Renewable Energy Systems 10 Buchla, Kissell, Floyd
Chapter Outline Geothermal Power Generation 10 Buchla, Kissell, Floyd 10-1 TYPES OF GEOTHERMAL RESOURCES 10-2 GEOTHERMAL ELECTRICAL POWER 10-3 LOW-TEMPERATURE GEOTHERMAL HEAT 10-4 GROUND-SOURCE HEAT PUMPS 10-5 ENVIRONMENTAL IMPACT
10-1 Types of Geothermal Sources Three attributes of a high quality geothermal source are heat, water, and permeability of the ground. Locations with these attributes are called highenthalpy resources. Enthalpy is defined as the amount of energy in a system capable of doing mechanical work; it is a function of temperature, pressure, and volume.
10-1 Types of Geothermal Sources The geothermal gradient refers to the increasing temperature at increasing depths within the earth. The thermal gradient varies widely according to location; it is about 25 o C/km in most locations but can be as much as 200 o C/km.
10-1 Types of Geothermal Sources Three temperature classifications of geothermal sources are: 1. High temperature sources are >150 o C. These sources are useful for generating electricity. 2. Moderate temperature sources are between 90 o C and 150 o C. These sources are useful for space heating, drying, and industrial heat and in some cases are used for electricity production. 3. Low temperature sources are <90 o C. Low temperature sources are useful for heating buildings, fish farms, and bathing.
Source: NREL 10-1 Types of Geothermal Sources To exploit high temperature hot dry rock, water is generally injected from nearby wells. The water boils and the steam is used to drive a steam turbine. One plant at the 750 MW Geysers in Northern CA, is shown here. Recycled water from nearby Santa Rosa is piped in to be injected to create steam for the turbine.
Coprid/Fotolia 10-1 Types of Geothermal Sources Geopressurized reservoirs consist of high-pressure, hot brine in deep underground reservoirs. They are characterized by dissolved methane. The best known areas for geopressurized reservoirs are along the Texas and Louisiana Gulf Coast. Energy can be obtained from the thermal energy, from the hydraulic high pressure, or from burning the dissolved methane.
Source: Courtesy of Chena Hot Springs 10-1 Types of Geothermal Sources Hot water reservoirs are natural steam reservoirs that are typically found along fault lines in the earth. The temperatures are high enough to create steam, which can be used to drive turbines. For moderate temperature reservoirs, Organic Rankine Cycle (ORC) generators are used. ORC generators are shown at the Chena Hot Springs Resort in AK.
Source: NREL 10-1 Types of Geothermal Sources Normal ground temperature sources are useful for underground geothermal heat pumps. In most locations in the world, the ground is a constant 10 o C at relatively shallow depths. This is sufficient to keep heat pumps working efficiently even when the outdoor temperature is freezing. The photo shows drilling of a geothermal well for a heat pump in Klamath Falls, OR.
10-2 Geothermal Electrical Power Three methods for generating geothermal electricity: 1. Dry steam
10-2 Geothermal Electrical Power Three methods for generating geothermal electricity: 2a. Flash steam (single flash)
10-2 Geothermal Electrical Power Three methods for generating geothermal electricity: 2b. Flash steam (double flash)
10-2 Geothermal Electrical Power Three methods for generating geothermal electricity: 3. Binary-cycle
Source: David Buchla 10-2 Geothermal Electrical Power An Enhanced Geothermal System (EGS) is one in which a geothermal site is made productive by artificial means including fracturing networks to increase permeability and improved drilling methods including horizontal drilling and laser drilling. EGS has successfully produced an additional 1.7 megawatts from an existing well field using EGS fracturing methods at Brady Hot Springs, NV (shown here). Other successes are at Newberry, OR and Desert Rock, NV.
10-2 Geothermal Electrical Power Another innovative proposal is to harness the heat in underground coal seam fires. These fires represent an ecological disaster as they can burn for tens of decades in a very wide area and emit tons of pollutants as well as consume a valuable resource. Most are caused by man and they have been nearly impossible to extinguish. One study has shown that there is significant energy that can be recovered by using standard methods developed in geothermal plants. The heat would be used to drive ORC turbines.
10-3 Low-Temperature Applications for Geothermal Heat Low-temperature sources can be used for space heating and cooling. Cooling is accomplished with an absorption type refrigeration system in four steps: 1 Generator: The generator heats the refrigerant (lithium bromide solution); this causes the water in solution to boil, creating water vapor at high pressure. 2 Condenser: The high pressure vapor is condensed to a liquid at higher pressure. Heat from compressing is removed. 3 Expansion valve: High pressure liquid water passes through an expansion valve and becomes a cold liquid vapor mix. The pressure (and the boiling point) are reduced. 4 Evaporator and Absorber: The cold water absorbs heat from the space to be cooled and is absorbed by the LiBr to form a weaker solution. The solution is returned to the generator.
