Power Recovery from Low Cost Two-phase Expanders

It is well known that power recovery from liquid geothermal brines could be considerably increased by means of efficient two-phase expanders. Hitherto proposals for them have not been economically viable either due to the low adiabatic efficiencies of the expanders or their relatively high manufacturing cost. It has already been reported how, as a result of a long term research and development programme, carried out at City University, London, screw machines have been designed, built and tested there which can expand two-phase fluids with adiabatic efficiencies of more than 70%. A more recent and major advance in this work is that by the development of special rotor profiles and the use of up to date bearing technology, such machines have been successfully operated without external timing gear to prevent rotor contact, and with process fluid lubrication of the bearings. By this means the need for an oil lubrication system and internal seals has been eliminated. These developments have led to a huge reduction in the cost of such machines and make it possible to consider the Trilateral Flash Cycle system as a viable alternative to Organic Rankine Cycle systems when considering geothermal power plant applications where binary systems are preferred to flash steam.