On Using Heat for Water Desalination and Purification

Abstract

The global freshwater crisis, intensified by population growth, climate change, and industrial demands, necessitates more energy-efficient and sustainable water treatment technologies. This dissertation explores how low-grade thermal energy can be harnessed to improve the performance and energy efficiency of membrane-based desalination systems. Four thermally integrated desalination strategies are investigated: thermally enhanced reverse osmosis, thermally driven reverse osmosis, membrane distillation, and ocean thermal membrane distillation.For thermally enhanced reverse osmosis, a detailed thermodynamic analysis reveals that preheating feedwater can reduce specific energy consumption by up to 24 for seawater and 33 for brackish water desalination. While higher feed temperatures reduce pumping work by enhancing membrane permeability and reducing polarization, the trade-off with thermal input is critically analyzed to identify net energy savings under high-efficiency thermal recovery conditions. We conclude that the benefit of thermal enhancement is marginal, as it requires a substantial amount of heat input to achieve only a slight reduction in pump work. For thermally driven reverse osmosis, a fundamentally new process is introduced where thermal energy directly drives mechanical desalination via a thermally driven piston system. A theoretical model evaluates the system’s efficiency under various design parameters, demonstrating competitive performance compared to conventional thermal methods when optimized for low-grade heat sources. A feasibility study of an ocean thermal membrane distillation (OTMD) system is conducted for application in remote islands, using cold deep-sea water as a thermal sink. Experimental and theoretical studies show that under optimized conditions, the OTMD system can outperform conventional reverse osmosis (RO) systems in specific energy consumption, offering a low-carbon, resilient solution for water-scarce regions. Finally, for membrane distillation, performance is further enhanced through real-time control strategies that adjust operating parameters, feed temperature, flow rates, and distillate conditions, to reduce specific energy consumption. Experimental results from a lab-scale direct contact membrane distillation system validate the proposed control method and report specific thermal energy consumptions of 274 kWh/m3 which is significantly lower than those reported in the literature. Overall, this dissertation demonstrates that, if properly integrated, heat can significantly reduce the energy use of membrane desalination systems, especially in locations with access to waste heat, solar thermal, or ocean thermal gradients.