downtown (Fig. E This enhances safety and has minimal costs (the original

Transcription

1 Geothermal energy Hot water from underground direct use In Klamath Falls, a geothermal deicer has being used on a section of local highway by the Oregon Department of Transportation since (79,81) Klamath Falls also deices downtown (Fig. E This enhances safety and has minimal costs (the original expense of drilling). The city is home to about 500 geothermal wells. The total easilytapped geothermal energy resource in Oregon alone is estimated at 2 GW. (81) These include hot water (as Klamath Falls uses) and hot, dry steam (similar to what is found at California s Geysers). Fig. E A geothermal district heating system keeps the sidewalks clear and dry after a snowfall in Klamath Falls, Oregon. The trees are protected with styrofoam insulation to keep them from budding in the winter during system operation. The district heating system melts snow on more than 4500 square meters (50,000 square feet) of sidewalks and crosswalks. (U.S. Department of Energy, National Renewable Energy Laboratory)

2 Energy, Ch. 25, extension 3 Geothermal energy 2 In Klamath Falls, the city financed a system that connected public buildings, and then private buildings, to a circulation system for a district heating system, originally built in 1981 to serve 14 buildings. The system heats more than 30 buildings government offices, churches, and small businesses in the downtown area. Its design capacity is around 6 MW of thermal energy. Controls in each building compare the hot water temperature from the system with the building s set temperature. If the water is colder than desired, a two-way valve is closed and a backup heating system used. In the buildings, hot water is circulated through specially selected commercial heating coils that maximize the temperature difference between input and outflow water. The water supply is monitored to maintain the lowest flow (minimizing pumping energy use) and highest thermal transfer possible. According to the National Renewable Energy Laboratory, tubing carrying an antifreeze solution was installed under city sidewalks in 1995 as part of an upgrade of the system. There is a heat exchanger connecting the antifreeze circulation system for sidewalk clearing with the return (low-temperature) water from the building heating system. This increases the overall thermal efficiency of the system, or, alternatively, we could say it supplies an additional benefit for no additional cost. Note the sidewalk trees in Fig. E25.3.1; to keep them from budding in winter, they are isolated from the warmed sidewalk by styrofoam insulation. While the system works well under normal winter conditions, it cannot stop the freezing of the sidewalk in times of extreme low temperatures. About 5600 m 2 (60,000 ft 2 ) of public sidewalk is cleared this way, and another 1,400 m 2 of sidewalk is served by private geothermal systems. For comparison, the building area warmed by the system is many times larger; the Government Center building alone has an area of over 3,400 m 2 (37,000 ft 2 ) space conditioned. Figure E shows how the city of

3 Energy, Ch. 25, extension 3 Geothermal energy 3 Klamath Falls uses the heated water from the geothermal system to keep bus stops clear of ice. Fig. E The Klamath Falls geothermal district heating system keeps the sidewalks clear and dry at the Basin Transit station after a snowfall. (U.S. Department of Energy, National Renewable Energy Laboratory) Other American cities use geothermal energy as well. Boise is expanding its distribution facilities for low-temperature hot water for space heating from geothermal wells (the first utilization in Idaho was in 1892). (82) Elko, Nevada, uses low-temperature hot water from wells to heat 14 downtown buildings and for irrigation, and San Bernardino, California, uses geothermal hot water for heating in the city s wastewater treatment plant and 30 industrial facilities. (79,80) Exploitation of Hawaii s geothermal resources is just beginning. There is a 3 MW e generator at Puna, near Mauna Loa (83) currently producing electricity.

