EGEE 101
Energy and the Environment

Geothermal Electricity

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In lesson 1, we discussed geothermal heat pumps as a method of producing home heating, cooling, and hot water by taking advantage of the solar energy captured by the first meter or so of the earth's surface. Here we are considering the energy that is contained within the earth, specifically the hot molten core for electricity generation (not home heating and cooling!!!!). The planet is comprised of a molten core surrounded by a crust. Unfortunately, the amount of energy coming to the surface of the earth from geothermal is small in most locations. Currently geothermal contributes 0.5% to the electricity generation in the U.S.

Watch

The following video 3:47 minute video. It provides an excellent overview of geothermal energy production.

Click here for a transcript of the Geothermal Energy video.

You may have relaxed in a natural hot springs pool or seen the Old Faithful geyser blasting hot water into the air at Yellowstone National Park. But have you ever thought of where all that heat comes from? Well, it comes from deep beneath the surface of the Earth. And it's called geothermal energy. And we can use it to generate clean, renewable electricity.

OK, here's how geothermal works. Heat from the Earth's crust warms water that has seeped into underground reservoirs. In some places, when water becomes hot enough, it can break through the Earth's surface as steam or hot water. This usually happens where the Earth's crust or plates meet and shift. In the past, taking advantage of geothermal energy was limited to areas where hot water flowed near the surface. But as geothermal technologies advance, we can leverage even more of these natural renewable energy sources.

Engineers have developed a few different ways to produce power from geothermal wells drilled into the ground. Have a look at this. It's a dry steam geothermal power plant. And it's the most common type of geothermal technology used today. Underground steam flows directly to a turbine to drive a generator that produces electricity. Pretty straightforward.

Another geothermal technology is called a flash steam power plant. A pump pushes hot fluid into a tank at the surface where it cools. As it cools, the fluid flashes or quickly turns into vapor. The vapor then drives a turbine and powers a generator.

A binary cycle plant works differently. It uses two types of fluid. Hot fluid from underground heats a second fluid called a heat transfer fluid in a giant heat exchanger. The second fluid has a much lower boiling point than the first fluid. And so it flashes into vapor at a lower temperature. When the second fluid flashes, it spins a turbine that drives a generator.

The environmental benefits of this clean, round-the-clock, renewable energy source are substantial. Low emissions, small physical footprint, and minimal environmental impact. The few byproducts that can come up are often reinjected underground.

Geothermal energy can also help recycle wastewater. In California, wastewater from the city of Santa Rosa is injected into the ground to generate more geothermal energy. Some plants do produce solid waste. But that solid waste may contain minerals that we can remove and sell, which lowers the cost of this energy source.

The US Geological Survey estimates that untapped geothermal resources in the United States if developed could supply the equivalent of 10% of today's energy needs and cut our dependence on fossil fuels. In fact, electricity generated by geothermal energy already provides about 60% of the power along the Northern California coast. From The Golden Gate Bridge to the Oregon State line, geothermal energy-- helping to push America toward energy independence and a clean, renewable way to meet our growing energy demands.

Credit: U.S. Department of Energy

Read

Another nice explanation about Geothermal Basics can be found on the DOE's Office of Energy Efficiency & Renewable Energy website.

Averaged over the earth's surface, the heat energy flow is 0.06 Watts per square meter (500 times less than the incoming solar energy flux). This is much smaller than incoming solar energy and so for most locations extracting geothermal energy (other than surface heat pump applications) will be too costly to drill down deep towards the core of the planet. In some locations, however, geothermal energy is far more concentrated and accessible. Locations such as Jellystone (oops, Yogi Bear slip), Yellowstone National Park's "Old Faithful" are recognizable and spectacular examples of geothermal activity.

As can be seen in the map below, certain locations have easy access to geothermal energy. Many of the island chains owe their existence to volcanic activity. Iceland, New Zealand and Hawaii have ample geothermal energy and use this renewable energy for the generation of electricity, home heating, hot water, etc. In the US, geothermal accounts for only about 0.4% of our electricity.

Graphic of a world map outlining the tectonic plates.
The tectonic plates sit on top of the magma and are constantly moving.  At the junctions of the plates, the magma has an easier access to the surface, spectacularly via volcanoes (red dots). The ring of fire goes up the West coast of Mexico, the US and across and down along Asia. This is where there is a higher risk of earthquakes and volcanic eruptions.
Credit:  Living Earth and Mark Wherley

How is geothermal energy turned into electricity?

How the electricity is generated from geothermal follows the same principles as the techniques already covered in this course. In an open-loop system, water and steam are separated. The high temperature, high-pressure steam turns a turbine, that spins a generator, that produces electricity. The steam is cooled and the water injected back into the ground to ensure that the system is renewable. In closed-loop systems, water is injected into the ground in a pipe where heat exchange warms it up and returns it as steam or hot water. In a binary system, ammonia is used in place of water as the working fluid. Ammonia will be a liquid at normal conditions but can easily be converted into ammonia gas (it has a low boiling point). The ammonia is used to turn the turbine (this technique is also used in Ocean Thermal Energy Conversion (OTEC), more on that later). The advantage of this method is that it can be used when the thermal gradient is not as great. Most of the geothermal plants use the open-loop system.

Geysers Geothermal Field

Here is a domestic geothermal power plant located in California (one of the 18 plants operating within the Geysers geothermal field). Multiple wells were drilled to supply the steam to the power plants (some as deep as 3 km to reach the higher temperature water). This is a mature site with over 60-years of operation. It was approaching the end of life as the water resource had been depleted (and water is expensive in California) but has been extended by injecting wastewater. There are geothermal pools in the region but interestingly no actual geysers!

 Image of a geothermal site on a hill.
The Geysers geothermal field in California producing electricity. The cooling towers are an obvious sign of electricity generation via steam.

Advantages of Geothermal Energy

  • Geothermal is a renewable energy source and can operate 24 hours a day
  • Much less air pollution is generated than electricity generated from fossil fuels although they do emit some pollutants
  • Small footprint - it doesn't require a great deal of land
  • Enhances national security and trade deficit reduction
  • It's a cheap electricity source. If interested you can also look at this site: 5 things to know about geothermal power.

Disadvantages of Geothermal Energy

  • There are limited locations where it is a viable option. The western United States is the most likely area for generating geothermal energy on the US mainland.
  • It emits sulfur dioxide (sulfur is the yellow-colored material in some of the geyser basins), and NOx as well as greenhouse gases such as CO2