Geothermal: Technology Overview
Geothermal resources can provide energy for power production or a wide variety of direct use applications. Geothermal power plants use heat from the earth to generate steam and drive turbine generators for the production of electricity. There are three basic types of geothermal technology: dry steam, flash steam, and binary cycle steam. Dry steam power plants are suitable where the geothermal steam is not mixed with water, and operate at high temperatures of between 180º-350ºC (356º-662ºF). Flash steam power plants tap into reservoirs of water with temperatures greater than 182ºC (360ºF). Binary cycle power plants operate on water at lower temperatures of 107º-182ºC (225º-360ºF).
The current global installed capacity for geothermal power plants is approximately 8,227 MWe. An additional 15,580 MWth is used in direct heat applications. It is estimated that geothermal resources using today’s technology could support between 35,448 and 72,392 MWe of electrical generating capacity. Using enhanced technology that is currently under development (permeability enhancement, drilling improvements) geothermal resources have the potential to support between 65,576 and 138,131 MWe. (Renewable Energy World, 2002)
Applications
In addition to generation of electricity and direct space heating applications, hot water and saturated steam from a geothermal resource can be used for process heat applications such as fish hatching, soil warming, mushroom growing, refrigeration, washing and drying of wool, drying and curing of light aggregate cement slabs, evaporation in sugar refining, canning of food, drying of timber, and digestion of paper pulp. (Geothermal Resources Council)
Resource Availability
Geothermal power is limited to locations where geothermal pressure reserves are found. Well temperature profiles determine the potential for geothermal development and the type of geothermal power plant installed. The Blue Book on Geothermal Resources prepared for the European Commission in 1999 contains some the most current information on worldwide geothermal resources. The figure below shows the locations of low and high energy geothermal resources in the COO. High energy sites are suitable for electricity production, while low energy sites are suitable for direct heating.
[picture]
Environmental Impacts
Dissolved minerals and hazardous non-condensable gases in geothermal fluids can be an environmental concern if not handled correctly (fluid reinjection addresses many concerns). Geothermal power plants with modern emission control technologies have minimal environmental impact. They emit less than 0.2 percent of the carbon dioxide, less than 1 percent of the sulfur dioxide and less than 0.1 percent of the particulates of the cleanest fossil fuel plant.
There is the potential for geothermal production to cause ground subsidence. This is rare in dry steam resources, but possible in liquid-dominated fields (for example, Wairakai, New Zealand). However, carefully applied reinjection techniques can effectively mitigate this risk.