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