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Just Scratching the Surface: A Beginner’s Guide to Geothermal Energy

By Sustainability Office on July 19, 2017
-Chaveli Miles ’19

If you’ve ever seen a volcano erupt or a geyser spew hot water and steam, you’ve seen geothermal energy at work! Source: History.com

Underneath the Earth’s surface, large energy reservoirs cradled in subsurface geological formations can be used to illuminate and heat our modern world without the pollution caused by burning fossil fuels. The Earth’s natural heat, geothermal energy, can be harnessed in two ways: deep (geothermal power plants and direct-use) and shallow (geothermal heat pumps). While direct-use is still in its experimental stages, geothermal power plants and geothermal heat pumps have been generating clean, renewable energy for decades.

Geothermal power plants utilize both hydro (water) and thermal (heat) sources to power a turbine. As the turbine spins it generates electricity. This technology first occurred in Larderello, Italy in 1904. In the present day, geothermal energy is created in three ways: dry steam, flash, and binary. As defined by National Geographic, dry steam utilizes naturally occurring steam to directly drive a turbine. This is oldest form of geothermal energy production. Flash geothermal energy pulls high-pressure hot water into low-pressure cooler water, generating steam which can then be used to drive a turbine. As of right now, flash plants are the most common.

Lastly, in binary geothermal energy, hot water is mixed with a “secondary fluid” with a much lower boiling point than water. This causes the secondary fluid to turn to vapor, which then drives a turbine. Secondary fluids are usually hydrocarbons such as isopentane or a refrigerant. Future geothermal plants will likely be binary as this process does not release geothermal fluids such as nitrogen, hydrogen sulfide, and ammonia into the environment which can occur with older geothermal methods. In addition, the water and the secondary fluid are kept separated during the whole process, resulting in little or no air emissions.

Unlike geothermal power plants that are used to generate electricity, geothermal heat pumps are used for heating and cooling. They can also be used to provide hot water all year round. Here at Colgate, Chapel House – Sanctuary and Spiritual Retreat Center –  is heated and cooled using a vertical, closed-loop geothermal pump. Below ground, the temperature remains at about 45 to 75 degrees fahrenheit. Geothermal heat pumps rely on this constant temperature to regulate indoor heating and cooling. A system of polyethylene pipes called a loop transfers heat in or out of a building, depending on the season.

In a vertical closed-loop heat pump, the loop is drilled down approximately 100 to 400 feet deep and conjoined into one continuous pipe. A water solution (usually water in a open-loop system and a mix of water and environmentally-safe antifreeze in a closed-loop system) runs through the loop, transferring heat to and from the ground through a heat pump which is connected to an indoor air duct system. The heat pump contains a refrigerant which undergoes variations in temperature and pressure, alternating between a liquid and gaseous state. This effectively heats and cools the air indoors. The water solution and refrigerant never mix, the refrigerant remains in the heat pump while the water solution remains in the loop.  
A geothermal heat pump simply collects energy and redistributes it unlike a standard gas furnace. For every kilowatt of energy consumed by a geothermal heat pump, approximately 4 kilowatts of energy is generated, making not only more environmentally friendly but also significantly more efficient and economically rewarding than fossil fuels.

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