It's no coincidence that most steam-turbine electricity generating plants are built close to, if not on, a river or lake? These plants require so much water that water availability is one of the top criteria when selecting a location for a new plant and an ongoing environmental concern.
Enhanced Geothermal Systems, or EGS, are a renewable energy source that utilizes steam turbines to generate electricity. However, many of the better locations for Enhanced Geothermal Systems are found in areas where water is scarce or its use is restricted. To get around these water issues, EGS employs several techniques that naturally keep water usage to an absolute minimum.
The vast majority of the water usage in conventional steam-turbine plants goes to condensing low-energy steam back into water. After giving up most of its energy, steam exiting the turbine goes into an a condenser (actually a huge heat exchanger), where cooled water absorbs heat to turn the steam back into water. The condensed water is then pressurized and returned to the boiler to be turned back into steam. The energy absorbed by the water used to condense the steam has to go somewhere either into a large body of water or into the atmosphere. Heat released to the atmosphere provides the most iconic feature of steam-turbine plants: huge plumes of steam billowing from an evaporative cooling tower.
EGS uses a closed-loop water system to extract the heat from hot rock formations underground (see http://en.wikipedia.org/wiki/Enhanced_geothermal_system) . Water is pumped down to the hot rock formations through an injection well and the heated water is captured through a production well or wells. The heated water is sent back down the injection well for reheating once the energy has been extracted from it on the surface.
Perhaps most interesting is the fact that in a ‘binary’ Enhanced Geothermal System a fluid other than water can also be used to power the steam turbine. The water heated by the hot rock formations in EGS often don't reach temperatures above 200°… not nearly hot enough to turn water into steam. Therefore, Enhanced geothermal Systems use what is termed a binary fluid (one with a much lower boiling temperature than water) to drive the turbine. In a binary system, the water heated underground passes through a heat exchanger on the surface where it flashes the binary fluid to a vapor (the equivalent of steam). The vaporized binary fluid then acts (as steam would) to power the turbine to generate electricity. Environmentally benign butane and pentane-based fluids are commonly used in the closed binary systems. Since these binary fluids have such a relatively low boiling temperatures, binary "steam" can be condensed in air-cooled (rather than water-cooled) condensers and that saves water use.
All of this means that in addition to being a very clean, renewable energy source, binary Enhanced Geothermal Systems are very earth-friendly when it comes to water consumption and total usage.