System Application Benefits and Impacts

HDR systems generate baseload electricity, but might also be used in load-following modes. An experiment conducted at Fenton Hill, New Mexico, in 1995 demonstrated that an HDR reservoir is capable of a significant, rapid increas e in thermal power output on demand. In other words, an HDR electric plant could continuously generate power 24 hours a day and supply additional peak load power for a few hours each day. Los Alamos National Laborator y estimates that the thermal output could be increased by 65% for four hours each day without requiring additional wells or a larger reservoir [2]. Additional capital expense would be incurred to size the power plant and reinjection pump s to handle the increased output. However, it is possible that a price premium for the peaking power would exceed th e additional costs, improving the economics of the system. An analysis of this mode of operation is not included in this study.

The Hot Dry Rock resource is important in that it is an untapped class of resource that could one day provide the nation with a significant amount of clean, reliable, economic energy. Its potential lies in its broad geographical distributio n and its size. Hot dry rock is believed to exist in all geographic locations, but at different depths, depending on loca l geology. In the U.S., the higher grade (shallower) HDR resources exist in the western states, including Hawaii. A 1990 study conducted by the Massachusetts Institute of Technology [3] concluded the nation's high grade (gradien t > 70oC/km) HDR resources could potentially produce 2,875 GW at an average price below 10 0/kWh using curren t technology. This is over 400 times the world's current installed geothermal electric capacity.

The HDR resource is much larger and more widespread than hydrothermal resources and is probably, therefore, th e future of geothermal energy in this country. The natural progression of hydrothermal development has been to utilize the higher quality resources first. As the higher quality sites are expended and the technology matures, a minimum cost will be achieved, and the cost of developing new hydrothermal resource sites will begin increasing. The minimum cost for HDR will likely occur later than that for hydrothermal (see Figure 2), and at some point the curves will probabl y intersect, meaning it will become less expensive to develop HDR resources than the remaining low qualit y hydrothermal resources. The shape of the curves or their relationship to each other in Figure 2 are not exact. They are merely intended to illustrate the possibility that HDR will one day be less expensive than hydrothermal and that th e historical minimum cost for hydrothermal binary will probably be less than, and occur before that, for HDR binary . It is the authors' estimate that the historical minimum cost for HDR will be approximately twice that for hydrothermal and will occur 15 to 20 years later.

Figure 2. Hypothetical minimum cost curves for hydrothermal and HDR resources.

The environmental impacts of generating electricity from geothermal resources are benign relative to conventiona l power generation options. Geothermal power generation does not produce the federally regulated air contaminant s commonly associated with other power generation such as sulfur dioxide, particulates, carbon monoxide, hydrocarbons, and photochemical oxidants. Some, but not all, hydrothermal fluids contain hydrogen sulfide and/or high levels o f dissolved solids, such as sodium chloride. Thus, with geothermal hydrothermal power generation, the bigges t environmental concerns are the possible emissions of hydrogen sulfide and contamination of fresh water supplies with geothermal brines. Hydrogen sulfide emissions are abated, when necessary, with environmental control technology , and ground water contamination is avoided through protective well completion practices. Generally, there is les s possibility of adverse environmental impacts with hydrothermal binary generation than with hydrothermal flas h generation because the hotter fuids used in flash plants tend to have greater concentrations of chemical contaminant s than do less hot fluids typically used in binary plants. Also, in binary plants that employ dry, rather than wet, cooling systems, the geothermal fuid remains in a closed system and is never exposed to the atmosphere before it is injecte d back into the reservoir. See the characterization of geothermal hydrothermal technology elsewhere in this documen t for additional information.

The possible environmental impacts from a HDR binary electrical generating system are likely to be considerably less than those from a hydrothermal system employing binary technology. The water used in the HDR system is from a shallow ground water well or other source of water with low levels of dissolved solids and no hydrogen sulfide. Al l the water in a system with dry cooling remains in a closed loop and is never exposed to the atmosphere, limitin g emissions to possible minor leaks of the working fluid around valves and pipe joints. If a wet cooling system is used , there will be some evaporation into the atmosphere with possible minor emissions, the level of which will depend o n the original water quality and any chemical changes the water may experience in the reservoir. However, suc h emissi ons would be quite small compared to emissions from even the best fossil fuel electric generating technologies .

Although some water loss in the reservoir is expected with HDR systems, ground water contamination is not a concern for two reasons. First, it is probable that fresh water will be used in the system. Second, the depth and relativ e impermea bility of the reservoir will lower the probability that the water used would migrate to shallow fresh wate r reservoirs.

Water consumption is a concern with HDR plants since they will likely be located in arid areas of the western U.S . Leakage around the boundaries of the reservoir may be anywhere from 5% to about 15% of the injection flow rate [4]. This would constitute water consumption of about 2 to 6 m3/MWh in a mature 30 MW system. Larger losses are possible depending on the original permeability of the reservoir rock. Larger losses could render a project uneconomic depending on the availability and cost of water.

Siting HDR plants is complicated by the need for the plant to be located at the site of the resource. This may impac t the use of other resources (cultural, agricultural, mining, etc.) at the same location. It would not be unusual for HD R resources to be co-located with mining or agricultural resources.

Land use for an HDR binary plant is expected to be minimal - ranging from about 6.1 ha (15 acres) for a 5 MW plant up to 10 ha (25 acres) for a 25 MW plant. Land disruption, erosion and sedimentation, and increased levels of nois e and human activity may adversely impact biological systems in the immediate vicinity of the plant and wells.

Adverse visual impacts are also possible with HDR developments and would be of concern in inhabited areas an d scenic areas. However, binary geothermal power plants are compact and have a very low profile compared to othe r industrial facilities. A combination of the low profile, landscaping, and color camouflage was used to successfull y mitigate visual impacts at the 30 MW Mammoth Lakes binary power plant in California. It is located within abou t three miles of one of California's major ski resorts in a county that depends heavily on tourism.

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