Statkraft currently generates enough osmotic power at its prototype power plant in Tofte, the world’s first, to barely run an electric kettle. The estimated technical potential of osmotic power globally is similar to half of Europe’s annual electricity consumption.
A key milestone in osmotic power technology dates back to the 1960s in the US with Sidney Loeb. He created a membrane technology that could be used to make freshwater out of saltwater, also known as desalination. However, he quickly saw the potential of utilizing the energy created from the reverse process when freshwater mixes with saltwater from the sea.
“Kennedy realized producing affordable drinking water from seawater was one of the main challenges in the new age ahead,” said Stein Erik Skilhagen, Statkraft head of osmotic power. “Early on (Loeb) saw it as a reverse way to make energy.”
The technology is based on exploiting the difference in entropy. When freshwater in a river meets seawater, there is a difference in salinity. Nature wants to equalize the concentration and energy is then released. The temperature increase due to the change in entropy is very small, only about 0.1 degrees Celsius. But this energy can also be exploited for generating electricity.
Loeb patented pressure retarded osmosis (PRO) technology, which uses a plastic membrane in between freshwater and seawater. The salt in the seawater draws freshwater through the membrane, causing the pressure on the seawater side to increase. This pressure corresponds to a water column of 120 metres, or a large waterfall, that can be utilized in a turbine to generate electricity.
|The world’s first osmotic power prototype is situated at Tofte, one hour south of Oslo in Norway.
© Damian Heinisch/Statkraft
However, the technology was not cost effective at the time, said Skilhagen. Energy prices were low and the technology immature. During the 1980s, membrane efficiency improved more than tenfold and prices decreased, but membranes that worked for desalination, did not work for pressure retarded osmosis (PRO).
“We thought it didn’t make a difference, but it made a big difference,” said Skilhagen.
A breakthrough came in 1996 when Statkraft read an article on osmotic power by Trondheim-based SINTEF, the largest independent research organization in Scandinavia. Statkraft decided to contact SINTEF and the two formed a research partnership on membrane technology for osmotic power. Statkraft has also worked together with many international research partners in Portugal, Finland, Germany, the Netherlands and the US to develop pressure retarded osmosis.
The culmination of this technology came in November 2009, when Statkraft opened the world’s first prototype osmotic power plant in Tofte, one hour outside Oslo. Currently, the plant uses more energy than it produces. However the technology verified here could be used towards building the first full-scale osmotic power plant by 2017, possibly the world’s first. Such a plant could provide 25 MW of energy, similar to about 10 windmills and enough to power 7,000 homes in Norway and three times as many in the UK.
“We need to develop more of our mature technology like hydropower and onshore wind power, and we have to develop more immature technology, which this osmotic power plant is an example of,” said Terje Riis-Johansen, Norway’s petroleum and energy minister, at the opening ceremony in Tofte. “Tomorrow, salt will be a part of the solution. The potential is huge: 12 TWh in Norway and 1600-1700 TWh worldwide.”
|Terje Riis-Johansen, Crown Princess Mette Marit and Statkraft chief executive Bård Mikkelsen share a cup of tea cooked with osmotic power.
© Kim Laland/StatoilHydro
There are still many challenges with the technology. An average osmotic power plant would require 5 million square metres of membrane, equivalent to 10% of annual membrane production today, according to Skilhagen. In addition, the installation would be quite expensive due to the large amounts of water that need to be handled. Other mature renewable energy sources, such as hydropower, are more cost competitive at the moment.
Statkraft has invested more than NOK 100 million towards the development of osmotic power. Its goal is to produce 5 watts of electricity per square metre of membrane. To do that, it needs to increase the membrane element size and reduce the energy loss. It is working with several international research partners in the US, such as the University of Kentucky and Sante Fe Science and Technology, and in Europe with the GKSS Research Centre and Desalogics in Germany, among others.
Although it is cost prohibitive at the moment, Statkraft believes the technology could be competitive by 2030. By then, osmotic power should have a levelised cost of energy (LCOE) of EUR 50-100 per MWh, making it more affordable than solar or offshore wind power. Plus, being produced from running rivers, it will be a based load and good contribution to the energy mix with the non-flexible energy sources, such as solar and wind.
“In 2030, we believe a certain number of plants can already have been built,” said Skilhagen. “A technical potential of 1,600-1,700 TWh of power produced per year is technically feasible on a global perspective. That is similar to half of the electricity consumed in Europe annually. The European potential is estimated to 180 TWh, which is similar to one third of the necessary increase in renewables in Europe to reach the (EU’s) 2020 goal of 600 new TWh.”
Another part of the challenge is to prepare a supply chain from membrane manufacturers, mostly located in the US and Japan, and to establish a predictable customer base for the different technology suppliers. The desalination market for membranes, by comparison, currently grows about 30% per year.
Its other goal is to push governments to realize the potential of osmotic power as a renewable energy source. Statkraft has so far been successful in lobbying the EU to recognize salinity gradients along with other sources of ocean energy in the EU’s Renewable Energy Sources Directive.