Geothermal power |


Hot prospect for future growth


Geothermal power is a renewable energy source with a number of advantages, including ability to deliver baseload power, high capacity factors and small footprint relative to other types of power plant. It is expected to enjoy significant growth in the coming years


Luca Rizzo, ABB Global Industry Manager for HV Motors and Generators, Water and Power


Geothermal power generation is truly renewable since heat radiates continuously from the Earth’s core and will carry on doing so for billions of years. Some of this heat originates from the friction and gravitational pull created when the planet was formed more than four billion years ago. But the majority results from the continuous decay of naturally occurring radioactive isotopes in the Earth’s core, which is the hottest part of the planet, located around 2900 km below the surface. At this point the temperature is estimated to be in the range 2700-4000°C. The most favourable locations for utilising geothermal energy are in the ‘hotspots’ – regions where high temperatures are found at relatively shallow depths. Depth is important because drilling is expensive and typically accounts for around half the total cost of a power generating facility. These regions – such as the “Ring of Fire” around the Pacific Ocean – are typically associated with volcanic and seismic activity (See Figure 1).


Currently, geothermal power has a low share in the overall global energy mix (see Figure 2). It is highest in the Asia Pacific region, while the largest numbers of geothermal power stations are in North America and Southern Europe.


0.45% 0.39% 0.34% 0.35% 0.33% 0.32% 0.31%


Above: Figure 1. Geothermal ‘hotspots’, where higher temperatures are relatively near to the Earth’s surface


Presently, there are 26 countries that generate power from geothermal sources. Those where it accounts for more than 15% of the total energy mix include Costa Rica, El Salvador, Iceland, Kenya, New Zealand and the Philippines. Indonesia is considered to be the country with the world’s foremost geothermal resources. Currently it is using less than 5% of its overall potential but is increasing its capacity rapidly.


Global geothermal installed capacity amounted to 14.2 GW in 2019, of which 2.6 GW or 18.3% was in the USA. By 2021 capacity had increased to 16 GW and geothermal plants generated a total of 96.7 TWh of electricity. The expected annual growth rate in capacity is 7% up to 2030. Based on the IEA’s Net Zero Scenario, the growth rate should be double this figure, and capacity should reach 98 GW by 2030 and 126 GW by 2050.


2005 2010 2015 2020


 Europe


Middle East and Africa North America


South and Central America


Above: Figure 2. Geothermal generation (GWh) worldwide and percentage within the global energy mix. Its highest share is in the Asia Pacific region


2025 2030 2035


It is estimated that only around 7% of the total global potential for geothermal power is being used now, providing plenty of scope to increase utilisation.


In some cases, geothermal heat is in the form of steam or hot water and can be utilised directly. However, in most cases, water is injected, ie, pumped down, into the heat pocket to produce steam. This process is known as “enhancing”.


Geothermal resources are generally classified as low or high temperature, based on the temperature of the fluid that is utilised. High


30 | March 2024| www.modernpowersystems.com


temperature resources are those with a fluid temperature over 150°C.


There are four types of geothermal power plant currently in commercial use: dry steam, flash, binary, and flash/binary combined:


• Dry steam power plant (Figure 3). The dry steam process is straightforward and efficient. But it is the oldest type of power plant and few remain in use today. Steam at a temperature of 150°C or higher is brought to the surface directly from the geothermal reservoir and passed through the turbine to power the generator. The steam is then fed into a condenser where it turns into water. It is subsequently injected back into the geothermal reservoir where it is reheated to produce more steam.


• Flash steam power plant (Figure 4).Water at high pressure and at least 180°C flows under its own pressure into a separator vessel. The drop in pressure causes some of it to “flash” to steam, which separates from the water and powers the turbine. The remaining water and condensed steam are cooled and injected back into the geothermal reservoir. • Binary cycle power plant (Figure 5). Binary cycle plants offer the major advantage of being able to generate power using low temperature fluids, down to as low as 57°C. Hot water from the geothermal reservoir is pumped through a heat exchanger, with the cooled water being injected back into the reservoir.


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