Screw Expanders Increase Output and Decrease the Cost of Geothermal Binary Power Plant Systems

A study has been carried out on the feasibility of using screw expanders, as an alternative to turbines, in three types of binary systems, suitable for recovering power from liquid dominated brines. The maximum power that one machine can generate is of the order of 1-3 MW per unit, depending on the type of system, and the brine and cooling conditions. They are shown to be roughly as efficient as turbines, when expanding dry vapour, and they can be coupled directly to a generator, without an intermediate gearbox. They have similar characteristics as two-phase expanders, when their efficiencies are not much reduced. This latter feature enables improvements to be made in the conversion of heat to power, of the order of 30% over conventional ORC systems, from brines in the 100 – 140C temperature range. The costs of these machines are very competitive and it is shown that in a cycle designed for power recovery from brines in the 170210C range, the cost per unit output of the first expander stage may be less than 50% of that of a radial inflow turbine-generator set.