Exergoeconomic and exergoenvironmental analyses of an off-grid reverse osmosis system with internal combustion engine and waste heat recovery

Abstract

Recognizing the intensive energy consumption of desalination and the importance of the water-energy nexus, integrated energy schemes can reduce energy costs and carbon emissions while simultaneously increasing energy efficiency. This study presents exergoeconomic and exergoenvironmental assessments for a stand-alone, off-grid desalination plant, constituted by an internal combustion engine, a Rankine-based heat recovery unit, and a seawater reverse osmosis system. The use of fossil diesel is compared to soybean biodiesel. The exergoeconomic analysis follows the SPECO method, and the exergoenvironmental analysis employs environmental data obtained from a Life Cycle Assessment using the Eco-indicator 99 impact assessment method. The exergy efficiency of the plant is 6.9% and 6.7% for the fossil diesel and biodiesel scenarios, respectively. The highest destructions of exergy occurred in the engine, followed by the reverse osmosis system, for both fuel scenarios. Results of the exergoeconomic and exergoenvironmental assessments suggest that the reverse osmosis system and the engine should be the first focuses of actions aimed at reducing overall costs and environmental impacts. The levelized cost of freshwater is 1.312 $/m³ and 2.164 $/m³ (within literature values), and the environmental impact of freshwater is 207.5 mPt/m³ and 70.4 mPt/m³ for the fossil diesel and biodiesel scenarios, respectively. The environmental impacts associated with freshwater are 75% lower for biodiesel because the impacts associated with its production are significantly lower. The system proposed herein is technically viable and an alternative for off-grid seawater desalination.