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Statkraft's Osmotic Power Facility At Tofte, Norway

The concept [ of Osmotic Power ] has been little explored since the 1970s — until now. Part of the reason, in addition to a growing push for renewable energy, has to do with membrane technology.

Stein Erik Skilhagan
Vice President, Osmotic Power
Statkraft

Oslo, Norway

Redefining The Meaning Of Sea Power

The physics have been understood for nearly a century. The concept of capturing the energy potential of salt water  by means of osmosis has been known since the 1970s. But doing so, at least commercially, has never been practical, or even remotely economical. Until now — for a small pilot project demonstrating the potential of osmotic power has been completed near Tofte, Norway that seems destined to forever change how the world thinks about generating electricity.

Scientists have understood osmotics for nearly 60 years, so some wonder what’s so new about Statkraft’s technology that warranted building a demonstration plant. The answer lies, not in the scientific principals, but in a semi-permeable membrane that easily passes fresh water but blocks salt water. Thus, Statkraft’s power generation technology is made feasible by their highly refined membrane technology and pre-filtering systems — both of which have immense potential for future large scale applications.

Now that the pilot plant is operational, and the economics of scale revealed, the potential for widespread application seems limited only by the number of locales where both fresh and salt water are found in abundance. While the economics of large scale use are not entirely clear, the benefits of a baseload generating facility absent either fossil fuels or gaseous discharges carries immense ecologic benefits.

The Pilot Project

Stein Erik Skilhagen, Statkraft vp Osmotic Power

Osmotic Schematic

Statkraft Osmotic Power Plant At Tofte, Norway.

Membrane Separated Osmosis Cylinders

Pressure Exchangers

Pressure Containment Vessels

Pressure Driven Turbine

Statkraft, a Norwegian company developing renewable energy projects, has constructed a small scale osmotic  generating plant that generates electricity without using fossil fuels, or creating air pollution. Statkraft’s $8 million pilot project generates about 4 kilowatts of electricity — enough to power an electric oven.

According to Stein Erik Skilhagan, vp for osmotic power at Statkraft, the Tofte  ( one hour south of Oslo ) facility demonstrates more than the feasibility of osmotic power, for unlike any other renewable energy technology, osmotic power can deliver baseload energy, that is electricity available continuously around the clock, 365 days a year without interruption.

Skilhagen claims the proximity of fresh water, in rivers, to salt water in the sea, could theoretically provide half or more of Europe’s electrical energy demand in the future. Not only would osmotic power be cleaner, he suggests, but at substantially less cost. The company’s vision on the economics and practicality of osmotic power generation in Europe is garnering wide interest both in Europe and the United States.

The Science

Here’s how Statkraft CEO, Bård Mikkelsen describes the  underlying science on the company website:

The energy is based on the natural phenomenon osmosis, defined as being the transport of water through a semi-permeable membrane. This is how plants can absorb moisture through their leaves – and retain it. When fresh water meets salt water, for instance where a river runs into the sea, enormous amounts of energy are released. This energy can be utilized for the generation of power through osmosis.

At the osmotic power plant, fresh water and salt water are guided into separate chambers, divided by an artificial membrane. The salt molecules in the sea water pulls the freshwater through the membrane, increasing the pressure on the sea water side. The pressure equals a 120 metre water column, or a significant waterfall, and be utilized in a power generating turbine.

Power Production Model

Statkraft’s osmotic power generation system channels freshwater and saltwater through filters that eliminate contaminants and solids that might damage equipment or clog piping or the osmotic membrane.  Both streams are channeled into long osmotic cylinders in which the two liquids remain entirely separate on opposite sides of a precision made polymer membrane.

When pressures are similar on both sides of the membrane the salt molecules in the ocean water stream attract fresh water through the separating membrane from the river stream water. The migration of fresh water from the river stream to the ocean stream reduces pressure on the river stream due to the departing fresh water molecules.

When the river stream molecules reach the ocean water stream they are instantly diffused with salt. Increasing numbers of river stream molecules add volume to the ocean water stream which produces an increase in pressure.

Statkraft’s Skilhagen says the resulting pressure differential between the two streams is equivalent to a water column of 120 meters or, roughly the equivalent standpipe pressure found in some hydroelectric generating systems.

The pressure differential provides the energy required to run a turbine driving an electric generator. The flow capacity, measured in terms of gallons/minute at the required pressure differential, determines the power generating capacity. For the demonstration plant at Tofte, energy production is limited largely by the amount of water delivered to the turbine(s).

For very large capacity plants, the water flow would approach that observed at hydroelectric facilities. But, unlike the hydro plants, which return only one stream, osmotic power plants return two separate streams from the osmosis cylinders. One, the slightly lower salt ( high pressure ) stream flows back to the ocean while the fresh water source can be partially recirculated with incoming river water.

Harley Blank contributed to this report.