geothermal electricity and combined heat & power - European ...

Binary cycle power plantsBinary, known also as organic Rankinecycle (ORC), plants operate usually withwaters in the 100 to 180°C temperaturerange. In these plants, the heat is recoveredfrom the geothermal fluid, via a heat exchanger,to vaporize a low boiling point organic fluidand drive an organic vapour turbine. The heatdepleted geothermal brine is pumped back intothe source reservoir, thus securing sustainableresource exploitation. Since the geothermaland working fluids are kept separated duringthe process there are little if any, atmosphericemissions. Adequate working fluid selection mayallow to extend the former design temperaturerange from 180°C to 75°C.Upper and lower temperature limits depend onthe organic fluid stability and techno-economicconsiderations respectively. At low temperaturesthe size required by the heat exchangers coulddefeat project economic viability. Apart from lowto medium temperature utilisation, geothermaland waste fluid binary processes can be contemplatedto avoid well scaling damage further toin-hole flashing, in which case submersiblepumps are used to produce the geothermal fluidunder pressure.Binary powerplants.Binary processes are emerging as a cost effectiveconversion technology for recoveringpower from, water dominated, geothermalfields at temperatures below 180°C. Recently,a new candidate binary process, known as theKalina cycle, has been developed, displayingattractive conversion efficiencies. It’s distinctivefeatures address an ammonia-water workingfluid mixture and regenerative heating. In brief,it takes advantage of the low boiling point of thewater-ammonia mixture to allow a significantfraction of it to be vaporised by the excessheat available at turbine exhaust. The gainsover conventional ORC plant efficiencies havebeen estimated at 40%, although its reliabilityand performances are yet to be demonstratedon long term plant service.Reclamation of low temperature geothermalsources, achievable via binary cycles, can significantlyincrease the overall worldwide exploitationpotential. Small scale geothermal binary plants(< 5 MW eratings) can be widely implemented inrural areas and, more generally, respond to anincreasingly disseminated human demand.6 EGEC | Geothermal Electricity and Combined Heat & Power