Power from geothermal systems can be produced in different ways. The technology used depends from the reservoir temperature and the geological conditions of the rock hosting the geothermal resource. Generally, geothermal systems are divided into low enthalpy (hydrothermal systems) and high enthalpy regimes (conventional systems).
Conventional geothermal systems host a magmatic source, e. g. an active volcanic system, magma chambers, rift valleys, island arcs, hot spots, etc. These systems are characterized by high temperatures > 180 °C that normally produce dry-steam to super-heated water-steam phases. It enables the direct use of dry-steam or flash-steam driven power plants. In cases where aggressive magmatic fluids are abundant or the required temperature is at the minimum power will be produced by binary cycle such as Organic Rankine Cycle (ORC) plants that work the more efficient the higher the temperature of the extracted fluid is.
Hydrothermal systems are mostly located in regions where no magmatic systems are present, such as on the old continental shields (e.g. central Europe, Africa, Asia). The temperatures of the upper and middle earth’s crust are low (30 °C/km depth on average). These conditions do not allow the formation of vapor-controlled geothermal systems. The geothermal fluid is extracted as hot thermal water passing through the permeable jointed wall rock. Depending on the geological and tectonic conditions, temperature of the geothermal fluid ranges between 80 °C and 160 °C. These systems allow an efficient power production via binary systems such as ORC plants or ammonium-driven Kalina cycles.
Geothermal systems in Tanzania allow both the direct use of hot steam at Ngozi area as well as implementing binary cycles at Mbaka Area for power production.
The following Animation shows how a conventional geothermal power plant works (Flash process):