Flgure 2. Artists impression of how an osmotic plant can be constructed below ground or in rock caverns operating conditions can generate a hydrostatic pressure equivalent to a water head of over 100 m which can generale electricity in a hydropower station. This technology is called pressure retarded osmosis (PRO) and has a vasi unexploited potential. Estimates suggest the global power production potential is in the order of 2000 TWh per year. in Norway alone the available energy is estimated at more than 25 TWh each year. Background Osmotic power is a relatively unknown energy conversion technology even though osmosis, a spontaneous process which takes place in all living organisms, has been known for several hundred years. Technological development for the use of osmotic pressure in power generation goes back only 30-35 years when scientists proposed methods for osmotic energy conversion. A number of patents were filed and, some research was done at the time until it was realized !hat suitable membranes were stili unavailable. However, in the eighties and nineties membrane technology was introduced successfully in many industrial applications including seawater reverse osmosis desalination and water purification. Cheap semi-permeable membranes with an efficient retention of salt and a high permeability of water became available. in the lale nineties the development of pressure exchangers made progress which meant that efficient transfer of mechanical energy between fluids was possible. One now had all the technology components necessary for efficient osmotic power production. in the lale nineties, this observation spurred Statkraft SF and the Norwegian research institute SINTEF to make preliminary studies of the potential for cost-effective power production. The results were remarkable: lf a membrane could be developed which has about the same efficiency in forward ARTICLE / MAKALE osmosis as current reverse osmosis membranes one could start tapping in to a huge and environmentally friendly energy source! How does osmotic power work? A typical simplified PRO process is shown in Figure 1. Filtered fresh water is pumped into modules containing spiral wound membranes. in the module fresh water migrates through the membrane and into the pressurized seawater. The flow of diluted and pressurized sea water is then split in two streams where one is depressurized by a hydropower turbine to generale power, and the other passes through a pressure exchanger in order to pressurize the incoming seawater. As can be seen in the figure, the plant has a high pressure loop for the salt and brackish water and a low pressure side for the freshwater. The two key components in a traditional PRO planı are the pressure exchanger and the membrane. it is very important for the energy cost that both these components are very energy efficient. When !his is achieved, a PRO power plant generates about 1 MW from each cubic metre per second or freshwater !hat passes through the membranes. The process shown in Figure 1. is basically similar to a saltwater reverse osmosis (SWRO) desalination plant running backwards. AII the components are thus well known and proven technology in the water supply industry. Obviously, the membrane and modules significantly adds to the cost of the otherwise quite ordinary hydropower planı. A low cost membrane with a long operating life is thus essential for the economy or such a power planı. Decades of experience from the water treatment industry shows !hat a lifetime of 7-1 O years is normal and it is expected that !his will be achieved in a PRO power planı as well, enough to provide economically competitive power generation. Statkraft is also investigating other concepts for PRO power production and it has been found !hat certain process designs can reduce the cost of energy significantly while increasing the planı efficiency. Environmental performance Osmotic power is in many aspects a new and advanced way to generale hydropower, but without the need of a waterfall. The location of the planı is thus not very important as long as freshwater and seawater is available. Figure 2. shows an artists impression of how the power planı can be built below ground or in rock caverns. Feed water intake arrangements can in principle be concealed below the surface of the water. Because of the properties of the power plant, the need for dams and water management systems on the freshwater side is less than in traditional hydropower and the effects on fish and other wild life will be minimal. Features like these reduce the impact of the osmotic power planı on its surroundings and the flexibility in planı layout makes it possible to adap! the plant very well to local conditions such ENERJi & KOJENERASYON DÜNYASI 59
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