Geothermal Energy : Potential, Cost and Environment Issues

Transcription

1 Geothermal Energy : Potential, Cost and Environment Issues Vijaya Singh Former General Manager, Central Railway and Former Managing Director, Tata Projects Ltd. Prologue Enormous amount of heat energy at high temperature is locked up inside the earth. It remains confined by the earth s crust, which is a good heat insulator, but escapes at the edges of tectonic plates in the form of volcanic eruptions, hot springs and geysers. The energy coming out in the form of hot springs and geysers has been used for heating and bathing since ancient times and is now being used for generation of electricity also. Invention of geothermal heat pumps has greatly increased the scope of using this energy and it is now being used in more than 70 countries. Development of Enhanced Geothermal Systems (EGS), has greatly enhanced the potential for extraction and use of geothermal energy which is available in unlimited quantity at greater depths. These high initial cost systems are viable and the potential for generation of power from them is unlimited. It is hoped that this source will offer a clean and inexhaustible alternative for meeting the global demand for energy in the near future. - Editor Introduction Earth was born as a ball of fire millions of years ago. It cooled down and a solid crust has formed on its surface while the interior remains hot having temperatures of about C which is almost equal to the surface temperature of the sun. Earth s crust is an excellent heat insulator and the surface temperature of earth remains low which makes it suitable for survival of life. The average temperature gradient through the crust is 25 0 C to 30 0 C per kilometer, though it varies at different points depending upon the nature of rock, thickness of the crust, proximity of tectonic edges and other factors. RITES Journal 8.1 January 2010

2 8.2 Geothermal Energy : Potential, Cost and Environment Issues Heat does travel through the crust to the earth s surface and is radiated outwards from there. The total heat lost in this manner is estimated to be about 44.2 TW but its intensity is only 0.1 Watts per square meter of surface area which is too low to be of any practical use. It is insignificant as compared to the intensity of energy received from the sun which is about 1370 Watts per square meter of earth s cross sectional area. The heat lost by earth in the above manner is replenished by the nuclear disintegration which goes on at the core of the earth where the temperature is very high and it is estimated that the energy generated in this manner is about 30 TW. The earth s crust is not continuous and it has cracks which divide it into sections called plates. The major fault lines are shown in Fig. 1. Fig. 1 : Tectonic Plates & Fault lines The hot molten material inside the earth finds escape through the cracks in the crust in the form of volcanic eruptions. The molten material is accompanied by superheated steam, CO 2, H 2 S and other gases. Energy released in this explosive manner causes a lot of destruction and it can not be harnessed to generate power or for any other purpose. In some cases the molten lava comes up through the cracks and instead of emerging as a volcanic eruption, it solidifies near the surface forming a crust which is much thinner than the plates.the temperature of this crust is high and if there is subsoil water in the area it gets heated giving rise to hot springs. In some cases the

3 Vijaya Singh 8.3 temperature is very high and a mixture of steam and hot water emerges in the form of jets which are called geysers. In areas where the earth is dry, the hot earth can be used for heating water by embedding a network of pipes in it and circulating water through them. The temperature of hot springs and geysers varies from a few degrees above atmospheric temperature to well above the boiling point of water and some of the geysers discharge superheated steam. The water from hot springs and geysers and the heat of dry hot rocks can be used directly for bathing, cooking, heating of buildings and also for generation of electricity. Volcanic eruptions and hot springs also occur in ocean beds and they give rise to disturbances in oceans. The temperature gradient in the oceans caused as a result of volcanic activity or on account of seasonal variations can also be harnessed for direct heating or cooling of buildings or for generation of electricity. Even in areas which do not have hot springs or geysers, advantage can be taken of the fact that the temperature of earth remains constant throughout the year below a depth of about 10 m and is not affected by seasonal variations which affect the surface temperature. In India, traditionally, people take bath in fresh water taken out from wells and hand pumps as this water is warmer than the atmospheric temperature during winters and cooler than the atmospheric temperature during summers. Water taken out from greater depths can be used for heating of buildings and other purposes. Advantage can also be taken of the fact that the top layers of the earth get heated by the sun during summers. The atmospheric temperature goes down rapidly as winter sets in but it takes a much longer time for the earth to cool down and the higher temperature of the top layers of earth during winter can be harnessed for heating of houses. During summers, the earth is cooler as compared to the atmospheric temperature and water circulated through it can be used for cooling the buildings. Use of this temperature difference at shallow depths has received a boost with the invention of geothermic heat pumps. Presently, extensive use is being made of water from hot springs and geysers and from geothermic heat pumps for direct heating and for generation of electricity in more than 70 countries. The total energy harnessed for direct heating was about 28,000 MW in 2007 and for generation of power it was about 10,000 MW. The use of energy from hot springs and geysers is confined to countries which are in the fault zones and are having volcanic activity. The possibility of harnessing geothermal energy in areas which do not have volcanic activity is now being explored taking advantage of the fact that at a depth of 3 to 4 Km the temperature of earth is higher than the boiling point of water and in some locations as high as C to C. A hole is drilled to this depth, water is pumped into it to raise steam which

