Geothermal energy is an alternative of fossil fuels. The word “Geothermal” consists of two words Geo which means Earth and thermal means the heat energy. Thus, the conversion of natural heat of Earth into electrical energy is known as geothermal energy. Useful applications of geothermal energy is possible from the combination of geologic knowledge and engineering technology. The idea of using geothermal energy is not new. This type of resource is not uniform throughout the globe. To utilise the potential of geothermal reservoirs, it must have a minimum of 40°C/km geothermal gradient.
Some parts of the Earth experience high flow of heat as compared to others. This type of areas are mostly along plate boundaries. For example- Mid Oceanic Ridge (Divergent Plate Boundaries) and also Convergent Plate Boundaries. The suitable sites for exploration are areas which reflects the near surface geothermal gradient such as geysers and hot springs, also areas of recent volcanic activity. Sometimes there can be no surficial expression of the underlying geothermal reservoir. Therefore, geophysical exploration serves as the best technique to locate the area of interest. Techniques like gravimetric method can be used.
Hot magma near the surface, heats up the groundwater. If the temperature is high enough and is able to form steam from water, vapour is used to run turbines. If the temperature is not high and is unable to form steam, warm water can also be utilized for generating electricity.
If molten magma is not present, but igneous rocks are present. Even hot, dry igneous rocks can serve as geothermal reservoirs. Some different and new approach is needed for utilizing the heat as they are not capable of convection like processes. In such conditions, hydro fracturing techniques are to be employed. Cold water is injected into the rock from one point and is pumped from the other point. This will help in elevating the temperature of water because it will consume the heat inside the rock.
Sources of Geothermal Energy
There are mainly three types of source depending on the geologic criteria
- Hydrothermal convection systems.
- Igneous systems.
- Geo pressured systems.
- Hydrothermal convection systems are one of the most significant source of heat loss from the Earth. The system is more active close to Mid Oceanic Ridge and Subduction zones. It is characterized by a permeable layer in which variable amount of hot water circulates. There are two types of hydrothermal circulation system-
Vapour dominated system- such systems are not very common. Such geothermal reservoirs contains steam and water at depth and near the surface, the pressure is less and superheated steam is present over there. This superheated steam can be tapped and can be directly use for running turbine to generate electricity. However, these systems have slow recharge rate of groundwater and thus, the hot rocks are capable of converting the water into steam.
Hot water system- such systems have high surface water temperature (up to 150°C). It has a zone of circulating hot water which when tapped, the pressure gets reduced and yields a mixture of steam and hot water. The steam is used to run the turbine. Thus, water is to be removed from the steam before utilizing it. This will lead to the issue of water disposal. However, in such cases water is reinjected into the geothermal reservoir.
- Igneous systems are other source or reservoir of geothermal energy. It may contain hot magma. The temperature can range from 650° C to 1200° C depending on the type of magma. Such igneous masses are capable of converting the groundwater into vapours. And such vapours are used to run turbines for generating electricity. Moreover, the dry, hot rocks are also capable of heat loss. Such systems may store heat per unit volume more than any of the other geothermal systems. Hydro fracturing method is to be employed in such conditions.
- Geo pressured systems are where the heat flow from the Earth is suppressed due to the above impermeable layers might be of clay. Such impermeable layers acts as resistors and resists the heat flow. Rapid sedimentation and regional surface subsidence are the favourable conditions for the formation of Geo pressured systems. Water trapped in the sediments develops considerable pressure and the temperature gets elevated. Such systems are able to produce electricity for some reasons
Such systems contain hot water thermal energy that can be easily extracted and utilized.
High mechanical energy is provided through the water that is at high pressure and thus it is successful in running hydraulic turbine.
Such systems are also capable of generating methane up to certain extent. Methane is generated by the decay of organic debris accompanied by rapid sedimentation and subsidence.
Pros and cons of using Geothermal Energy.
- It is a pollution free source for the generation of electricity.
- Sulphur is emitted from magma which gets mixed with steam but it do not pose any serious and harmful environmental effects.
- There is no problem of ash, radioactive wastes and carbon dioxide problems associated with other fossil fuels.
- It does not require extensive transportation either for raw materials or for refining.
- Such a system is economically competitive as compared to other fossil fuels.
- The hot water may contain many dissolved salts which can be the source of contamination of surface water and ultimately the groundwater.
- Such dissolved salts are also capable of corroding pipes.
- Such a resource can be used for certain time period and should be used in a sustainable way.
- Geo thermal power plants are stationary and is found near the source. Transmission is not economically favourable.
- One of the most important issue is the subsidence problem. When the hot water is removed from the rocks, the rocks starts to cools and contracts and give rise to the problem of surface subsidence.
- Although such a system do not have such harmful effects as other fossil fuels but still there are some impacts. Geothermal energy is developed at particular site and can cause problems like on site noise, gas emissions and industrial scars.
- Except for vapour dominated systems, such systems are capable of generating thermal pollution from hot waste water. Such waters may contain dissolved solids and thus it can be saline and corrosive. Reinjected the water back into the reservoir can activate the fracture systems in Earth and can cause Earthquakes.