A spray-assisted multi-effect distillation system driven by ocean thermocline energy

Abstract

Ocean thermocline energy is a sustainable and reliable heat source for seawater desalination in remote islands that has no access to primary energy. To adapt to the unique features of thermocline energy, i.e., a sensible heat source with a small temperature gradient, we propose a novel desalination system combining the direct-spray technology and multi-effect distillation (MED). The steam generator of a conventional MED is replaced by a tubeless spray evaporator, which eliminates the thermal resistance across the heat transfer surface. The merits of this system, which is named spray-assisted multi-effect distillation (SMED), include reduced internal losses, higher productivity, and smaller heat transfer areas. To quantify the potential of the thermocline-driven SMED system, we firstly conducted a thermodynamic analysis. Comparing with MED, SMED boosts freshwater productivity by 35%, while the pumping power consumption and required heat transfer area are reduced by 58% and 17%, respectively. Based on the thermodynamic performances, the economic potential of SMED is assessed. The SMED system demonstrates significant economic benefits due to the reduced heat transfer area and higher productivity. The optimal cost of freshwater is $1.53/m3, 55% lower than MED under the same operating conditions. The originality and novelty of this study can be summarized as follows: (a) a novel desalination system that maximizes thermocline energy utilization is proposed; (b) a detailed process model is developed and validated for system design and optimization; and (c) a thermo-economic analysis is conducted to maximize the desalination performance. The derived results highlight the thermodynamic and economic potentials of the proposed SMED system, making it a competitive and appealing solution for thermocline desalination.