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Statkraft, an energy utility owned by the Norwegian government, is today the largest generator of renewable energy in Europe. With generation capacity within hydropower, wind power, gas power, and soon also solar power, the company has a large portfolio of environmental energy solutions. But it is clear to the company that to maintain a leading position within renewable energy it is necessary to focus on innovation with a clear ambition to deliver the energy solutions of the future. With over 100 years of tradition in hydropower, working with pressurized water and sustainable project development, it was natural that Statkraft turned the focus toward PRO already in 1997.

When Statkraft started working on PRO, the first efforts were to understand the realistic potential of this concept provided the technology would be made available. Calculations and surveys of the availability of the resources - freshwater and seawater - were executed, and the result showed that a significant amount of clean, renewable energy could be produced by

*The information given in this section is provided by and property of the company Statkraft AS, Norway and used with permission. The authors would like to acknowledge Statkraft AS for the contribution.

osmotic power. In addition, there are specific characteristics of this technology that give it its unique character not only among the new sources ofrenewable energy that are currently under development, such as tidal and wave power, but also in regard to more established technologies such as solar and wind. Since the generation of power is based on the availability of freshwater and seawater, resources that usually will be available all year round, osmotic power has the characteristics of a base load source of renewable energy. This is very different from the other technologies that are dependent on the present weather conditions, hence require back up supplies from other sources.

Another interesting characteristic is that after making a survey of the rivers running into the ocean worldwide, one found that these sites usually also have either settlements or industry, and mostly both. This means that the consumer of the electricity produced by osmotic power will be just next door to the power plant. When reflecting on the situation that most new sources of renewable energy, such as wind, wave, and so on, usually have huge challenges and significant investments related to the connection of the power generation device to the grid, this adds another advantage for the generation of osmotic power as a contribution to the total energy mix.

Based on the previously stated advantages of this new technology, Statkraft made a detailed study of the state of the technology necessary to exploit these possibilities of PRO. Although there is a lot of resemblance with components used in other processes, it became clear that the membranes are one of the crucial components, where significant improvements both in efficiency and in cost were necessary. The membranes produced at that time were not in a position to produce power at a competitive level. Hence extensive efforts to design a membrane suitable for PRO were made, and this was done together with partners with long experience in membrane development both in the United States and in Europe. As described earlier, this is not an easy task, but today the best results produced by Statkraft are in the range of 3 W/m2. This result shows the significant progress made in membrane development and it made Statkraft to decide to expand their efforts to the maximum towards a full-scale osmotic power system.

In the fall of 2007, Statkraft decided, due to the promising improvement in the critical components, such as membrane and pressure recovery devices, the time had come for a full-scale proof of the concept for a complete PRO system. A plant with a sufficiently large amount of membrane area is currently built to transfer the salinity gradient into work and also further into electricity. At the same time, the interface for, and integration of, all the components in the system can be studied together in operation, not only as individual parts of a system.

After a little more than a year of development and construction, the world's first prototype plant has been put into operation in spring 2009 in the southeast of Norway (Fig. 6). The location is within the facility of a pulp factory in operation, which simplifies the approval process and at the same time gives good access to the existing infrastructure. In addition, the location has good access to seawater from the ocean and freshwater from a nearby lake.

The prototype plant is designed as a typical plant placed at sea level. Freshwater is taken from a river close to its outlet. Seawater is fed into the plant by underground pipes, and the brackish water is led to the natural brackish water zone.

The main objectives of the prototype PRO plant are twofold. First, it confirms that the designed system can produce power on a reliable 24-h/ day production. Second, the plant will be used for further testing of the technology achieved from parallel research activities to substantially increase the efficiency. These activities will mainly be focused on membrane modules, pressure exchanger equipment, and power generation (turbine and generator). In addition, there will be a focus on further development of

Figure 6 Prototype PRO plant at the east coast of Norway.

control systems, water pretreatment equipment, as well as infrastructure with regard to water inlets and outlets.

The plant is equipped with 2000 m2 of specially designed PRO membranes. A miniature hydropower turbine and devices for recovery of hydraulic pressure are installed. Although the design capacity is in the range of 10 kW, the expectations for the capacity in the first phase are somewhat less. The membranes have room for improvement, and there are high expectations for optimizations for the full system as such.

Since this is the first plant built for PRO operation, several precautions have been taken to make sure that possible pollution in the water does not destroy the membranes (Fig. 7). For the seawater regular pressure screens are used, and for the freshwater from the lake the pretreatment is similar to that being used for drinking water. The ambition is that the freshwater can be treated similar to the seawater. This will however be based on the operational experiences.

After the start-up, operation, and further testing the experience gained will be based on both operational changes as well as changes to the system and replacement of parts. This is in order to increase the efficiency and optimize the power generation. In a longer perspective, this would be used as a basis to develop a power plant with an installed capacity between 1 and 2 MW, bringing the technology one step further toward commercialization.

The prototype plant put into operation during 2009 is also intended as a meeting place for parties from both government and industry with ambitions in osmotic power. With the increasing focus on the environmental

Figure 7 Prototype PRO plant illustration.

challenges and the need for more renewable energy, this can give a significant contribution to increase the momentum in development of new clean technologies.

Statkraft has specified that in order to be competitive to other new, renewable sources of energy, a power output of 5 W/m2 for flat-sheet membranes is required, whereas due to the higher packing densities obtainable, a target in the range of 3 W/m2 should be sufficient for hollow fiber membranes. This is based on the water flux trough the membrane, in relation to the salt retention that creates the driving force. The estimated costs of producing one MW based on a number of detailed investment analyses are that osmotic power will be able to produce electricity at a cost level of Euro 50-100 MW"1, which is in a similar range as other renewable technologies such as wind power, wave and tidal power, and power based on biomass.

These calculations are based on existing hydropower knowledge, general RO desalination engineering information, and with a membrane target as a prerequisite. The capital costs of installed capacity are high compared to other renewable energy sources. However, each MW installed is very productive, with an average operation time above 8000 h a year. This should generate approximately twice the energy supplied (GWh) per installed MW per year compared to a wind mill.

To achieve competitiveness, given the large volumes of membranes, the membrane pricing is important. For an average 25 MW plant, it is calculated that 5 million m2 of membrane area is required, meaning that the industry would see a demand of PRO membranes exceeding the current RO membrane market.

There are still significant improvements and verifications of the technology required before osmotic power can be represented among the currently commercial renewable energy technologies. But it is not only the technology itself that need to be put into place to exploit this huge potential; in the following sections some of the major topics to be assessed will be discussed, and it is known from the history of developing both wind power and solar power that these topics are not trivial. For wind and solar power, the technology was long past the proof of concept, but it took still several decades before these were able to gain a significant market share.

A new technology such as osmotic power can only be developed to a certain level by researchers and especially dedicated companies such as Statkraft. But to exploit the full potential of such a technology, one will be dependent on external factors as well, such as that several organizations have sufficient demand for this specific power technology. When several companies and governments around the world commit themselves to utilize the technology, whether it is solar, wind, or osmotic power, this provides strong signals to the supplier industry and the competition for developing and supplying the best solution will go up to full speed.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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