Amidst the deteriorating climatic condition and exhaustion of existing conventional energy sources, mankind is on a constant lookout for a renewable and clean energy source. Geothermal energy seems to be a suitable contender capable of contributing to our fight against climate change and fulfilling the ever-increasing energy demand. The field has come to a new light with the invention of new tools and developed technology, however, geothermal energy has been a reliable source of heating systems for centuries as evidenced by archaeological research. The naturally occurring hot springs have been a famous retreat and recoup site for people since long ago. Not just this, the United Kingdom has the famous hot spring spas, one of the oldest of its kind, and Romans built steam rooms and pools sometime around 60 CE using the naturally occurring hot low-temperature geothermal pockets. France is home to the world-famous hot springs of Chaudes Aigues, popular among tourists. The United States’ first formal geothermal system for heating homes started in 1892 at Boise, Idaho. Technology has slowly made its way into the global market and is now one of the potential means to fight global warming. At present Iceland is the leading producer and consumer of geothermal energy. A review to understand geothermal energy and its implications in climate mitigations.
What is geothermal Energy?
To quote literally, geothermal means earth (geo) and heat (thermal). Thus, geothermal energy is the heat (energy) present within the Earth. The Earth consists of three parts the crust, the mantle, and the core. The curst form its outermost part followed by the mantle and the core is the innermost part. Located approximately 2900 feet deep, the core is the hottest part because of the continuous decay of the radioactive elements, leftover heat from the planetary formation, the molten iron alloy present in the outer core, the friction, and the gravitational pull created by the Earth’s surface. The estimated temperature of the core is around 5000 ℃ (9000℉). As we ascend to the Earth’s surface the temperature decreases gradually. The heat from the core makes the rocks and underground aquifers hot. These are the sources of geothermal energy. It may be applied to a heating/ cooling system or to generate electricity using steam-run turbines.
Technologies in the geothermal Energy
Geothermal heat pumps – Geothermal heat pumps (GHP) are also known as geo-exchange, earth-coupled, ground-source, or water-source heat pumps. The first such pumps were used in 1940. These pumps use the heat from inside the Earth where the temperature remains relatively the same despite the changes in atmospheric temperature. The GHPs have a pipe connected to them that circulates underground as well as in the buildings. This pipe carries hot water. The heat is transferred to the surroundings through a duct system. The duct system has the ability to raise the ambient temperature during the winter and make the surroundings cool during the summer. The GHPs are only 3 to 90 meters deep and do not disturb the bedrock. The GHPs are the most efficient and carbon-free means of heating or cooling buildings.
Electricity generation using geothermal energy – The heat supplied by the GHPs is not enough for the generation of electricity. This requires deeper digging below Earth’s surface except in the regions with natural hot water/ spring pockets at the surface. The present oil wells active or inactive can also be used for the co-production of geothermal energy. Depending upon the geological makeup of the region and the resources, there are different types of geothermal power plants.
- Dry-steam power plant – It uses naturally occurring underground sources of steam to run the turbines for the generation of electricity. The first dry-steam power plant was built in LarderEllo, Italy, in 1911. It is functional to date and supplies electricity to the entire town. The dry-steam power plants are suitable for regions with hot springs/ pockets naturally. Other examples of the dry-steam power plant are the Geyser, California, and Yellowstone national park, Wyoming.
- Flash-steam power plant – The Flash-steam power plant uses underground aquifers of hot water; the flash of water evaporates into steam which is used to run the turbine and generate electricity. This geothermal power plant is most suitable for tectonically active regions. Iceland is the best example here; the nation uses geothermal heat to meet most of its energy demand.
- Binary cycle power plant – It is an innovative technology that produces electricity and also conserves water. The method involves an additional step in which the hot water from underground runs in the pipe heating an organic liquid with a boiling temperature lower than that of water. The steam created by the heating of the organic liquid is used to power the turbine and generate electricity. The method helps in the conservation of water as the water in the pipe is recycled. This technology is presently used at the Beowave Geothermal Facility, Nevada, USA.
Enhanced geothermal systems – This technique was developed to install geothermal power plants in areas without the supply of underground water, and areas that have dry hot rocks underground. The technique uses an injection well to release cold water under high pressure in the deep rocks creating an underground water reservoir. The water absorbs the heat from the rock, the hot water is then pumped out and circulated through pipes generating steam to run the turbines and generate electricity.
Share in the global energy market
With the advancement in the techniques used in geothermal power plants as described above and increased investment in the sector, the share of energy produced by this source is increasing gradually. The Earth’s core constantly emits heat making geothermal energy a renewable and reliable source. Moreover, the energy is fossil-fuel-free (zero carbon footprints) and does not have geographical constrain for extraction as the Earth’s core emits an equal amount of heat everywhere irrespective of the climatic condition of the region. At present, geothermal power plants are responsible for the production of 8.3% of total global electricity annually with the potential to generate much more. As to the direct use of geothermal energy for heating/ cooling systems in buildings, the prevalence is much more and is at present used in almost 70 countries.
Geothermal energy and the developing countries
Geothermal energy provides an excellent alternative source of energy, especially for developing countries. The major constraint in its implementation is the demand for large-scale investment and infrastructural installations during the early stages. However, with the increasing prices of fossil-fuel-based energy sources, developing countries are exploring this alternative source. India has over 300 sites with naturally occurring hot spring pockets spread across the country. The potential geothermal provinces of India are parts Himalayas, Sahara, Cambay basin, Son-Narmada-Tapi lineament belt, West coast, Godavari basin, and Mahanadi basin. Among its efforts to reach net zero emissions by 2070, the government of India has started its first geothermal power plant project at Puga Valley, Leh. The region alone has about 80 hot springs. At the initial stage, the power plant is estimated to produce 1 megawatt (MW) of electricity, sufficient to power almost 200 homes in the area. The work is being carried out in collaboration with ONGC ltd. More such power plants are likely to be set up depending upon the success and output of the present project.
The future with geothermal energy
Geothermal energy is a sustainable and green source of energy and offers innumerable advantages. It is safe for the environment and does not have geographical barriers like solar or wind energy. Among its countless advantages, geothermal energy does not need huge pieces of land and sophisticated instrumentations, thus having low footprints of facilities. The three important components to generate electricity using geothermal energy are heat, water, and turbines. Moreover, unlike Solar and wind energy, it does not need to be stored thus cutting the storage expenditure. The only concern raised in the extraction of geothermal energy is the likelihood of geological disturbances and increased seismic activities in the region. The field needs detailed pre-drilling research and the use of stress field analysis modelling in order to cut down the cost and have precise techniques for use of geothermal heat to avoid any geological accidents. All in all, geothermal energy is going to form an important contributor to renewable and clean energy generation in the future. The total energy production using geothermal sources is predicted to be 140 GWe by 2050, thus, it has great potential to assist us in designing our climate mitigation policies and achieving net zero goals at the earliest.