Energy, exergy and thermoeconomic analysis of an industrial solar pond

Abstract

This study evaluates the viability of an industrial salinity gradient solar pond during two operating seasons (2014 and 2015). The Granada solar pond was built to supply low-temperature heat (up to 60 °C) to satisfy the temperature requirements of the flotation unit in a mineral processing plant (Solvay Minerales in Granada (Spain)). Energy analysis indicates that the non-convective zone (NCZ) and the upper convective zone (UCZ) have low efficiencies because these zones have a low capacity to store heat, and no heat extractions are carried out from these zones. In contrast, heat extraction in the lower convective zone (LCZ) has a positive impact on the system because it increases the capacity of the solar pond to store energy. Higher efficiencies were achieved in the second operation period in the LCZ, since higher amount of heat was extracted during this period. The overall exergy efficiency of the LCZ after the first and second operation periods was 1.6% and 2.3%, respectively. The solar pond works at temperatures close to room temperature and thus the exergetic efficiency decreases significantly. Regarding the thermoeconomic analysis, the cost of exergy stored is studied in this work, using two different approaches, by estimating the minimum price for exergy stored or the minimum surface area that ensure the thermoeconomic viability. The price of the stored exergy tends to decrease inversely to the price of fuel oil. Thus, the cost of the stored exergy must be four to five times higher than the price of fuel oil for the solar pond system to be feasible. On the other hand, solar pond technology is affected by economies of scale; the larger the solar pond, the higher the inflation rates and the lower the cost reduction rates that can be accommodated. This implies that estimating the cost of the stored exergy is complex due to the variability of the processes involved and can lead to erroneous interpretations when evaluating the viability of a solar pond from a thermoeconomic perspective.