Salt: Bad For Your Diet, Good For Renewable Energy

While the world is busy marveling over solar arrays, wind farms and giant geothermal installations, something rather more clever is going on in northern Europe. A power plant opened in late 2009 in the village of Tofte in Hurum, Norway, about 40 miles south of Oslo. The plant, run by the Norwegian state-owned electric company, Statkraft, is the world’s first osmotic power plant.


For those of us who once again slept through high school physics, the power plant runs on the Physics 101 science of osmosis, which governs the movement of solvent molecules through a semi-permeable membrane in order to equalize concentrations of the solvent. In the simplest example, if fresh water and salt water meet through a membrane through which molecules can pass, the differences in salt concentration draws the fresh water (called the “hypotonic solution”) through the membrane and into the salt water chamber (the “hypertonic solution”) with no additional input energy needed (and therefore no fuel costs) in order to equalize the salt concentrations in both samples of water. This creates pressure on the salt side of the membrane.

Osmosis, first observed by a clever French monk named Jean-Antoine Nollet in 1748, is why you get wrinkly fingers and toes after a long soak in a bath tub or a pool. Monsieur Nollet, however, probably never dreamed that the process could produce clean, renewable electricity on a large scale.

The Statkraft plant generates osmotic pressure power (also referred to as salinity gradient power) by introducing fresh water and salt

Statkraft Osmotic Power Prototype at Tofte (Hurum), Norway

water, bringing the two together with nothing but a semi-permeable membrane separating them. The salt water pulls the fresh water through the membrane. Pressure increases in the salt water side, and that resulting pressure can be harnessed to run a turbine, which in turn produces electricity. It’s the first-ever renewable power source that relies entirely on natural forces.

The Stratkraft plant is a prototype – the first of its kind – developed together with Norwegian technology research group Sintef. It uses 10 liters of fresh water and 20 liters of salt water each second. While the prototype is running as an experiment, it’s only producing about two to four kilowatts of electricity: about enough to power a coffee pot. The plan is to test and simulate extensively in an attempt to determine ways to keep production costs down when the plant is ramped up to full-scale power production.

The goal of the test project is to improve the efficiency of the membrane from its current one watt per square meter of membrane to about five watts per square meter. (The company has developed a membrane that will produce two to three watts per square meter once installed, but it’s finishing system checks with the current membrane). The plant, as it stands now, uses about 2,000 square meters of membrane.

Once they achieve this greater efficiency of five watts per square meter, Statkraft says would make osmotic power costs comparable to those from other renewable sources. Development of the membrane has happened in conjunction with NASA: the U.S. space agency is interested in the technology as a way of filtering and recycling water for astronauts on extended missions.

Norwegian researchers believe that osmotic power, or “salinity power” could eventually provide about 10 per cent of Norway’s power needs, or around 12 terawatt-hours of electricity per year, according to Gizmag.

“While salt might not save the world alone, we believe osmotic power will be an interesting part of the renewable energy mix of the future,” said Statkraft Chief Executive Baard Mikkelsen at the time of the plant’s opening. The company knows of what it speaks: it draws more energy from renewable sources – primarily hydropower right now – than any other utility company in Europe, which means it has stiff competition.

Though the Stratkraft plant is currently operating largely for research purposes, it is expected to make a commercial debut by 2015, at which time Statkraft will begin construction of more osmotic power plants. Future full-scale plants could produce about 25 MW of electricity each: enough to provide power for to 30,000 households. The plants would be about the size of a football stadium and would require about five million square metres of membrane, according to Statkraft.

In the meantime, a group in the Netherlands called Wetsus, or the Centre for Sustainable Water Technology, is also racing to bring osmotic power to commercial viability. Unlike the Stratkraft project, which is using simple osmosis (salt water and fresh water meeting through a membrane) or “pressure retarded osmosis” (PRO), the Wetsus project is using a process called reverse electrodialysis (RED). Though the process uses membranes, like PRO, the membranes are impermeable to water. They do allow, however, for ions to pass through them. Positively charged sodium ions from sea water pass through one membrane, while negatively charged chloride ions (which also occur in seawater) move through the other membrane. The resulting reaction can then be converted into electrical energy and channeled into a special kind of “salt battery.” Wetsus refers to the process as “blue energy,” and they think they can ultimately create enough electrical energy – over a gigawatt – to power about 650,000 homes across the Netherlands.

