Geothermal energy is the natural heat of the earth. Earth’s interior heat originated from its fiery consolidation of dust and gas over 4 billion years ago. It is continually regenerated by the decay of radioactive elements, that occur in all rocks.
From the surface down through the crust, the normal temperature gradient – the increase of temperature with the increase of depth – in the Earth’s crust is 17 °C — 30 °C per kilometer of depth (50 °F — 87 °F per mile).
Below the crust is the mantle, made of highly viscous, partially molten rocks with temperatures between 650 °C — 1250 °C (1200 °F — 2280 °F). At the Earth’s core, which consists of a liquid outer core and a solid inner core, temperatures vary from 4000 °C — 7000 °C (7200 °F– 12600 °F).
Major geothermal fields are situated in circum-pacific margins, rift zones of East Africa, North Africa, Mediterranean basin of Europe, across Asia to Pacific (Figure 1).
Geothermal reserves up to depths of 10 km are estimated at 403X106 Quads. The world average geothermal heat flow is 0.06 W/m2
There are four major types of Geothermal energy resources.
Hot dry rocks
Currently, hydrothermal energy is being commercially used for electricity generation and for meeting thermal energy requirements. In 1997, The world’s geothermal electricity generation capacity was 8000 MW and another 12000 MW for thermal applications.
Italy, New Zealand, USA, Japan, Mexico, Philippines, Indonesia are some of the countries which are using geothermal energy for electricity generation and thermal applications. Exploration of geothermal fields needs knowledge of geology, geochemistry, seismology, hydrology and reservoir engineering.
In India, exploration and study of geothermal fields started in 1970. The GSI (Geological Survey of India) has identified 350 geothermal energy locations in the country. The most promising of these is in Puga valley of Ladakh. The estimated potential for geothermal energy in India is about 10000 MW.
There are seven geothermal provinces in India : the Himalayas, Sohana, West coast, Cambay, Son-Narmada-Tapi (SONATA), Godavari, and Mahanadi.
The important sites being explored in India are shown in the map of India (Figure 2) .
Technology for electricity generation
There are two types of the plants.
1. Flash steam plants
When the geothermal energy is available at 150 °C and above temperature, the fluids can be used directly to generate electricity. In some cases, direct steam is available from the geothermal reservoir; otherwise the steam is separated and turbines are used for power generation.
2. Binary plant
These plants are used when geothermal temperature is between 100 °C and 150 °C. The fluid is extracted and circulated through a heat exchanger where the heat is transferred to the low boiling point organic liquid. This gets converted into high pressure vapour, which drives organic fluid turbines (Figure 3b).
These systems are useful for heating of houses and living spaces like offices, commercial complexes etc.
Central Electricity Authority
Geological Survey of India
Indian Institute of Technology, Mumbai
Regional Research Laboratory, Jammu
National Geophysical Research Institute, Hyderabad
Oil and Natural Gas Corporation, Dehradun
Ongoing Projects in India:
Magneto-telluric investigations in Tattapani geothermal area in Madhya Pradesh
Magneto-telluric investigations in Puga geothermal area in Ladakh region, Jammu & Kashmir
Geothermal Atlas of India, prepared by the Geological Survey of India(GSI) gives information/data for more than 300 geothermal potential sites. This Atlas is being updated by GSI with the support from MNES.
Applications of geothermal energy for small-scale power generation and thermal applications are being explored.
Use in greenhouse cultivation