Performance Assessment of Model Organic Rankine Cycle Power Plant Components for Application at Olkaria-I Geothermal Power Plant

Abstract

Energy consumption is an indispensable component for the continued development of the human population. Developing countries have higher economic growth rates and hence the energy consumption rate increases. Kenya is among the fast developing countries. According to the Kenya National Bureau of Statistics report 2012, the electricity consumption of the country has increased by over 5900 GWh in the years 2008 to 2011. The increase in energy demand, high cost of fossil fuels and the link between energy utilization and environmental impacts have resulted in the need for a sustainable approach to the utilization of the earth's energy resources. The Olkaria I geothermal power plant in Kenya uses a single ash condensing turbine type which constitutes three units each with a capacity of 15 MW. Moreover the plant does not operate at its full capacity. Reports on the available energy analysis of the power plant indicated that more than 17 MW power is lost as thermal energy. Currently, the hot water (brine) from the steam separator of the power plant is discharged to the surrounding while still at high temperatures and high pressure in which a signi cant amount of available energy is lost. Despite the simplicity and low cost, a single ash steam geothermal power plant has the lowest e ciency mainly due to the untapped available energy in the brine. In Olkaria-I geothermal power plant the discharged brine has a temperature of 160 C and mass ow rate of 197 metric tons per hour at an average pressure of 7 bars. This rejected high temperature brine results in wastage of signi cant amount of available energy which warrants further investigation to develop mechanisms for energy recovery. Moreover, the Organic Rankine Cycle (ORC) power plant is an advantageous technology that has been applied in generation of power from low temperature brine of geothermal reservoirs but has not been used at Olkaria-I. The power plant utilizes oa low boiling organic working uid such as a refrigerant or a hydrocarbon. Researches indicated that the performance of ORC power plant is highly dependent upon factors such as proper organic working uid selection, types of heat exchangers (condenser and evaporator) and turbine used. Despite a high pressure drop, shell-tube heat exchangers have satisfactory performance for ORC power plants. The project involved the design, fabrication and performance assessment of the components of a model Organic Rankine Cycle power plant to utilize the geothermal brine at Olkaria I geothermal power plant. Two shell and tube heat exchangers (evaporator and condenser) and a single stage impulse turbine have been designed, fabricated and the performance assessment of each component has been conducted. The interacting

uids in the heat exchanger were pentane as a working uid and hot water simulating the geothermal brine. The performances of the heat exchangers were evaluated in terms of the overall heat transfer coe cients in the ow streams, heat energy transferred and pressure drops across the heat exchangers. The turbine was coupled to a car alternator and the performance was evaluated in terms of shaft speed (RPM) and power generated using compressed air at di erent pressures to the inlet. The results of the experiment indicated that the increase in mass ow rate of hot water by 0.08 kg/s caused a rise in overall heat transfer coe cient of the evaporator by 17.33% and the heat transferred was increased by 6.74%. In the condenser, the increase of cooling water ow rate from 0.15 kg/s to 0.35 kg/s increased the overall heat transfer coe cient by 1.21% and heat transferred was increased by 4.26%. Moreover, the performance of the single stage impulse turbine was tested using compressed air at di erent pressures. The shaft speed varied from 1585 to 4590 rpm as inlet pressure was varied from 0.5 to 5.0 bar and power generated was varying from 4.34 to 14.46 W. The results of the experiments indicated that the performance of each componentof the model Organic Rankine Cycle power plant operating at low temperature heat resources was satisfactory. It is anticipated that, an optimum design of an ORC will help increase the second law e ciency of existing open geothermal power plants. This will be recovery of low grade heat before re-injection of the geo- uid back to the earth.