Feasibility study for implementation of a binary power plan in Lake Abhé geothermal area with a particular hot arid climate, Djibouti

Abstract

Lake Abhe, situated in Southwestern part of Djibouti is one of promising geothermal field and was recently the subject of a complete surface exploration. Located in a hot arid climatic zone, Djibouti possesses several medium-enthalpy resources distributed in different parts of the country. This particular climate makes it necessary to find new ways in modelling common geothermal power plants. The objective here is to determine how the medium-enthalpy resource in Lake Abhe geothermal field would be best utilized, both technically and commercially. The backbone of this paper will be how to deal with the hot, arid climate in order to maximize the net power output of the plant. A thermodynamic model was developed using Engineering Equation Solver (EES) to evaluate the performance of ORC geothermal power plants standalone with different cooling system, and an ORC assisted by a parabolic trough solar concentrating collector field. The water cooled condenser got 6,1% of the net power output more than the air cooled condenser (ACC) and was found to have more negative effects on the environment. Water usage was calculated to be 1324 kg/s or 41.8 million tons per year corresponding 1.4 % of the total lake water.

This system draws energy out of the working fluid before expelling the water back into the Lake Abhe with an increase in water temperature of 17°C. The ACC and hybrid solar-geothermal designs were selected and the NPV and IRR of these designs were modelled to allow an economic comparison. This study estimates a geothermal fluid mass flow of 443 kg/s and temperature of 145.7°C. Under Djiboutian climatic conditions with an average ambient temperature of 30.04°C, the air-cooled condenser basic binary model produces 10,924 kWe of gross power output with an auxiliary power consumption of 22.6% of the total gross output power. The fan power represents 51.8% of the parasitic power. The cycle efficiency of the ACC is 10.44%. For the hybrid solar-geothermal power plant, the net power output is 13,865 kWe with 20.6% for the use of the auxiliary components. The cycle efficiency of the hybrid is 10.18%. The hybrid system shows higher power output (up to 21.24% difference) compared to ACC. This study finds the hybrid system to be a better option than individual geothermal system at all ambient temperatures. It is demonstrated that the hybrid is the most economically attractive scenario, providing the highest NPV of US $9,900,000 and the fastest payback period of 18 years with the highest IRR of 13%.