GEOTHERMAL ENERGY

                                                                          INTRODUCTION

WHAT IS GEOTHERMAL ENERGY?

"Geothermal" comes from the Greek words geo (earth) and thermal (heat). So, geothermal means earth heat. The thermal energy contained in the interior of the earth is called geothermal energy. Geothermal heat originates from earth’s fiery consolidation of dust and gas over four billion years ago. The geothermal energy is enormous and will last for several millions of years and is therefore called renewable.     

EARTH’S HEAT AND VOLCANIC REGIONS

It is almost 6,500 kilometers (4,000 miles) from the surface to the center of the Earth, and the deeper you go, the hotter it gets. The outer layer, the crust, is three to 35 miles thick and insulates us from the hot interior.

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 - 30°C per kilometer of depth (50-87°F per mile). Below the crust is the mantle, made of highly viscous, partially molten rock with temperatures between 650 and 1,250°C (1,200-2,280°F). At Earth's core, which consists of a liquid outer core and a solid inner core, temperatures may reach 4,000-7,000°C (7,200 to 12,600°F).

Since heat always moves from hotter regions to colder regions, the Earth’s heat flows from its interior toward the surface. This outward flow of heat from Earth’s interior drives convective motion in the mantle rock which in turn drives plate tectonics -- the "drift" of Earth's crustal plates that occurs at 1 to 5 cm per year (about the rate our fingernails grow). Where plates move apart, magma rises up into the rift, forming new crust. Where plates collide, one plate is generally forced (subducted) beneath the other. As a subducted plate slides slowly downward into regions of ever-increasing heat, it can reach conditions of pressure, temperature and water content that cause melting, forming magma. Plumes of magma ascend by buoyancy and force themselves up into (intrude) the crust, bringing up vast quantities of heat.

HOW DOES GEOTHERMAL HEAT GET UP TO EARTH'S SURFACE?

The heat from the earth's core continuously flows outward. It transfers (conducts) to the surrounding layer of rock, the mantle. When temperatures and pressures become high enough, some mantle rock melts, becoming magma. Then, because it is lighter (less dense) than the surrounding rock, the magma rises (convects), moving slowly up toward the earth's crust, carrying the heat from below.

GEOTHERMAL RESOURCES

Understanding geothermal energy begins with an understanding of the source of this energy—the earth’s internal heat. The Earth’s temperature increases with depth, with the temperature at the center reaching more than 4200 °C (7600 °F). A portion of this heat is a relic of the planet’s formation about 4.5 billion years ago, and a portion is generated by the continuing decay of radioactive isotopes. Heat naturally moves from hotter to cooler regions, so Earth’s heat flows from its interior toward the surface.

Because the geologic processes known as plate tectonics, the Earth’s crust has been broken into 12 huge plates that move apart or push together at a rate of millimeters per year. Where two plates collide, one plate can thrust below the other, producing extraordinary phenomena such as ocean trenches or strong earthquakes. At great depth, just above the down going plate, temperatures become high enough to melt rock, forming magma.3 Because magma is less dense than surrounding rocks, it moves up toward the earth’s crust and carries heat from below. Sometimes magma rises to the surface through thin or fractured crust as lava.

However, most magma remains below earth’s crust and heats the surrounding rocks and subterranean water. Some of this water comes all the way up to the surface through faults and cracks in the earth as hot springs or geysers. When this rising hot water and steam is trapped in permeable rocks under a layer of impermeable rocks, it is called a geothermal reservoir. These reservoirs are sources of geothermal energy that can potentially be tapped for electricity generation or direct use. Figure 1 is a schematic of a typical geothermal power plant showing the location of magma and a geothermal reservoir.4 Here, the production well withdraws heated geothermal fluid, and the injection well returns cooled fluids to the reservoir.