10-3 Low-Temperature Applications for Geothermal Heat Source: NREL Snow melting is an application for low-temperature heat in areas with ample geothermal sources like Iceland. The idea is being tested also in other locations on bridges and overpasses. Tubing for snow melting is installed before concrete is poured. This installation is in Klamath Falls, OR. The city has more than 50,000 sidewalks and crosswalks with district heating.
10-3 Low-Temperature Applications for Geothermal Heat Source: NREL Greenhouses and fish farms are another application for low-temperature geothermal heat. Farming operations include aquatic plants and animals as well as certain food drying applications (including onion, garlic, carrots, and celery dehydrators). Alligators are raised in Colorado at an elevation of over 7500 ft (2300 m). These are part of the Colorado Gators Reptile Park, which also has other reptiles and birds on display. A geothermal well keeps the water warm year round.
10-4 Geothermal Heat Pumps A conventional heat pump operates in a standard refrigeration cycle. Here it is operating in the heat cycle:
10-4 Geothermal Heat Pumps A geothermal heat pump uses the same principle of operation but absorbs heat from the ground into a glycol solution.
10-4 Geothermal Heat Pumps The coefficient of performance (COP) is a measure of efficiency for heat pumps and is the ratio of heat energy delivered to energy supplied to the heat pump. Geothermal heat pumps are very efficient. A heat pump rated for 5 kw is on for 30 min and delivers 40,000 BTU of heat in that time. What is the COP? The energy supplied in 30 min is (5 kw)(0.5 h) = 2.5 kwh The energy delivered = 40,000 BTU/(3413 BTU/kWh) = 11.7 kwh The COP = 11.7 kwh/2.5 kwh = 4.69
Source: NREL 10-5 Environmental Impact Water Geothermal plants use steam and need to recycle water to maintain the source, so monitoring of the quality of injected water and flow is important. Plants like the Geysers use treated wastewater for injection. Monitoring of ground water can be done by injecting chemical tracers. Tracer tests are used to evaluate the injected water flow through the reservoir and the amount of injection-derived steam that is produced.
Source: NREL 10-5 Environmental Impact Air Air pollution from geothermal plants is minor and is less that 1% of an equivalent fossil-fueled plant. Small amounts of hydrogen sulfide (H 2 S) may be in the steam; most can be removed at the plant. Other gases include small amounts of CO 2 and NO x.
Selected Key Terms Binary-cycle plant Coefficient of performance (COP) Double-flash steam plant Dry-steam plant A geothermal power plant that uses the brine water or steam from the geothermal reservoir to heat and vaporize a secondary fluid with a lower boiling point to drive the turbine and generator. A measure of efficiency for a heat pump; it is the ratio of the heat produced to the energy consumed and it varies with the outside temperature. A geothermal plant with two pressure reducing stages to create high pressure and low pressure steam. The high and low pressure steam is routed to two different turbines, which turn a generator. A geothermal electrical plant that uses superheated dry steam from a geothermal reservoir and routes it directly to a steam turbine and generator to produce electricity.
Selected Key Terms Enhanced Geothermal System Enthalpy Solar module Flash steam plant A system in which a geothermal site that is deficient in water or permeability is made to be productive by artificial means. The amount of energy in a system capable of doing mechanical work; it is a function of temperature, pressure, and volume. A geothermal plant that creates steam from high pressure hot water (brine) using a special control valve or orifice plate to reduce the pressure and cause some of the liquid to boil (or flash) into steam, which is used to drive a steam turbine and generator to produce electricity.
true/false quiz 1. A high enthalpy site for geothermal power is one that has high heat, water, and permeability.
true/false quiz 2. Low-temperature geothermal sources are useful for generating electricity.
true/false quiz 3. Hot water reservoirs are not found in cold climates.
true/false quiz 4. Most locations in the world can use geothermal energy for heat pumps.
true/false quiz 5. An advantage of a flash power plant is that it isolates underground water from the turbines.
true/false quiz 6. The Organic Rankine cycle is used with double-flash power plants.
true/false quiz 7. Enhanced Geothermal Systems do not use artificial means to improve output.
true/false quiz 8. Low-temperature geothermal sources can be used to cool buildings.
true/false quiz 9. Geothermal heat pumps tend to be more efficient than standard heat pumps.
true/false quiz 10. Tracers can track the flow of water through a reservoir and the amount of steam that is produced.
true/false quiz Answers: 1.T 2.F 3.F 4.T 5.F 6.F 7.F 8.T 9.T 10. T