Geothermal Energy


Technology

Electricity can be generated through the usage of geothermal power. This is done by harnessing intense heat that is available within rocks deep inside the Earth's crust. This heat, by its very nature, continuously flows outwards and away from the centre of the Earth. The heat may usually be generated as a result of the radioactivity in the rocks deep within the Earth's crust. However, geothermal energy primarily originates from the hot molten core of the Earth, which consists of metals, at high temperatures exceeding several thousand degrees Celsius. Geothermal energy therefore is one another form of renewable energy. However, at present, it contributes to less than 2% of the world's energy requirements. Currently, the installed capacity of geothermal energy across more than 20 countries in the world is around 8000 megawatts.

Types

Heat from the Earth, also known as geothermal energy, heats water that has seeped into underground reservoirs. These reservoirs can be tapped for a variety of uses depending on the temperature of the water. The energy from high temperature reservoirs (225-600F) can be used to produce electricity. There are currently three types of geothermal power plants:

  • Dry steam - Dry steam plants use steam from underground wells to rotate a turbine, which activates a generator to produce electricity. There are only two known underground resources of steam in the United States namely The Geysers in northern California and Old Faithful in Yellowstone National Park. The power plants at The Geysers are the only dry steam plants in the country considering that Yellowstone is protected from development
  • Flash steam - Flash steam plants are the most common type of geothermal power plants. They use waters at temperatures greater than 360F. As this hot water flows up through wells in the ground, the decrease in pressure causes some of the water to boil into steam. The steam is then used to power a generator and any leftover water and condensed steam is returned to the reservoir
  • Binary cycle - Binary cycle plants use the heat from lower-temperature reservoirs (225-360F) to boil a working fluid, which is then vaporized in a heat exchanger and used to power a generator. The water, which never comes into direct contact with the working fluid, is then injected back into the ground to be reheated

There are three different ways for tapping geothermal energy:

  • The first method involves the usage of geothermal heat found near the surface of the Earth to directly heat buildings. The most well known example is district heating. This system supplies communities with hot water or heating. It is also applied for a number of commercial and industrial uses.
  • The second method is based on geothermal heat pumps that can pump the relatively constant temperature of the top 15 metres of the Earth's surface (or ground water) to heat or cool buildings indirectly. The pump uses a series of pipes to circulate fluid through the warm ground. In the winter, when the ground is warmer than the buildings above, the liquid absorbs heat from the ground, which is then concentrated and transferred to the buildings. This can also be used to heat domestic water. In the summer, when the ground is cooler, the pump transfers heat from the buildings back into the ground.
  • The third method revolves around Electricity production. Electricity can be produced through this method through 3 different types of power plants, all of which  convert geothermal energy to electricity, depending on the temperature of the geothermal fluid used. The design of all three power plants is based on the usage of a turbine which is driven by steam that then drives a generator to produce electricity.

Benefits

The primary key benefit of geothermal energy is that it is non-polluting and in sync with nature and the planet. Apart from being a clean energy source, it is also widely available and free. Moreover, geothermal energy can be tapped not only in warm climates but also in cold climates since it is below the surface of the ground where temperatures are stable across Earth. The applications are numerous and include heating and cooling as the need arises. This form of energy also includes quiet operation since the systems are either underground or indoors inside a building. Additional geothermal systems involve low maintenance costs and high efficiency. Further, this energy doesn’t contribute to global warming since it is part of the natural systems of the planet and is environmentally friendly. It is also long lasting when compared to conventional fuels like coal and oil which are expected to run out in the next few decades. There is also no hazardous waster per se from this type of renewable energy.

Barriers

Geothermal systems are naturally handicapped in cooler climates since they need backup heat sources. This makes them less feasible when compared to other sources of energy. Also, the quality of air indoors is a general concern which includes gases such as hydrogen sulfide. This gas is usually released from the ground geothermal drilling is implemented for deep energy sources. It is also important reinjection of materials should be done without polluting ground water when geothermal systems are employed through drilling to harness energy from steam or water from Earth. One of the primary concerns however is the depletion of geysers and hot springs when this energy is channeled. If more steam or water is removed than is actually advised, then it could lead to damage of the ecosystems. Similarly, implementing geothermal systems may impact the land stability in nearby areas. The release of carbon dioxide when drilling can actually lead to addition of greenhouse gases though it may be infinitesimal. Certain waste is also produced as part of the process that needs to be disposed off carefully.

Performance and Efficiency

The energy performance of a GHP system can be influenced by three primary factors: the heat pump machine, the circulating pump or well pump, and the ground-coupling or groundwater characteristics. The heat pump is the largest energy consumer in the system. Its performance is a function of two things: the rated efficiency of the machine and the water temperature produced by the ground-coupling (either in heating or cooling mode).The most important strategy in assembling an efficient system is to start with an efficient heat pump. It is difficult and expensive to enlarge a ground-coupling to improve the performance of an inefficient heat pump.

