Worth its salt: Norway trials osmotic power

16 August 2010 | By Stuart Nathan

Back in the early days of experimental science in pre-Revolution France, a priest-turned-physicist called Jean-Antoine Nollet performed a very odd, if not perverse, experiment. For reasons that must surely have baffled anyone visiting his laboratory, he filled a pig’s bladder with wine and then dunked it into a barrel of water. He was surprised, as anyone would have been, when the submerged bladder swelled up and finally burst.

Nollet would surely have been castigated for wasting perfectly good wine, but he’d discovered the phenomenon of osmotic pressure: the tendency for the concentration of two solutions, separated by a permeable membrane, to equalise. Basically, fresh water had diffused through the walls of the bladder to dilute the wine inside.

Unlikely as it may seem, this phenomenon is currently being investigated as a new form of marine power. Water-based energy generation conjures up pictures of towering hydro-electric dams, submerged tidal turbines and bobbing wave-energy converters. But while the energy embodied in moving water – which all of these technologies exploit – is one way to make electricity from the sea, it isn’t the only one. In Scandinavia, a pilot power station is demonstrating that another of the sea’s defining characteristics, saltiness, could also be harnessed to provide electricity.

The plant, built and operated by Norwegian energy company Statkraft not far from Oslo, employs a technique known as osmotic power generation, which uses the difference in salinity between seawater and fresh water to drive an electricity-generating turbine. First discovered in the 1970s, osmotic power was considered an interesting but impractical phenomenon for commercial energy production for many years, but increasing energy prices and advances in materials are now making it a technology worth watching.

Osmotic power is a blanket term for two techniques: reverse electrodialysis (RED) and pressure-retarded osmosis (PRO). Both rely on harnessing the energy dissipated when fresh water flows into salt water. The other factor they have in common is the use of membranes, but here the techniques differ significantly.

RED uses two types of membrane to create what is essentially a battery, with the differences between ion concentration between seawater and river water setting up a small electrical voltage across each membrane. Stacking the cells up with alternating cathode and anode membranes – that is, membranes that only let positively or negatively charged ions through – creates a system capable of generating reasonable voltages. A stack the size of a shipping container could generate 250kW.

Small-scale studies of RED are currently under way in the Netherlands, where the meeting of seawater and fresh water has been a fact of life for centuries. Energy consultancy KEMA is leading research into preventing the biofouling of membranes using a 50kW installation, with plans to scale up in subsequent projects.

PRO, meanwhile, uses a form of osmotic generation more closely related to Nollet’s original experiments. Seawater and fresh water enter the system through separate pipes, filtered along the way to remove sediments and other particles. They are fed into opposite sides of a chamber separated by a membrane that is permeable to water but not to the dissolved salt in the seawater.

In accordance with the laws of osmosis discovered by Nollet, the system tries to equalise the concentration of salt, and the only way this can happen is by fresh water diffusing through the membrane into the salty side. As this happens, pressure builds up on the seawater side. This pressure is relieved by letting water escape, routing it through a turbine. The result is brackish water – saltier than seawater but less salty than river water – discharged back to the sea and a supply of carbon-free electricity.

Anything that sounds too good to be true generally is, and, of course, there are drawbacks. Notably, the membranes that power this process have always been far too expensive to make commercial-scale power plants a practical proposition. Torleif Holt and Thor Thorsen, working for Norwegian research organisation SINTEF, began looking into this problem during the 1980s and became involved with Statkraft in 1997, aiming to develop lower-cost membranes based on electrically modified polyethylene. Funding from the European Union opened up in 2001 and finally – with a contribution from NASA (see panel) – put the technology into the realms of possibility.

The Statkraft plant uses a polyimide membrane capable of generating a watt per square metre of membrane, which requires a flow of 10 litres of water per second flowing through the membrane at a pressure of 10bar. The capacity of the plant is tiny: just 4kW. Yet, according to Statkraft, the potential for osmotic power is huge. The company claims that 1,600-1,700TWh of electricity could be generated globally each year. ’In Norway alone, we will be able to generate 12TWh per year, equivalent to 10 per cent of our total power consumption,’ it said.

These figures might seem fanciful, until you consider what historically happens where big rivers meet the sea: people tend to build ports and large cities grow up around them. Estuaries and river mouths are very often the site of major industrial and urban development, creating demand for power in the very place that osmotic power could provide a supply. ’A power plant the size of a football stadium could supply 15,000 households with electricity,’ said Statkraft, adding that such stations could be built underneath parks or municipal buildings, reducing their visual impact.

The cost of the membranes is, although reduced, still significant, especially if the process is to be scaled up; the pilot plant has 2,000m2 of membrane and a full-scale 25MW plant would need five million square metres, according to Stein Eric Skillhagen, Statkraft’s head of osmotic power.

Statkraft is continuing to develop the technology. ’Only once membranes with a particular efficiency have been developed will osmotic power become a competitive source of renewable energy,’ said the company, which has the ambitious aim of developing a high-performance, low-cost membrane to enable commercial-scale osmotic power by 2015. Its performance target is 5W/m2 of membrane – five times the performance of the pilot plant.

To this end, Statkraft is investigating membranes and is also testing samples developed by other organisations, including cellulose-based materials and biomimetic, protein-based substances. The company recently went on a ’membrane-hunting expedition’ to the US, organising an osmotic power membrane conference in San Diego along with the American Membrane Technology Association. ’We will meet players such as General Electric, Hydranautics, Dow and Toray,’ said Skillhagen. ’It is very important to engage in a dialogue with these players and it is also important for us to demonstrate that we are at the forefront of development of osmotic power.’

So far, he added, Statkraft’s membranes are yet to exceed 3W/m2 in laboratory tests. However, the company is continuing trials, with membrane manufacturers looking at a potential new source of high demand for their products.

There is a pleasing historical circularity to these events. Back in 18th-century Paris, experiments with pigs’ bladders and wine were something of a sideline for Nollet. He was best known, and is best remembered, as a pioneer in the research of electricity. He is especially known for discovering how an electric shock could travel along a line of people holding hands, leading to the understanding of electric current. This particular shock could travel even further.