The expanded use of geothermal energy has run into some opposition from environmental groups, which protest that such plants are dangerous, dirty, noisy, and unsightly. One of the problems of a geothermal plant is emission of noxious gases, such as hydrogen sulfide (H2S), which smells like rotten eggs. Carbon dioxide is also emitted in geothermal processes, although significantly less is emitted than from any other fossil-fuel plants with the same output. The steam from dry steam fields contains minerals that can contaminate the groundwater and poison fish and other aquatic life after it condenses (Fig. 18.8). In wet steam fields, the mineral and salt content of the hot water (brines) can be as high as 20 to 30% dissolved solids. Damage from corrosion can occur to the turbine blades, and the piping can become clogged.
There are additional problems with liquid waste disposal. Procedures have been developed in which the hot water would be evaporated after use so that the minerals could be extracted.
Another problem in some geothermal regions is that the removal of steam from the reservoirs can cause subsidence (setting or slumping) of the land above. One of Mexico’s steam plants reported a subsidence of 13 cm. This problem might be remedied by reinjecting the waste water from the fields into the area via injection wells. In hot dry rock processes, the fluid from the hot reservoir is reinjected into the ground, making this process environmentally attractive.
The prospects for geothermal power are certainly promising, although restricted to specific geographical areas. Following are the different forms of geothermal resources:
• Hydrothermal resources contain steam or hot water reservoirs that can be tapped by drilling.
• Hot dry rock resources store heat in largely impermeable rocks. These can be used by injecting cold water into the well.
• Geopressured resources are deeply buried brines that contain energy in thermal, mechanical, and chemical forms.
For accelerated exploitation of this resource to occur, several problems have to be overcome:
• Lack of reliable information about geothermal resources, such as location, lifetime, and energy available. This is especially true for hydrothermal systems for which much of the technology is available. Investors don’t want to build a power plant unless they can be assured of a reliable supply of geothermal fluids over the lifetime of the plant.
• Lack of proven technology for extraction and use of the resource. This is especially true for geopressurized systems.
• Lack of complete knowledge about environmental impacts, such as air pollution and subsidence.
• Regulatory requirements and complexity of leasing regulations. Most of the currently exploitable geothermal resources are on public land, and leases have been available for only about twenty-five years. (Federal law prohibits the exploitation of geothermal resources in national parks.)
• Lack of substantial funding to research these concerns.
The absence of information in these areas will hinder private industry from committing capital for the development of these large resources. The economics certainly appear to be good. The costs of geothermal energy are one-half to three-quarters of those for fossil-fuel plants in similar locations.
HOW WOULD YOU CHOOSE
Heat from the earth is usually not of very high quality; that is, it is at a low temperature. Locations away from earthquake zones and volcanic activity (refer to Fig. 18.2) are even less likely candidates for geothermal. The investors should also understand environmental concerns that might be expressed at the public hearing for the geothermal power plant. Based on what you have learned in this chapter, would you remain with the city you chose? Why or why not?
1. List the different types of sources of geothermal power.
2. Why is the vast amount of thermal energy lying beneath the surface of the earth not a very useful resource?
3. Describe the similarity between the geysers discussed in this chapter and a percolator coffee pot.
4. Why are the most likely geothermal sites located in regions of high volcanic activity?
5. What are the environmental impacts of geothermal energy?
6. If a geothermal plant runs at one-third its Carnot efficiency, how much more heat will be added to the atmosphere than from a fossil-fuel plant operating at two – thirds Carnot efficiency? Take the condenser temperature to be 27°C (300 K) for both systems; for the geothermal plant, the boiler temperature is at 150°C, and for the fossil plant, it is 550°C. Assume the electrical output from both plants is the same.
7. Investigate the manner in which geothermal energy is used to heat homes in Reykjavik, Iceland, or businesses in San Bernardino, California.
8. An interesting review of electricity generation is raised in the introduction of this chapter. If installed capacity of wind is 10 times more than geothermal, why is its output only twice as large?