Geothermal Electricity Production

Most power plants need steam to generate electricity. The steam rotates a turbine that activates a generator, which produces electricity. Many power plants still use fossil fuels to boil water for steam. Geothermal power plants, however, use steam produced from reservoirs of hot water found a couple of miles or more below the Earth's surface. There are three types of geothermal power plants:dry steam, flash steam, and binary cycle.

Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant, where it is directed into a turbine/generator unit. There are only two known underground resources of steam in the United States: The Geysers in northern California and Yellowstone National Park in Wyoming, where there's a well-known geyser called Old Faithful. Since Yellowstone is protected from development, the only dry steam plants in the country are at The Geysers.

Flash steam power plants are the most common. They use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource.

Binary cycle power plants operate on water at lower temperatures of about 225°-360°F (107°-182°C). These plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions.

Small-scale geothermal power plants (under 5 megawatts) have the potential for widespread application in rural areas, possibly even as distributed energy resources. Distributed energy resources refer to a variety of small, modular power-generating technologies that can be combined to improve the operation of the electricity delivery system.

Low Temerature and Direct Geothermal Energy Use

According to the National Renewable Energy Laboratory, "The Earth houses a vast energy supply in the form of geothermal resources. Domestic resources are equivalent to a 30,000-year energy supply at our current rate for the United States! In fact, geothermal energy is used in all 50 U.S. states today. But geothermal energy has not reached its full potential as a clean, secure energy alternative because of issues with resources, technology, historically low natural gas prices, and public policies. These issues affect the economic competitiveness of geothermal energy."

Besides use of geothermal energy in power plants, hot water from geothermal resources can be used for a number of purposes (also see Figure 1) including:

• Heating buildings or districts (either individually or whole towns)
• Raising plants in greenhouses
• Drying crops
• Heating water at fish farms
• Industrial processes, such as pasteurizing milk

Geothermal direct use dates back thousands of years, when people began using hot springs for bathing, cooking food, and loosening feathers and skin from game. Today, hot springs are still used as spas, but there are now more sophisticated ways of using this geothermal resource.

In modern direct-use systems, a well is drilled into a geothermal reservoir to provide a steady stream of hot water. The water is brought up through the well, and a mechanical system -- piping, a heat exchanger, and controls -- delivers the heat directly for its intended use. A disposal system then either injects the cooled water underground or disposes of it on the surface. In the United States, most geothermal reservoirs are located in the western states, Alaska, and Hawaii.

The direct-use geothermal reservoirs have relatively low to moderate temperatures - 68° to 302°F (20° to 150°C)

Direct use of geothermal energy in homes and commercial operations is much less expensive than using traditional fuels. Savings can be as much as 80% over fossil fuels. Direct use is also very clean, producing only a small percentage (and in many cases none) of the air pollutants emitted by burning fossil fuels.

District and Space Heating

The primary uses of low-temperature geothermal resources are in district and space heating, greenhouses, and aquaculture facilities. A 1996 survey found that these applications were using nearly 5.8 billion megajoules of geothermal energy each year - the energy equivalent of nearly 1.6 million barrels of oil!

In the U.S., more than 120 operations, with hundreds of individual systems at some sites, are using geothermal energy for district and space heating. District systems distribute hydrothermal water from one or more geothermal wells through a series of pipes to several individual houses and buildings, or blocks of buildings. Space heating uses one well per structure. In both types, the geothermal production well and distribution piping replace the fossil-fuel-burning heat source of the traditional heating system.

Geothermal district heating systems can save consumers 30% to 50% of the cost of natural gas heating. The tremendous potential for district heating in the western U.S. was illustrated in a 1980s inventory which identified 1,277 geothermal sites within 5 miles of 373 cities in 8 states.

Greenhouse and Aquaculture Facilities

Greenhouses and aquaculture (fish farming) are the two primary uses of geothermal energy in the agribusiness industry. Thirty-eight greenhouses, many covering several acres, are raising vegetables, flowers, houseplants, and tree seedlings in 8 western states. Twenty-eight aquaculture operations are active in 10 states.

Most greenhouse operators estimate that using geothermal resources instead of traditional energy sources saves about 80% of fuel costs - about 5% to 8% of total operating costs. The relatively rural location of most geothermal resources also offers advantages, including clean air, few disease problems, clean water, a stable workforce, and, often, low taxes.

Industrial and Commercial Uses

Industrial applications include food dehydration, laundries, gold mining, milk pasteurizing, spas, and others. Dehydration, or the drying of vegetable and fruit products, is the most common industrial use of geothermal energy. The earliest commercial use of geothermal energy was for swimming pools and spas. In 1990, 218 resorts were using geothermal hot water.

