SMALL GEOTHERMAL RESOURCES - A REVIEW

Geothermal resources are present, in different forms and quantities, all over the world. The high-enthalpy resources (temperatures above 225-degrees-C) and part of the medium enthalpy resources (between 225-degrees and 125-degrees-C) are concentrated mainly along belts that coincide with the margins of the lithospheric plates. The low-enthalpy resources (less than 125-degrees-C) are present in all countries. Small geothermal resources belong for the most part to the category of medium-low enthalpy resources and are capable of feeding electric power plants of a maximum capacity of 10 MW(e), and any type of nonelectric plant. Electricity generation is the best known and most important utilization of geothermal energy. In 1993 the total installed geothermoelectric capacity in the world had reached 5915 MW(e). This figure includes the approximately 80 MW(e) coming from small geothermal resources For the year 2000 the total installed geothermoelectric capacity is expected to reach 10,197 MW(e). Electricity generation can be achieved utilizing fluids at temperatures above 150-degrees-C in conventional condensing or back-pressure power plants, and fluids with a temperature as low as 85-degrees-C, in binary-cycle plants. Direct applications of geothermal heat (nonelectric uses) are a viable option with resources of any temperature. At the moment there are nonelectric plants in the world to a total of more than 11,000 MW(t). The most common nonelectric uses of geothermal energy are district heating, agriculture (greenhouse heating and animal husbandry), aquaculture, and industrial processes. There are, however, numerous other forms of utilization that have been in operation for many years (such as oil recovery) or are now being experimented (such as fog dispersal systems on airport runways). The environmental impact of geothermal energy is related mainly to the atmosphere (chemical pollution) and the surface and underground waters (chemical and thermal pollution). It is a relatively minor impact, especially with regard to the nonelectric uses, and is certainly lower than the pollution deriving from fossil fuel plants.