Geothermal energy is generally classified as falling within three temperature classifications: low (< 90oC or < 194oF); moderate (90o - 150oC or 194o - 302oF); or high (> 150oC or > 302oC) (Oregon Institute of Technology). Geothermal energy can also be developed with three broad applications in mind. These included ground source or geothermal heat pumps, direct use applications, and electrical power generation. Each of these applications requires a different temperature range for the subsurface rock strata and fluid for optimum use of this geothermal energy resource. Let's take a brief look at each of these applications.
GEOTHERMAL HEAT PUMPS (GHP)
Anyone who has every gone underground into a cavern knows that the temperature is constant and very pleasant year round. In the winter time the cavern air temperature is warmer than the outside air and in the summer the cavern temperature is cooler. The geothermal heat pump uses this same idea of exchanging heat back and forth within the upper 200 to 300 feet of the ground surface for heating and cooling within a commercial building or a residential structure. During the summer a GHP extracts heat from a building and transfers it to a circulating liquid in an underground loop system, where the heat is transferred into the cooler earth. During the winter the liquid that circulates through the underground piping system absorbs heat from the earth and is used to warm the air of the building. The presence of ground water only enhances the overall efficiency of the process.
The importance of GHPs is that they can cut your heating and cooling costs by 30% to 60% (or more) while typically delivering three to four times more energy than they consume. They can provide hot water if needed and they require 50% to 80% less space for equipment rooms. The smaller units can be either floor or ceiling mounted. They are low to nearly maintenance free. The ground heat exchanger will last 40+ years and the GHP typically lasts over 20 years.
There are various ways to store and capture this heat for GHP use in buildings. Four of these methods are briefly discussed here. Vertical and horizontal energy loop systems utilize the natural thermal properties of the earth for heat exchange by circulating water or antifreeze through a closed loop into the ground and then back into the GHP within the building. In the vertical configuration, vertical wells are drilled usually to between 100 to 300 foot depths, depending upon the subsurface temperatures and other related factors. This approach is good when land area is limited. The horizontal configuration lays plastic pipe within horizontal trenches at depths of 6 to 8 feet and in lengths of 75 to 400 feet per ton. A little more land is necessary when using this technique, but this approach can be attractive in cost as well as operating efficiency.
Two other methods can also be applied when water is available either as ground water or as surface water. When an aquifer is present the ground water energy system will draw water through a supply well from the aquifer, run it through GHP heat exchanger where heat is either absorbed or rejected, then returned to the ground. This is a high efficiency system that maintains a constant temperature year round regardless of the outside air temperature. These systems are ideal for a home that has an existing water well. An even more economical approach is a closed loop system that uses a body of surface water such as a pond or lake. The plastic piping is submerged into a body of water where it can use the consistent temperature and heat transfer characteristics of the water. Wells are not required and only minimal trenching is necessary, thus cutting installation costs.
DIRECT USE APPLICATIONS
Geothermal reservoirs within the low to moderate temperature range can provide heat for residential, industrial, and commercial use. The Energy Efficiency and Renewable Energy (EERE) division of the U.S. Department of Energy reports that savings can be as much as 80% over the use of fossil fuels. This form of energy is also very clean, with far fewer air pollutants emitted when compared to fossil fuels.
The employment of direct use geothermal energy requires that a certain infrastructure be established for proper handling of this resource. First, a production facility, usually a well, will bring the hot water to the ground surface. In many areas of Texas, existing wells drilled for oil and gas can be converted to hot water extraction sites. Additionally, hot water presently being produced by oil and gas operators may actually have a market for selling that hot water to an end user for its heat content, thus turning a perceived liability into an economic asset. Second, a mechanical system to deliver the heat to a space or process must be developed. This means the piping, heat exchanger, and controls infrastructure for heat extraction. Siting a direct use facility in proximity with existing boreholes from the oil and gas industry can decrease the economic cost for moving the water over distance, as well as decrease the loss of the heat resource due to heat conduction with the surrounding environment. Third, there must be a disposal system, such as an injection well or storage pond, that can receive the cooled geothermal fluid.
