By Ragnar Burenius and Abraham Nilsson
Summary
The inquire for renewable energy resources has never been bigger then today. A relative unknown, renewable energy resource is osmosis. It's based on the materials striving after equality. For example a warm body emits heat to the surrounding environment, until they have the same temperature. In the osmotic process two solutions with different salt concentrations is separated by a semi permeable membrane. The membrane only let small molecules like water molecules to pass while the salt stay on one side. The water strives after equality so the water flows through the membrane to the side with the highest salt concentration and creates a pressure. That pressure can be used to drive a turbine and gain electricity. There are different types of osmosis power plant. Both land-based plants and plants anchored to the sea floor. The biggest advantages is that it's renewable and don't have almost ant impact on the environment. The disadvantages are unfortunately bigger. The worst one is probably the cost, but building a power plant under the water isn't easy either. Although osmotic power generation in itself might not be the future the membranes are interesting since they can be used for many other purposes. They've made of a thin film of polymeric material cast on fabric support and the selection of molecules they let through is based on the molecules size and charge. This development makes membranes an interesting market with a very good future potential.
Table of Contents
Introduction
The need of renewable energy recourses has never been more urgent than nowadays, but not many have heard about the osmotic theory. This report will explain what it is, and what it's used for.
The Osmotic process
Osmotic energy is based on the natural osmosis effect. The nature tries to even things out to make them equal. Temperatures are a good example. If you stick together two objects with different temperatures the hotter one always transfer heat to the colder until the temperatures match. Osmosis is based on the same effect. If you got two solutions with different salinity, e.g. salt water and fresh water, then the solutions try to mix to even out the salinity. Osmosis is also used in plants and animals. If you put an animal cell in a very dilute solution, with much fresh water, the cell will swell and eventually burst. In the opposite case, when the cell is put into a concentrated solution, water is sucked out of the cell and the cell shrinks.
This osmotic effect can be used to extract energy. You just have to control the process. To do this you usually use a container with two chambers separated by a semipermeable membrane. This is an organic filter that only will let certain molecules pass through. The selection of which molecules is based on the size and the charge of the molecule. This way you can allow small molecules like water, oxygen, carbon dioxide, ammonia etc. to pass through and larger molecules like sucrose, starch and protein not to.
Example #1
In the first example we have a simple container with two sections separated by a semipermeable membrane. On the left side we put fresh water and on the right salt water. The water will then start to flow from the left side to the right until either the salinity is the same on both sides (which won't happen since we got fresh water on the left side) or until the pressure of the water pillar is bigger then the pressure created by the osmotic flow (the so-called osmotic head pressure).
Example #2
The problem with the setup in ex #1 is that the process will eventually stop. To make a system with constant movement we need a constant flow of salt and fresh water. We have this in Example #2. Here we pour in fresh water on the left side and salt water on the right. In the upper right corner we then have a water turbine that's driven by the osmotic pressure/flow. This way we can get energy from the process as long as we give it water, both salt and fresh. The theoretic potential of osmotic energy is rather big. A difference in salinity of 3% corresponds to the potentiel energy of a 250-meter waterfall.
Reversed osmosis
This doesn't really have anything to do with osmotic energy, but since this is the most usual application of the osmotic effect we'll explain the basics here anyway. Reversed osmosis is the opposite of producing energy by osmosis. Instead of letting the fresh water flow through the membrane and create a pressure we put a pressure on the salt water. When this pressure is equal to the osmotic pressure no water will flow through the filter. If we increase the pressure further, the water will start flowing through the membrane in the opposite direction. This forces the salt to separate from the water since only water molecules are allowed to pass through the filter. As you might have understood this is a method of making fresh water out of salt water. The picture to the right is however somewhat simplified. In reality you use special cross-flow techniques to prevent the salt to obstruct the filter. Reversed osmosis is used in many applications, for example sewage-treatment.
Osmotic power plants
Osmosis power plants is built on the principle that they take charge of the power that the water produce when it flows trough the membrane. When the water flow trough the membrane it increase the pressure on one side of the membrane and decrease the pressure on the other side. The pressure difference can be used to drive a turbine and consequently get electricity.
