Geothermal Energy

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.

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.

1.  Hydrothermal 

2.  Geopressurised brines

3.  Hot dry rocks

4.  Magma

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.

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.

Indian organisations working in geothermal energy:

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 

India has reasonably good potential for geothermal; the potential geothermal provinces can produce 10,600 MW of power.
Though India has been one of the earliest countries to begin geothermal projects way back in the 1970s, but at present there are no operational geothermal plants in India. There is also no installed geothermal electricity generating capacity as of now and only direct uses (eg.Drying) have been detailed.
Thermax, a capital goods manufacturer based in Pune, has entered an agreement with Icelandic firm Reykjavík Geothermal. Thermax is planning to set up a 3 MW pilot project in Puga Valley, Ladakh (Jammu & Kashmir). Reykjavík Geothermal will assist Thermax in exploration and drilling of the site.
India’s Gujarat state is drafting a policy to promote geothermal energy

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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 

Achievements:

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. 

Potential Applications:

Power generation 
Cooking 
Space heating 
Use in greenhouse cultivation 
Crop drying 

ADVANTAGES OF GEOTHERMAL ENERGY

The  first  advantage  of  using  geothermal  heat  to  power  a  power  station  is  that,  unlike  most  power stations, a  geothermal  system does  not create any pollution.  It may once  in a while  release  some gases  from deep down inside the earth, that may be slightly harmful, but  these can be contained quite easily. Geothermal power plants have sulphur-emissions rates that average only a few percent of those from fossil -fuel alternatives. The  newest  generation  of  geothermal  power  plants  emits  only  ~135  gm  of  carbon  (as  carbon  dioxide)  per megawatt-hour  (MW-hr) of electricity  generated. This  figure compares with 128  kg  /MW-hr of carbon  for a plant  operating  on  natural  gas  (methane)  and  225  kg/MW-hr  of  carbon  for  a  plant  using  bituminous  coal. Nitrogen oxide emissions are much lower in geothermal power plants  than  in  fossil power plants. Nitrogen-oxides combine with hydrocarbon vapours in the atmosphere to produce ground-level ozone, a gas  that causes adverse health effects and crop losses as well as smog.
The cost of  the  land  to build a geothermal power plant on,  is  usually  less expensive  than  if  you were planning  to  construct  an;  oil,  gas,  coal,  or  nuclear  power  plant. The  main  reason  for  this  is land  space,  as geothermal plants  take up very little room, so you don’ t need to purchase a larger area of land.
Another  factor that  comes  into  this  is  that  because  geothermal  energy  is  very  clean,  you  may  receive  tax  cuts,  and/or  no environmental bills or quotas to comply with the countries carbon emission scheme (if they have one).
No fuel is used to generate  the power, which in return, means  the running costs  for the plants are very low as there are no costs for purchasing,  transporting, or cleaning up of fuels you may consider purchasing to generate the power.
The overall financial aspect of these plants is outstanding, you only need to provide power to the water pumps, which can be generated by the power plant itself anyway.  Because they are modular, then can be  transported conveniently to any site. Both baseline and peaking power can be generated.
Construction time can be as little as 6 months for plants in the range 0.5 to 10 MW and as little as 2 years for clusters of plants totalling 250 MW or more.

DISADVANTAGES OF GEOTHERMAL ENERGY

Fluids drawn from the deep earth carry a mixture of gases, notably carbon dioxide (CO2), hydrogen sulfide (H2S), methane (CH4) and ammonia (NH3). These pollutants contribute to global warming, acid rain, and noxious smells if released. Existing geothermal electric plants emit an average of 122 kilograms (269 lb) of CO2 per megawatt-hour (MW·h) of electricity, a small fraction of the emission intensity of conventional fossil fuel plants. Plants that experience high levels of acids and volatile chemicals are usually equipped with emission-control systems to reduce the exhaust.
In addition to dissolved gases, hot water from geothermal sources may hold in solution trace amounts of toxic chemicals such as mercury, arsenic, boron, and antimony. These chemicals precipitate as the water cools, and can cause environmental damage if released. The modern practice of injecting cooled geothermal fluids back into the Earth to stimulate production has the side benefit of reducing this environmental risk.
Plant construction can adversely affect land stability. Subsidence has occurred in the Wairakei field in New Zealand and in Staufen im Breisgau, Germany. Enhanced geothermal systems can trigger earthquakes as part of hydraulic fracturing. The project in Basel, Switzerland was suspended because more than 10,000 seismic events measuring up to 3.4 on the Richter Scale occurred over the first 6 days of water injection.

SUSTAINABILITY

Geothermal power is considered to be sustainable because any projected heat extraction is small compared to the Earth’s heat content. The Earth has an internal heat content of 10 joules (3·1015 TW·hr). About 20% of this is residual heat from planetary accretion, and the remainder is attributed to higher radioactive decay rates that existed in the past. Natural heat flows are not in equilibrium, and the planet is slowly cooling down on geologic timescales. Human extraction taps a minute fraction of the natural outflow, often without accelerating it.

Even though geothermal power is globally sustainable, extraction must still be monitored to avoid local depletion. Over the course of decades, individual wells draw down local temperatures and water levels until a new equilibrium is reached with natural flows. The three oldest sites, at Larderello, Wairakei, and the Geysers have experienced reduced output because of local depletion. Heat and water, in uncertain proportions, were extracted faster than they were replenished. If production is reduced and water is reinjected, these wells could theoretically recover their full potential. Such mitigation strategies have already been implemented at some sites. The long-term sustainability of geothermal energy has been demonstrated at the Lardarello field in Italy since 1913, at the Wairakei field in New Zealand since 1958, and at The Geysers field in California since 1960.

The extinction of several geyser fields has also been attributed to geothermal power development.