Geothermal energy is a natural and renewable source of energy that comes from the ground. This energy is also visible in our magnificent nature; hot springs, geysers and magma from the volcanoes are all manifestations of geothermal energy. People want to take advantage of this spectacular energy to generate electricity for a long time. Here, we explore the main advantages and disadvantages of geothermal energy and the way we should regard it.
What is Geothermal Energy?
A geothermal power plant works similarly to a coal-fired power station or a nuclear power plant; it generates electricity by channelling steam to rotate turbines. However, steam in a geothermal power plant is not manually generated through burning coal or nuclear fission but is rather produced by natural heat under the Earth. As we know, the structure of the Earth is made up of four different layers: the crust, the mantle, the outer core, and the inner core. The estimated temperature of the inner core is about 5,200C, and a geothermal power plant utilises this natural thermal energy from the core to get the steam. Most geothermal power plants are built along the boundaries of the tectonic plates where the crust is thinnest at the boundaries, and heat reaches the surface more easily – usually in the form of magma.
Typically, there are three types of geothermal power plants: dry steam, flash steam, and binary steam. Dry steam is the most straightforward method to obtain geothermal energy in which steam is directly extracted from the ground and sent to turbines that generate electricity.
But flash steam is the most commonly applied method. The boiling point of water is higher when it is under greater pressure; since the pressure under the ground is higher, the boiling point of water is therefore also higher. When the highly-pressurised hot water under the ground is extracted to the surface, its boiling point drops significantly, then the water evaporates and “flashes” instantly into steam to drive the turbines. Once the steam cools, it turns back into liquid water and is injected once again into the ground to ensure that there is enough water underground to carry heat.
Lastly, binary steam is the most complicated of the three methods. It uses hot water from the ground as a mediator to heat another liquid to be converted into steam instead. The secondary fluid needs to have a lower boiling point than water as well as to be safe and environmentally-friendly. In this case, pressurised butane and pentane are typically used as the fluid.
The United States is the country that generates the most electricity with geothermal energy. Most of the geothermal power plants in the US are located in western states and Hawaii since they are closer to the boundaries of the tectonic plates. Despite being the world’s largest producer, geothermal energy only accounts for about 0.4% of the total generation of electricity in the US.
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What are Geothermal Energy Advantages and Disadvantages?
Carbon dioxide is not created during the production of geothermal energy, but it can emit a slight t amount of greenhouse gases and air pollution depending on the method applied to extract heat. In dry steam or flash steam power plants, small amounts of greenhouse gases and pollutants that are naturally present underground will be released into the atmosphere during extraction. On the other hand, in binary steam power plants, carbon emissions can be limited to zero since the steam does not directly come from underground. It is estimated that the average global CO2 emission of geothermal power plants is 122 g/kWh, about 10 times less than coal and petroleum.
The result in Iceland, a country that currently generates 25% of its electricity with geothermal energy, is even more phenomenal, given its average CO2 emission being only 34 g/kWh. However, the CO2 emission of geothermal power plants comes with a huge regional difference. In countries like Italy and Turkey, the CO2 emission of geothermal power plants can reach 1,300 g/kWh, surpassing even the emission of coal-powered stations. This huge regional difference can be ascribed to the differences in the methods of production, the levels of technology in this domain, and most importantly, the quality of geothermal reservoirs. Some geothermal reservoirs are naturally filled with more non-condensable gases (NCG), mostly CO2. Geothermal energy is definitely renewable, but is not necessarily nor always ‘green’.
One of the biggest disadvantages of geothermal energy is that its adoption has many limitations: the location of power plants needs to be close to the boundaries of the tectonic plates while making sure that the level of CO2 inside the geothermal reservoirs is low. At the same time, plants may trigger earthquakes as it alters the Earth’s structure by digging. Therefore, geothermal power plants must be kept away from populations, which reduces the options for sites.
Yet, how efficient is it for those countries that enjoy the geographical advantage? The energy return on investment (EROI) of geothermal energy is about nine, meaning that nine units of output can be produced by one unit of input. The score is not particularly high, considering that solar energy has a score of 10, and wind energy has a score of 18. Nevertheless, geothermal energy possesses some unique advantages, such as its high reliability. Geothermal energy does not face the problem of intermittency that bothers solar and wind power, as the heat of the Earth’s core is always consistent. As a result, we can accurately calculate and predict the electricity generation of geothermal power plants.
Each energy has its pros and cons; some sources are effective in one country, and some are not. What we should be doing is not comparing the efficiency of different renewable energy sources in a shallow manner but comparing their efficiency according to the relative advantages of each specific location. Seeing that the use of geothermal energy has been constantly increasing at a growth rate of 2% per year while the cost of operations have been decreasing, it is projected that global geothermal energy can provide around 800-1300 TWh per year in 2050, contributing 2-3% to global electricity generation. Geothermal energy has its advantages and disadvantages, it will unlikely play a huge role in the transition to renewable energy – though it is still an indispensable part of it.
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