Hybridization of cooling system of Olkaria ii geothermal power plant: utilization of energy and exergy analysis concepts

Abstract

Improved energy supply is the main challenge facing most African countries because energy is fundamental for the sustainable development of all sectors of an economy. Currently, 65% of the Kenyan population do not have access to electricity. Therefore, there is need to reverse this situation by investing in energy generating projects and improving the efficiency of the current power plants by adopting or researching on new technologies to reduce energy and exergy losses. In this study, we have focused on geothermal energy with the aim of improving the power output by lowering the exergy losses in the condenser. Condensers of geothermal power plants need to operate at low pressures to ensure optimal use of energy resources. To optimize the condenser pressure, cooling water temperature is varied to determine the value that gives a higher condenser efficiency resulting in more power output. This study focused on modelling the cooling system towards lowering the temperature of cooling water in which an absorption chiller was integrated into the system. The relevant energy and exergy balance, and efficiency equations for the Olkaria II (Kenya) geothermal power plant subsystem were derived. Codes were then developed from the equations and solved using the Engineering Equation Solver software. The evaporator temperature was varied for a constant refrigerant load temperature and changes in condenser exergy efficiency and turbine output were recorded. Simulation results shows that by adoption of an absorption chiller as the secondary cooling system, cooling water temperature reduced to 16oC from the current temperature of 25oC. The exergy destroyed in the condenser and turbine decreased from 2.89 MW to 2.3 MW and 7.7 MW to 7.3 MW respectively. The power output increased by approximately 1.6 MWh which translates to an annual cash flow of US $ 981,120 having taken the electricity sale price at 0.07 US $/kWh. The economic analysis of investing in the absorption refrigeration system in terms of discounted payback period resulted in payback years being approximately 9.4 years. Therefore, investing in the absorption chiller system for Geothermal Power Plants is economically feasible