Cost Effective Small Scale ORC Systems for Power Recovery from Low Enthalpy Geothermal Resources

Abstract

Small scale geothermal power generation has, until recently, been inhibited by the relatively high cost of the power plant. Developments in the design and manufacture of screw machines have made it possible to modify standard mass produced air compressors to act as expanders in place of turbines in ORC units of up to approximately 500kWe output. Such machines have adiabatic efficiencies close to those of turbines of similar output but with a number of advantages. These include, very low cost, the ability to enhance the cycle efficiency by admitting wet vapor, direct coupling to standard 3-phase generators, and the elimination of traditional lubricating systems. Combining them with standard heat exchangers has made it possible to manufacture small ORC systems for cost effective power production at outputs of as little as 20-50kWe. An experimental unit, containing all these features, has been designed, built and tested. Preliminary results showed good agreement with performance predictions with the screw expander achieving an adiabatic efficiency of 74% at an output of only 22kW. This compares well with turbines of similar power. A detailed design study has shown that a 50 kW air cooled system, receiving heat from brine at only 100oC can be built and installed for a total cost of only $1500-2,000/kWe of net output, if water cooled, and approximately $2,500/kWe of net output, if air cooled. Extrapolating these figures to a 200kW unit, it is estimated that up to approximately 500kWe output, such screw driven ORC units are 30% cheaper than recently publicized low cost, turbine driven systems.