Geothermal flash, binary and hybrid power generation

The hot geothermal brine (170 - 260 °C) is kept from boiling because of the pressure deep down in the well. The brine is pumped up to the surface by multistage well pumps - the drop in pressure causes the hot brine to “flash” to steam. A separator is used to separate the steam from the condensed brine. The separated steam (5 - 10 bar, 150 - 220 °C) is used to propel a steam turbine. In hybrid plants the remaining energy of the steam leaving the turbine is used in an ORC (organic Rankine cycle) to evaporate the working fluid (typically Pentane). The mixed brine & steam condensate is reinjected into the geothermal field by high pressure pumps.

The organic Rankine cycle is a closed cycle using an organic working fluid (iso-pentane or iso-butane) – these have a lower evaporation temperature than water. The loop is kept isolated from the brine through the heat exchanger design of the pre-heater and evaporator. The working fluid is first pre-heated and then evaporated - the gas (approx. 20 bar, at 150 °C) drives the turbine, generating electricity. The exploited gas (approx. 4 bar 75 °C) is returned, as a condensed liquid again, in the condenser. The feed pump closes the loop and starts the cycle again.

Geothermal fluids contain entrained non condensable gases (NCGs) which need to be removed from the process for higher efficiencies. Steam ejectors or vacuum pumps ae used to remove NCGs from the system. Geothermal brine is hot and corrosive: it contains H2S, silica and other unwanted substances. Vacuum may occur in the lines, along with elevated temperatures. Geothermal steam is close to saturation or even wet.

KROHNE has the best suitable measurement technology for flow, pressure, level, temperature and other variables. We can act as main instrument vendor for complete projects. Our dedicated wet gas system measures the quality and flow of Geothermal steam reliably and in real time without any sampling and maintenance effort and thus is a powerful system for the integrity monitoring.