4 Energy, Ch. 25, extension 3 Geothermal energy 4 More exploitation of the field appears likely, since the field has an estimated 3 GWcenturies of thermal energy available. (83) Hot water (brine) from underground for electricity Some hot water geothermal electricity generating installations are operational. It is necessary to check the gas content of the steam to see if the exhaust from the turbine can be vented directly to the atmosphere (limited to < 5 MW e turbines), or if the system should be run on a condensing cycle (more efficient, but more expensive). (75,84) In most large-scale hot water systems for producing electricity, there are several stages in which the steam runs a turbine; at each successive stage, the pressure is lowered to allow more steam to escape (this depressurization is called a flash ). There are also 10 MW e installations in Brawley and on the Salton Sea near Niland, California; there is a 45 MW e binary cycle (that is, heat exchanging) facility near El Centro, and a 47 MW e double flash facility (in which the steam runs two turbines in sequence) near the same location. (85) All of these resources appear in the form of huge pockets of hot brine. The Salton Sea water is approximately one-quarter salt, which is extremely corrosive and clogs the interior of the installation s pipes (silting, scaling, corrosion are major problems). Nevertheless, since the brine is at such high temperature, 230 C, it has the capability of producing considerable energy, and engineers are confident that they can develop the resource.

5 Energy, Ch. 25, extension 3 Geothermal energy 5 Fig. E The Leathers geothermal power plant is located in the Salton Sea (geothermal resource area) located in southeastern California (just 60 miles east of San Diego). It is part of the Imperial Valley Partnership Project and is under contract to sell power to Southern California Edison Company under a 30- year power purchase agreement. The combined capacity at Imperial Valley is about 268 net MW, or about 90 MW of electricity. (U.S. Department of Energy, National Renewable Energy Laboratory) A hot brine facility in the Imperial Valley of California is shown in Fig. E It sells its energy to Southern California Edison under a 30-year contract. If the pumped-up brine

6 Energy, Ch. 25, extension 3 Geothermal energy 6 is dumped onto the ground, it could poison plants and pollute groundwater, so all the brine is reinjected back into the ground, the normal practice in geothermal exploitation. (84,85) Such high salinity as is found in these areas near the Salton Sea is rarely found in other geothermal fields. Fig. E Hot Springs Lodge and Pool in Glenwood Springs, Colorado, uses a geothermal heat exchanger system with its hot surface spring to provide space heating, domestic hot water, and snow melting. The hot spring heats fresh water within copper tubing coils, which are piped in a series or parallel with other copper coil heat exchangers. Three heat exchanger systems are included: one for space heating, one for domestic hot water, and another for snow melting. This is just one of about 200 American resorts using geothermal energy resources. (U.S. Department of Energy, National Renewable Energy Laboratory)

7 Energy, Ch. 25, extension 3 Geothermal energy 7 Everyone who thinks about Yellowstone Park and Old Faithful geyser knows that California is not the only place where hot groundwater is close to the surface. Geothermal energy has a place in warming open swimming pools (like the one in Glenwood Springs, Colorado shown in Fig. E25.3.4). It is even being used in New York State to decrease energy costs of the prestigious Westchester Country Club in Rye, New York. (86) The Club will save $130,000 and 775,000 kilowatthours annually at a capital cost of $7 million. The project was subsidized by the state of New York; it may not be economic overall without the subsidy. Hot dry steam how geothermal energy is extracted In most cases, there is some indication near the surface of the presence of heated water from underground, such as geysers or hot springs. Once found, these can be tapped.

8 Energy, Ch. 25, extension 3 Geothermal energy 8 Fig. E Diagram of the operation of a geothermal energy installation running on dry steam. (U.S. Department of Energy, National Renewable Energy Laboratory, Ref. 87, p. 4). In a steam system, the steam escapes from a production pipe, a pipe leading to the reservoir of water in hot rock, which is at higher pressure than near the turbine (Fig. E25.3.5). The water turns to steam at the lower pressure around the pipe, comes up through the production pipe, and turns the turbine to run the generator. The spent steam is either released directly to the atmosphere or condensed to water and disposed of (in which case it might be hazardous waste depending on whether there are any contaminants carried up with the steam) or, as is most commonly done nowadays, reinjected back into the original formation.

