Using Salt Gradient Energy and Thermal Energy to Enhance Reverse Osmosis Desalination

Abstract

Improving desalination energy efficiency is crucial for meeting rising global water demands. Reverse osmosis (RO) is a common desalination process that uses an applied pressure to overcome the natural osmotic potential of seawater to drive nearly pure water permeate through a semipermeable membrane. However, it has high specific energy consumption ranging from 4-5 kWh/m³ and environmental issues associated with discharging the highly concentrated brine that is left over after separation. This work investigates two methods of improving the energy efficiency of RO desalination: (1) Recovering salt gradient energy from desalination brine, and (2) Using thermal energy to pre-heat RO feed water and reduce mechanical pump work.

Through this work several scientific and technical advances are made. (1) A new maximum power point control strategy is developed for salt gradient energy systems, improving power by up to 44%. (2) Lab-scale implementation of this MPPT control strategy is shown to boost power density over threefold, from 0.6 W//m² to 2 W//m² for an osmotic energy system recovering energy from RO brine. (3) Using low grade heat, the power density of salt gradient energy systems is increased by up to 110%. (4) By analyzing the energy balance of thermally enhanced RO, it is shown that the tradeoff between mechanical energy savings and thermal energy input can be favorable, and desalination energy was reduced by 12%