RESOURCE IDENTIFICATION

Geological, hydrogeological, geophysical, and geochemical techniques are used to identify and quantify geothermal resources. Geological and hydrogeological studies involve mapping any hot springs or other surface thermal features and the identification of favorable geological structures. These studies are used to recommend where production wells can be drilled with the highest probability of tapping into the geothermal resource. Geophysical surveys are implemented to figure the shape, size, depth and other important characteristics of the deep geological structures by using the following parameters: temperature (thermal survey), electrical conductivity (electrical and electromagnetic methods), propagation velocity of elastic waves (seismic survey), density (gravity survey), and magnetic susceptibility (magnetic survey).5 Geochemical surveys (including isotope geochemistry) are a useful means of determining whether the geothermal system is water or vapor-dominated, of estimating the minimum temperature expected at depth, of estimating the homogeneity of the water supply and, of determining the source of recharge water.

Geothermal exploration addresses at least nine objectives:6

• Identification of geothermal phenomena
• Ascertaining that a useful geothermal production field exists
• Estimation of the size of the resource
• Classification of the geothermal field
• Location of productive zones
• Determination of the heat content of the fluids that will be discharged by the wells in the geothermal field
• Compilation of a body of data against which the results of future monitoring can be viewed
• Assessment of the pre-exploitation values of environmentally sensitive parameters
• Determination of any characteristics that might cause problems during field development

DRILLING

Once potential geothermal resources have been identified, exploratory drilling is carried out to further quantify the resource. Because of the high temperature and corrosive nature of geothermal fluids, as well as the hard and abrasive nature of reservoir rocks found in geothermal environments, geothermal drilling is much more difficult and expensive than conventional petroleum drilling. Each geothermal well costs $1–4 million to drill, and a geothermal field may consist of 10–100 wells. Drilling can account for 30–50% of a geothermal project’s total cost.

 Typically, geothermal wells are drilled to depths ranging rom 200 to 1,500 meters depth for low- and medium-temperature systems, and from 700 to 3,000 meters depth for high-temperature systems. Wells can be drilled vertically or at an angle. Wells are drilled in a series of stages, with each stage being of smaller diameter than the previous stage, and each being secured by steel casings, which are cemented in place before drilling the subsequent stage. The final production sections of the well use an uncemented perforated liner, allowing the geothermal fluid to pass into the pipe. The objectives of this phase are to prove the existence of an exploitable resource and to delineate the extent and the characteristics of the resource. An exploratory drilling program may include shallow temperature-gradient wells, “slim-hole” exploration wells, and production-sized exploration/production wells. Temperature-gradient wells are often drilled from 2–200 meters in depth with diameters of 50–150 mm. Slim-hole exploration wells are usually drilled from 200 to 3000 meters in depth with bottom-hole diameters of 100 to 220 mm. The size and objective of the development will determine the number and type of wells to be included in exploratory drilling programs

APPLICATIONS OF GEOTHERMAL ENERGY:

1. POWER GENERATION:

Utility-scale geothermal power production employs three main technologies. These are known as dry steam, flash steam and binary cycle systems. The technology employed depends on the temperature and pressure of the geothermal reservoir. Unlike solar, wind, and hydro-based renewable power, geothermal power plant operation is independent of fluctuations in daily and seasonal weather.

DRY STEAM:

Dry steam power plants use very hot (>455 °F, or >235 °C) steam and little water from the geothermal reservoir.12 The steam goes directly through a pipe to a turbine to spin a generator that produces electricity. This type of geothermal power plant is the oldest, first being used at Lardarello, Italy, in 1904.13 Figure 2 is a schematic of a typical dry steam power plant.14

 
Figure 1. Dry Steam Power Plant Schematic  
Source: National Renewable Energy Laboratory (NREL)

