28-04-2011, 04:32 PM
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Summary
The need of new energy sources has led to a number of alternatives. Some better then others. One of those alternatives is energy created by transportation of solutions, osmotic energy or salinity gradient energy. In the osmotic process two solutions with different salt-concentrations are involved (often freshwater and salt-water). A semipermeable membrane, which is an organic filter, separates the solutions. The membrane only lets small molecules like water-molecules pass. The water aspires to decrease the salt-concentration on the side of the membrane that contains most salt. The water therefor streams through the membrane and creates a pressure on the other side. This pressure can be utilised in order to gain energy, for example by using a turbine and a generator.
There are several different types of power plants using osmosis (the osmotic process); both land-based plants and plants anchored to the sea floor. The thing the plants we have studied have in common is that osmosis is not directly used to generate power. What the osmosis does is that it creates a flow through the plant and it is that flow that forces the turbine to rotate.
Energy created by osmosis has very little impact on the environment and that is of course an important fact to consider when it comes to determine whether osmotic energy is something to invest in or not. Another advantage is that osmotic energy is renewable, since the process does not "consume" the salt. (Salt-water evaporation leads to precipitation over land.)
The major fact when it comes to the disadvantages is the high cost. Osmotic-produced power is much more expensive than for example fossil fuels. There are also engineering problems to be overcome. The high cost has made us draw the conclusion that osmotic energy is not something for ABB Alstom Power to invest in, at least not in the nearest future, since no one wants to buy the energy when it is so expensive.
The possibility to use osmotic power from our oceans lies within the technology that needs to be developed. There are many possible ways to exploit energy from salinity gradients. It seems, as osmotic pressure will be crucial with each of the possibilities.
Introduction
We can't continue using several of our energy sources we gain energy from today. For example fossil fuels contaminate our environment and we are also running out of them. It is therefor necessary to find other ways of producing energy. This report focuses on one of those alternatives, osmotic energy.
Osmosis means passage of water from a region of high water concentration (often freshwater) through a semipermeable membrane to a region of low water concentration (often NaCl). The membrane only lets water molecules pass. Salt molecules, sand, silt and other contaminants are prevented to do so.
Several physiological processes use this osmotic effect. For instance, our body uses it to bring water back from the kidneys, and plants use osmosis to keep the water pressure inside the plant at a fixed level.
Since scientists have found a way to build semipermeable membranes, we can use the osmotic effect and convert it to mechanical energy. We will give examples of different ways of doing this later on in the report. But first we will explain how osmosis really works.
The osmotic process
The main thing with osmotic energy is transportation of solutions (often pure water and salt-water), separated by a special filter, a membrane. In the osmotic process it is not possible to use an ordinary filter. You need a"Semipermeable membrane".
A semipermeable membrane is an organic filter with extremely small holes. The membrane will only allow small molecules, like water molecules, to pass through. The thin layers of material cause this and that is what the osmotic energy process is all about.
The picture here on the right shows a simple test rigg for this process. The left side contains pure water. The right side contains a solvent with water and salt (NaCl). The only thing that separates them now is the semipermeable membrane. The process is about to begin.
When the process gets started the pure water on the left side aspires to decrease the salt-concentration on the right side of the membrane. The amount of water on the right side will now increase and create an "Osmotic head pressure". We can use this pressure, for example, to force a water- turbine to rotate.
The amount of freshwater that will pass through the membrane depends on the salt-concentration in the salt-water, before the osmotic process begins. For instance, if the salt-concentration from the beginning is 3,5%, the osmotic pressure will be about 28 bars.
The problem with the test rigg is that the salt-concentration in the salt-water will decrease and the process will slow down. The only way to fix this is to continuously, empty and refill both the left and the right side. This must be done very quickly to avoid run-interference.
Another problem is that the membrane can, and will wear out because of all silt and other contamination that will get stuck in the membrane. If we don't consider this fact a membrane's length of use is about 6 months. This sort of process could not only be used for energy purpose. The main use area today is Reverse Osmosis, where you create a pressure larger than the osmotic head pressure and push the salt water through the membrane. From this process you gain fresh water out of salt-water.
Different power plants using osmosis
SHEOPP Converter
The picture below shows a SHEOPP Converter, which is a submarine hydroelectric power plant anchored to the sea floor. Fresh surface water, from a river mouth or an aqueduct, is conveyed through a penstock (standpipe) to a hydraulic turbine. After generating electric power, the fresh water is discharched and depressurised into a submarine tank. Finally the fresh water diffuses out in the sea by osmosis, through a barrier of semipermiable membranes.
For pure fresh water and perfect semipermeable membranes a flushing pump would not be necessary and the electric power produced in the SHEOPP Converter would be maximised. In real situations, however, the fresh water will generally contain non-tolerable amounts of dissolved salts and particles like sand, silt and other contaminants. It may then be necessary to pretreat the fresh water and a flushing pump would be required to prevent accumulation of unwanted solutes and contamination on the fresh water side of the membranes, to keep them in good working condition for as long as possible.
The efficiency for the SHEOPP will reach its maximum at a depth of 110 meters.
Underground PRO plant
If an osmotic flow passes through a semipermeable membrane, which separates the two solutions and forces a turbine to rotate, the process is called pressure-retarded osmosis, PRO. Both these plants described here use PRO, but the plant below is land-based while the SHEOPP Converter is anchored to the sea floor.
Fresh water at sea level flows vertically downward through a penstock. The lower end of the penstock is situated about 90 meters below the sea surface where the pressure is 9 bars. This pressure forces a turbine to rotate and the pressure drops to 0 bar. Seawater is pumped from the surface to a barrier of semipermeable membranes (an osmotic unit). By osmosis the fresh water is driven through the membranes, trying to even out the amount of dissolved salt in the seawater. The flushing solution is pressurised to 9 bars and is pumped up to the surface. The diluted solution returns to the seawater by the osmotic pressure.
The osmotic effect is thus used to force the turbine to move. When the water is pressed out through the membranes a sucking effect, a stream appears. It is that stream, created by osmosis that makes the turbine spin. Thus, in neither of these plants osmosis is used for the direct generation of electric power. It is the sucking effect, the flow, which generates electric power.
