10 Geothermal Energy

Dr. Dhanya M.S M.S and Mr. Yogendra Singh Singh

1. Introduction

Many countries anticipating the threats caused by climate change realize the values of geothermal power as a baseload and sometimes flexible source of renewable energy. The availability of geothermal power is most environment-friendly power, available around the clock (round the year 24×7), independent of the surface climatic conditions. Geothermal Energy is an abundant resource that has great potential to provide low-cost energy and mitigate climate change, but despite being a mature technology it is still largely untapped due to the high up-front cost of resource exploration and growing at only a modest pace of 3 to 4 percent per year. The geothermal power becomes cost effective in the long run as the cost of fuel is negligible though the exploration activity is expensive.

In the last few decades, there has been an increase in the use of geothermal energy all over the world. Around 340 geothermal hot springs with surface temperatures of 37°- 90°C were identified by Geological Survey of India. Majority of the hot springs areof low temperature hot water resources and can be best utilized for direct thermal applications rather than electrical power generation.

2. What is “Geothermal Energy”?

The word “Geothermal” comes from two Greek words ‘geo’ and ‘therme’ means earth and heat respectively. Geothermal energy is the heat contained in the rock and fluid that fills the fractures and pores within the rock in the earth crust. The shallow ground water and hot rock in deep earth’s crust to magma at interior of the earth are examples of geothermal energy resources.

3. Energy Inside the Earth

The temperature of the earth’s core (at 4000 miles) with hot molten iron ranged from 2,800 to 6,000 degrees Celsius (°C). The geothermal energy (heat) continuously produced within the earth by slow radioactive decay of minerals which is natural in all rocks. The middle layer is mantle (1800 miles thick) which is partly rock and partly magma. The outer layer of the earth is crust with continents (15 to 35 miles thick) and ocean floors (3-5 miles thick).

The volcanoes are present in that part of crust which is thinned, faulted, or fractured by plate tectonics and magma come near the surface, on eruption lava moves out. The heat from magma was absorbed by rocks and water at depth. The heat transfer to water led to generation of geothermal energy. The temperature of the rocks and water increases with depth.

4. Formation of Geothermal Reservoir

Geothermal energy is called a renewable energy source because the water is replenished by rainfall, and the heat is continuously produced by the earth. The Earth’s temperature increases with depth and it is higher than 4200 °C (7600 °F) at centre. The heat at the interior of earth is produced partly during earth’s formation about 4.5 billion years ago and partly by continuous decay of radioactive isotopes. Due to plate tectonics, 12 huge plates are formed from the earth’s crust and move apart or push together at a rate of millimeters per year. The collision of plates results in ocean trenches or strong earthquakes. The temperature at higher depth is capable of melting rock forming magma. The less dense hot magma reaches the earth’s crust or as lava and carries heat energy.

The most portion of magma retains in earth crust and its high temperature heats the surrounding rocks and subterranean water. This hot water comes out to earth surface through faults and cracks as hot springs or geysers. The hot water and steam trapped in permeable rocks under a layer of impermeable rocks and is known as a geothermal reservoir. These reservoirs can act as sources of geothermal energy with potential to generate electricity or direct use application.

5. Classification of Geothermal Resources

The geothermal resources are classified based on enthalpy of the geothermal fluids that act as the carrier transporting heat from the deep hot rocks to the surface. Geothermal resources are divided into low, medium and high enthalpy resources.

6. Uses of Geothermal Energy

Geothermal energy is a base load power source similar to nuclear and coal-fired plants as it produces energy at a constant rate and so its power output remains consistent nearly 24 hours a day. It also has higher capacity factor than solar or wind power, which are dependent on sun to shine or the wind to blow, respectively.

Some applications of geothermal energy use the earth’s temperatures near the surface, while others require drilling miles into the earth. The three main uses of geothermal energy are:

6.1 Direct Use as Heating Systems which use hot water from springs or reservoirs near the surface.

The thermal energy (heat) of earth is used directly as heat, rather than being used to generate electricity. The hot springs with low reservoir temperatures, i.e., <150 °C is utilized for direct heat uses. The hot water of 60° – 130 °C can be used for direct heat utilization if the difference in temperature with the surroundings is adequate to extract the heat content. Direct uses can be made available at the sites where the transportation and transmission of electricity and water may be difficult due to remoteness of the area. Direct uses are useful in partial saving of the energy required in small-scale industrial uses like vegetable drying, concrete block curing, etc. It also has a major societal impact in the form of bathing centres, skin cure centres, spas used for tourist attraction, green housing and cold storage for utility of local population and farming industry. Geothermal Heating/Cooling technologies by retrofitting of existing HVAC systems by Geo exchange based technologies for space heating in mountainous regions and severe winters, and for space cooling / air-conditioning.

6.2 Electricity generation in a power plant requires water or steam at very high temperature (150 to 350 degrees Celsius). Geothermal power plants are generally built where geothermal reservoirs are located within a mile or two of the surface.

6.3 Geothermal heat pumps use stable ground or water temperatures near the earth’s surface for comfort cooling or heating of buildings

7. Types of Geothermal Power plants

Geothermal power plants use hydrothermal resources for electricity generation from the heat source (magma) within the earth’s crust. It consists of water (hydro) and heat (thermal). The high temperature (150 to 350 degrees Celsius) in hydrothermal resources is harnessed either by drilling wells (dry steam wells or hot water wells) into the earth and piping the steam or hot water to the surface. The power plants use steam to power a turbine for generation of electricity. The geothermal power plants are categorized into three groups.

a) Dry steam geothermal Plant

b) Flash steam geothermal plants

c) Binary power plants

7.1 Dry Steam Geothermal Plant

A dry steam geothermal plant is a type of geothermal power plant in which steam, generated underground by geothermal heat, is used directly, thus eliminating the need for boilers and boiler fuel that characterizes other steam-power-generating technologies. Conventional turbine generators are used with dry steam resources. It is also known as vapour dominated plant.

