Twenty-four countries generated a total of 56,786 gigawatt-hours (GWh) (204 PJ) of electricity from geothermal power in 2005, accounting for 0.3% of worldwide electricity consumption. Output is growing by 3% annually, because of a growing number of plants and improvements in their capacity factors. Because geothermal power plant does not rely on variable sources of energy, unlike, for example, wind or solar, its capacity factor can be quite large – up to 96% has been demonstrated. The global average was 73% in 2005. The global installed capacity was 10 gigawatts (GW) in 2007.
The thermal efficiency of geothermal electric plants is low, around 10-23%, because geothermal fluids are at a low temperature compared to steam from boilers. By the laws of thermodynamics this low temperature limits the efficiency of heat engines in extracting useful energy during the generation of electricity. Exhaust heat is wasted, unless it can be used directly and locally, for example in greenhouses, timber mills, and district heating. The efficiency of the system does not affect operational costs as it would for a coal or other fossil fuel plant, but it does factor into the viability of the plant. In order to produce more energy than the pumps consume, electricity generation requires high temperature geothermal fields and specialized heat cycles:
1. Dry steam plants are the simplest and oldest design. They directly use geothermal steam of 150°C or more to turn turbines.
2. Flash steam plants pull deep, high-pressure hot water into lower-pressure tanks and use the resulting flashed steam to drive turbines. They require fluid temperatures of at least 180°C, usually more. This is the most common type of plant in operation today.
3. Binary cycle power plants are the most recent development, and can accept fluid temperatures as low as 57°C. The moderately hot geothermal water is passed by a secondary fluid with a much lower boiling point than water. This causes the secondary fluid to flash to vapor, which then drives the turbines. This is the most common type of geothermal electricity plant being built today. Both Organic Rankine and Kalina cycles are used. The thermal efficiency is typically about 10%.