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| Fig. 1: Solvent travels across the semipermeable membrane from a volume of low concentration to a volume of high concentration with osmotic pressure as the driving force. (Image Source: M. Swint) |
Picture a two-cell system, where there is an equal volume of solution in either cell. The two cells are separated by a semipermeable membrane, where only the solvent is able to cross between the two cells. The cells have the same solvent but differing amounts of solute dissolved in them. The system, with two equal volumes of liquid, looks to be at physical equilibrium but it is not chemically. Allowing the system to sit for some time, the amount of liquid in the more concentrated cell increases and the liquid level in the other cell decreases. The system comes to equilibrium with more equal concentrations of the solute but a different amount of total liquid in each of the cells as a result of the changing amount of solvent. This process, shown in Fig. 1, is an example of osmosis and it is done entirely without inputting energy through a pump, heating, etc.
This flow of solvent to equilibrium can be stopped by exerting a certain amount of pressure on the solute-containing cell, called osmotic pressure. Osmotic pressure overcomes hydrostatic pressure to create a difference in volumes that can be harnessed to produce energy. Pretend the two-cell example shown in Figure 1 is now an open system, where the cells have two streams of varying concentrations flowing past the semipermeable membrane. The difference in concentration actively moves solvent from the less concentrated stream into the more concentrated stream, increasing the volume of liquid in the more concentrated stream. The more concentrated stream now has an increased pressure in its part of the system and it is able to turn a turbine and generate power. With the combination of a turbine for power generation, this system is called pressure retarded osmosis (PRO). The only power required in this process is the energy needed to initially pump the streams across the membrane.
This system is potentially applicable where two streams of varying concentration mix, like where a freshwater river mixes with the ocean. Statkraft, a Norwegian energy company, sought to harness this application, opening a prototype osmotic power plant near Oslo in 2009 in which it tested various membranes. [1] With a membrane size of 2000 m2 with an efficiency between 1-2 W/m2, the 2009 prototype produced between 2 and 4 kW. [2]
For the full-scale plant on a footprint the size of a football field, Statkraft planned on a membrane efficiency of 5 W/m2 and a total membrane area of 5 million-m2. [2] Through simple calculation, the full-scale plant should have produced 25 megawatts of power. And assuming the physical footprint of the plant to be the size of a National Football League (NFL) field at 5350 m2, the power per plant area turns out to be 4.67 kW/m2.
Picture a small research university, such as Caltech, that seeks to increase its on-site energy production with osmotic energy. In 2022, Caltech consumed 117,334 MWh of electricity, 71% of which was generated on the 124-acre campus. [3] To produce the remaining 29% of grid energy consumed 34027 MWh, or 3.88 MW over the full year assume the potential osmosis plant would match the 5 W/m2 membrane efficiency and power per plant area of 4.6 kW/m2 of the proposed commercial-scale Statkraft plan. To produce the 3.88 MW Caltech consumed from the grid in 2022, 844 m2 of land would have to be dedicated to an osmotic power plant. This area is only 15.8% of a football field - seems reasonable.
However, Caltech already produced 24% of its energy in 2022 through Bloom fuel cells, equating to approximately 28,160 MWh, or 3.21 MW over the year. [3,4] Using the specifications of the Bloom Energy Server 6.5 white paper, a solid oxide fuel cell powered by natural gas, biogas, or hydrogen/hydrogen mixture, the 325 kW server has a footprint of 11.7 m2. [5] To produce the additional 29% of power consumed by Caltech, the institute would only have to dedicate 140 m2 - only 16% of the footprint a power-equivalent osmotic energy plant would require. For an area-constrained entity like Caltech, additional fuel cells would prove the better option to produce that 3.88 MW. As an aside, in terms of practicality of this example, Caltech is located in the water-starved Los Angeles, CA area. As the osmosis power technology requires a large amount of water, this drought- prone region is not the ideal location for an osmotic plant.
In 2013, Statkraft announced they would no longer be following PRO, stating that the technology was not on track to be developed to a competitive level for the time being. [1] With the comparative footprint of the technology to a fuel cell and water stress as a increasing global issue, this choice makes sense. However, it is disappointing to see the leading osmotic-power company cease research in this field.
© Maddie Swint. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. Patel, "Statkraft Shelves Osmotic Power Project," Power Magazine, 1 Mar 2014.
[2] N. El Bassam, M. Schlichting and D. Pagani, Distributed Renewable Energies For Off-Grid Communities, 2nd Ed. (Elsevier Inc., 2021), Ch. 18.
[3] "Caltech 2022 Sustainability Report," California Institute of Technology, Aprl 2023.
[4] K. Von Emster, "Caltech's Path to Decarbonization," The California Tech, 12 Mar 24.
[5] "The Bloom Energy Server 6.5," Bloom Energy, 2024.