While seawater desalination is a renewable water source, it can’t be truly renewable without minimized energy needs. This means desalination needs two things: to be extremely energy efficient, and to be highly compatible with renewables. This begs the question: what are the most energy efficient desalination technologies? And second, how can these technologies be integrated with renewable power?
For energy efficiency, reverse osmosis desalination, which uses high pressure membranes that only pass pure water, is by far the most efficient using electrical energy[1]. The electrical energy powers pumps (often above 90% efficient), and the pressurized flow must overcome the osmotic pressure to surpass the thermodynamic minimum energy limit [2]. Therefore, the most efficient technology revolves around minimizing pumping energy. Obviously pump efficiency is key, but as these are already very efficient, the main approaches come into play in reducing wasted or excess pressure. The wasted energy forms come from three areas, in descending order: 1) excess pressure above the osmotic pressure [2,3,4], 2) pressure required to overcome membrane permeability [5,6], 3) concentration boundary layers, aka concentration polarization[4], and 4) pressure losses through membrane modules and pipes [3].
For the first main loss of energy, pumping pressure, the aim has been to follow the osmotic curve as closely as possible [2]. Multistaging pumps to step along the curve is promising, but the stepwise reductions means that to fully eliminate this energy loss, infinite pumps (with infinite cost) are required. Furthermore, efficient pumps that can handle high pressure inlets are rare and difficult to manufacture. In contrast, batch systems, which vary over time, can very closely follow these osmotic pressure curves, largely eliminating this wasted energy [3]. However, the membrane modules have finite recovery that limits this reduction, and causes batch systems to have an energy tradeoff between membrane recovery per pass and pumping recirculation energy [3]. Still, these systems have shown excellent promise, predicting 25% improvements in efficiency vs single stage systems [2]. While batch technologies are new, semibatch systems have achieved the lowest seawater desalination energy needs yet, at about 2 kWh/m3. Our preliminary results show batch systems could reach down to 1.4 kWh/m3 [3] for seawater. Furthermore, osmotically assisted batch processes could achieve such excess pressure improvements, and thus high efficiencies, for much higher salinities [3,6,7,8,9].
For the second energy loss category, ultrapermeable membranes have been studied extensively to reduce the energy required [10]. The most permeable membranes developed can increase permeability multiple orders of magnitude. However, today commercial thin-film-composite membranes achieving permeabilities above 2 L/m2hrbar, returns are diminished as concentration effects take over [6].
The next critical efficiency concern is concentration boundary layers. New spacer designs have been proposed that can reduce this effect by 2-5x, with a tradeoff in some pressure drop. Optimizing these designs with fouling and pressure concerns remains an important step in further progress on reverse osmosis.
The fourth and final main category of pressure drops is hydraulic pressure drops through pipes [10]. Simple engineering and larger pipe diameters can help with larger pipes, and better membrane spacers and module inlet designs may assist further. Pressure drops related to hydraulic components including pumps and energy recovery devices remain larger and more important.
Overall, high efficiency has been demonstrated with sources of losses working separately, but high-quality engineering bringing all of these together can still provide impressive benefits. For seawater at 50% water recovery, these means demonstrating systems operating below 2 kWh/m3, and eventually approaching the 1.09 kWh/m3 limit.
[1] J. H. Lienhard V, G. P. Thiel, D. M. Warsinger, and L. D. Banchik, Low Carbon Desalination: Status and Research, Development, and Demonstration Needs. Abdul Latif Jameel World Water and Food Security Lab, MIT, 2016. (link)
[2] D. M. Warsinger1, E. W. Tow1, K. Nayar, and J. H. Lienhard V, “Energy efficiency of batch and semi-batch (CCRO) reverse osmosis desalination,” Water Research, vol. 106, pp. 272-282, 2016. https://doi.org/10.1016/j.watres.2016.09.029 (preprint)
[3] S. Cordoba, A. Das, D. Warsinger, Desalination, Double-acting piston batch reverse osmosis configuration for best-in-class efficiency and low downtime, vol. 506, pp. 114959, 2021 https://doi.org/10.1016/j.desal.2021.114959
[4] A. Das, D. Warsinger, Batch Counterflow Reverse Osmosis, Desalination, vol. 507, pp. 115008, 2021 https://doi.org/10.1016/j.desal.2021.115008
[5] D. M. Warsinger, E. W. Tow, L. A. Maswadeh, G. Connors, J. Swaminathan, and John H. Lienhard, “Inorganic fouling mitigation by salinity cycling in batch reverse osmosis,” Water Research, vol. 137, pp. 384-394, 2018. https://doi.org/10.1016/j.watres.2018.01.060
[6] D. M. Warsinger, J. Swaminathan, and J. H. Lienhard V, “Ultrapermeable membranes for batch desalination: maximum desalination energy efficiency, and system cost,” IDA 2017 World Congress on Water Reuse and Desalination, São Paulo, Brazil, October 15-20, 2017. (preprint)
[7] S. P. Córdoba, A. Das, D. M. Warsinger, Improved Batch Reverse Osmosis Configuration For Better Energy Efficiency, IDA 2019 World Congress World Congress on Water Reuse and Desalination, Dubai, UAE, October 20-24, 2019.
[8] J. Swaminathan, E. W. Tow, D. M. Warsinger, and J. H. Lienhard V, “Effect of practical losses on optimal design of batch RO systems,” IDA 2017 World Congress on Water Reuse and Desalination, São Paulo, Brazil, October 15-20, 2017. (preprint)
[9] D. M. Warsinger, E. W. Tow, R. McGovern, G. Thiel, and J. H. Lienhard V. Batch Pressure-Driven Membrane Separation with Closed-Flow Loop and Reservoir. Full Patent US ,US10166510B, 2 previously No. 15/350,064 November 2016 (link)
[10] Rao, Akshay K., Owen R Li, Luke Wrede, Stephen M. Coan, George Elias, Sandra Cordoba, Michael Roggenberg, Luciano Castillo, and D.M. Warsinger. 2021. “A Framework for Blue Energy Enabled Energy Storage in Reverse Osmosis Processes.” Desalination, vol 511, pp 115088. https://doi.org/10.1016/j.desal.2021.115088