Exergetic and exergoeconomic analysis of a renewable polygeneration system serving a small isolated community

A great interest has been arising in the last decade for the sustainable supply of energy and fresh water, also due to the rapidly growing demand from developing countries. Facing this demand by traditional technologies and fossil energy sources implies evident difficulties and risks, because of the scarcity and high cost of fuels and their relevant environmental impact in terms of local pollution and global warming emissions. Then, the efforts of researchers have been focusing on alternative solutions, either based on the use of renewable energy sources and on innovative plant schemes for the combined and efficient production of several energy and material flows. In this paper an innovative renewable polygeneration system is considered, which includes: a solar field based on concentrating parabolic trough photovoltaic/thermal collectors; a biomass heater; a single-stage absorption chiller; a Multiple Effect Distillation desalination plant; two sensible heat storage tanks; Balance of Plant and auxiliary components, like pumps, mixers, diverters, valves, controllers, plate-fin heat exchangers, etc. The system is sized to cover the very base load of an isolated small community. The plant operation has been analysed in detail in previous papers; in this paper an exergetic and exergoeconomic analysis is made. As concerns the exergetic analysis, it is intended to: i) quantify the exergy destructions occurring along the whole complex energy conversion process; ii) identify the steps that mostly affect the overall exergy efficiency of the system and thus propose possible improvements. Also, as the system actually operates under dynamic conditions, the analyses are performed by adopting daily integral values of energy flows, comparing the results obtained in the summer, winter and intermediate (spring/autumn) seasons. Finally, being the installation and maintenance costs of plant components known, an exergoeconomic analysis is also performed, based on the exergy costing method. The study is applied to integral energy flows; then, the cost accounting procedure will allow one to assign a monetary value to each energy or material flow, thus representing a good premise for the future definition of selling prices.