Fluid transport through micro- and nano-porous barriers can be exploited for energy and thermal management applications. For example, porous barriers show promise for separating energetic fuels like hydrogen and methane from gas mixtures. Porous barriers can also facilitate cooling of electronics or people via evaporation and transmission of heat-carrying vapor through the barrier. While these first two applications remain laboratory curiosities, a third application that harnesses porous barriers for energy generation, osmotic power, has just achieved a critical milestone toward commercialization.
Osmotic power harnesses the salinity difference between fresh water and salt water to generate pressure. Imagine pouring a glass of salt water into a glass of fresh water. Through random motion, the molecules seek equilibrium by arranging themselves to give the combined fluids equal salinity concentration throughout. Now imagine instead that a semi-permeable barrier is placed between a fresh reservoir and a brackish reservoir, and the barrier’s pores are sized to prevent dissolved salt ions from passing through. The tendency to seek equilibrium still persists, and molecules from the fresh water side pass through the barrier to reduce the salinity on the salt water side. However, the dissolved salt ions cannot permeate across the barrier in the other direction to increase salinity on the fresh water side. The resulting net flux of water molecules into the brackish reservoir induces a pressure increase there. This increase is then harnessed to turn a hydraulic turbine to produce power, and that’s how the system functions. A good technical overview of this process is given at the Osmotic Energy Web site.
Osmotic power is a renewable energy source that relies only on pre-existing salinity differences. Power plants can be established at locations where fresh water rivers run into the salty ocean. These locations carry two key advantages. First, the mouths of many major rivers already accommodate large population centers owing to trade up the river; so, the generated power does not need to be transported long distances. Second, unlike transient and weather-dependent wind and solar energy sources, river/ocean salinity gradients are nearly continuous, providing a predictable, uninterrupted flow of renewable energy.
According to a Guardian article, “Power of osmosis used to deliver eco-friendly energy,” the first osmotic power plant in the world is now operating at Tofte, Norway, one hour south of Oslo. The Norwegian energy company Statkraft is operating the prototype plant, which currently generates only two to four kilowatts, according to the company. Statkraft estimates, however, that a commercial-scale installation, which it plans to bring on-line between 2015 and 2020, could produce 25 megawatts.
What is the global potential for osmotic power? According to a short article, “Osmosis Power“, appearing in Mechanical Engineering Magazine’s News & Notes section, the fresh water / sea water osmotic pressure difference is equivalent to 240 meters of hydraulic head, which exceeds that of Hoover Dam. The article estimates that global deployment of this technology could harness 2.6 terawatts of power, which at the moment is simply being dumped into the ocean.
Micro- and nano-porous barriers present many exciting opportunities for energy generation and conservation, and it is thrilling to see one promising application come off the drawing board to take shape within a functioning prototype power plant.