Lifetime design strategy for binary geothermal plants considering degradation of geothermal resource productivity

This work proposes a lifetime design strategy for binary geothermal plants which takes into account heat resource degradation. A model of the resource temperature and mass flow rate decline over a 30 year plant life is developed from a survey of data. The standard approach to optimise a basic subcritical cycle of n-pentane working fluid and select component sizes is used for the resource characteristics in years 1, 7, 15 and 30. The performances of the four plants designed for the different resource conditions are then simulated over the plant life to obtain the best lifetime design. The net present value and energy return on investment are selected as the measures of merit. The production history of a real geothermal well in the Taupo Volcanic Zone, New Zealand, is used as a case study for the lifetime design strategy. The results indicate that the operational parameters (such as mass flow rate of n-pentane, inlet turbine pressure and air mass flow rate) and plant performance (net power output) decrease over the whole plant life. The best lifetime plant design was at year 7 with partly degraded conditions. This condition has the highest net present value at USD 6,894,615 and energy return on investment at 4.15. Detailed thermo-economic analysis was carried out with the aim of improving the plant performance to overcome the resource degradation in two ways: operational parameters adjustments and adaptable designs. The results shows that mass flow rates of n-pentane and air cooling should be adjusted to maintain the performance over the plant life. The plant design can also be adapted by installing a recuperator and reducing the heat transfer area of preheater and vaporizer.