The Oslofjord inlet of Tofte, Norway is currently home to not much more than a waterfront cellulose factory. Atop that coast also lies our world’s first osmotic energy plant which had been in operation for over three years, manned by the Norwegian energy company Statkraft.
When you consider renewable energy, you probably think of: solar, wind, or hydraulic sources. What has yet to gain widespread attention is the potential of osmotic power, also called “salinity gradient” power, which relies on a relatively elementary physical process: diffusion.
Current renewable energy sources such as solar, wind, or hydraulic have met a new match. Osmotic power, also called “salinity gradient” power, has yet to gain widespread attention which relies on a relatively elementary physical process: diffusion.
Statkraft has high hopes for their experimental power plant. Their decidedly small plant holds a meager two to four kilowatts of capacity, which is barely an adequate amount of energy to foam a cappuccino. Favorably, recognition of their embryonic potential is held not only in Statkraft’s hands, but also those of the Norwegian Center for Renewable Energy, a group who clinches the global potential of osmotic power to be about 1,370 terawatt-hours per year (approximately equivalent to the current electric consumption of 520 million people). Independent experts as well as everyday observers note the clean, carbon-free process. Questions do remain, concerning future large-scale operations and their brackish byproducts, and how their pretreatment processes might impact local marine organisms.
Statkraft estimates that they spent more than ten years and the equivalent of over $12 million in developmental research funding for one osmotic technique: Pressure Retarded Osmosis (PRO), in the prototype facility at Tofte.
Osmosis is a broad term to use when describing the process by which Statkraft harvests their “salinity gradient” power. Osmosis is simply the tendency of solvent molecules to diffuse through a semipermeable membrane from an area of low solute concentration to an area of higher solute concentration. When applied to the salt-water energy pant, this process involves the mixing of saline water with fresh water.
When the river meets the sea, the diffusion of salt water molecules into fresh water begins, and enormous amounts of energy are released. The energy procured through this diffusion can be utilized for the generation of power by strategically placing a semipermeable membrane barrier between the salt and fresh waters.
Reverse osmosis is a water purification process that has been used for decades. American chemical engineer, Sidney Loeb, was the first to coin a practical reverse osmosis process in the 1950s, and later ensued a technique for capturing the energy of the rush of salt water to the fresh water side of the membrane.
The dilemma that poses as most vital is that there is not enough work happening to develop inexpensive membranes tailored for the process, which in turn would allow more hypothetical stimulations that could later permit vast usage of the saline osmosis energy project. This major obstacle reduces the likelihood of a full scale osmotic power plant for now, until efficient membranes designed to optimize the process are developed and readily available.
Fortunately, interest in renewable energy sources has become somewhat of an international hot topic. NASA has been pursuing osmotic systems for wastewater treatment abroad spacecraft, and is now investigating the PRO method with tertiary treatment, with the goal to develop a system that can purify water and obtain energy simultaneously.
Hydro-Québec, the most extensive electricity generator in Canada and the largest producer of hydroelectric power in the world, is partnering with Statkraft on next-stage augmentation of PRO technology. The two operatives are musing the feasibility of osmotic energy along the coast of Canada.
The Tokyo Institute of Technology in Japan opened its own Osmotic Power Research Centre in 2010, the year before a devastating earthquake and tsunami crippled the Fukushima Daiichi nuclear plant and led to reconsideration of the country’s energy-future. Akihiko Tanioka, a researcher leading the osmotic effort there, persists that the flow volume of Japan’s rivers contain the potential energy capacity to replace five or six nuclear reactors if osmotic plants were established where rivers deluge into the sea.
Not only has this process gained replicative attention, it also sparked new research and development that might veer in an alternative direction. Researchers in the Netherlands are cultivating an alternative method to PRO, that is, Reverse Electrodialysis, or RED, which is essentially creating a natural battery.
The RED approach, osmotic energy of mingling fresh and salt water is seized by directing the solution through an oscillating series of positively and negatively charged exchange membranes. The result is a chemical potential difference that creates a voltage over each membrane which leads to the production of direct electric energy.
Though the RED technique has racked up far less hours for development, it may eventually surpass the practicality of the PRO method, because of its lower initial cost structure which does not include the complex machinery the PRO method does (chambers, turbines and generators). Rather, RED technology produces electricity directly from the difference in fresh and salt water. Infrastructure for the process is what is holding back the expansion of osmotic power for everyday use. Statkraft estimated that a PRO plant with the ability to supply energy for 30,000 homes would be the size of a sports stadium and require 5 million square meters of membrane. Another challenge would be the development and maintenance of a primary filtration system for incoming water, so as to prevent fouling of the membranes.
Ideally, the development of osmotic power will follow a curve similar to those of other green energy sources that will allow it to mature and become globally useful. For now, osmotic power plants are in their dormant/developmental stages, and we will have to wait and see if they can jump their challenges and hurdles.