Multi-objective optimization of evaporator of organic Rankine cycle (ORC) for low temperature geothermal heat source

• Development of thermal and hydraulic design model of plate type evaporator.
• Optimization of plate type evaporator for minimum cost and pressure drop.
• NSGA-II was employed for optimization based on the evaporator geometry.
• Investigation of the effect of geometrical parameter on pressure drop & cost.
• Analyzed the effect of pressure drop on ORC net power & cost of evaporator.

Present study deals with the development of hydraulic and thermal design model of chevron type plate evaporator and optimization of its geometrical parameters for a low temperature geothermal ORC system. The optimization is performed using Non-dominated Sorting Genetic Algorithm-II (NSGA-II). The primary geometrical parameters of evaporator are selected as decision variables which include length, width and plate spacing. The minimum cost of evaporator and minimum pressure drop are chosen as objective functions under restriction of constant amount of heat transfer. Since the objective functions are conflicting, a single value of decision variables cannot satisfy both objective functions simultaneously. Therefore optimization results are presented in the form of Pareto Front solution which is trade-off between pressure drop and cost of evaporator. The minimum cost evaporator is 1570$ corresponding to pressure drop of 125 kPa while the maximum cost is 6988$ corresponding to a pressure drop of 5.2 kPa. The sensitivity analysis shows that the plate length has promising effect on pressure drop and cost of evaporator. Furthermore, the effect of pressure drop on cost of evaporator and net power of ORC system is also investigated. The optimum value of allowable pressure drop is 30–40 kPa, corresponding to net power of 73–74 kW of ORC system and cost of evaporator 3000–3500$.