Power from osmosis

Wherever saltwater and freshwater collide, energy is there for the taking. We take a look at Statkraft’s efforts to harness this new energy source.

In the push towards renewable methods of power generation, a whole array of different options are being investigated. Although humanity has been using the movement of water for centuries to generate mechanical power via the watermill and more recently, electricity via hydropower, there is another option which has vast potential waiting to be tapped.

Wherever a river meets the sea, the collision of freshwater and saltwater could one day be used to generate electricity, through the process of osmosis (see overleaf). Norwegian company Statkraft has calculated that worldwide, there is the potential to generate 1600-1700TWh of electricity through this method, which is equivalent to China’s entire power consumption in 2002. The unrealised resource available is thought to be approximately 180TWh for Europe as a whole, while Norway’s resource is a more modest 12TWh, or 10 per cent of the country’s annual power consumption. The technology is particularly interesting for Scandinavia, Russia and Canada in the northern hemisphere due to their extensive water resources, along with Africa and South America. Importantly from an environmental perspective, osmotic power plants lack any significant environmental impact in-situ in contrast to conventional hydropower. They are essentially noiseless, produce no emissions and can be integrated into existing industrial zones, for example, in the basements of industrial buildings, thereby minimising their visual impact. Of obvious interest to power utilities is the fact that output from an osmotic power plant does not suffer from the problem of intermittency which affects other green methods of electricity generation such as wind and solar.

The original concept was first discovered in the 1970s by Sidney Loeb, who pioneered the use of membranes for the desalination of seawater. Theorectical work was then performed by Torleif Holt and Thor Thorsen at SINTEF during the 1980s, but it wasn’t until 1995 that they were able to obtain the finance for a feasibility project. A watershed moment occured when a report by Thorsen became widely referenced in the scientific community, alerting Statkraft to the potential of this new and exciting technology. A collaboration project between Holt, Thorsen and Statkraft was formed in 1997 and in 2001, it received EU funding, boosting the pace of research. In the same year, they performed their first environmental study. The latest milestone in terms of research efforts was the granting of Statkraft’s first patent for the osmotic power membranes in 2003 and the opening of its test facility at Sunndalsøra.

Working principles

The general working principles of an osmotic power plant are as follows: freshwater and seawater are fed into two chambers separated by a semi-permeable membrane, after being filtered to remove any particulates that could block the membranes. Water flows along the concentration gradient, causing the pressure on the seawater side to increase, until it reaches a level corresponding to a water column 120m in height.The membrane system in Statkraft’s prototype is in the form of a series of spiral coils. The pressure is used to force water through a turbine, generating electricity in the process.

Towards a prototype

For its prototype, Statkraft has leased premises from the paper pulp factory Södra Cell Tofte in the municipality of Hurum by the Oslo fjord. The location gives it access to plentiful supplies of both fresh water and salt water. Although Statkraft is responsible for the bulk of the financing, the project is also receiving financial assistance from Enova and the Research Council of Norway. Operation of the plant will be shared by Statkraft and Sintef.

The plant was officially opened on Tuesday, November 24, 2009 by Her Royal Highness Crown Princess Mette-Marit of Norway. “In an era of major climate change and an increasing need for clean energy, we are proud to be presenting a renewable energy source which has never been harnessed until now. We are also most grateful that the Crown Princess wishes to lend her support to this milestone in our development of osmotic power,” says Statkraft CEO, Bård Mikkelsen.

Figure 1: Schematic of an osmotic power plant

The prototype is made up of membranes, pipes, a cleaning unit, pressure exchangers and a turbine. The plant is modular in design and consists of 66 pressure pipes with rolled-up membranes on the inside. The advantage of a modular design is that parts of the facility can be taken out of operation for cleaning and maintenance while the rest of the facility remains in operation. In addition, once the technology has matured, a modular approach enables individual plants to be easily scaled up or down, depending on available resources. The prototype holds a total of 2000m² of membranes, which will initially have an efficiency of less than 1W/m², but Statkraft plans to install membranes with efficiencies of 2-3W/m² once the plant has been in operation for a while, with the goal of eventually reaching an efficiency of 5W/m².

The prototype plant will initially generate 2-4kW of electricity, enough to operate a coffee machine, but this will be increased to its design capacity of 10kW. Despite this modest size, the prototype will still be connected to the Hurum Energiverk power grid. The plant is expected to operate for two to three years, after which it will be replaced by a 1-2MW pilot facility. Assuming the smaller figure, this would require mixing 1m³ of freshwater with 2m³ of seawater per second.

Osmotic future

Although Statkraft has managed to increase the efficiency of its membranes many times over since it started researching osmotic power back in 1997, there is still a long way to go before the technology is mature enough to be a competitive source of renewable power. Aside from the goal of boosting membrane efficiency to 5W/m², much work needs to be done on reducing the energy losses incurred when transferring pressure to the turbine. Despite these considerable hurdles, Statkraft remains confident that it can commercialise the technology and has set itself the aim of commissioning a full-scale osmotic power plant by 2015.

Increasingly active

Statkraft is becoming increasingly active on the world stage, and its joint venture, SN Power, has recently signed an agreement with India’s Tata Power to develop hydropower projects in India and Nepal. It believes there is plenty of power for further hydro and wind power projects closer to home and continued an ambitious NOK90m (US$16.1m) renewable investment plan despite the financial crisis of 2008 and the ensuing recession. Statkraft is also looking to tap into the carbon trading market. It has already installed trading desks in Santiago, Chile, and Singapore and is hoping to open another in the US over the course of 2010.

For more information, consider visiting the following websites:
www.statkraft.com
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html/