4 8.4 Geothermal Energy : Potential, Cost and Environment Issues comes out through another adjacent hole and is used for generation of electricity. Technology for deep drilling is already available and deep drilling up to a depth of 13 Km has been done for exploration and extraction of oil. In order to get a large surface area for heating the water, areas having shattered rock are selected or cracks are created in solid rock having hair cracks, by pumping water at high pressure. Projects for generation of power are called Enhanced Geothermal Systems (EGS) in USA and Hot Dry Rock (HDR) systems in Europe. Pilot EGS projects have been taken up in Australia, New Zealand, USA, Germany, France and Switzerland and if they are successful and are found to be safe and financially viable, geothermal energy, which is clean and is inexhaustible, will become the alternative source of energy capable of meeting all the global energy requirements for all time to come. In the following paragraphs, the potential of geothermal energy for direct heating and generation of electricity, the economics of using it and its impact on environment will be discussed. Geothermal Energy from Geysers and Hot Springs Hot springs exist in many countries having temperatures ranging from a few degrees above the atmospheric temperature to boiling point of water. There are geysers which discharge water mixed with superheated steam at high temperature and pressure in the form of a jet. We also have Fumaroles in nature having hot water and steam in them at the ground level. When these are mixed with mud they are called Mud Pots. The rate of discharge from the hot springs and geysers varies widely, ranging from a few drops per second to as much as 250 liters per second. There are also groups of springs existing in close vicinity to each other having very high aggregate yields. The Dalhousie Springs Complex in Australia had a peak total flow of more than 23,000 liters per second in 1915, which has now come down to 17,170 liters per second. The Oita Prefecture Complex has 4763 hot springs with total flow of 4437 liters per second. Another complex of 2850 hot springs of Beppu in Japan has a total flow of 1592 liters per second. There are also hot springs with high flow rates in Iceland, Brazil, Indonesia and states of Arkansas, New Mexico, Idaho, Colorado and Queensland in the USA. Hot springs generally have a high level of dissolved salts of Calcium, Lithium and in some cases even Radium and dissolved gases like CO 2 and H 2 S. However, these dissolved impurities are a disadvantage in using this water for generation of power. Use of Geothermal Energy for Direct Heating Hot water from hot springs has been used since ancient times. Romans used it for bathing and for heating their houses. In India there are hot springs which

