What is Geothermal Energy?
Geothermal energy comes from the natural heat generated and stored in the Earth. Temperatures below the Earth's surface increase with depth due to heat from the Earth's core and the radioactive decay of elements within rocks and soil. This Geothermal Power can be accessed and harnessed to produce electricity or to directly heat buildings and water sources. Areas with higher geothermal potential are found near tectonic plate boundaries where hot rocks or water can be found at shallower depths below the earth's crust.
Sources of Geothermal Energy
There are two main sources of geothermal energy that can be exploited for power generation - hydrothermal resources and geothermal or "hot rock" reservoirs.
Hydrothermal resources consist of underground water that has been heated by the earth's core to above boiling point and remains liquid due to the tremendous pressure of the overlying rock. This superheated water, or steam, rises up through permeable underground reservoirs towards the surface. Where it emerges, it can be accessed through geothermal wells and used to drive steam turbines for power production. Areas with known hydrothermal activity include the US Western states, Central America and Southeast Asia.
Geothermal or "hot rock" reservoirs exist where temperatures are elevated due to the earth's internal heat but lack liquid water. Engineers are researching enhanced geothermal systems (EGS) technologies to artificially fracture hot underground rocks and circulate water through these engineered reservoirs to extract heat for power production. EGS has the potential to expand geothermal electricity generation worldwide.
Generating Electricity from Geothermal Resources
Electricity from geothermal resources is generated in much the same way as geothermal power plants generate electricity from hydrothermal reservoirs. Wells tap into the underground reservoirs and either water or steam is extracted and piped to a power station at the surface. Here it turns turbine generators which produce electricity.
At hydrothermal sites, naturally occurring steam or hot water rises up production wells and is piped directly to the power plant. The steam turns turbine generators which drive electric generators. Any water is collected and either re-injected into the reservoir or used for other purposes. Re-injection of fluid helps maintain reservoir pressure for long-term power production.
At EGS sites, water is pumped down injection wells into hot fractured rock where it is heated up. It is then extracted via production wells as hot water or steam and piped to the power station. Here the heat is used to power a closed-loop binary system where the geothermal fluid heats a secondary working fluid with a lower boiling point, like isobutane or propane. The vapors from this fluid drive turbines to produce electricity before being condensed and re-circulated.
Binary plants can generate power from lower temperature resources compared to dry steam plants and have higher thermal efficiencies of around 15-20%. Overall baseload availability from geothermal plants is around 90-95% with minimal greenhouse gas emissions.
Direct Geothermal Heating Applications
As well as generating electricity, the natural heat of geothermal resources can be directly utilized for space heating, greenhouses, aquaculture and industrial processes. Low temperature geothermal districts heating schemes pipe hot water at 70-90°C from shallow geothermal wells to heat buildings, providing an efficient, renewable alternative to gas or electric heating.
Geothermal heat pumps use the constant underground temperature to heat and cool buildings via a closed loop ground source heat exchanger. During winter, heat is extracted from the ground and concentrated via a heat pump for space heating. In summer this process is reversed for air conditioning. As well as high efficiency, these systems have significantly lower lifetime costs than air source heat pumps due to the stable underground temperature.
Geothermal bathing also takes advantage of natural hot springs emerging from the earth. Hot springs are popular tourist attractions and support local spas, bath houses and swimming pools. Their stable, mineral-rich waters provide therapeutic health benefits. Geothermal aquaculture, greenhousing and industrial applications also utilize the direct use of underground heat for fish farming, plant cultivation and manufacturing processes.
Environmental and Economic Benefits
Geothermal energy harnesses a renewable heat source with very low carbon emissions and land use impacts compared to other energy sources. Power generation emits 50-90% less carbon dioxide than fossil fuel plants. There are no emissions of particulates, sulphur or nitrous oxides which cause acid rain and smog.
Geothermal plants also require little fuel, providing continuous baseload power independent of weather conditions. Maintenance costs are low due to few moving parts. The resource is sustainable as heat continuously flows up from the earth's interior and fluid reinjection maintains reservoir pressures to replenish extracted resources over decades.
Geothermal energy is a promising source of clean, domestic power generation able to supply electricity and heating to local communities for many years to come. While upfront development costs of plants and wells are higher than some renewable technologies, geothermal has among the lowest levelized costs of energy once constructed due to its very high capacity and availability factors. As technologies advance, geothermal power and direct use applications will play an increasingly important role in transitioning to sustainable energy economies worldwide.
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