Thermo-economic assessment of reverse osmosis desalination system driven by the organic Rankine cycle

A lack of fresh water can be observed in many countries with the increase of the world population. Therefore, waste heat was recovered from a diesel engine and used to run an organic Rankine cycle to supply power to a high-pressure pump, which pumps feed water to a reverse osmosis (RO) system. This study was done to increase the mass flow rate of fresh water production from the seawater. In order to increase the fresh water, a higher value of power is required which increases the total annual cost (TAC) of the system. The fresh water mass flow rate is defined as the efficiency of the system that increases by the increase of the power generated in the turbine. By considering the fresh water mass flow rate and cost as the two objective functions, an enhancement in one function destroys the other function. Due to the conflict between the functions, multi-objective optimization is required to apply to increase the thermal efficiency, and decrease the TAC. For these purposes, seven design parameters that some of the are turbine pressure, condenser pressure were selected, and a set of solutions were obtained for the optimized system parameters to find the effects of system design parameters on the system efficiency, TAC and consequently fresh water production. It was concluded that increasing the turbine pressure enhances the fresh water production, but increasing the condenser pressure decreases the mass flow rate of fresh water. In addition, increasing the feed water temperature and mass flow rate of feed water has positive effects on the RO recovery ratio and the mass flow rate of fresh water. Finally, a single solution is introduced as the final optimum point to evaluated different design parameter's effects on the system performance and fresh water production. The optimum magnitude for the system thermal efficiency was 37.99% with a TAC of 40,785 $/y as well as 954.67 kg/s of fresh water.