5 Vijaya Singh 8.5 attract a large number of tourists who take bath in them for pleasure and for their therapeutic value. In some of the springs the temperature is so high that rice can be cooked just by tying it up in a cloth and keeping it dipped in water for a few minutes. Systematic use of geothermal energy is being made for central heating of whole towns by circulating hot water from the springs through a network of pipe lines to individual buildings. The same pipe lines are also used for supplying hot running water for bathing and other uses. Even in areas in which hot springs do not exist, pipes are laid deep inside the earth where the temperature is high and water is pumped through them to get it heated. The same system is adopted in non-volcanic areas also taking advantage of the high temperature as we go deep inside the earth and geothermal heat pumps are used to extract heat from the earth using water as the carrier. It is estimated that in 2007 geothermal energy was being used for direct heating in 70 countries and the total capacity of small and large facilities was 28,000 MW. This included about 12,000 MW of the capacity of heat pumps which were invented by Lord Kelvin in The idea was to draw heat from the ground using water as the medium and the concept was patented in Switzerland in However, it was only in 1946 that the geothermal heat pump was successfully implemented. J.D.Knockner designed the first geothermal heat pump to heat the Commonwealth Building in Portland, Oregon and Professor Carl Nielsen of Ohio State University built the first residential version two years later. The technology got a fillip as a result of the oil crisis of 1973 and was used extensively in Sweden and subsequently in other countries. Development of polybutylene pipe in 1979 reduced the cost of heat pumps and increased their popularity. As of 2004, there were over a million heat pumps installed worldwide providing about 12,000 MW of aggregate capacity. Each year about 80,000 units are being installed in USA and about 20,000 in Sweden. Different variations of heat pumps are used to suit local conditions. The following are the important systems in use: Open Loop This system is used where ample subsoil water is available at the required temperature. Hot water is pumped out by constructing normal tube wells and circulated through radiators installed in the building to be heated. The cool water coming out of the system is made available for irrigation or for domestic use. The same system can be used for cooling the buildings during summers when the temperature of subsoil water is lower than the atmospheric temperature. Direct Exchange In this system copper pipes are laid in a closed loop with pipes buried inside the earth at one end and the radiators for heating the building at the other end. Two

6 8.6 Geothermal Energy : Potential, Cost and Environment Issues vertical pipes having lengths ranging from 25m to 150m, depending upon temperature conditions and the extent of heating required to be done and connected to each other at the bottom by a U-shaped pipe, are buried in the ground to be in contact with hot earth. Three to four such pairs of pipes laid at interval of 4 to 5 meters would be sufficient for a house of three bed rooms.the bore holes in which these pipes are placed, are filled with bentonite grout in order to fecilitate heat transfer. Alternatively, taking advantage of the temperature difference between top layers of earth and the atmospheric temperature, m long pipes are laid horizontally by digging a 1 to 2 m deep trench. The temperature at a depth of two meters may be as low as 7 0 C during summer months and as high as 24 0 C during winters due to the time lag between the change of atmospheric temperature and the temperature of the top layers of earth with change of seasons. The refrigerant is circulated in the closed loop to pick up heat from the earth and to release it in the radiators as it goes round the closed loop. The entire loop is made of copper pipes and the quantity of refrigerant required is also high resulting in high cost of the system. Closed Loop In this system, water mixed with antifreeze compounds circulates in a closed loop as in the Direct Exchange System, through pipes buried in earth at one end but at the other end it passes through a heat exchanger and heats the refrigerant. The refrigerant in turn feeds the radiators which heat the building. Polybutylene pipes are used for the water loop, which bring down the cost, while for the refrigerant loop copper pipes are used. In this system, two pumps are required, one for the water loop and the other for the refrigerant loop. The initial cost of geothermal pumps is high but the operating cost is low and the system causes much lesser pollution as compared to systems using fossil fuels or electricity for heating. The other advantage is that the same system can be used for cooling during summer months as the temperature of the earth is lower than that of the atmospheric temperature. The capacity factor of direct use of geothermal energy in most cases is very low (about 20%) as it is generally used only for heating in winter months. Generation of Electricity Geothermal energy can be used much more efficiently for generation of electricity as it can be done round the clock and throughout the year as against direct heating which has low Capacity Factor. Generation of electricity using geothermal energy scores over wind energy and also solar energy as they have low and uncertain Capacity Factors.