Permits for Direct Use

Direct use projects are not regulated by the California Energy Commission, and usually fall under the jurisdiction of local government unless on federal lands or tribal lands. Depending on the type of project and the specifics of the resource such as temperature, flow and chemistry there may be variations in the permitting requirements. The local government most likely would require a Conditional Use Permit. Additional permits would depend on the type of use. If there are structures, local building permits may be needed. Balneology may require a public pool permit from a local health department, district heating systems may need permits for disposal (National Pollution Discharge Elimination Permit - NPDES) of the resource from the Regional Water Quality Control Board if there is not an injection well, etc. These permits can take just as long as two years in the case of the NPDES permit and are in addition to the CEQA or National Environmental Policy Act environmental review process needed.

Sources (all accessed 9/22/08):

• NREL: Learning - Geothermal Direct Use, http://www.nrel.gov/learning/re_geo_direct_use.html
• Geothermal Direct-Use Case Studies, Geothermal Direct-Use Case Studies, http://geoheat.oit.edu/casestudies.htm
• Direct Use of Geothermal Energy, http://www1.eere.energy.gov/geothermal/pdfs/directuse.pdf
• Geothermal Technologies Program: Direct Use of Geothermal Energy, http://www1.eere.energy.gov/geothermal/directuse.html
• Geothermal Direct Use - Geothermal Energy, http://www.renewableenergyworld.com/rea/tech/geodirectuse

Types of Geothermal Power Plants

All geothermal power plants use steam to turn large turbines, which run electrical generators. In the Geysers Geothermal area, dry steam from below ground is used directly in the steam turbines. In other areas of the state, super-hot water is "flashed" into steam within the power plant, and that steam turns the turbine.

 

Direct Dry Steam

Steam plants use hydrothermal fluids that are primarily steam. The steam goes directly to a turbine, which drives a generator that produces electricity. The steam eliminates the need to burn fossil fuels to run the turbine. (Also eliminating the need to transport and store fuels!)

This is the oldest type of geothermal power plant. It was first used at Lardarello in Italy in 1904. Steam technology is used today at The Geysers in northern California, the world's largest single source of geothermal electricity. These plants emit only excess steam and very minor amounts of gases.

Flash and Double Flash Cycle

Hydrothermal fluids above 360°F (182°C) can be used in flash plants to make electricity.

Fluid is sprayed into a tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly vaporize, or "flash." The vapor then drives a turbine, which drives a generator.

If any liquid remains in the tank, it can be flashed again in a second tank (double flash) to extract even more energy.

Binary Cycle

Most geothermal areas contain moderate-temperature water (below 400°F). Energy is extracted from these fluids in binary-cycle power plants.

Hot geothermal fluid and a secondary (hence, "binary") fluid with a much lower boiling point than water pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines.

Because this is a closed-loop system, virtually nothing is emitted to the atmosphere. Moderate-temperature water is by far the more common geothermal resource, and most geothermal power plants in the future will be binary-cycle plants.

Text and graphics from U.S. Department of Energy's Energy Efficiency and Renewable Energy Program.

Geothermal Heat Pump Basics

Geothermal heat pumps take advantage of the nearly constant temperature of the Earth to heat and cool buildings. The shallow ground, or the upper 10 feet of the Earth, maintains a temperature between 50° and 60°F (10°–16°C). This temperature is warmer than the air above it in the winter and cooler in the summer.

Geothermal heat pump systems consist of three parts: the ground heat exchanger, the heat pump unit, and the air delivery system (ductwork). The heat exchanger is a system of pipes called a loop, which is buried in the shallow ground near the building. A fluid (usually water or a mixture of water and antifreeze) circulates through the pipes to absorb or relinquish heat within the ground.

Heat pumps work much like refrigerators, which make a cool place (the inside of the refrigerator) cooler by transferring heat to a relatively warm place (the surrounding room), making it warmer. In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system, moving heat from the ground to the building's interior. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger, effectively moving the heat from indoors to the ground. The heat removed from the indoor air during the summer can also be used to heat water, providing a free source of hot water.

Geothermal heat pumps use much less energy than conventional heating systems, since they draw heat from the ground. They are also more efficient when cooling your home. Not only does this save energy and money, it reduces air pollution.

Geothermal Heat Pumps

The shallow ground, the upper 10 feet of the Earth, maintains a nearly constant temperature between 50° and 60°F (10°-16°C). Like a cave, this ground temperature is warmer than the air above it in the winter and cooler than the air in the summer. Geothermal heat pumps take advantage of this resource to heat and cool buildings.

Geothermal heat pump systems consist of basically three parts: the ground heat exchanger, the heat pump unit, and the air delivery system (ductwork). The heat exchanger is basically a system of pipes called a loop, which is buried in the shallow ground near the building. A fluid (usually water or a mixture of water and antifreeze) circulates through the pipes to absorb or relinquish heat within the ground.

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 heat water, providing a free source of hot water.

Geothermal heat pumps use much less energy than conventional heating systems, since they draw heat from the ground. They are also more efficient when cooling your home. Not only does this save energy and money, it reduces air pollution.