9 Energy, Ch. 25, extension 3 Geothermal energy 9 In areas where there is hot water, but not steam, the process is very similar. Hot water is drawn up from the zone of hot rock, the pressure is lowered and it flashes into steam. From there on, the steam turns the turbine and runs the generator, just as for hot, dry steam. Just as for hot, dry steam, the condensed steam is reinjected. Before it is reinjected, the spent steam is condensed in a heat exchanger (this is the lowest temperature in the heat engine cycle). Typically, the spent steam travels by pipes containing cold water, and condenses out on the surface, then drips off and collects in a pool. It is then pumped back into the ground. In some cases, there is a heat exchanger used between the hot water and the working fluid in the turbine. In this case, the hot water from below boils pure water (or possibly some other liquid), which produces steam that runs the turbine and generator. The steam is condensed and returned to the cycle. As in all the systems discussed, while the fuel is free, the initial cost is great because of the investment required in wells, pipes, and turbines to produce energy from geothermal reservoirs: about $2 million per megawatt. (81) In many systems, the remaining hot water can be used again in a heat exchanger to heat a working fluid with a much lower boiling point than water; this working fluid then runs another turbine. A Heber, California, plant uses geothermal brine at 182 C to boil a mixture of 90% isobutane and 10% pentane in the heat exchanger. (84) (This working fluid could easily be ignited.) The Geysers, California The Geysers area of California has energy resources underground as hot, dry steam. The rock is so hot that groundwater is vaporized. Clearly, the dry steam geothermal resources

10 Energy, Ch. 25, extension 3 Geothermal energy 10 are hotter, and hot, dry steam geothermal resources are thermodynamically more efficient than the hot water resources. The Geysers Power Plant is shown in Fig. E Fig. E The Calpine plant at the Geysers, California. (U.S. Department of Energy, National Renewable Energy Laboratory) The Geysers Power facility was originally developed by Pacific Gas & Electric Company, but as part of California deregulation, PG&E had to sell its energy facilities. Calpine bought out the PG&E geothermal facility. As steam has been used for electricity generation, the number of working wells has declined from 500 to 360. The pressure drops as steam is released; outlying wells are capped to keep the pressure at the remaining wells from declining further. PG&E had looked for a way to get the resource restored. It found that a local sanitation district had wastewater and worked with the district to pipe the water about 40 km to its site. The water is pumped down into the

11 Energy, Ch. 25, extension 3 Geothermal energy 11 formation replenishing the steam. The $45 million project was completed in 1995, and brings 11 million liters a day to the wells. (87) Calpine is building a new pipeline to bring 42 million liters a day of wastewater from Santa Rosa, a distance of 66 km. (87,88) The water will be injected into the rock, replenishing the supply of water in the rock formation. Calpine plans to spend some $140 million for the pipeline and renewing wells, as well as drilling a number of new wells. (88) All this extra water should prolong the life of the Geysers energy resource for a long time. The National Renewable Energy Laboratory (NREL) tested an advanced direct contact condenser system at the Geysers facility, with such success that NREL won an award for its innovative work. (87) The spent steam is mixed directly with the cooling water in an open chamber on geometric surfaces called packing structures. The direct contact condenser is much more efficient than current condensers, allowing a 17% increase in power at the plant at which it was installed along with a decrease in emission treatment costs. (87) The technology has been licensed and will be applied in other situations in which steam must be condensed. Geothermal energy supplies about 5% of California s energy demand, the highest proportion of any state. The United States has about 2.8 GW of total capacity. (87) As of 1997, about 7 GW e of base-load generating capacity and over 15 GW t of heating capacity from high-grade geothermal resources are in commercial use worldwide, much of it from hot, dry steam. (87,89) The Department of Energy has an ambitious set of goals for the near future. It plans to double the number of states with geothermal electric power facilities to eight by 2006; to

