Water-energy nexus: A thermoeconomic analysis of polygeneration systems for small Mediterranean islands

This paper focuses on the energy-water nexus, aiming at developing novel systems producing simultaneously energy and water. This work investigates two solar polygeneration plants for the production of thermal and cooling energy, electricity, and desalinated water for two small Mediterranean islands. In this case, seawater and solar energy are largely available, whereas freshwater is scarce and extremely expensive. The work also aims to compare different technologies included in the polygeneration systems. In particular, the first plant is based on concentrating photovoltaic/thermal solar collectors, producing electric and thermal energy. The thermal energy is used to produce space heating, domestic hot water and space cooling by means a single-stage Lithium Bromide/Water absorption chiller. An electric auxiliary chiller is also included. A multi-effect distillation unit is included for freshwater production supplied by the concentrating photovoltaic/thermal collectors solar energy and an auxiliary biomass-fired heater. In the second plant, a photovoltaic field is coupled with electric driven technologies, such as heat pumps for space heating, cooling and domestic hot water production and a reverse osmosis unit. The solar electrical energy excess is delivered to the grid. The third polygeneration plant includes the same components as the first layout but it is equipped with a reverse osmosis unit. Two main case studies, Favignana and Salina islands (South Italy) are selected. The heating, cooling and electric hourly loads of some buildings located in both investigated weather zones are calculated in detail. In particular, space heating and cooling loads are calculated by means of the Type 56 of TRNSYS (version 17), coupled to the Google SketchUp TRNSYS3d plug-in. The buildings geometry, envelope, windows, lighting, machineries heat gains schedule, as well as the buildings users' occupation and activity are simulated by means of the Type 56. TRNSYS is also used to accurately model all of the plant components. The work also includes comprehensive energy, environmental and economic analyses to maximize the plants profitability, evaluated by considering both operating and capital costs. Sensitivity analyses aiming at establishing the optimal values of the most important design parameters are also performed. The developed plants achieve important savings in terms of carbon dioxide emissions due to the use of renewable energy sources and the high efficiency of the included technologies. The best economic indexes are obtained for the layout using electricity-driven technologies, resulting in very profitable operation with a payback period of about 6.2 years.