Water boils underground in a hydrothermal resource when it has pressure of about 7 atm and temperature of about 165oC. The dry steam fields are located at the Geysers region of California and Matsukawa region of Japan.

The plant consists of production well to extract steam from the hydrothermal resource, a centrifuge separator to remove solid matter from the steam, a turbine to convert thermal energy into mechanical energy, a generator coupled to turbine to generate electric power, a condenser to condense wet steam exited from turbine into water by direct contact with cooling water and a cooling tower to cool warm water exited from the condenser and returning the cooled water to the condenser. The turbines of the generators are operated by the steam piped directly from a geothermal reservoir for electricity production.

7.2 Flash Steam Geothermal Plant

A flash-steam geothermal plant is a type of geothermal steam plant where liquid is flashed to steam and the steam is separated from the remaining liquid used to drive a turbine generator. When the temperature of the hydrothermal liquids is over 350°F (177°C), flash-steam technology is generally employed. It is also known as wet steam or liquid dominated geothermal plant. The dual-flash cycle which separates the steam at two different pressures return water to geothermal reservoir improves the economics of most hydrothermal flash plant. The dual-flash cycle produces 20% to 30% more power than a single-flash system at the same fluid flow.

These plants harness high-pressure hot water from deep inside the earth and convert it to steam to drive the generator turbines. The water is condensed after cooling of steam and is injected back into the ground for recycling. Most of the geothermal power plants are flash steam plants.

7.3 Binary Cycle based Geothermal Plant

Binary cycle power plants are used with hot water that is at a lower temperature (100°–300°F). The hot water is passed through a heat exchanger in conjunction with a secondary fluid with a lower boiling point. Generally a hydrocarbon such as iso-butane or iso-pentane I used as secondary fluid. The secondary fluid vaporizes, which turns the turbines, which drive the generators. The remaining secondary fluid is simply recycled through the heat exchanger. The geothermal fluid is condensed and returned to the reservoir.

8. Enhanced Geothermal System

An Enhanced Geothermal System (EGS) also known as engineered geothermal systems is a variety of engineering techniques used to artificially create hydrothermal resources (underground steam and hot water) that can be used to generate electricity. In an EGS, fluid is injected into the subsurface under carefully controlled conditions, which cause pre-existing fractures to re-open, creating permeability. Enhanced geothermal systems utilize advanced, often experimental, drilling and fluid injection techniques to augment and expand the availability of geothermal resources, which can be used to generate electricity from the heat in the earth’s crust. Enhanced geothermal systems, when recharged, can provide near continuous output, making the technology a renewable, zero-carbon option for supplying base load electricity generation.

9. Ground Source Heat Pump (GSHP’s)

Ground Source Heat Pumps (GSHP’s) use the earth’s relatively constant temperature between the 16-29°C all year round at a depth of 20 feet. The ground source heat pumps (GSHP) are also known as Ground Heat Pumps-GHP, Ground Coupled Heat Pump-GCHP, Groundwater Heat Pump-GWHP and Geo-exchange Pumps. GSHP provides heating, cooling, and hot water for homes and commercial buildings. GSHPs use the earth as a heat source (in the winter) or a heat sink (in the summer). The basic principle on which the GHP works is “refrigeration cycle”. The refrigerant carries the heat from one “space” to another.

10. Geothermal potential in India

The estimated total installed capacity of geothermal power generation in the world was around 12.8 gigawatts (GW). USA, Philippines, Indonesia, Mexico and New Zealand are the major geothermal power generating countries. The total installed capacity of direct utilization of geothermal energy is around 70.3 GW and China, USA, Sweden, Turkey, Germany, France, Japan and Iceland are leading countries.

India is in low and medium heat enthalpy zone (100-180 °C) and 340 hot springs have identified in different parts of India with surface temperature ranges from 35 °C to as much as 98 °C. Puga, Manikaran, Tatapani, Tapovan, Cambay are potential geothermal locations in India.

The hot springs present in the country are grouped into seven geothermal provinces    :

(i) Himalayan – Puga Chhumthang Province

(ii) Sohana Valley

(iii) Cambay Basin

(iv) Son-Narmada-Tapi (SONATA) lineament belt

(v) West Coast

(vi)Godavari basin and

(vii) Mahanadi basin.

The thermal characteristic of some of the above potential geothermal provinces are given below:

Research, development and demonstration programme for utilization of geothermal energy for power generation and direct heat applications was supported by Ministry of New and Renewable Energy (MNRE) through different organizations like Indian Institute of Technology (IIT), Delhi; National Aeronautical Laboratory (NAL), Bangalore; Geological Survey of India; National Geophysical Research Institute, Hyderabad; and National Hydroelectric Power Corporation, Faridabad.

11. Advantages of geothermal energy

• It is reliable source of energy
• It is in continuous supply
• Its availability id independent of weather
• It has an inherent storage capability and it does not require a storage device
• Small space is required o install the plant
• Least polluting
• It does not require any supply fuel to generate heat

12. Disadvantages of geothermal energy

• Geothermal energy is available as low grade heat(that is temperature of geothermal fluid is low)
• Geothermal fluids also bring dissolved gases and solute which lead to air and land pollution
• Removal of heated water from the hydrothermal reservoir may lead to land subsidence or seismic imbalance
• Geothermal energy cannot be transported for very long distance (less than 30 kms)
• The life of plant equipment is limited due to corrosive and abrasive nature of geothermal fluids.