All areas of the United States have nearly constant shallow-ground temperatures, which are suitable for geothermal heat pumps.

Extreme Spring-Time Energy Savings with Geothermal Pool Heating

Spring is here, and the cooling season is quickly approaching. Pools that have been out of commission after our very cold winter are likely to stay that way unless we turn the heat on.  That can get expensive.

Stop for a moment and think of how many blow dryers, computers, cooking appliances, lights and people there are in your home that add to the cooling load.  These are all a potential source of energy that can provide domestic hot water and be used to heat spas and pools. With standard air source heat pumps or air conditioners, the heat generated inside your home typically goes out the return air ductwork and ultimately is exhausted through outside air exchange.

Like geothermal sourced cooling and heating systems, geothermal pool heat pumps bring your swimming pool and potable water heating in harmony with nature, while providing high energy efficiency. They do that by working together with the stable earth temperatures to provide heating for your pool throughout your desired swimming season.

With just a little twist to a home’s geothermal heating and cooling system, the waste heat from appliances and devices can be channeled into usable heat for domestic hot water needs, swimming pools, and spas.

There are a few different ways that a pool is normally heated; Fossil fuel, electric resistance, solar, or heat pump (either “air sourced” or “geothermal sourced”).

Solar-thermal is the most energy efficient and renewable source for potable water and pool heating, but solar is dependent upon the cooperation of the weather. Cloudy and cool days can mean a cold pool necessitating the need for backup heating sources during those times.

Fossil fuel heating of potable water, pools and spas is an old favorite. First cost is relatively low, but that comes at a higher price environmentally and monetarily as you move forward.  In addition to high costs for propane and other fuels, there are safety issues when fossil fuels are used as in this unfortunate story of carbon monoxide poisoning to guests at a hotel from a pool heater, March 21, in the LA Times.

Electric resistance heating uses raw electricity to warm heating elements over which the water passes, providing a clean and safe water heating alternative, but it can be extremely expensive. Using the Coefficient of Performance (COP) rating system (used internationally) for heating equipment, electric heating has a COP of 1.0, meaning that 1 unit of heat is provided for each unit of electricity, a one-to-one ratio, or 100% efficient in the COP rating system.

Air source heat pumps designed for pool and potable water heating are environmentally friendly and use outside air, pumping heat out of the ambient air into your pool or hot water tank. However, they too rely somewhat on cooperative weather conditions, that is; air temperatures being warm enough to facilitate extraction of heat to transfer to potable water and/or pool heating needs.  Air source heat pump efficiencies are in the 3.0 COP ranges (300% efficient).

For swimming pool and spa heating, the best scenario is attained with a geothermal sourced water to water heat pumps, pulling heat from a dependable, steady and renewable energy source; the earth.  Geothermalheat pumps can be about 5.0 COP (500% efficient).

Outside, temperatures fluctuate with the changing seasons, but underground temperatures don't change nearly as dramatically, thanks to the mass of the earth. Just 4 to 6 feet below the ground, the temperature remains relatively constant year round (about 45F to 75F in the US). A geothermal sourced water to water heat pump, which  can work in tandem with your geothermal HVAC system, typically consists of water sourced heat pump and a buried system of pipes called an earth loop, and/or a pump to reinjection (Class V thermal exchange process illustrated here) well. This geothermal source can be shared between the building’s HVAC and water heating systems.

Think of it like this: While providing power to run your home’s cooling system, you are also paying for the energy to run computers, lights, toasters, and home entertainment items like your big flat-screen TV. Your home’s cooling system must use power to remove the heat created by all of these internal gains on top of the occupant loads (one occupant presents a load of 1200 BTU’s each hour, or 10% of 1 ton of air). You pay for energy twice to remove this waste heat through the process of cooling your building. Why not channel that heat somewhere else where it’s needed? There is a way, and it’s easy with a geothermal sourced building:

Among the benefits that you can get from a geothermal HVAC system is the ability to channel and use this waste heat energy.  That’s because unlike the widely used air sourced cooling equipment (those that have an outside condenser that discharge waste heat), geothermal systems discharge the heat through a discharge water line.  Most manufacturers of geothermal heat pumps even have a factory installed hot water generator available.  This option gives you two extra connections labeled DHW (Domestic Hot Water) “In” and “Out” that may be connected to most any hot water tank, and the geothermal heat pump can turn waste heat into usable hot water.

There are thousands of geothermal heated pools around in the US.  There is a good chance that the local YMCA, hotel, health club or community pool near you already has geothermal sourced pool heating, because they maintain heated pools year round.  They know geothermal is dependable and ultra-efficient.   Surprisingly, many of these still have air sourced cooling systems that could be converted to geothermal (and likely will be) during the normal course of HVAC equipment attrition and upgrade.

Stop paying two and three times to move energy, and share the loads in your home; enjoy true thermal load sharing with a geothermal HVAC system.