7 Vijaya Singh 8.7 The first geothermal power generator with enough capacity to light four light bulbs was set up at Larderello dry steam field on 4 th July Later in 1911, the world s first commercial geothermal power plant was built there. It was the world s only industrial producer of geothermal electricity until New Zealand built a plant in As many as 24 countries are now generating electricity using geothermal energy. These 24 countries generated a total energy of 56,786 Gega Watt hours (GWh) of electricity in 2005 accounting for 0.3% of worldwide electricity consumption. Generation of electricity is growing at the rate of about 3% per annum. At present, generation of electricity is confined to edges of tectonic plates where there is intense volcanic activity. Further, generation of electricity was being done till recently only from hot springs, geysers and dry steam sources where the temperature is very high. With the introduction of Binary Systems it has now become possible to generate power even from hot springs and geysers having temperatures in the range of C to C. Use is being made of hot springs having even lower temperature and in Chena Hot Spring in Alaska, generation of electricity has been successfully achieved by the Binary System using water at 57 0 C. Under the Binary System, water from the hot spring is fed into a heat exchanger where heat is transferred to a fluid having low boiling point. The fluids commonly used are Ammonia, Butane, Pentane and Isopentane. Hot water flowing through the heat exchanger causes the low boiling point fluid to evaporate into gas at high pressure and the same is used to drive a turbine to generate electricity. USA is the leader in the direct use of geothermal energy and also in generation of electricity from it. The total capacity for generation of electricity in 2007 was 2687 MW, which generated 15 billion KWh of power in one year. The capacity has now gone up to 3040 MW. The first geothermal generation plant having capacity of 11 MW was set up at Geysers in California in 1960 and it has been in operation successfully since then. The aggregate generation capacity in Geysers has been progressively increased to 1360 MW and after deducting internal utilization, the system generates about 750 MW of net power. Geysers which is located 116 Km north of San Francisco, is the largest dry steam field in the world. The complex comprises 21 plants which work in close coordination with each other. The steam which is used for power generation is replenished by injecting treated affluent from the city of Santa Rosa, which used to be discharged in rivers and streams earlier. Another major geothermal area is located in south-central California on the southeast side of Salton Sea near cities of Niland and Calipatria.15 geothermal plants having aggregate capacity of about 570 MW are operating in the area. New plants are coming up rapidly in Nevada, Brady/ Desert Peak, Dixie Valley, Soda Lake, Stillwater and Beowawe. The present generation from these plants is 235 MW. As of Aug.2008, 103 new projects were under construction in 13 US

8 8.8 Geothermal Energy : Potential, Cost and Environment Issues states which have the potential of supplying 3979 MW of power when developed fully. It is expected that at the present rate of development the generation of geothermal power in USA may exceed 15,000 MW in Development of geothermal power is being encouraged by the government and it is now treated at par with wind power and projects using biomass for power generation. All these projects are fully exempted from Federal Taxes. The government is also funding research on geothermal power development in view of its advantages over other sources of energy. It is clean and has a high Capacity Factor unlike wind power and solar power. The source of energy is inexhaustible and there is no limit to its development once the technology for generation of geothermal power in areas far removed from tectonic plate edges is developed and becomes commercially viable. The total installed global capacity for generation of geothermal power in 2007 was 9732 MW. The installed capacity of geothermal power in different countries is shown in the table below: Table 1 : Installed Geothermal Electricity Capacity 2007 Country MW Country MW USA 2687 Russia 79 Philippines 1970 Papua New Guinea 56 Indonesia 992 Guatemala 53 Mexico 953 Turkey 38 Italy 811 China 28 Japan 535 Portugal 23 New Zealand 472 France 15 Iceland 421 Germany 8 El Salvador 204 Ethiopia 7 Costa Rica 163 Austria 1 Kenya 129 Thailand 0.3 Nicaragua 87 Australia 0.2 Enhanced Geothermal Systems (EGS) These systems are also known as Hot Dry Geothermal (HDR) energy systems in Europe. As has been stated above, the temperature of the earth s crust increases as we go down deeper and deeper inside the earth. In areas away from tectonic plate boundaries, the average temperature gradient is about 25 0 C to 30 0 C per kilometer depth. In areas close to tectonic plate edges and in some areas away from them

