Energy & Exergy Analysis for Proposed Olkaria II Binary Power Plantin Relation to Sustainability Index for Different Working Fluids

Abstract

Geothermal energy is a renewable energy source with low carbon footprint. In Olkaria geothermal field - Kenya, single flash geothermal power plants have been analyzed mainly by energy criteria. Olkaria II geothermal power plant generates 105 MWe and reinjects the brine which contains the exergy of 19,685 kW. This paper applied energy and exergy analysis techniques to investigate a proposed binary power plant using Engineering Equation Solver (EES), by introducing sustainability index (SI) and isentropic effectiveness. Eight working fluids were selected for the study. The paper investigated the exergy destruction in the plant under ambient conditions (86 kPa and 20°C) with varying turbine inlet pressure and reinjection temperatures. Heat exchangers contributed about 60% (between 2,900 - 4,200 kW) of total exergy destruction. The second utilization efficiencies were between 26-45%. Sustainability indices were between 1.24 and 1.53 with reinjection temperatures above 78°C. Isobutane and R600a had sustainability indices of 1.52 and 1.53 with power generation of 6,791 and 6,792 kW, respectively. Trans-2-butene as a working fluid enerated 5,790 kWe with thermal, exergy, and second utilization efficiencies of 11.67%, 29.41% and 45.18%, respectively with a better reinjection temperature of 99.14°C. It was revealed that using trans-2-butene working fluid resulted in the lowest exergy destruction. Grassman exergy flow diagram showed reinjected exergy and network generated were the main proportions of the total exergy into the system. Also, it was observed that, for both the isobutane and R600a systems, the optimum operating condition for maximum net output power are similar.