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What is Geothermal Energy?

Geothermal Energy

Geothermal energy is heat that is generated within the Earth and is a renewable resource. About 2,900 km below Earth’s crust, or surface, is the core which is hottest part of our planet. Although most of the heat is constantly generated by the decay of radioactive isotopes (i.e. potassium-40 and thorium-232), a small portion of the heat comes from the friction and gravitational pull that was formed when Earth was created more than 4 billion years ago. Geothermal energy can be used to heat structures such as buildings, parking lots, and sidewalks [1].

Geothermal Gradient

This is the gradual change in temperature. In most parts of the world, the geothermal gradient is about 25°C per 1 km of depth [1]. The main source of geothermal energy is from magma (partly melted rock permeated by gas and gas bubbles) which heats nearby rocks and underground aquifers. Hot water can be released through geysers, hot springs, steam vents, underwater hydrothermal vents, and mud pots [1].

What are the Advantages?

• Renewable energy [1]

• Accessed and harvested anywhere in the world

• Using this energy is relatively clean with most systems only emitting water vapor

• Geothermal power plants can last for decades if managed properly

• The system is “baseload” unlike other energy sources which means they can work in the summer or winter and are not dependent on changing factors such as the presence of wind or sun while producing electricity or heat 24 hours a day, 7 days a week

• Power plant is compact in size

• System is adaptable to many different conditions

What are the Disadvantages?

• The process of injecting high-pressure stream of water into the Earth in order to access the energy can result in minor seismic activity or small earthquakes [1]
• Geothermal plants have been linked to subsidence (slow sinking of land which damages pipelines, roadways, buildings, and natural drainage systems)
• The plants can release small amounts of greenhouse gases such as hydrogen sulfide and carbon dioxide
• Water flowing through underground reservoirs can pick up trace amounts of toxic elements which can be leaked to water sources if the system is not properly insulated
• Initial expensive cost

Technologies/Methods for Harvesting Geothermal Energy

Low-Temperature Geothermal Energy

This energy is obtained from pockets of heat about 150°C and can be used immediately as a source of heat. Most pockets of low-temperature geothermal energy are found a few meters below ground. People use this energy for engineering, comfort, healing, and cooking [1].

Co-Produced Geothermal Energy

This technology relies on other energy sources. This form of energy uses water that has been heated as a byproduct in oil and gas wells. The steam created can be used to generate electricity that can be used immediately or sold to the grid [1].

Geothermal Heat Pumps

Geothermal heat pumps (GHPs) can be used anywhere in the world as they are drilled about 3 to 90 meters deep (much shallower than most oil and natural gas wells). GHPs do not require fracturing bedrock to reach their energy source. In the winter, the liquid in the system carries the heat upward through the building and gives off warmth through a duct system. In the summer, the system is in reverse: the liquid in the pipes is warmed from the heat in the building or parking lot which carries the heat to be cooled underground [1].

Enhanced Geothermal Systems

Enhanced Geothermal Systems (EGS) uses drilling, fracturing, and injection to provide fluid and permeability in areas that are not hydrothermal. Hydrothermal are areas underground that are hot, contain liquid, and are permeable (allow liquid and gases to pass through) [1].

Heating Mode

1. 1. 1. Circulation: above ground pump moves water (or another fluid) through a series of buried pipes or ground loops [2].

      2. Heat absorption: absorbs heat from the fluid passing through the ground loop through warmer soil, rock, or ground water around it.

      3. Heat exchange and use: heated fluid returns to the building through a heat exchanger that transfers heat into the building's air handling, distribution, and ventilation system.

      4. Recirculation: once the fluid transfers its heat to the building, it returns at a lower temperature to the ground loop to be heated again.

Cooling Mode

1. 1. 1. Heat exchange and absorption: fluids absorb heat from the air inside the building through a heat exchanger (e.g. air conditioner) [2].

      2. Circulation: the above ground heat pump moves the heated fluid through a series of buried pipes or ground loops.

      3. Heat discharge: as the heated fluid passes through the ground loop, it gives off heat to the relatively colder soil, rock, or ground water around it.

      4. Recirculation: once the fluid transfers its heat to the ground, the fluid returns at a lower temperature to the building where it reabsorbs the heat and repeats the process again.

Dry-Steam Power Plants

This plant uses natural underground sources of steam which is piped directly to a power plant where it is used to fuel turbines and generate electricity. This is also the oldest type of power plant to generate electricity using geothermal energy [1].

Flash-Steam Power Plants

Flash-Steam Power Plants use naturally occurring sources of underground hot water and steam. Water that is hotter than 182°C is pumped into a low-pressure area. Some of the water “flashes” or evaporates rapidly into steam and is funneled out to power a turbine to generate electricity. Any remaining water is placed into a separate tank to extract more energy. This is the most common type of geothermal power plant [1].

Binary Cycle Power Plants

This plant conserves water to generate heat. Water is heating underground to about 107°C – 105°C which is contained in a pipe that cycles above ground. The hot water heats a liquid organic compound that has a lower boiling point than water. The organic liquid then creates steam which flows through a turbine to power a generator, creating electricity. The only emission created in this process is steam since the water in the pipe is recycled back to the ground to be re-heated and complete the cycle again [1].

Improving Geothermal Energy

Cascaded Technologies

Geothermal fluid cascades from the highest available temperature for creating electricity down through direct use applications that require successively less heat, including spas, industrial processes and snow melting (each with distinct temperature ranges) [4].

Harvesting Valuable Materials from the Geothermal Fluid

“Developing technologies to profitably extract strategic minerals from brines through geothermal mining. The purpose is validate improves lithium extraction techniques to transform mined materials into saleable products cost-effectively.” [4].

Improving Efficiency and Reducing the Cost of Power Generation Technologies

“The DOE’s Pacific Northwest National Laboratory is in the process of developing microporous metal-organic solids as the primary heat carrier and heat transfer medium that could increase power generation at binary plants by 15%.” [4].

Additional Reading

Biomass Energy

Hydropower

Solar Energy

Wind Energy