12 Energy, Ch. 25, extension 3 Geothermal energy 12 reduce the cost of geothermal energy to $0.03 to $0.05 per kilowatthour by 2007 (windmills deliver electricity for about $0.05/kWh); and to supply the electrical or thermal energy needs of 7 million homes and businesses in the United States by (87) Geothermal energy elsewhere As mentioned in the chapter itself, there are sites available for tapping geothermal energy in many places in the world. Development has proceeded in New Zealand, Iceland, Mexico, the Philippines, and other places. Over 60 countries obtain some of their energy from geothermal resources. Sometimes these are very small projects, such as the 450 kw geothermal power plant used to power the Aachen University s Student Center. (90) Perhaps small is a misnomer the how water is 2500 m deep. Aachen was a Roman spa renowned for its healing hot aprings, snd the same underground heat responsible for Aachen s hot springs is what makes this project possible. Some are larger. One such larger project is the controversial Calpine plan to build a 48 MW geothermal power plant at Telephone Flat near Medicine Lake in northern California. The project is to cost $120 million to complete. (91) This is a closed-cycle plant, so the water pumped up to run the turbines is reinjected to be reheated and reused. The controversy stems from the opposition of Indian tribes, who regard Medicine Lake as sacred. (92) The Clinton administration denied Calpine the permit, but it was later approved by the Bush administration after Calpine threatened to sue. From Australia comes a plan to use hot dry rock 1000 kilometers north of Adelaide in the small town of Innimincka. A small demonstration plant (13 kw) is to be scaled up to a

13 Energy, Ch. 25, extension 3 Geothermal energy MW plant eventually. (93) The total energy resource underground is about half as large in size as the oil of Kuwait. The supply well is drilled almost 5 kilometers down into rock that is at a temperature around 300 C. (93) Fig. E The Wairakei Power Station draws hot dry steam from a depth of 2.5 km to the surface to run a thermal power plant producing 161 MW e. (Courtesy of Chip Scott) In the Chapter, we discussed the situation in New Zealand, which gets about 10% of its energy from geothermal sources. Figure E (next page) shows this in the perspective of total New Zealand energy production. Figure E (next page) shows New Zealand s geothermal energy production. It is variable from year to year, but has generally increased between 1974 and Geothermal energy supplies are assessed as electricity supplied to the grid rather than the original thermal energy used to make that electricity. Geothermal energy supplies total about one-third that from (imported) oil, which means that they are roughly equal in effect (there is a factor of three loss in converting thermal energy from oil into electricity; see Chapter 7).

14 Energy, Ch. 25, extension 3 Geothermal energy 14 Energy Production (PJ) Year Fig. E New Zealand total energy production (black) and the portion from geothermal energy (red) between 1974 and It varies from 14% to 11% (New Zealand Ministry of Economic Development, Energy Data File January 2005) Geothermal energy produced (PJ) Year Fig. E New Zealand production of geothermal energy between 1974 and (New Zealand Ministry of Economic Development, Energy Data File January 2005)

15 Energy, Ch. 25, extension 3 Geothermal energy 15 Figures E , 11, and 12 show several views of the Contact Energy, Ltd. Wairakei Power Station on the North Island of New Zealand. Figure E shows the facility from a distance. Figure E is a closer look at the turbines. Figure E shows the main generator hall of the plant. Fig. E The Wairakei Power Station complex as seen from about 1 kilometer away. (Courtesy of Chip Scott) The Philippines gets over one-fifth of its energy from its geothermal resources (the country lies over part of the Pacific ring of fire ). The islands area contains several active volcanoes. Kenya gets around 5% of its electricity from geothermal resources. It lies in the Rift Valley, a geologically active region. It is the only area above ground where the crust is spreading apart. The World Bank and the United Nations Environmental Program are

16 Energy, Ch. 25, extension 3 Geothermal energy 16 planning to build many geothermal energy plants in the area to supply resource-poor African countries with a cheap, reliable source of energy. (94) Fig. E A bird s eye view of the steam system and the reinjection area at Wairakei Power Station. (Courtesy of Chip Scott) Study is taking place in Britain to see whether a geothermal system producing 10 GW (more power than Britain currently uses) might be built in Cornwall. If the project goes

17 Energy, Ch. 25, extension 3 Geothermal energy 17 forward, it will cost approximately $115 billion. Construction would stretch over decades. (95) Fig. E The main generator hall at Wairakei Power Station. (Courtesy of Chip Scott)