cooling costs significantly for most buildings. Super-heated water from below the earth’s surface is also a useful source of energy.
Geothermal Energy Extraction: Small Scale Systems
The most common type of geothermal system in use today is the geothermal heat pump, also known as the ground source heat pump. This is an underground loop that allows air, water or refrigerant to transfer heat from below the surface of the ground. Geothermal heat pumps can be used for both heating and cooling, and they are especially effective in extreme climates. Geothermal heat pumps provide a constant, moderate temperature that is easily brought into line with human comfort and they can pre-heat water, as well. Under most circumstances, geothermal heat pumps do not eliminate the need for conventional heating systems, but they can reduce energy use significantly.
Geothermal Energy Extraction: Large Scale Systems
Large scale geothermal energy projects are already in use in many geologically active areas, and most of these projects are dependent on water and steam from geysers and hot springs, which can provide direct heating or be used to generate electricity. For example, Iceland heats almost all its buildings using water from hot springs, while northern California generates more than half its electricity using steam from underground geysers.
New technologies are being developed to tap the vast store of energy underground. Enhanced Geothermal Systems (EGS) can produce power from dry heat and do not require local geysers or hot springs. Methods of co-producing geothermal energy at the sites of oil and gas wells are also being developed.
Geothermal power can lower energy bills in the short term, but it also has the potential to replace fossil fuels in large-scale electricity production. Both solar power and wind energy can be variable and difficult to predict. The geothermal power that lies below ground all over the world has the potential to provide a predictable and almost inexhaustible supply of renewable energy.