Batch design cuts excess energy consumption in reverse osmosis desalination by 82%

As climate change worsens, water scarcity becomes a more pressing issue. Desalinating seawater is a possible solution; however, because of its massive energy demands, many believe there isn’t much room for improvements in the desalination process. Purdue University researchers have proven otherwise. They have published research showing that up to 82% of excess energy can be saved by combining batch reverse osmosis and other emerging innovations.
Desalination plants use a lot of energy. With new technologies and processes, they could become much more energy efficient. Photo credit/Mike Blake

 

David Warsinger, assistant professor of mechanical engineering, developed the concept of batch reverse osmosis in 2015. Instead of a continuous flow of seawater at high pressure, as is the case in most seawater treatment plants, a batch process takes in a set quantity of water at one time; processes it; discharges it; and then repeats that process over with another batch of seawater. His previous theoretical research indicated that batch and semi-batch reverse osmosis are the key to reducing energy consumption in the desalination process.

They decided to apply their findings to real-world desalination sites. In a recent publication, they evaluated 39 seawater reverse osmosis facilities by finding the average energy consumption with efficiency metrics across different technologies and operating conditions. They then divided that data into four hypothetical energy consumption bands: current reported energy consumption, state-of-the-art, practical minimum, and thermodynamic minimum. State-of-the-art was the minimum energy required when best available equipment and technology was adopted, and practical minimum was the minimum energy required when applying research and development technologies. Warsinger and team also calculated the thermodynamic minimum which stood as the theoretical outcome under ideal conditions, which is impossible to achieve in real-world systems.

From this evaluation, Warsinger and team were able to confidently suggest the use of batch and semi-batch reverse osmosis technology to reduce energy levels. Although energy hasn’t decreased much in past years, these technologies offer substantial improvement, when many thought there was no room for improvement left. Semi-batch proves to be a viable commercial option, as it could cut excess energy levels by 69%; where full batch could cut 82% of excess energy with future technologies.

Their research has been published in the journal Joule.

David Warsinger's team members (L to R, Mateo Roldan Carvajal, Sandra Cordoba, Ali Naderi, David Warsinger, Sultan Alnajdi, and Abhimanyu Das) showcase a membrane (white cylinder in front) and a reciprocating piston tank (black cylinder at rear left), the key components to a desalination process called double-acting batch reverse osmosis. (Purdue University photo/Jared Pike)

 

Their next step is to test these findings. In May 2023, Warsinger’s lab received a $1.67 million grant from the U.S. Department of Energy to start a small pilot project.  This year, another test site is being built at the Colorado School of Mines that is 40 times bigger than the previous model. Here, semi-batch, batch, and continuous configurations will be tested head-to-head to verify their findings and indicate which energy-efficient desalination process should be pursued in the future.

 

Writer: Julia Davis, juliadavis@purdue.edu

Source: David Warsinger, dwarsing@purdue.edu

 

Practical minimum energy use of seawater reverse osmosis

Sultan Alnajdi, Ali Naderi Beni, Albraa A. Alsaati, Mitul Luhar, Amy E. Childress, David M. Warsinger

https://doi.org/10.1016/j.joule.2024.08.005

ABSTRACT: Increasing the energy efficiency in seawater reverse osmosis (SWRO) is crucial to address worsening climate change and water scarcity. This study uses data from 39 facilities and detailed modeling to identify configurations for conventional, state-of-the-art, and practical minimum energy use. Performance benchmarks for pump efficiency, membrane permeability, membrane spacer mass-transfer coefficient, and pre- and post treatment were developed. Current systems use substantially more energy than the thermodynamic least work; 69% of this excess energy can be eliminated using state-of-the-art methods, and 82% with future technologies like batch reverse osmosis (RO). Additionally, isobaric energy recovery devices (ERDs) can save significant energy in conventional designs. We also map out the impact on energy of a wide range of operating conditions, including salinity, water flux, and water recovery. The most impactful high-efficiency solutions include using batch and semi-batch configurations, using the most efficient pumps, and operating at lower flux.