Not to be outdone by the Europeans, the technology has caught the eye of IBM and its Almaden Laboratory, the company’s pure research arm. IBM, which had already developed some advanced membrane materials for the purpose of water desalination, has spotted the potential for the materials for the purpose of osmotic pressure energy generation, as well, and is said to be tinkering with the technology.

Thus far, one of the only major issues holding osmotic power back is limitations with the membranes used in the processes. Membranes, even as their efficiency improves, remain prone to “biofouling,” or collecting silt and aquatic plants that impede the flow of molecules or ions (depending on the process) through the membranes. Many research groups are currently working to build a better membrane, or at least come up with a process that will regularly and effectively clear the membranes of debris: no small feat when you’re looking at five million square meters of membrane per power plant.

The reason osmotic pressure power is so incredibly “green” is that it requires no energy input, and the only by-product of the process is brackish water: that is, water with a low salination level. Of course, no claims can be made that the emission of brackish water couldn’t harm an environment: changing the salinity of a body of water could absolutely cause irreparable damage to the plant life and wildlife that occupies it, not to mention humans living nearby.

For this reason, researchers have proposed building osmotic pressure plants in places where there is an abundance of naturally occurring fresh and salt water, and large existing emissions of brackish water: anywhere a large river runs into the sea. The Wetsus group has their eye on a spot in the Netherlands near Rotterdam where the Rhine river flows into the North Sea, providing all the natural resources the process needs: fresh water and salt water flowing together. The Statkraft project is located in Norway, the land of many fjords that empty into the sea, and therefore also ideal for salt energy projects.

While the Statkraft and Westus projects are looking to produce energy at a grand scale, other researchers are looking at “salt power” as a way to provide for more localized energy needs. One method is by the use of something called a salinity gradient solar pond (SGSP). Essentially, it’s a big reservoir of salt water that collects and stores solar energy on sunny days. The solar pond is made up of three layers: an upper convection zone, a gradient zone, and the thermal zone, or the bottom layer where the solar heat is stored. While heat would normally escape from most ponds of water via convection – just as it escapes from your coffee cup if you leave it too long – the layers of three different salt concentrations (the salt content increases with the water’s depth) actually trap the heat at the bottom of the pond. The hot water can’t rise and convect because it’s saltier, and therefore heavier, than the water in the layers above it. Essentially, the less salty water on top of the pool acts as an insulator for the hot salty water at the bottom of the pool. An industrial site with a gradient solar pond could tap this hot water as energy by pumping the solar heated salt water from the bottom of the pond into a heat exchanger or an evaporator, removing the heat and turning it into power for industrial use. It’s a way of storing and using solar heat without the need for batteries.

The first salinity gradient solar pond installation in the U.S. was built by Bruce Foods Corporation in El Paso, Texas. The company operates a 0.8-acre solar pond that provides about 20 percent of the energy required by the company for operations. Israel has had some commercial success with large solar ponds, as well: the country actually contains a large number of naturally occurring salinity gradient solar ponds.

While the electrical conversion efficiencies are low with solar ponds, if the body of water is large enough, it can provide a significant amount of energy: enough to powerfully offset the energy use of a large organization, for example, or a small village in a rural part of the world (though not a part of the world where fresh water is scarce: solar ponds require constant replenishing of the top fresh water layer, since much is lost through evaporation). Some have suggested that this drawback can be overcome if the energy from the solar pond is used for the purpose of desalination. With a large source of seawater nearby, the pond can be used to create fresh water out of salt water, which would allow it to replenish its top layer of fresh water while at the same time providing clean drinking water for a village.

Obviously, these processes – osmotic and saline/thermal – have their drawbacks and they aren’t going to be able to power the entire earth with renewable energy. Osmotic power is only really practicable in parts of the world that have coastlines and abundant rivers. Some researchers estimate that salinity power may only ever be able to supply one percent of the planet’s energy needs. Rather than a global solution, the technology will only be one tool in the great global toolbox of clean energy solutions.

And your doctor told you salt was bad for you.

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