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Abstract In this paper a thermoeconomic analysis of a novel hybrid Renewable Polygeneration System connected to a district heating and cooling network is presented. The plant is powered simultaneously by solar and geothermal sources, producing electricity, desalinated water, heat and cooling energy. System layout includes Parabolic Through Collector (PTC) field, geothermal wells, Organic Rankine Cycle (ORC) unit and a Multi-Effect Desalination (MED) system. Cooling and thermal demands are calculated by suitable building dynamic simulation models, calibrated for Pantelleria Island. Electrical demand is obtained by measured data. A detailed control strategy has been implemented in order to prevent any heat dissipation, to match the appropriate operating temperature levels in each component, to avoid a too low temperature of geothermal fluid reinjected in the wells and to manage the priority of space heating and cooling process. A 1-year dynamic simulation has been performed and results analyzed on daily, monthly and yearly basis. The system achieved an SPB equal to 8.50 and it resulted capable to cover the energy demands of a small community. Moreover, the plant is capable to cover the fresh water demand of the Pantelleria Island.

Abstract The paper presents a thermoeconomic comparison between two different solar thermal technologies, namely Linear Fresnel Reflector (LFR) and evacuated tube solar collectors (ETC), integrated into a polygeneration plant. The system produces space heating and cooling, domestic hot water and drinkable desalinated water, by means of a multi-effect distillation (MED) system. In the ETC layout, a single-effect LiBr H2O absorption chiller (ACH) is included; in the second layout, based on LFR collectors, a double-effect ACH is considered. An auxiliary biomass-fired heater is used to supply the additional heat required by the MED unit, in case of low availability of solar radiation. Both plants are simulated by means of a zero-dimensional dynamic simulation energy model, developed in TRNSYS environment. The model also includes detailed thermo-economic calculations. The results show that in some winter weeks, the solar fraction for freshwater production ranges between 15% and 20% for the ETC-based system, whereas is zero in case of LFR, when the MED unit is supplied only by the biomass auxiliary heater. Therefore, for the analysed case study, ETCs resulted more profitable than LFRs, achieving simple pay-back periods of about 4–5 years.

Abstract A great interest has recently arisen for the sustainable supply of energy and fresh water, due to the growing demand from developing countries. Facing this demand by traditional technologies implies evident risks related with the high cost of fossil fuels and their environmental impact. Then, alternative solutions based on the use of renewable sources and innovative technologies must be considered. In this paper a renewable polygeneration system is examined, which includes a solar field based on parabolic trough photovoltaic/thermal collectors, a biomass heater, an absorption chiller and a Multiple Effect Distillation desalination unit. Plant operation under dynamic conditions has been analysed in previous papers; in this paper an exergetic and exergoeconomic analysis is carried out. The exergetic analysis is intended to identify the steps that mostly affect the overall plant exergy efficiency, so as to propose possible improvements. The exergoeconomic cost accounting is aimed at assigning a monetary value to each energy or material flow, thus providing a rational basis for price assignment. Both the exergetic and exergoeconomic analyses are applied to integral values of energy flows, comparing the results obtained in the summer and winter season. Finally, economic viability of the system in different context scenarios is discussed.

Abstract The purpose of the present paper is to propose a methodology for the fluid selection of an Organic Rankine Cycle for low-temperature waste heat recovery. The selection of an optimal working fluid is carried out by an optimization process, using the Genetic Algorithm. Three decision variables are considered: the working fluid, the evaporation temperature and the condensation temperature. These variables are subjected to some constraints that take into account the good operation of the heat exchangers and the expander. The defect of efficiency and the total heat exchange area per unit of power output are selected as the objective functions to be minimized. The heat recovery is made possible by a hot water source, which assumes inlet temperatures of 100 °C and 150 °C. The water mass flow rate is fixed to 1.0 kg/s. The results show that fluids with low value of critical temperature, like Novec649, RE347mcc, R245fa, optimize the defect of efficiency, whilst, in order to minimize the total heat exchange area per unit of power output, fluids with high value of thermal conductivity and latent heat of vaporization must be selected. This work offers a tool to selected an optimal working fluid, among all possible candidates, for this type of applications.

Abstract Waste management and energy production are becoming critical issues in geographically disadvantaged areas, like small islands. For waste disposal the most common strategy is shipping to the mainland, due to the scarcity of land and local suitable treatments. Electricity generation is based on the importation of fossil fuels for local production, due to the lack of connections with continental energy networks. Both, waste shipping and fuels importation determine strong dependence on the mainland, as well as high economic and environmental cost for small islands. Therefore, using local renewable energy sources is extremely attracting. In this work, geothermal energy is considered for thermal drying of wastewater sludge and electricity supply of the whole wastewater treatment. The system is analysed for the case study of Pantelleria, a small island in Southern Italy, where sludge is currently dewatered to a final water content of 70–80%, and then shipped to mainland. The proposed system decreases wastewater sludge to be transported and disposed by 73.3%. Its profitability is demonstrated by several economic indicators, showing a Simple Payback time equal to 8.34 years and a Net Present Value of 502 k€. In addition, a sensitivity analysis for the main parameters affecting plant operation is carried out.