9 Vijaya Singh 8.9 also, the temperature gradient is much steeper. Such areas are preferred for the installation of EGS systems. A hole is drilled to a depth of 3 km to 4 km to reach a point where the temperature is C to C. Water is pumped through this hole which is converted into high temperature high pressure steam. This steam comes out through another hole drilled close by and is fed into a turbine to generate power. Favourable conditions for getting high temperature at lesser depth are presented by granite rocks overlaid by a thick layer of sedimentary rock which is a good insulator of heat. It is necessary to have a large surface area to transmit heat to the water pumped into the hole. This is achieved by selecting areas having shattered rock at the point where the water is to be injected. Water percolates into the crevices of the shattered rock and gets heated. In areas having solid rock, water is pumped at very high pressure in order to open up the hair cracks existing in the rock and the surface area for heating is enlarged in this manner. The rock temperature gradually comes down with usage as the rate at which heat is withdrawn is higher than its replenishment. The temperature may drop by about 10 0 C in 20 to 30 years and at that stage operations may be discontinued at that site and the rock allowed to recoup its temperature. This process may take several years. The technology for drilling deep holes is already well established and oil companies have successfully done drilling up to a depth of 13 km for exploration and extraction of oil. The cost of drilling is, however, very high and it constitutes a major portion of the initial investment for generation of power by EGS technology. The cost may be anything between $ 3 to 8 million per MW of power to be generated depending upon the strata conditions and the depth to which the drilling is required to be done. The operating cost of generation of EGS power is also very high, ranging from $ 0.06 to 0.15 per KWh. This makes the power prohitively costly and a lot of research work will have to be carried out in order to bring down the cost and to eliminate the risks involved in the generation of EGS power. Environmental Issues Geothermal electricity is comparatively clean. Small quantities of CO 2 and H 2 S and some other gases are discharged along with steam when deep drilling is done and steam is generated by pumping water deep inside the earth. Existing EGS plants release on an average of 120 Kg. of CO 2 per MWh of electricity which is a small fraction of CO 2 discharged by thermal power stations using fossil fuels. There may also be dissolved salts in the mixture of steam and water which is discharged. Some of these salts such as salts of mercury, arsenic and antimony, are injurious to health and effluents from the power plants will have to be disposed of with great care. The best method may be to pump them back deep inside the earth. Construction of EGS plants may also upset land stability as a result of pumping of water at high pressure deep inside the earth and formation of steam at high pressure.

10 8.10 Geothermal Energy : Potential, Cost and Environment Issues Subsidence of earth occurred in the Wairakie field in New Zealand and in Stanfen in Breisgau, Germany. EGS operations can also trigger earthquakes. The project in Basel in Switzerland had to be suspended when more than 1000 seismic events measuring up to 3.4 on the Richter scale occurred during the first six days of water injection. Geothermal power generation requires much less land area as compared to thermal power houses. The requirement of land for a geothermal power plant may range from 1-8 acres per MW as compared to 5-10 acres for a nuclear power plant and about 19 acres per MW for a thermal power plant. Potential of EGS for Power Generation The potential for power generation of EGS is unlimited. There is an enormous reservoir of heat inside the earth which is continuously getting replenished by nuclear decay. The only constraint is its high cost and the safety of operations. A 2006 report of Massachusetts Institute of Technology (MIT) estimated that by investing one billion US Dollars on research and development over 15 years, it would be possible to create 100,000 MW of generating capacity by 2050 in USA alone. Further, the MIT report projected that over 200 ZetaJoules (ZJ) would be extractable with further potential of increasing this to 2000 ZJ with technology improvement, which would be sufficient to meet the earth s present level of energy needs for several millennia. The report also concluded that the cost of generation could be brought down to 3.9 Cents per MWh by making correct choices of the depth of the bore and temperature, rate of flow of water and by bringing down the cost of drilling. Global Development Activities EGS/HDR systems are being developed at present in France, Germany, Switzerland, USA, Japan and Australia. The largest EGS project in the world is being developed in Cooper Basin, Australia with potential to generate MW of electricity. Development of geothermal power generation has been taken up in a big way in Australia. Exploration work is being carried out to locate blocks of Hot Rocks with fracture systems which are suitable for efficient power generation by injecting water into them. It has been found as a result of exploration that there are vast deep seated granite systems in Central Australia having high temperatures at a depth of 3 to 4 km. Three wells were drilled to a depth of over 4 km near Innamincka, South Australia in early At this level, the rock temperature was C. Geodynamic, who are the pioneers in this field intend to develop a 1 MW demonstration power station based on these wells. Another company, Petrotherm, has drilled two wells and deepened one to a depth of 3.6 km, where the rock temperature is C. They also propose to carry out flow tests as a proof- of- concept.

