Varieties of Geothermal Power Plants | Engineering Influencers to Follow
Types of geothermal power plants
By: Temitayo Oketola | Originally published on 8/29/2023
In the search for alternative energy sources, geothermal energy has emerged as a promising option for power generation. According to the International Renewable Energy Agency (IRENA), the total global installed geothermal capacity reached about 15.96 GW as of 2021.
Geothermal power plants harness the heat stored deep within the Earth’s crust and convert it into electricity. These plants typically utilize vapor from hot water reservoirs to drive a turbine that generates electrical power. However, there are several kinds of geothermal plants, each with a unique mode of operation depending on the state of the geothermal reservoir and application requirements.
Geothermal power plants can be broadly categorized into three types:
#1 Dry steam geothermal power plant
A dry-steam geothermal power plant harnesses energy from geothermal reservoirs that produce dry saturated steam or superheated steam at very high pressure. These plants are typically designed to utilize the dry steam directly from the ground without the need for additional heat exchange or water conversion processes.
The operation of dry steam power plants starts with steam extraction. High-pressure steam is naturally released from the reservoir and flows up through the well to the surface due to pressure difference. This steam at the surface is then channeled to a turbine, where it causes the turbine blades to spin and, in turn, generate electricity. After the steam condenses, it is frequently reinjected into the geothermal reservoir through injection wells.
Dry steam geothermal sources are rare and usually occur at the deepest drilling ranges but are often times the most economical to tap. The dry steam power plant technology is used today in northern California at the Geysers, the world’s largest single source of geothermal power.
#2 Flash steam geothermal plants
Flash steam geothermal plants are the most common type of geothermal plant in operation today. Unlike dry steam geothermal plants that utilize high-pressure steam directly, flash steam geothermal plants work with high-temperature water from the reservoir.
The operation of flash steam geothermal plants starts with pumping the high-temperature water under pressure from deep underground to a low-pressure tank at the Earth’s surface. As the high-pressure water is rapidly released into a low-pressure environment, a portion of the water suddenly transforms (or flashes) into steam. The steam is then separated from the remaining hot water before being channeled to the power plant to drive the turbine and generate electricity.
Geothermal reservoirs for flash steam plants typically produce pressurized water in the range of 180° C to 370° C and are typically more abundant than dry steam reservoirs. In areas where the geothermal reservoirs have a lower temperature, the binary-cycle geothermal power plants are typically used for power generation.
#3 Binary-cycle geothermal power plant
A binary-cycle geothermal power plant uses a secondary fluid to generate electricity. Its operation starts with the extraction of hot water or steam from the geothermal reservoir to the surface of the Earth. This hot water is then directed through a heat exchanger, where the heat is transferred to a secondary fluid.
Due to the lower boiling point of this secondary fluid, even modest heat from the geothermal fluid causes the secondary fluid to flash into vapor. This vapor is then used to drive a turbine and generate electricity.
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Some basic calculations
There are several parameters used in designing, specifying and analyzing the performance of geothermal power plants. An example of such a parameter is the heat input, which describes the thermal energy extracted from the geothermal fluid. It can be calculated using:
is the temperature difference (K)
For instance, consider a geothermal plant with a mass flow rate of geothermal fluid of 50 kg/s. If the specific heat capacity of the fluid is 4,000 J/kgK and the temperature difference of the geothermal fluid at the plant’s inlet and outlet is 50 K, then the heat input to the geothermal plant would be 10,000,000 J/s (or 10 MW). This value represents the amount of energy extracted from the geothermal fluid per unit of time, which serves as the input for electricity generation in the plant.
Conclusion
Geothermal power plants are capable of meeting energy demands, provided they are accurately designed and specified for a particular application. While this article presents the basic introduction and calculations on geothermal power plants, there are several other things to consider when designing these systems. As such, it is recommended to reach out to geothermal steam turbine and heat pump manufacturers to discuss application requirements.
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