Overview
Hybrid process in geothermal power plants
Full solution supplier for geothermal processes
Hybrid geothermal power plants combine two processes in one plant in order to increase energy exploitation: these are a flash steam process, and the binary or ORC process. The hot geothermal brine (170 - 260 °C) is kept from boiling because of the pressure deep down in the well. The brine is pumped 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 washed, dried and 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 boiling 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 liquid again, in the condenser. The feed pump closes the loop and starts the cycle again.
Geothermal steam contains NCGs and is close to saturation or even wet. Its reliable and accurate measurement is challenging. KROHNE offers dedicated flow measurement solutions designed for the specific steam conditions based on factory calibrated OPTIBAR Venturi tubes with heated annular chambers and high grade stainless steel inlet cone and throat sections together with the OPTIBAR DB 7060 C differential pressure transmitters and further components as a full flow metering solution. By applying KROHNEs dedicated wet gas system based on the industry proven pressure loss ratio method it is possible to online monitor process changes for example sudden changes of steam wetness and liquid carryover to the steam turbine. This sufficient tool is a powerful system for the integrity monitoring of the plant, improves process safety and increases turbine life.
Geothermal brine is hot and corrosive: it contains H2S, silica and other unwanted substances. Vacuum may occur in the lines, along with elevated temperatures. KROHNE has the best suitable measurement technology for every measuring point based on dedicated OPTIFLEX electromagnetic flow meters with special liners that offer low diffusion and vacuum resistance. Pentane, butane and other low boiling fluids are used as the ORC fluids in binary and hybrid plants. Flows and levels in the organic Rankine cycle need to be measured for efficient and safe process control. OPTIMASS Coriolis mass flow meters and OPTISONIC 3400 ultrasonic flowmeters are ideally suited for these Rankine cycle applications. These meters also supply additional process information about the fluid density for process diagnostics. Liquid levels can be measured by directly applying OPTIFLEX 2200 guided radar level meters or via bypass gauges type BM 26 which can incorporate a variety of different measurement sensor technologies, which can also be installed in redundant configurations.