The term geothermal originates from the Greek words; Geo, which means earth and Thermal, which means heat. This derivation quickly points to the definition of geothermal energy, which is heat emanating from underneath the surface of the earth. The energy inside the earth was formed by the decay of minerals and forests several years ago. Traditionally, it was used for bathing and heating purposes but today it is also used for generating electricity.
It’s a renewable energy source, meaning it’s inexhaustible to humans. It’s also a green source of energy, meaning it does not emit greenhouse gasses that are hazardous to human and environmental health.
Sources of Geothermal Energy Heat
Scientists have constantly parted ways on the real source of heat for the generation of geothermal energy. But successive rigorous research has converged to this explanation. Approximately 4000 miles underneath the surface of the earth, this phenomenon called geothermal energy is produced deep inside the earth’s core.
The earth’s core is composed of three layers; the outer silicate and solid crust, a highly viscous mantle, and a liquid outer core. The outer core consists of extremely hot magma or melted rock wrapping around a solid iron center known as the inner core. The slow decay of radioactive material continually generates extremely high temperatures inside the earth.
This is a natural process in all rocks. Wrapping around the outer core is a layer called the mantle. The mantle is approximately 1800 miles thick and mainly composed of magma and rock. The crust is the outermost layer of the earth’s core. The crust forms the bulk of continents and ocean floors that run approximately three to five miles thick beneath the oceans and 15 to 35 miles thick on the continents.
The earth’s crust is split into numerous parts known as plates. It is at the edges of these plates that magma finds way near the surface of the earth. It is in these areas that volcanoes are prevalent. When a volcano occurs, lava erupts from underneath. This lava is partly magma. Underneath the earth’s surface, the water and rocks absorb heat from the magma.
As the depth increases, so do the temperatures of the underground water and rocks. Individuals across the world take advantage of the underground energy to heat their homes and generate electricity by digging up deep wells and subsequently pumping the hot underground water or steam to the earth’s surface.
Geothermal energy dates far back in the ancient times where it was utilized for heating and bathing. Today, hot springs across the world are still being utilized for bathing.
How is Geothermal Energy Converted into Electricity?
Utilizing geothermal energy to generate electricity is a considerably new industry, which manifested in 1904 in Italy. Italians first powered a turbine generator using natural steam erupting from beneath the earth. The year 1960 heralded the first successful operation of the large-scale geothermal electricity generation plant at the Geysers, North California. A lot of American geothermal power plants are spread across California, while the rest are located in Hawaii, Nevada, Utah, Idaho and Montana.
The conversion of geothermal energy into electricity occurs through a geothermal power plant. The power plant harnesses the steam from the hot water beneath the earth’s surface to turn turbines, which later activates a generator to produce electricity. Some geothermal power plants utilize steam to directly turn the turbine. Others utilize the steam to heat a liquid that is used to turn the turbine.
Main Types of Geothermal Power Plants
Geothermal power plants come in 3 main types:
- Binary cycle power plants
- Dry steam power plants
- Flash steam power plants
These three kinds of geothermal power plants have one thing in common; they utilize steam turbines to produce electricity. This concept is pretty much comparable to other thermal power plants that use alternative sources of energy other than geothermal.
Dry steam power plants
As the name suggests, these geothermal power plants utilize ‘’dry steam” to generate electricity. Dry steam is, essentially, water vapor or water in gaseous state. The geothermal power plant companies drill two separate wells to the extremely hot water reservoir under the earth’s surface; the production well and injection well. The production well extracts steam with a temperature of at least 150°C (300°F) from the hot water reservoir below and directs it to the turbine.
The steam turns the turbine, which turns a shaft connected to a generator. With the turning, the generator converts the energy into electricity, which goes through power lines to a power grid and eventually supplied to homes, institutions, and industries. The used steam finds its way to the condenser, where it’s converted into water and sent back down to the hot water reservoir through the injection well and the cycle continues.
Dry steam power plant is the old kind of geothermal power plant. The first dry steam power plant was set up in 1904 in Larderello, Italy. In the U.S., this type of geothermal power generation is only utilized in high volcanic mountain areas in California.
Flash steam power plants
This kind of geothermal power plant utilizes water at temperatures of at least 182°C. As the name suggests, it uses flash steam to generate electricity. Flash steaming is the process whereby extremely high-pressure hot water is flashed or vaporized into steam in a flash tank by reducing the pressure. The steam is then directed to turn turbines, which turns a shaft connected to a generator leading to production of electricity.
Flash steam power plants are the most common types of geothermal power plants in the modern world. The Wairakei Power Station, built in 1958 in New Zealand, was the first geothermal power plant that utilized flash steam.
Binary cycle power plants
This geothermal power plant is advantageous compared to the flash steam and dry steam power plants because it requires slightly cooler water (as low as 57°C) to heat a separate fluid (binary fluid) that has a lower boiling point.
The power plant enables cooler geothermal reservoirs to be utilized than is necessary for the flash steam and dry steam power plants. We have learned that the flash steam and dry steam use water at temperatures higher than 182 °C, which is pumped up under extremely high pressure to the electricity generation plant at the surface.
However, with binary cycle power plants, companies use pumps to pump up hot water from the hot water reservoir below through the production well, and the slightly cooler water is allowed to return to the reservoir below. A separate fluid with a lower boiling point known as the binary fluid, normally a pentane hydrocarbon or butane, is pumped up at considerably high pressure via the heat exchanger.
At the heat exchanger, the binary fluid is vaporized and directed to turn a turbine, which turns a shaft connected to a generator and electricity is generated. The vapor used to turn the turbine is then converted to water by cold air radiators and allowed to go back to the reservoir below through the injection well.
In a nutshell, binary cycle power plants allow individuals to harvest geothermal energy from hot water reservoirs that flash steam, and dry steam plants would not permit. However, the binary cycle power plants have an efficiency rate of just 10-13%. Russia is accredited with setting up a successful binary cycle power project in 1967.
Geothermal electricity production does bear the brunt of low thermal efficiency rates. However, the warm water, exhaust heat, and by-products have numerous uses. Transportation of geothermal electricity is the question that lingers in most people’s minds.
The mode of transportation of geothermal electricity has a lot in common with other sources of power like solar and wind. Voltage is normally enhanced to reduce losses, and the power conveyed onto the electrical grid. Transporting power over long distances needs a highly insulated piping system, which adds upfront costs to the overall geothermal electricity generation.
Future Of Geothermal Energy
The future of geothermal energy depends on three factors: it’s demand, supply and it’s competitiveness among other renewable resources in terms of cost, availability, reliability etc.. Demand for geothermal energy is going to increase and increase with the increase in the population and extinction of other non-renewable sources. Moreover, today government also support the resources which are cleaner and do not spoil the environment.
Supply of geothermal energy is limited and confined to certain areas only. The entire resource of geothermal energy is fairly bigger than that of coal, oil and gas. Geothermal energy can be made more widely available if the methods and technologies used to extract it are improved. Geothermal energy is still not explored fully. Several miles below the earth surface is hot, dry rock being heated by the molten magma directly below it.