Fuel Types

Applications:

Power plant technologies are being used to convert hydrothermal fluids to electricity. The type of conversion used depends on the state of the fluid (whether steam or water) and its temperature.

Geothermal reservoirs of low-to moderate-temperature water — 68°F to 302°F (20°C to 150°C) — provide direct heat for residential, industrial, and commercial uses. This resource is widespread in the U.S., and is used to heat homes and offices, commercial greenhouses, fish farms, food processing facilities, gold mining operations, and a variety of other applications.

The geothermal heat pump, also known as the ground source heat pump, is a highly efficient renewable energy technology that is gaining wide acceptance for both residential and commercial buildings. Geothermal heat pumps are used for space heating and cooling, as well as water heating. Its great advantage is that it works by concentrating naturally existing heat, rather than by producing heat through combustion of fossil fuels.



FERREIRA RESIDENCE: Pump system for a water to water geothermal system with radiant floor heating, second stage air heating and central air conditioning.

Advantages and Disadvantages

Advantages

Disadvantages

  • Geothermal energy offers a number of advantages over traditional fossil fuel based sources. clean and safe for the surrounding environment.
  • Construction of the power plants can adversely affect land stability in the surrounding region. This is mainly a concern with Enhanced Geothermal Systems, where water is injected into hot dry rock where no water was before.
  • It is also sustainable because the hot water used in the geothermal process can be re-injected into the ground to produce more steam.
  • Dry steam and flash steam power plants also emit low levels of carbon dioxide, nitric oxide, and sulfur, although at roughly 5% of the levels emitted by fossil fuel power plants.
  • Geothermal power plants are unaffected by changing weather conditions.
  • Geothermal plants can be built with emissions-controlling systems that can inject these gases back into the earth, thereby reducing carbon emissions to less than 0.1% of those from fossil fuel power plants.
  • Geothermal power plants work continually, day and night, making them base load power plants.
  • The system was designed too large for the site, since there is only so much energy that can be stored and replenished in a given volume of earth.
  • From an economic view, geothermal energy is extremely price competitive in some areas and reduces reliance on fossil fuels and their inherent price unpredictability.
  • A specific geothermal location can undergo depletion.
  • It offers a degree of scalability: a large geothermal plant can power entire cities while smaller power plants can supply more remote sites such as rural villages.
  • Though geothermal energy is supposed to be renewable, water injection may be required due to reduced content of water in the reservoirs.

Economic Performance

Ultimately, the costs of small geothermal plants will determine their potential market. The two types of cost evidence here are reported costs form small geothermal plants at large geothermal developments and modeled cost estimates. Reported costs from small geothermal plants are rare. Few small units in the GRC database list initial power prices. Those that do are located at large fields and are in the $0.05_$0.07/k Wh range, for units in the 1-%MW range (GRC 1998).

Entingh et al. (1994 a and b) developed a model called GT-SMALL for small, binary geothermal systems in the 100-1000-KW size range. The accuracy of GT-SMALL is difficult to evaluate given the scarcity of remote applications of small systems. The $0.05-$0.07/K Wh prices reported in the GRC database are comparable to the modeled cost estimates at the 1-MW size.

Assumptions about the exploration costs, resource quality, and financing costs determine the modeled cost results. In their modeling study, Entingh et al. (1994 a and b) describe as example system that serves as a reference point. The characteristics of the example system are shown in below table.



An economically competitive geothermal power plant can cost as low as $2800 per kilowatt installed. While the cost of a new for geothermal power plant is higher than that of a comparable natural gas facility, for natural gas construction costs account for only one third of the total price of the facility, while the cost of the fuel at a natural gas facility represents two thirds of the cost. The initial construction costs of a geothermal facility, in contrast, represent two thirds or more of total costs. So while initial investment is high for geothermal, natural gas and geothermal are still economically comparable over a long term.

Geothermal power plants can produce electricity as cheaply as some conventional power plants. It costs 4.5 to seven cents per kWh to produce electricity from hydrothermal systems. In comparison, new coal-fired plants produce electricity at about four cents per kWh.

Initial construction costs for geothermal power plants are high because geothermal wells and power plants must be constructed at the same time.

The cost of producing electricity over time is lower because the price and availability of the fuel is stable and predictable. The fuel does not have to be imported or transported to the power plant. The power plant literally sits on top of its fuel source.

Geothermal power plants are excellent sources of baseload power. Baseload power is power that electric utility companies must deliver all day long. Baseload geothermal plants sell electricity all the time, not only during peak use times when the demand for electricity is high.

Utilities were required to buy the least-cost electricity, without regard to environmental impacts. Federal and state energy and environmental agencies are studying ways to give preference to nonpolluting energy sources such as geothermal energy.

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