In this next part of our renewable energy series, we’ll focus on geothermal energy after previously discussing solar and wind power. Geothermal energy comes from the natural heat at the Earth’s core created by the planet’s compression 4.6 million years ago. It is not uniformly distributed and is conducted through solid rock and fluid convection. The temperature gradient also varies throughout the structure of the Earth which can be seen in the diagram below. The Earth’s crust helps insulate the heat enabling heat storage beneath the surface. Geothermal energy can be harnessed in the form of hot water or steam.
Source: Energy BC
Geothermal power plants can use either steam from a heat reservoir or a working fluid that vaporizes at a different temperature to water to power a turbine. A generator is then used to transform this kinetic energy into electrical. Geothermal heat pumps can be used to both heat and cool buildings using an air delivery system and a heat exchanger system buried underground. The process of heat delivery and extraction to and from the building can be reversed depending on the temperature required in the building.
For heating, the fluid is circulated through pipes underground and heat is absorbed from the warmer ground. The fluid then passes into the building where it is used for heating or as a hot water supply. The same fluid (if not used) can be recirculated underground at a lower temperature and the cycle can continue allowing heat transfer from under to above ground. For cooling, the fluid absorbs heat from above ground using a heat exchanger and circulates it underground where it is released into the cooler ground. Like with the heating cycle, this process can be repeated allowing cooling to buildings.
Source: United States Environmental Protection Agency
The heat transferred to the fluid within heat exchanger pipes, Qh (J), can be calculated using the formula below where ṁ is the mass flow rate of air (kg/s), Cp is the specific heat of air (J/kg K), Tout and Tout are the inlet and outlet temperatures respectively (°C).
There is potential for widespread use of geothermal energy in areas where the heat in closer the the Earth’s crust making heat extraction easier and less costly. Geothermal energy plants can be costly to build but this can be offset over time as the operating costs are low, with the installation of underground pipes accounting for most of the cost.. Geothermal energy is also not intermittent like alternative sources such as wind and solar power. A constant energy supply is predictable and preferable. Geothermal plants can cause land stability problems due to the nature of the drilling deep into the ground. Areas ideal for capturing geothermal energy are often subject to greater volcanic and seismic activity (earthquakes).
As well as heating buildings, geothermal energy can be used for growing crops, heating water at fish farms and in industrial processes. The Nesjavellir Geothermal Power Station (NGPS) is the second largest geothermal power station in Iceland and produces 120 MW of electrical power and over 1000 litres of hot water per second.
Additionally, thank you to Nia Hughes for her contribution towards this blog series.