11 Vijaya Singh 8.11 In 2007 there were 19 companies in Australia carrying out exploration work at 141 sites. Another 12 companies propose to take up exploration work in 116 areas in South Australia. The results of exploration are expected to become available by 2010 and once the viability of the concept is proved, construction of power generation plants will be taken up. It is expected that at least three demonstration projects will become operational by Efforts are being made to pool the experience and the data collected by geothermal, mining and petroleum industries, each one of which zealously safeguards the data generated by it. All the companies involved in the business will benefit and the country will benefit if the data is pooled and duplication of effort avoided. The Federal Government of Australia is supporting the development of geothermal power generation and has announced a grant of $50 million to the industry for making the generation of geothermal power viable. The Opposition Party has promised to step up the support if it comes into power. The government in Germany is also supporting the efforts of the industry to generate geothermal electricity. They have fixed 23 Cents/KWh Feed-in Tariff (FIT) for generation of power. It is acting as a great incentive and many companies are showing interest in its development. The Landau partial EGS project, which is supplying power under FIT of 23 Cents/MWh, is working at a profit. The table below gives details of EGS projects currently in operation or under construction. Table 2 : Current EGS Projects: Project Type Country Size Plant Depth Developer Status (MW) Type (km) Soultz R&D France 1.5 Binary 4.2 ENGINE Operational Desert R&D United Binary DOE,Ormat Development Peak States Geotherm- Ex Landau Commercial Germany 3 Binary 3.3 Operational Paralana Commercial Australia 7-30 Binary Petratherm Drilling (Phase-1) Cooper Commercial Australia 250- Kalina 4.3 Geo- Drilling Basin 500 dynamic The R&D/ USA Flash AltaRock Drilling Geysers Commercial Energy, NCPA

12 8.12 Geothermal Energy : Potential, Cost and Environment Issues Conclusion Geothermal energy from hot springs and geysers is being used directly for space heating and bathing since ancient times. The invention and use of geothermal heat pumps has greatly enhanced the use of geothermal energy for direct heating. Natural dry steam fields and geysers are being used for generation of electricity in 24 countries and new capacity is being added every year. However, the scope of using geothermal energy which is available at the surface or at shallow depths is limited. The scope of developing geothermal generation of power has been greatly enhanced by the development of Enhanced Geothermal Systems (EGS)/HRD technology which would enable extraction of unlimited amount of heat energy locked up in the interior of the earth. This technology has the potential of developing into a clean source of renewable energy capable of meeting the entire energy requirements of the world for all times to come. For this to happen, it is necessary to carry out intensive research to bring down the cost, most of which comprises the cost of drilling, and to make the system safe from subsidence and tremors caused by disturbance deep inside the earth. References 1. Hot Spring: Wikipedia the free encyclopedia. spring 2. Geothermal power: Wikipedia the free encyclopedia Enhanced geothermal systems: Wikipedia the free encyclopedia Geothermal power in Australia: Wikipedia the free encyclopedia Geothermal energy in the United States: Wikipedia the free encyclopedia Geothermal heat pump: Wikipedia the free encyclopedia Geothermal Heat Pumps. *****