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. However, 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.
Currently, geothermal power has a low share in the overall global energy mix. It is highest in the Asia Pacific region, while the largest numbers of geothermal power stations are in North America and Southern Europe.
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.
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-temperature resources are those with a fluid temperature over 150°C.
There are four types of geothermal power plants currently in commercial use: dry steam, flash, binary, and flash/binary combined:
Dry steam power plant: 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: 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: 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.
In the heat exchanger heat from the water transfers to a secondary or working fluid such as isobutane, which has a lower boiling point than water. This expands into a vapour, and the resulting force drives the turbine. The vapour then returns to liquid form in a condenser and passes to the heat exchanger to begin its next circuit.
Flash/binary combined cycle power plant: Water at high pressure and high temperature from the geothermal well is fed into a separator, where lower pressure causes some of it to flash to steam. The steam drives the plant’s level I turbine. The remaining water passes to a preheater where it heats the secondary working fluid in a binary system. The working fluid then enters a vaporiser where it is further heated by the steam leaving the level I turbine. The vaporised working fluid drives the level II turbine.
Geothermal power plants generate consistently and can deliver baseload power. They also have a high capacity factor. Globally, geothermal has a mean capacity factor of 74%, which compares favourably with other renewables. The figure for biomass is 55%, hydropower 43% and solar 11%. The equivalent figure for nuclear is 79%, and for fossil fuels 46%.
Geothermal power is highly scalable, and facilities require relatively little space compared to other types of power plants.
Geothermal power generation is generally clean with the only emission being water vapour. However, in some cases, geothermal systems can emit small amounts of the greenhouse gases hydrogen sulphide and carbon dioxide, as well as very small amounts of sulphur dioxide, nitrous oxides, and particulates. There is also a risk that water flowing through underground reservoirs can become contaminated with trace amounts of elements such as arsenic, mercury, and selenium. If the geothermal system is not isolated effectively, these elements can leak into water sources.
Equipment for geothermal power plant
Large motors and generators play a variety of roles across the geothermal value chain.
Motors
Typical motor-driven applications include condensate, hotwell, re-injection and cooling water circulation pumps, cooling tower fans and gas re-injection? compressors.
Induction motors are robust, reliable solutions for high power density, efficiency and optimal performance. Built-in flexibility enables them to be configured to match the plant’s specific needs.
Permanent magnet motors offer premium efficiency. They can be combined with a variable speed drive (VSD) or used in a direct drive configuration to maximise system efficiency.
Generators
Generally, synchronous 4-pole steam turbine generators are used in high-voltage versions up to 70 MVA and are tailored to match the installation.
Motors and generators are often specified as part of a complete package including the cooling system, main terminal box, maintenance tools, VSDs, low voltage motors and control equipment as well as system monitoring and protection solutions.
Efficient equipment is key to maximising the power generated from geothermal resources. High efficiency means lower losses, lower operating costs and greater power output from the same resource. Even a small increase in efficiency can make a big difference in terms of profitability. As geothermal plants replace fossil fuel generation, efficient motors and generators also help to reduce CO2 emissions.
Reliability is enhanced by designing motors and generators for cooler operating temperatures and low vibration. High reliability reduces unexpected plant downtime and therefore boosts productivity.
For geothermal plants where motors and generators operate in areas with the presence of hydrogen sulphide gas, special anti-corrosion protection might be needed. It is important to protect all metal parts, either with proper painting or coating, or by tinning or other means. Active parts can be further protected by pressurising the enclosure to prevent hydrogen sulphide from entering. The measures required for a particular plant will depend on site-specific conditions.
Kaishan projects
Typical examples of geothermal projects where ABB has provided key equipment are those constructed by Kaishan, a diversified engineering company based in China, which has developed innovative modular geothermal power stations
Kaishan has built two geothermal power plants based on this modular technology in Indonesia. The Sorik Marapi project in Sumatra has a capacity of 240 MW, while the Sokoria project in East Nusa has a capacity of 30 MW. ABB has supplied synchronous steam turbine generators rated at 12 MVA for each plant. The specification required an efficiency level of 97% and this was surpassed in type testing, where the actual efficiency level was shown to be 98.37%.
Moving to lower-temperature heat sources
Geothermal energy can produce baseload electricity and is an excellent complement to intermittent renewables like wind and solar. It is set to make a useful contribution to the world’s efforts to boost sustainability while cutting the emissions of greenhouse gases.
New technologies for generating electricity from geothermal energy have been developed over the years, with four types of power plants currently in commercial use. As newer technologies enable the utilisation of lower-temperature heat sources, a growing number of countries around the world can take advantage of their geothermal resources.
This article first appeared in Modern Power Systems magazine.
