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from the 20 sites with more than 100 MW installed gener- ating capacity (Fig. 1). The cumulative worldwide installed capacity [13] is about 7974 MW and the growth of capacity has been 17% from 1995–2000. VIII. GLOBAL POTENTIAL Geothermal energy is available in the earth to produce many orders of magnitude more electric power than the 49 TWh/yr currently being produced, once the technology, reg- ulatory environment and marketplace are favorable. Roughly joules of heat energy are stored in the earth [14] and the global rate of heat loss is estimated at joules per year [2], of which 70% is lost in the oceans and 30% from the conti- nents. The thermal energy stored in the upper 10 km of the earth’s crust below the U.S., relative to surface ambient tem- perature, is estimated [15] at 3 10 joules, or 10 times the annual energy demand in the U.S. At this time, only 6 10 joules per year of geothermal power are being generated in the U.S. A survey of geothermal experts [16] was conducted in 1999 to estimate the country-by-country potential of geothermal power, and the results indicated that 70 GW , or roughly nine times the existing installed capacity, could be produced with current technology. With enhanced tech- nology, the survey suggested that this could be doubled to 140 GW from known high-temperature reservoirs, resulting in 1000 TWh of potential geothermal power generation annually. In practice however, local market and other economic factors limit the expansion from known sources. For example, Indonesia has an abundance of geothermal resources but after several years of aggressive exploration and developmental activity in the 1990s, investment in geothermal projects virtually stopped in 1998 because of financial and political instability in that country. The cost of geothermal power from a typical high tem- perature, high permeability liquid-dominated reservoir, using conventional condensing turbines, is estimated to be in the range $2000/kW to $2500/kW for a project of roughly 100 MW in size. A breakdown of the cost components is ap- proximately as follows. Exploration & Development Drilling: 35%. Infrastructure: 10%. Surface steam and brine facilities: 15%. Power Plant: 40%. Rapid expansion beyond known sources can be conceived once it becomes cost-effective to explore for “hidden” sys- tems. The term “hidden” is here meant to depict those hy- drothermal systems at 200 C–350 C located within 3 km depth, which have no surface expression of hot springs, fu- maroles or gas seeps, and whose detection would have to rely principally on geological and geophysical prospecting methods. In the oil and gas industry, the technology for de- tecting reservoirs with no surface “leaks” is well developed, but the geothermal industry has yet to develop a suite of exploration tools to adequately lower the dry hole risk in prospects which have no thermal fluids at the surface. WILLIAMSON et al.: GEOTHERMAL POWER TECHNOLOGY The greatest promise for the long term lies in “Heat Mining”—extracting energy from rock with insufficient temperature or permeability to permit vigorous natural con- vection, where artificial stimulation is required to enhance permeability, and a surface source of fluid is required as the circulating medium. Resource temperature has a strong influence on the conversion efficiency of heat to electricity and Armstead and Tester [17] estimated that the conversion efficiency increased from less than 5% at 100 C to more than 25% at 300 C rock temperature. The depth at which 240 C is reached by drilling depends upon the local value of terrestrial heat flow, the presence or absence of groundwater convection and the thermal conductivity of the rocks. In most locations the ranges of these parameters dictate that 240 C will be encountered shallower than 10 km. This promise of vast untapped pollution-free energy re- sources encouraged both the U.S. DOE and later the U.K. to invest heavily in HDR demonstration projects in the 1980s and 1990s, but limited technical success and funding problems brought both projects to a halt. The EC is now proceeding with a demonstration project at Soultz-sous-Forêts, France, where a 5 km well was recently drilled to reach 200 C. IX. SUSTAINABILITY OF GEOTHERMAL ENERGY The rate of heat extraction from a 100 MW power project during production is of the order of 100 W /m from the drilled area, typically an order of magni- tude more than pre-exploitation surface heat flows. So at commercial withdrawal rates, the heat input from below is insufficient to sustain production at the fully developed rate. From an investor’s standpoint, maximizing withdrawal over a 10–20 year period is a more attractive option than operating at a lower capacity for hundreds or thousands of years. Commercial developments typically ramp up production in the first few years, maintain it constant for a period of 15–25 years, and then allow a natural decline once the cost of makeup wells to maintain steam supply cannot be economically justified. After 30 years of production at fully developed commercial rates, much of the available heat still remains in the reservoir. This presents an opportunity for secondary recovery projects, such as the treated wastewater injection projects at The Gey- sers field, where production has depleted fluid mass from the reservoir much more efficiently than it has depleted the avail- able heat. Now 0.4 m /s (9 million gallons per day) of treated wastewater is pumped 46 km from a nearby town to The Gey- sers, and has been shown to increase production by 68 MW . A second water injection project currently under construction willpump0.5m /s(11milliongallonsperday)oftreatedwaste- water 66 km from the city of Santa Rosa to The Geysers. The question of whether geothermal is truly a sustain- able energy source therefore depends on the extraction rate chosen for each resource. Investment criteria using discount rates of 15% or more drive a commercial developer to maxi- mize field production over a period of 10–20 years, and allow a natural decline in later years. At a significantly lower rate the field may remain productive for hundreds of years. geothermal Authorized licensed use limited to: National United University. Downloaded on October 10, 2009 at 14:17 from IEEE Xplore. Restrictions apply. 1789PDF Image | Geothermal Power Technology
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