Hydropower

By the power of Oz: Osmotic generation technology

Issue 1 and Volume 11.

Norwegian utility company Statkraft has announced that it is to build the world’s first osmotic power plant. David Appleyard reports.

When a river runs into the ocean and fresh water mixes with sea water, huge amounts of energy are unleashed. However, when this process of mixing fresh water and sea water is carried out by controlling the pressure on the saltwater side, this energy can potentially be tapped.

The process is called pressure-retarded osmosis (PRO) and now, the world’s first prototype osmotic power plant that utilizes this novel renewable energy resource is to be constructed in Norway by Statkraft.

Based on the natural process of osmosis, in which water travels across a salt concentration gradient, in an osmotic power plant sea water and fresh water are separated by a semipermeable membrane. The sea water draws the fresh water through the membrane, diluting the sea water but increasing the pressure on the sea water side. The osmotic process increases the volumetric flow of high pressure water and is the key energy transfer in the plant. If the saltwater compartment has a fixed volume, the pressure will increase towards a theoretical maximum of 26 bar, a pressure equivalent to a 270 metre head.

First discovered by Sidney Loeb in 1973, PRO is used to produce power through a turbine or similar device. The pressure-retarded osmosis power plant is similar to a reverse osmosis desalination plant running backwards. However, instead of consuming power, the PRO plant generates power from fresh water. The global technical potential for osmotic power production is estimated at around 1600 TWh, including around 200 TWh in Europe and 12 TWh in Norway alone, some 10% of the country’s current power production.

Following Loeb’s discovery no particular progress was made during the 1970s and 1980s due to inefficient membranes. However, during the 1980s and 1990s a breakthrough was made regarding membranes for RO, and the membrane technology was successfully introduced into many industrial applications. Now, after ten years of research and development, including building the world’s first pilot plant for PRO, which began operation in June 2003, Statkraft believes the technology is at the prototype stage and is therefore stepping up its initiatives and investments to develop the technology further. This also follows a Salinity Power project (1999-2004) financed by the European Commission which resulted in the design and production of a semi-permeable membrane optimized for PRO. While all the acquired technology is currently in use in the water treatment industry, Statkraft has focused its efforts on membrane development and has achieved an increase in power generation from less than 0.1 W/m2 to almost 3 W/m2. According to the company, the osmotic process requires a membrane that has a high water flux and high salt retention, with commercial operation requiring a membrane performance of 5 W/m2.

WE’RE NOT IN KANSAS ANY MORE

The prototype plant will be built at the paper pulp manufacturer Soedra Cell Tofte’s plant at Hurum in Buskerud, Norway. The location will provide the osmotic plant with a good supply of fresh water and sea water, along with access to established infrastructure.The construction of the prototype is expected to be completed by the end of 2008 and the plant will produce between 2 and 4 kW.

It is hoped that the prototype, a necessary platform for the further development of the technology, will provide Statkraft with a better understanding of the challenges involved.

The decision brings the total invested in the research work to more than NOK100 million (US$18 million) – the work has also been supported by The Research Council of Norway.

Several plant designs have been developed for PRO power generation and the illustration (above) shows a typical plant located at sea level. Fresh water is taken from a river close to its outlet, fed into the plant and filtered before entering the membrane modules containing spiral wound or hollow fibre membranes. Sea water, volumetrically about twice that of the freshwater, is fed into the plant by underground pipes. In the membrane module, 80%-90% of the fresh water is transferred by osmosis across the membrane into the pressurized seawater. The brackish water from the membrane module is split into two flows.About one third of the water goes to the turbine to generate power while two thirds return to the pressure exchanger to pressurize the seawater feed.To optimize the power plant, the typical operating pressure is in the range of 11-15 bars.After use the diluted water is pumped back into the estuary, maintaining the flow of water in the river.

Some pre-treatment of the water is necessary, although experience from Norwegian water treatment plants shows that mechanical filtration down to 50 micrometres in combination with a standard cleaning and maintenance cycle is enough to sustain the membrane performance for 7-10 years. Similar lifetime data are assumed for osmotic power plants.

THERE’S NO PLACE LIKE HOME

A detailed survey of the environmental aspects related to construction and operation of an osmotic power plant has been made and, even by renewable energy standards, osmotic power appears low impact.The mixing of sea water and fresh water is a process that naturally occurs in all estuaries, and because cities are often located on river mouths, most of the osmotic power potential can be utilized in high demand areas. Such power plants can also be constructed underground, allowing them to be discreetly located in the local environment, and the water management processes can be designed without affecting the biotopes of the river, estuary and sea. In addition, osmotic power plants are expected to require significantly less area per unit energy when compared to other renewable energy sources, says Statkraft.

‘We take the task of providing pure energy seriously, and osmotic power is a very promising technology. It is clean and emission-free, and could become competitive within a few years,’ remarked Statkraft’s CEO, Bard Mikkelsen, referring to cost estimates made by Statkraft which show that osmotic power would be competitive at today’s energy price level.

With novel technologies such as this on the horizon, today’s glass of water may yet prove to be half full.

David Appleyard is Associate Editor of Renewable Energy World e-mail: [email protected]