There is a natural source of power found below the surface of the earth that has been around for centuries. Underground, far below us, there are pools of water heated by magma (or molten rocks). These pools of water make up our geothermal reservoirs. Harnessing the power of the earth’s temperatures to power, heat or cool our homes and businesses is the essence of geothermal power.
There are currently geothermal plants in over 80 countries according to the Geothermal Energy Association and although the United States is currently the global leader of geothermal power, other countries like Indonesia, Turkey and Kenya are all in the process of expanding their power capacities as well.
The first geothermal plant in the United States was built by Pacific Gas and Electric in 1960 at an area called The Geysers. Located in the Mayacamas Mountains north of San Francisco, California is the world’s largest geothermal field. It is now home to 22 geothermal power plants, known as The Geysers Complex, and is considered the largest geothermal plant in the world.
Geothermal power does not require the burning of any fossil fuels. The hot water or steam used is returned to the ground after it is used where it can be used again, which makes it a renewable energy source as well.
Geothermal Power Plants
There are three main types of geothermal energy plants that generate power in slightly different ways.
Dry steam plants are the most common types of geothermal power plants, accounting for about half of the installed geothermal plants. They work by piping hot steam from underground reservoirs directly into turbines from geothermal reservoirs, which power the generators to provide electricity. After powering the turbines, the steam condenses into water and is piped back into the earth via the injection well.
Flash steam plants differ from dry steam because they pump hot water, rather than steam, directly to the surface. These flash steam plants pump hot water at a high pressure from below the earth into a “flash tank” on the surface.
The flash tank is at a much lower temperature, causing the fluid to quickly “flash” into steam. The steam produced powers the turbines. The steam is cooled and condenses into water, where it is pumped back into the ground through the injection well.
In these binary cycle plants, the main difference is that the water or steam from below the earth never comes in direct contact with the turbines. Instead, water from geothermal reservoirs is pumped through a heat exchanger where it heats a second liquid—like isobutene (which boils at a lower temperature than water.)
This second liquid is heated into steam, which powers the turbines that drives a generator. The hot water from the earth is recycled into the earth through the injection well, and the second liquid is recycled through the turbine and back into the heat exchanger where it can be used again.
Geothermal Heat Pumps
Powering your home with a geothermal heat pump allows you to harness the temperatures below the surface of the earth to heat or cool a structure. Even though the temperatures above ground fluctuate during the seasons throughout the years, the temperature below the surface remains consistent between 50˚F – 60˚F year round.
There are four types of pumps, three closed-loop systems and open-loop systems. Each depends on the type of soil, climate conditions and land available.
Closed loop horizontal systems are the most cost-effective for residential areas. For larger commercial buildings, closed loop vertical systems are more often used. These can sometimes go down 400 feet deep. Closed loops constructed under or in a pond or lake are usually the cheapest.
In closed loop systems, a water/antifreeze mixture circulates through a loop of pipes underground (or beneath a body of water) and into a building. In the winter (as shown above), the temperatures underground are warmer than the air, so the fluid pumping in is warmer. Then the electric compressors and heat exchangers transfer the heat through ducts in the building.
In the summer, the pipes draw heat away from the building and it is absorbed into the earth or water. Since the fluid is already cool in the summer and warmer than the air in the winter, the heater/AC system doesn’t have to work nearly as hard.
In open loop systems, the water is taken directly from a water source and into the heat pump where it then can either be recycled back into the same source or pumped into another water source (without polluting). The only difference with the water going in and out is a slight change in temperature. Although these can be cheaper, they also require a steady flow of water capable of powering your home.
These four types of geothermal heat pumps can be used all over the country due to the constant temperature below the surface, but they vary in efficiency and cost savings.
One of the big advancements for the future of geothermal energy is called an Enhanced Geothermal System (EGS). Traditionally, geothermal power must be taken where geothermal reservoirs exist, which is mainly in the Western United States. In fact, geothermal energy already provides around 60% of the power along the Northern California coast, according to the U.S. Department of Energy. So, EGS creates engineered geothermal reservoirs by pumping cold water thousands of feet underground to gain access to hot water and produce steam needed to power plants on the surface.
Since geothermal energy is a renewable natural resource, think of it like a gift from the earth that keeps giving. Although over time it is often necessary to drill additional wells to maintain levels of energy production, the earth is constantly giving off heat that was generated when our planet was formed billions of years ago. Next time you see a geyser like Old Faithful in Yellowstone National Park firing hot steam and water high into the air, imagine that same power being used to generate electricity.