Direct use applications for geothermally heated waters is quite extensive. A number of operations use low-temperature geothermal resources for district and space heating, greenhouses, and aquaculture facilities. The EERE reports that over 120 operations are using geothermal energy for district and space heating. District systems distribute naturally heated water from one or more geothermal wells through a series of pipes to several houses and buildings, or blocks of buildings. Space heating uses one well per structure. In both of these systems, the geothermal heat is replacing fossil fuel burning as the heat source for the traditional heating system. District heating systems can save consumers 30% to 50% of the cost of natural gas heating.
Greenhouses and aquaculture (fish farming) are the two primary uses of geothermal heat in the agribusiness industry. There are at least 38 greenhouses, many covering several acres, that are raising vegetables, flowers, houseplants, and tree seedlings in 8 western states. New Mexico is presently one of the leading states in the geothermally heated nursery business. There are also 28 aquaculture operations active in 10 different states, including New Mexico. Most of the greenhouse operators estimate that their energy savings are about 80% of total fuel costs, or about 5% to 8% of total operating costs. Usually the rural location of most geothermal resources also offers advantages that include clean air, few disease problems, clean water, a stable workforce, and, often, low taxes.
Numerous other industrial and commercial uses are also possible. Industrial applications can include food dehydration, cement and aggregate drying, concrete block curing, milk pasteurizing, spas, and others. A more complete list is shown above in the image from the Geothermal Education Office. The EERE reports that dehydration, or vegetable and fruit product drying, is the most common industrial use of geothermal energy. The earliest commercial use of geothermal energy was for swimming pools and spas. As of 1990, 218 resorts reported using geothermally heated hot water.
ELECTRIC POWER GENERATION
The third broad application of geothermal energy is for electrical power generation. This application uses water within the moderate to high temperature range. Electric generation from geothermal heat has been conducted using one of three primary approaches: a dry steam power plant; a flash steam power plant; and a binary cycle power plant. Let's briefly look at each of these existing technologies presently in use.
A dry steam power plant uses naturally heated water that is at a sufficiently high temperature to create steam in the subsurface. The steam rises from within the ground through the tubing of a well and is directed at a turbine, which drives a generator that produces electricity. The steam can then be exhausted into the atmosphere, or the or condensed to liquid water and reinjected into the subsurface rock strata from which it came. This is the oldest type of geothermal power plant, and was first used at Lardarello in Italy in 1904. Steam technology is used in many places worldwide, such as at The Geysers in northern California. These plants emit excess steam and only minor amounts of other gases.
A flash steam plant is different. Water that has been naturally heated to temperatures above 182oC (360oF) is brought to the surface in a controlled manner as a liquid. The fluid is then sprayed into a tank that is held at a much lower pressure. The sudden drop in pressure causes the water to rapidly vaporize, or "flash". The resulting vapor drives a turbine that drives a generator to produce electricity. Any remaining liquid in the tank can be flashed again in a second tank to extract even more energy. This process is called a double flash system. The vapor can later be cooled and reinjected back into the subsurface to recharge the formation and to extract additional heat.
A binary cycle power plant is best used with hot water that is below about 204oC (400oC). Hot water is brought to the surface through a well with the water then directed into a heat exchanger. The heat is transferred to a secondary (hence, "binary") fluid that has a lower boiling point than water and is called the "working fluid". The secondary liquid may be ammonia, isopentane, or some similar low vapor point liquid. Once vaporized, the resulting gas is directed at turbines to turn a generator for electrical production. Because this is a closed-loop system, virtually nothing is lost to the atmosphere. The secondary fluid is the reliquified for reuse, and the more moderate-temperature water is reinjected into the subsurface for moving additional heat. As these temperatures are more common over a larger part of the globe, binary-cycle plants will probably become more common in the future. A variation on this approach is the hybrid-cycle power plant, which combines an onsite engine or turbine to burn natural gas found within the hot water along with the standard binary-cycle plant. More will be said about this approach under the heading 'Geothermal Energy In Gulf Coast Texas'.