| Title | Design and Analysis of Two-Phase Geothermal Energy Turbine in Project Combi-Gen |
|---|---|
| Authors | Sham RANE, Li HE, Zhibin YU and Guopeng YU |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | geothermal energy, two-phase turbine, total flow system, computational fluid dynamics, eulerian-eulerian multiphase, two-phase turbine test rig |
| Abstract | The Combi-Gen consortium has been established to design and develop geothermally sourced combined power and freshwater generation technology in eastern Africa. The proposed thermal plant consists of a total flow two-phase turbine and a passive thermal chimney for condenser operation. In a total flow system, the high pressure and temperature geothermal fluid is directly passed through the power turbine. During the expansion process, there is flash boiling, generating steam which has very high specific volume and an abrupt increase in fluid volume makes the turbine passage difficult to be designed. Due to this reason, all traditional geothermal turbines have been operated as flash steam turbine using nozzles or binary cycle plants using heat exchange with clean working cycle fluids resulting into high system losses. In this paper, a computational fluid dynamic model of the two-phase turbine has been presented. Flash boiling has been incorporated through the thermal phase change approach that is based on local thermal non-equilibrium with homogeneous nucleation in order to estimate the rate of mass transfer between the liquid and vapour phases. Using this CFD model, a Fabris turbine design has been analysed to reduce inlet and exit losses and optimize the specific work output. A 1.0 KW turbine operating at 500 – 800 kPa well head pressure and exhausting to atmospheric pressure has been designed with estimated fresh water recovery of 8% and isentropic power generation efficiency of 6.8%. The turbine has been analysed at three conditions of 20º, 10º subcooled and saturated inlet water temperature. The feed water flow rate was lower at higher inlet temperatures thus improving the specific power of the turbine. However, the improvement in isentropic efficiency was not observed as the available enthalpy at inlet is higher for higher feed water temperatures. Turbine power increased as the feed water temperature was increased. At saturated inlet condition, the maximum power that could be produced was 2.08 kW at 4623 rpm. Exit vapour quality improved with increase in feed water temperature and was found to be in the range 0.085 to 0.11. Full-scale Combi-Gen thermal plants will be installed at suitable wells identified in the eastern regions of Africa where active geothermal wells have been sourced by the partnering organisations. |