Reducing the specific energy use of seawater desalination with thermally enhanced reverse osmosis

Abstract

This study investigates the concept of using heat to enhance reverse osmosis (RO) desalination. An analytical model is used to evaluate the effect of temperature on water permeate flux, specific energy, permeate quality, and applied operating pressures. When feed is heated from 20 to 50 °C, specific energy savings of up to 24 % are observed for high flux seawater desalination and up to 33 % for brackish water. Such improvements are consistent with the literature, but until now the thermal energy input required to heat feed has been mostly neglected from analysis. For the first time, the overall energy balance of thermally-enhanced RO is considered to evaluate the tradeoff between savings in mechanical pump work and thermal energy input. Results suggest that this tradeoff is favorable under the right conditions. In particular, there is a need for high thermal efficiencies including both a very high heat pump coefficient of performance and very high heat exchanger efficiency for recycling thermal energy. Under these conditions, overall energy savings of up to 12 % are observed for seawater desalination when feed is heated from 20 to 41 °C, and up to 18 % for brackish water with feed heated to 46 °C.