Session: 11-02: Process Heat for Desalination and Industrial Decarbonization

Paper Number: 138605

138605 - Reducing the Specific Energy Use of Seawater Desalination With Thermally-Enhanced Reverse Osmosis 

Abstract: 

The world needs more clean water. Currently about ¼ of global population lacks access to safe drinking water, and this is only expected to worsen with climate change and population growth. Since freshwater resources are limited, engineering solutions are needed to produce freshwater from seawater and brackish water. For this reason, desalination plants are being developed at a rapid pace, with global water production capacity now exceeding 100 million m³ per day.

The vast majority of desalination is done via the process of reverse osmosis (RO). In RO, pressure is used to drive clean water across a semipermeable membrane, separating it from salts and other impurities that are present in the feed source. This desalination method is generally preferred over other alternatives such as thermo-driven distillation, because of its cost effectiveness. However, RO does have a significant energy footprint. A minimum of ~ 0.8 kWh of energy is required to produce 1 m³ of water permeate from seawater, and in most commercial desalination plants energy consumption typically far exceeds this.  This energy footprint is one of the primary costs of RO, and also has an associated environmental impact. Therefore, to reduce the cost and environmental impact of desalination, it is necessary to improve the energy efficiency of RO.

In this presentation, we analyze the concept of using thermal energy to pre-heat feed water in order to improve RO – a process we refer to as thermally-enhanced RO. 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 desalination. 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. Recommendations are provided for future applications of thermally-enhanced RO.

Presenting Author: Jonathan Maisonneuve Oakland University

Presenting Author Biography: Jonathan Maisonneuve is an Associate Professor in the Department of Mechanical Engineering at Oakland University (Rochester, MI) and a Visiting Professor in the Department of Bioresource Engineering at McGill University (Montreal QC). He obtained his Ph.D. degree in electrical engineering from Concordia University (Montreal, QC), his M.Eng. degree in building engineering also from Concordia University, and his B.Sc. degree in environmental science from McGill University. The goal of Prof. Maisonneuve's work is to improve access to critical energy, water, and food resources that are essential to life, while also protecting the environment. To do this, his research focuses on membrane process development at the nexus of energy, water, and the environment.

Authors:

Sanjana Yagnambhatt Oakland University
Saber Khanmohammadi Oakland University
Jonathan Maisonneuve Oakland University