Exergoeconomic Analysis for Optimized Combined Wet and Dry Cooling BinaryPower Plant at Olkaria I, Kenya

Abstract

This study investigated a complete exergoeconomic comparison of wet and dry-cooled binary power plants. Simple organic Rankine cycles (ORC) for Olkaria geothermal field in Kenya using eight isobutane types of working fluids were proposed for analysis by the thermo-economic concept and sustainability index (SI). Network generated per heat transfer surface area was the optimized objective function, f(obj). Variable metric optimization method implemented in Engineers Equation Solver (EES) was applied to optimize plants for average fuel cost for geothermal fluid at 1.3 $/GJ. The exergoeconomic of the cooling tower contributed mainly to the investment cost of a water-cooled plant. Network of 1,628 kWe to 2,594 kWe was generated in wet cooled unit with SI of 1.654 to 2.701 for f(obj) of 1.5 to 1.8. For air-cooled plant, SI ranges were from 1.286 to 1.612 for the network from 1,446 kWe to 2,469 kWe with utilization efficiency of 34.77% to 59.37% and f(obj) values of 0.56 to 0.89. The cost of products range was from 18.3 $/GJ to 20.76 $/GJ for wet system and from 20.87 $/GJ to 23.4 $/GJ for dry-cooling. Selection of the suitable power plant based on exergoeconomic would be air-cooled systems with lower plant's cost rates of between 76.92 $/hr and 135.8 $/hr. The heat capacities are 20,806-20,610 kW for isobutene, 24,153-23,928 kW for isobutane, 13,727-13,595 kW for cis-2-butene and, 16,772-16,611 kW for n-butane for ambient temperatures between 0°C and 40°C. Application of air-cooled units would be advisable where water is scarce and in colder regions. Thermo-economic concept concludes that complete exergoeconomic of power plants presents a better investment decision by variable metric optimization method.