Today, I’ll go over the basics of geothermal energy and tomorrow I will talk about the pros and cons.
There are four types of alternative energy that I remember learning about in school. I remember hearing about them as far back as fourth grade or so. Wind, solar, hydro, and geothermal. The first three I hear about quite a bit if not all the time and they seem to have evolved by leaps and bounds. But I don’t often hear about geothermal energy, so I was curious about how it has changed and whether it seems like reasonable alternative.
Geothermal energy was used by ancient people for heating and bathing. Even today, hot springs are used worldwide for bathing, and many people believe hot mineral waters have natural healing powers. I don’t know how many people have actually smelled one of those springs, but it doesn’t smell as pleasant as bowl full of roses. I am not totally sure if I am up for smelling that all the time, but I haven’t found any evidence that it is actually a side effect of collecting or burning geothermal energy, but honestly I don’t see how it couldn’t be.
Geothermal energy is the heat from the Earth. Resources of geothermal energy range from the shallow ground to hot water and hot rock found a few miles beneath the Earth’s surface, and down even deeper to the extremely high temperatures of molten rock called magma.
It turns out that the largest contributor to the heat below our feet is the decay of radioactive particles. We stand on nothing less than a planet-sized nuclear reactor that promises to continue generating heat for billions of years in the future. Now, that is a scary thought.
Almost everywhere, the shallow ground or upper 10 feet of the Earth’s surface maintains a nearly constant temperature between 50° and 60°F (10° and 16°C). Geothermal heat pumps can tap into this resource to heat and cool buildings. A geothermal heat pump system consists of a heat pump, an air delivery system (ductwork), and a heat exchanger-a system of pipes buried in the shallow ground near the building. In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger. The heat removed from the indoor air during the summer can also be used to provide a free source of hot water.
Some geothermal power plants use the steam from a reservoir to power a turbine/generator, while others use the hot water to boil a working fluid that vaporizes and then turns a turbine. Hot water near the surface of Earth can be used directly for heat. Direct-use applications include heating buildings, growing plants in greenhouses, drying crops, heating water at fish farms, and several industrial processes such as pasteurizing milk.
The most common current way of capturing the energy from geothermal sources is to tap into naturally occurring “hydrothermal convection” systems where cooler water seeps into Earth’s crust, is heated up, and then rises to the surface. When heated water is forced to the surface, it is a relatively simple matter to capture that steam and use it to drive electric generators. Geothermal power plants drill their own holes into the rock to more effectively capture the steam.
There are three designs for geothermal power plants, all of which pull hot water and steam from the ground, use it, and then return it as warm water to prolong the life of the heat source. In the simplest design, the steam goes directly through the turbine, then into a condenser where the steam is condensed into water.
In a second approach, very hot water is depressurized or “flashed” into steam which can then be used to drive the turbine.
In the third approach, called a binary system, the hot water is passed through a heat exchanger, where it heats a second liquid—such as isobutane—in a closed loop. The isobutane boils at a lower temperature than water, so it is more easily converted into steam to run the turbine.
Enhanced Geothermal Systems. Geothermal heat occurs everywhere under the surface of the earth, but the conditions that make water circulate to the surface are found only in less than 10 percent of Earth’s land area. An approach to capturing the heat in dry areas is known as enhanced geothermal systems (EGS) or “hot dry rock”. The hot rock reservoirs, typically at greater depths below the earth’s surface than conventional sources, are first broken up by pumping high-pressure water through them. The plants then pump more water through the broken hot rocks, where it heats up, returns to the surface as steam, and powers turbines to generate electricity. Finally, the water is returned to the reservoir through injection wells to complete the circulation loop. Plants that use a closed-loop binary cycle release no fluids or heat-trapping emissions other than water vapor, which may be used for cooling.
It is one of the few renewable energy technologies that can supply continuous, baseload power. The costs for electricity from geothermal facilities are declining to as little as 50 percent in the last thirty years. A considerable portion of potential geothermal resources will be able produce electricity for as little as 8 cents per kilowatt-hour (including a production tax credit), a cost level competitive with new conventional fossil fuel-fired power plants. It is also becoming available directly for homes and businesses everywhere.