FLASH STEAM 

Flash steam power plants use hot water (>360 ºF, or >182 ºC) from the geothermal reservoir.15 When the water is pumped to the generator, it is released from the pressure of the deep reservoir. The sudden drop in pressure causes some of the water to vaporize to steam, which spins a turbine to generate electricity. Both dry steam and flash steam power plants emit small amounts of carbon dioxide, nitric oxide, and sulfur, but generally 50 times less than traditional fossil-fuel power plants.16 Hot water not flashed into steam is returned to the geothermal reservoir through injection wells. Figure 3 is a schematic of a typical flash steam power plant.17

Figure 2. Flash Steam Power Plant Schematic

BINARY-CYCLE

Binary-cycle power plants use moderate-temperature water (225 ºF–360 ºF, or 107 ºC–182 ºC) from the geothermal reservoir. In binary systems, hot geothermal fluids are passed through one side of a heat exchanger to heat a working fluid in a separate adjacent pipe. The working fluid, usually an organic compound with a low boiling point such as Iso-butane or Iso-pentane, is vaporized and passed through a turbine to generate electricity. An ammonia-water working fluid is also used in what is known as the Kalina Cycle. Makers claim that the Kalina Cycle system boosts geothermal plant efficiency by 20–40 percent and reduces plant construction costs by 20–30 percent, thereby lowering the cost of geothermal power generation.

Figure 4. Binary Cycle Power Plant Schematic
Source: National Renewable Energy Laboratory (NREL)

The advantages of binary cycle systems are that the working fluid boils at a lower temperature than water does, so electricity can be generated from reservoirs with lower temperature, and the binary cycle system is self-contained and therefore, produces virtually no emissions. For these reasons, some geothermal experts believe binary cycle systems could be the dominant geothermal power plants of the future. Figure 4 is a schematic of a typical binary cycle power plant.18

2. GEOTHERMAL HEAT PUMPS:

The shallow ground, the upper 10 feet of the Earth, maintains a nearly constant temperature between 50° and 60°F (10°–16°C). Like a cave, this ground temperature is warmer than the air above it in the winter and cooler than the air in the summer. Geothermal heat pumps take advantage of this resource to heat and cool buildings.              

Geothermal heat pump systems consist of basically three parts: the ground heat exchanger, the heat pump unit, and the air delivery system (ductwork). The heat exchanger is basically a system of pipes called a loop, which is buried in the shallow ground near the building. A fluid (usually water or a mixture of water and antifreeze) circulates through the pipes to absorb or relinquish heat within the ground.

    

In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to heat water, providing a free source of hot water.

Geothermal heat pumps offer unmatched benefits over traditional heating and cooling systems, including:

• Lower operating costs: A geothermal system can cut utility bills by 30 to 50 percent compared to conventional heating and cooling systems.
• Environmental impact: Ground-source heat is naturally renewable and non-polluting.
• Lower maintenance costs: All equipment is protected indoors or underground.
• Life span: A geothermal system can have a life expectancy of up to 30 years; ground loops are often warranted for up to 50 years.
• Single system: Geothermal equipment provides both heating and cooling in one system.
• Indoor comfort: Geothermal systems eliminate the drafts common with conventional forced-air systems.
• Design flexibility: Geothermal systems can be easily and inexpensively subdivided or expanded to fit building remodeling or additions.
• Energy efficiency: A geothermal heat pump can move more than three units of heat energy for every one unit of electrical energy used to power the system.
• Safety: No dangers of gas leaks or carbon monoxide poisoning.

 3. DIRECT USE APPLICATIONS

Geothermal reservoirs within the low to moderate temperature range can provide heat for residential, industrial, and commercial use.  The Energy Efficiency and Renewable Energy (EERE) division of the U.S. Department of Energy reports that savings can be as much as 80% over the use of fossil fuels.  This form of energy is also very clean, with far fewer air pollutants emitted when compared to fossil fuels. 

The employment of direct use geothermal energy requires that a certain infrastructure be established for proper handling of this resource.  First, a production facility, usually a well, will bring the hot water to the ground surface.  Second, a mechanical system to deliver the heat to a space or process must be developed.  This means the piping, heat exchanger, and control infrastructure for heat extraction. Third, there must be a disposal system, such as an injection well or storage pond, that can receive the cooled geothermal fluid.

