Kinds of Geothermal Sources Hydrothermal systems
Hydrothermal systems are those in which water is heated by contact with the hot rock, as explained above. Hydrothermal systems are in turn subdivided into 1) Vapor-dominated and 2) Liquid-dominated systems.
In these systems the water is vaporized into steam that reaches the surface in relatively dry Condition at about 205°C and rarely above 8 bar. This steam is the most suitable for use in turboelectric power plants with the least cost. It does, however, suffer problems similar to those encountered by all geothermal systems, namely, the presence of corrosive gases and erosive material and environmental problems. Vapor-dominated systems, however, are a rarity; there are only five known sites in the world to date. These systems account for about 5 per cent of all U.S. geothermal resources. Example: Geysers plant (United States) and Larderello (Italy).
In these systems the hot water circulating and trapped underground is at a temperature range of 174 to 315°C. When tapped by wells drilled in the right places and to the right depths the water flows either naturally to the surface or is pumped up to it. The drop in pressure, usually to 8 bar or less, causes it to partially flash to a two-phase mixture of low quality, i.e., liquid-dominated. It contains relatively large concentration of dissolved solids ranging between 3000 to 25,000 ppm and sometimes higher. Power production is adversely affected by these solids because they precipitate and cause scaling in pipes and heat-exchange surfaces, thus reducing flow and heat transfer. Liquid-dominated systems, however, are much more plentiful than vapor-dominated systems and next to them, require the least extension of technology.
Geopressured systems are sources of water, or brine, that has been heated in a manner similar to hydrothermal water, except that geopressured water is trapped in much deeper underground acquifers, at depths between 2400 to 9100 m. This water is thought to be at the relatively low temperature of about 160°C and is under very high pressure, from the overlying formations above, of more than 1000 bars. It has a relatively high salinity of 4 to 10 percent and is often referred to as brine. In addition, it is saturated with natural gas, mostly methane CH4, thought to be the result of decomposition of organic matter.
Such water is thought to have thermal and mechanical potential to generate electricity. The temperature however, is not high enough and the depth so great that there is little economic justification of drilling this water for its thermal potential alone.
Magma lying relatively close to the earth’s surface heats overlying rock as previously explained. When no underground water exists, there is simply hot, dry rock (HDR). The known temperatures of HDR vary between 150 to 290°C. This energy, called petrothermal energy, represents by far the largest resource base of the United States. Other estimates put the ratio of steam: hot water: HDR at 1: 10: 1000.
Much of the HDR occurs at moderate depths, but it is largely impermeable. In order to extract thermal energy out of it, water (or other fluid, but water most likely) will have to be pumped into it and back out to the surface. It is necessary for the heat transport mechanism that a way be found to render the impermeable rock into a permeable structure with a large heat-transfer. A large surface is particularly necessary because of the low thermal conductivity of the rock. Rendering the rock permeable is to be done by fracturing it. Fracturing methods that have been considered involve drilling wells into the rock and then fracturing by 1) High-pressure water or 2) Nuclear explosives.
Fracturing by high-pressure water is done by injecting water into HDR at very high pressure. This water widens existing fractures and creates new ones through rock displacement. This method is successfully used by the oil industry to facilitate the path of underground oil.
Fracturing by nuclear explosives is a scheme that has been considered part of a programme for using such explosives for peaceful uses, such as natural gas and oil stimulation, creating cavities for gas storage, canal and harbor construction, and many other applications.
This method would require digging in shafts suitable for introducing and sealing nuclear explosives and the detonation of several such devices for each 200-MW plant.
The principle hazards associated with this are ground shocks, the danger of radioactivity releases to the environment, and the radioactive material that would surface with the heater water and steam.