Suspension bridge

Osmosis

Osmotic power is the latest in a long line of renewable possibilities, and one company wants to clean up.

Harnessing nature's power

When a river runs into the ocean and the fresh water mixes with the salt water, huge amounts of energy are unleashed. Unlike violent torrents in a waterfall or steaming hot geysers, the energy released when mixing water with different salinity cannot easily be seen from the banks of the estuary. Nevertheless, the energy is there and everyone who has tried to separate salt from sea water knows that large amounts of energy are needed.

"It is easiest to compare it with the process in nature - osmosis is a natural process," says Stein Erik Skilhagen, project manager innovation for New Energy at Statkraft AS.

"If you have two solutions that have different contents of, for example, salt, which we use here, they will always tend to mix with each other. If you separate the two liquids with a membrane which is only open for transport of water and not for salt then you will have a transport of the fresh water into the compartment where you have the salt.

"This is the same process that a tree uses to drink water,' Skilhagen says. "It uses sugar in the leaves and when you have an increase of sugar inside the tree, which is separated from its surroundings, which you have with a membrane in the roots. Then you will transfer water through the roots to the sugar in the trees. Even though the sugar could be in the top of a tree that is 100m high, the water will still be transported because it wants to dilute this sugar." This is how the process works in nature.

Statkraft has developed pressure-retarded osmosis (PRO) from an academic idea to a new, environmentally friendly power technology concept. When placing a semi-permeable membrane (i.e. a membrane that retains the salt ions, but allows water through) between reservoirs containing fresh water and sea water respectively, a net flow of water towards the salt water side will be observed. If the salt water compartment has a fixed volume the pressure will increase towards a theoretical maximum of 26 bars. This pressure is equivalent to a column of water 270m high.

The pro process

The energy from the available pressurised water can be used to generate environmentally friendly renewable energy. This is if the mixing can be carried out by controlling the pressure on the salt water side. The process is called pressure-retarded osmosis (PRO) and in a technically feasible process approximately half the theoretical energy can be transformed to electrical power, making osmotic power a significant new source of renewable energy.

The pressure-retarded osmosis power plant is similar to a reverse osmosis desalination plant running backwards. However, the PRO plant generates power from fresh water instead of consuming power.

Fresh water is fed into the plant and filtered before entering the membrane modules containing spiral wound or hollow fibre membranes. In the membrane module, 80-90 per cent of the fresh water is transferred by osmosis across the membrane into the pressurised sea water. The osmotic process increases the volumetric flow of high pressure water and is the key energy transfer in the plant. This requires a membrane that has a high water flux and high salt retention. Typical membrane performance should be in the range of 4-6W/m2.

The salt water that is pumped from the sea and filtered before it is pressurised and fed into the membrane module. In the module it is diluted by the fresh water coming through the membrane. The volumetric feed of sea water is about twice that of the fresh water.

The brackish water from the membrane module is split in two flows. About one-third of the water goes to the turbine to generate power, while two-thirds returns to the pressure exchanger to pressurise the feed of sea water. To optimise the power plant the typical operating pressure is in the range of 11-15 bars. This is equivalent to a water head of 100-145m in a hydropower plant, generating about 1MW/m3 of fresh water. The freshwater feed operates at ambient pressure.

The only thing holding back the technology is the development of an efficient membrane. The membranes used in reverse osmosis systems are not that efficient, "so we had to design our own," says Skilhagen. "We have been working on this and have made quite good progress. We have defined a goal where the efficiency of the membrane is 5W/m2 of membrane and production costs are in the same order as those today. We say that this will be a competitive renewable energy source.

"The most important development during the last few months is that we have had great progress in our membrane development, and have now developed and tested a special osmotic power membrane producing well above 3W/m2. With our target value of 5W/m2 this once again indicates that we are doing the right things and that the technology is closer to being introduced to the market.

"We are also planning to build a prototype of osmotic power, which will then be the world's first complete osmotic power system designed to produce power. Although the unit only will be designed for a power generation of 10kW, it still will be a very important step in our up scaling process. It makes a good site for testing each of the system components isolated and as part of a complete system. In addition, all new development will continuously be tested there."

He says that the company has a top team of researchers working full time to develop the membrane. "When we started the work was mostly funded by ourselves but we had some help from the Norwegian Research Council. This allowed us to put in an application for European Union funding, which we got and we had a project from 2001 to 2004, which was partly funded by the EU.

"Most of the development on the membrane side was made at that time. It focused in detail on the membrane and how it should work. Since then we have been looking at how to produce larger quantities of membranes. This is quite a big step for the membrane industry because we need between 10 and 100 times the current membrane production today to exploit this energy source. So it is a new membrane industry we are preparing here."

Reliable system

When it comes to the efficiency Skilhagen says, "it depends on what you call efficient. You have to put some energy in because you have to transport the water into the system and you also have some losses in the system itself.

"But what we have found is that if you produce one measurement of energy, 25 per cent of this will be lost in the process. So, if you produce one amount of energy in the turbine, you will lose 25 per cent of that energy to drive the water into the system again. So, you could say it is about 75 per cent efficient. It is very energy efficient and it has to be because you are transferring quite a lot of water and have a system containing quite of lot of membrane. So, the system is quite a big installation compared with the amount of energy it produces."

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

The system is also likely to be reliable and long-lasting, which, says Skilhagen, "is a major advantage when compared with every other renewable energy source. If you have windmills, some tidal power, wave power and so on, you will always have to have some additional power that will always needs to produce energy and this could work like that because it only needs a continuous flow of fresh water and salt water, then it could operate all the time just like a hydroelectric power station."

Interestingly, most rivers around the globe run into the ocean in a city or an industrial community. This means that most of the osmotic power potential can be utilised without constructing power plants in unspoiled areas. The power plants can be constructed partly or completely underground and would thus fit very well into the local environment.

An environmental optimisation and pre-environmental impact assessment of an osmotic power plant located at a river outlet has been conducted. The possible negative environmental impacts can probably be compensated by a combination of environmental flow requirements for the river and the osmotic power plant and environmental engineering of intake and outlet of brackish water.

The future market for osmotic power will clearly be depending on its cost competitiveness compared to other available new renewable energy sources in the future.

In general and in the long-term perspective, the price of renewable energy is expected to trend towards the marginal cost for new generation capacity, given a decline in subsidy levels.

As of today, we have an estimated break-even cost of osmotic power well within limits of the expected subsidised price levels in the years 2015 -2020. Volume production of system components and increased system efficiency will possibly allow for further reduction in the future generating cost towards a competitive cost base.

Given our cost expectations, the current indication is that osmotic power will therefore be very competitive compared to other available renewable energy sources.

Area effective energy production

The osmotic power plant is very area efficient. A 25MW plant would only require some 40,000m2 of land even if it is located above the ground. Compared to a wind farm or the area required to harvest biomass to produce the same amount of energy, the osmotic power plant is very compact.

The investment cost for an osmotic power plant is relatively high per installed power compared with other renewable energy sources as, for example, wind or solar power. The main difference is that osmotic power plants will be designed for base load operation and are thus qualitatively different from most other new Renewable energy sources.

This means that, although a high investment per installed MW, the annual energy cost per kWh is comparable and competitive with the other renewable energy sources. Although osmotic power development needs long term commitment, resent results represents leaps in the development of a potentially important future energy technology.

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