Direct use applications for geothermally heated waters is quite extensive.  A number of operations use low-temperature geothermal resources for district and space heating, greenhouses, and aquaculture facilities. District systems distribute naturally heated water from one or more geothermal wells through a series of pipes to several houses and buildings, or blocks of buildings.  Space heating uses one well per structure.  In both of these systems, the geothermal heat is replacing fossil fuel burning as the heat source for the traditional heating system.  District heating systems can save consumers 30% to 50% of the cost of natural gas heating.  

Numerous other industrial and commercial uses are also possible.  Industrial applications can include food dehydration, cement and aggregate drying, concrete block curing, milk pasteurizing, spas, and others

ADVANTAGES OF USING GEOTHERMAL ENERGY

• Clean.

Geothermal power plants, like wind and solar power plants, do not have to burn fuels to manufacture steam to turn the turbines. Generating electricity with geothermal energy helps to conserve nonrenewable fossil fuels, and by decreasing the use of these fuels, we reduce emissions that harm our atmosphere. There is no smoky air around geothermal power plants -- in fact some are built in the middle of farm crops and forests, and share land with cattle and local wildlife.

• Easy on the land.

 The land area required for geothermal power plants is smaller per megawatt than for almost every other type of power plant. Geothermal installations don't require damming of rivers or harvesting of forests -- and there are no mine shafts, tunnels, open pits, waste heaps or oil spills.

• Reliable.

Geothermal power plants are designed to run 24 hours a day, all year. A geothermal power plant sits right on top of its fuel source. It is resistant to interruptions of power generation due to weather, natural disasters or political rifts that can interrupt transportation of fuels.

• Flexible.

Geothermal power plants can have modular designs, with additional units installed in increments when needed to fit growing demand for electricity.

• Keeps Dollars at Home

Money does not have to be exported to import fuel for geothermal power plants. Geothermal "fuel'" - like the sun and the wind - is always where the power plant is; economic benefits remain in the region and there are no fuel price shocks.

FUTURE PROSPECTS IN INDIA.

In a global tectonic context, India is not particularly well placed as far as geothermal energy is concerned. However, due to anomalous nature of some segments of its lithosphere, it does contain a number of geothermal areas with temperature in the range of 30 degrees celsius to 100 degrees celsius. Most of them are intermediate temperature type and occur along certain tectonic boundaries.

The most promising geothermal areas include-(I) Puga-Chhumathang, Manikaran and Tapoban in New Himalayas, (2) Konkan, Cambay and Bombay Offshore, (3) Taptapani (Orissa), (4) Gondwanic grabens, and (5) Volcanic areas of Andaman-Nocobar chain.

Power deficient India is planning to use geothermal energy to produce 10,600 megawatts of power, five times more than the combined output from all non-conventional energy sources.

The first such plant is being set up in the Ladakh region of Jammu and Kashmir. Officials say this renewable energy would come at a throwaway cost, less than one third of hydropower. China on the other side of the border has already set up a geo-thermal plant.

                  Officials associated with the project say that they have already completed preliminary investigations in Ladakh. A five-member team led by Dr D.Chandrasekharam, head of the Earth Sciences Department at the Indian Institute of Technology, that visited the region recently is submitting a blue print to the government. According to Chandrasekharam, India has the capacity to produce 10,600 megawatts of geo-thermal power.

CONCLUSION:

Today is the era of non-conventional sources of energy. Till now India has tapped the potential of wind, tidal, solar and nuclear energy only. But geothermal technology is still to appear in India. The Indian government has taken few modest steps in this regard. Considering the potential applications of geothermal energy and the present Indian power scenario, it is high time that India exploits this source of energy to the fullest and makes a mark in the field of power production.