Power plants convert the heat to electricity. Geothermal energy as the name suggests it is the heat energy from the earth that is being utilized for electricity generation or direct heating and cooling processes. This energy comes from the original formation of planet Earth & from the radioactive decay of materials. It is contained in the rocks and fluids beneath the earth’s crust and can be found deep at the earth’s hot molten rock called magma.
Figure 1
Source: www.geo-energy.org
The difference in temperature between core of the planet (Approx. 4000 °C) & its surface, termed as Geothermal gradient which drives the conduction of the heat energy from the core to the surface of the planet earth. This high temperature and pressure in the earth’s interior causes some rock to melt and solid mantle to behave plastically and convecting upwards being lighter than surrounding rock. The temperature profile and heat transfer from the earth crust is shown in Figure 1.
To limit global temperature, rise to 1.5°C and bring CO2 emissions closer to net-zero by 2050, clean and sustainable energy technologies are to be prominently used. Geothermal has been a clean, environmentally friendly, sustainable, and expanding source of energy in recent years. These are usually located close to tectonically active regions in the earth’s crust. According to the International Renewable Energy Agency (IRENA), geothermal energy has grown steadily from around 10 GW globally in 2010 to 14 GW in 2020. The total installed capacity for geothermal direct heat utilization for heating/ cooling (excluding heat pumps) is around 23 GWth.
Figure 2
Source: http://www.globalgeothermalalliance.org/
The first geothermally generated power was produced in Larderello in Italy in 1904. The world geothermal map as indicated in figure 2 shows that the United States is the largest producer of geothermal energy in the world followed by Indonesia.
During the 21st Conference of the Parties to the United Nations Framework Convention on Climate Change (UNFCCC), held in Paris, France, in December 2015, the Global Geothermal Alliance (GGA) was formed of which India is a member, to increase the use of geothermal energy, both in power generation and direct use of heat. India has set a target of installing 175 GW of renewable energy by 2022 majorly by solar and wind energy. Geothermal energy in India is still at a nascent stage given site-specific nature, risk, and uncertainties with exploration and high capital cost.
A ‘hot springs committee’ consisting of the National geophysical research institute (NGRI), Geological Survey of India, and Jadavpur University, Kolkata was formed in 1963 by the ministry of power and irrigation to explore the utilization of thermal springs in India commercially. In 1971, India sought assistance from the United Nations in preparation for the report on the geothermal resource development project, and the final project document was updated in 1973. After, 1973, the Geological Survey of India (GSI) with CSIR – National Geophysical Research Institute (NGRI) carried out a preliminary resource assessment for the exploration and utilization of geothermal resources.
Figure 3
Source: https://www.sciencedirect.com/science/article/pii/S2577444120300617
There are about 300 thermal spring localities in India as per the Geological Survey of India (GSI) report 2002 and these localities were grouped into geothermal provinces having estimated potential to produce around 11 GW. Chief geothermal provinces in India are shown in figure 3 which includes the Himalayas, Naga-Lushai province, Sohana, West coast, Andaman-Nicobar Islands, Cambay, Son-Narmada-Tapi (SONATA), Godavari, and Mahanadi valleys.
In 2007, MOU between the Centre of Excellence for Geothermal energy (CEGE, Ahmedabad) India and Iceland geo survey (ISOR) Iceland took place for cooperation on geothermal development in India.
In 2015 MNRE launched a draft national policy on geothermal energy to establish India as a global leader in geothermal power by installing 1GW in the initial phase by 2022. After the Paris summit on climate change, India proposes to install 10 GW geothermal energy by 2030 through active international collaboration with the US, Philippines, Mexico, and New Zealand.
On 7Th Feb 2021, the Agreement for establishing India’s first-ever geothermal field development project was signed, a step towards the goal of carbon-neutral Ladakh and MOU was signed between union territory administration of Ladakh, Ladakh Autonomous hill development council (LAHDC), and Oil & Natural gas cooperation (ONGC) energy center. This project will put India on the geothermal power map of the world. Puga and Chhumathang in eastern Ladakh is the most promising geothermal field in India as per the GSI where the deepest borehole was drilled up to 385 m depth and 220 m depth respectively.
These geothermal provisions are classified as medium enthalpy (100 to 200 Deg C) and low enthalpy (less than 100 Deg C) geothermal systems depending on the temperature profiles. Puga and Chummathang in Ladakh and Tatapani in Chhattisgarh are medium enthalpy geothermal systems that can be used for power production. The utilization of geothermal energy depends on the quantity and quality of the fluid and its temperature and pressure. In Puga, Ladakh geothermal fluids are used for extraction and refinement of borax and sulfur along with experimental space heating for a long time through shallow wells drilled.
Figure 4
Source: https://www.ongcinida.com
Recently, ONGC has planned Puga field development geothermal project in Ladakh in three phases. Phase-I involves exploratory-cum-production drilling of wells up to 500 metres depth and setting up of a Pilot Plant of up to 1 MW power capacity. Phase-II would involve deeper and lateral exploration of geothermal reservoir by drilling of optimal number of wells and setting up of a higher capacity Demo Plant and preparing a Detailed Project Report. Phase-III would involve commercial development of the geothermal plant.
The low-temperature heat energy from the earth crust can be utilized directly through direct heating and cooling of buildings, drying processes and industrial process heating applications whereas the high-temperature heat energy of about 150 Deg C can be used to generate the uninterrupted electricity.
Figure 5
Source: https://www.energy.gov/eere/geothermal/
Direct Exploitation
Indirect Exploitation
Hot water or steam deep below the earth surface at high temperature are drilled out to obtain environmental friendly, continuous, carbon-free, uninterrupted clean energy in the geothermal power plants by the use of turbines, generators, and transformers as being done in conventional power plants.
Figure 6
Source: https://www.energy.gov/eere/geothermal/
Geothermal electric power generations can be classified into three types as shown in figure 6. Dry steam plants are the oldest form of geothermal technology and take the steam out of the ground through production well and use it to directly drive a turbine. It is used when high-temperature steam is available. Transportation, storage, and combustion of the fuel are eliminated leading to the least land footprint among various energy generation technologies. Flash steam plants use high-pressure hot water into cool, low-pressure water. Hot water flows up through the well in the ground under its own pressure. As it flows upwards, pressure is reduced and hot water boils into steam. Steam is separated from water in the flash tank and is further utilized to generate power. Separated water and condensed steam are injected back to the reservoir for reheating through an injection well. Binary cycle plants pass hot water through a heat exchanger where secondary liquid with a lower boiling point (organic compound), turns to vapor and is used to drive the turbine.
Also Read: Geothermal Energy Scope in India
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