Two different ways how an osmosis power plant can work
This power plant uses the overpressure to drive a turbine, and gain electricity. Fresh water flows in and passes through the membrane into the pressure chamber where it's mixed with the salt water. The pressure increases and the water force the turbine to rotate. It's important that the salinity in the pressure chamber don't fall to low, because then the osmosis effect won't work. That's why you need a salt water pumped too. Suitably, this kind of power plant is placed where a river enters the sea.
If the fresh water was clean and the membranes were very good this maybe would work. But the water contains particles of sand, salt and other tings. It would e necessary to pre-clean the fresh water and have some kind of membrane cleaner to prevent accumulation.
The picture shows a SHEOPP Converter, which is a submarine hydroelectric power plant anchored to the sea floor It uses the potential energy in the water to drive a turbine and gain electricity. As you can see on the picture fresh water, from a river mouth or an aqueduct, is transported trough a pipe down to the turbine. When the water has passed trough the turbine it flows into a tank. Finally the water diffuses out in the sea by osmosis.
The advantage to use this kind of power plant is that you don't need a very big difference in height to make it work satisfactory. For example where a river enters the sea. If the fresh water was totally clean and the membranes were good, this plant would work well too. But like the other power plant the water contains many particles of sand, salt and other bad stuff. There for it would be necessary to pre treat the water and a flushing pump would be required to prevent accumulation of unwanted solutes and contamination on the fresh water side of the membranes
The efficiency for the SHEOPP will reach its maximum at a depth of 110 meters.
Economical aspects of osmotic energy
It's hard to give an estimate of what it would cost to build an osmosis power plant, according to the fact that no one has ever built one. 1977 Osmotic inc. gave a rough estimate for the cost of the membranes. This amounted to about $0,20 /m2 if 2km2 of membrane area were produced. The power output for 1 km2 would, by 1977 amount to 1,62 MW. This number has been calculated from the values given by several tests on semi permeable membranes. To it comes the production cost for the turbine, installation and all machinery that is required. A calculation made by a scientist shows that the cost for each installed KW would be $36.000 and the price for the osmosis made electricity would be about 36 times today's prices. Conclusion: There is no one that will build an osmosis plant due to the costs. At least not here in Sweden were the salt concentration is very low.
Pros and Cons
There are many advantages using an osmosis power plant. The biggest one is of course that it's renewable. To that comes that there is no risk to run out of raw materials (salt and water), since the power plant does not consume any salt or water. And even if it would, the resources wouldn't be exhausted so fast. Another big advantage is that the process is very clean and accordingly to that, osmosis plants have a minimal environmental impact.
Unfortunately the disadvantages are more. The cost is probably the worst one. To that come the difficulties to build a large foundation 110m under the water surface, protect the fish and other water organisms to enter the turbine.
The membranes
The uses of semi permeable membranes have increased rapidly in the last couple of years. For quite some years these membranes have been used to create waterproof clothing, like Gore-Tex, but now the use have branched out in a wide range of new applications. The medical industry uses it because it has the same characteristics as the wall of the human cell. Therefore the semipermeble membrane can be used in the production of artificial organs. The membranes are also used in nicotine patches and for drug delivery devices. These devices are supposed to help people with medication requiring a regular dose. Another, not so technologically sexy, use is in the production of disposable diapers.
The material
The semipermeable membranes consist of a thin film of polymeric material cast of fabric support. The film is about one micrometer thick and has high quality requirements. First of all the membrane must have high water permeability and a high degree of semipermeability; that is; the rate of water transport must be much higher than the rate of transport of dissolved ions. They also have to remain stable over a wide range of pH and temperatures, and have good mechanical integrity. Usually they have a life span of 3 to 5 years. There are two major groups of polymeric materials that have the right qualifications to produce satisfactory membranes for osmosis and reversed osmosis. These are Cellulose Acetate (CAB) and Composite Polyamide (CPA). Depending on which group of material you use the membrane get significantly different performance etc.
Cellulose Acetate Membrane
The first membrane of this type was manufactured in the 1950's and was made from cellulose diacetate polymer. Today CAB membrane is usually made out of a blend of cellulose diacetate and triacetate. The membrane is made by casting a thin film acetone-based solution of cellulose acetate polymer with swelling additives onto a non-woven polyester fabric. That's followed by a cold bath and finally a high temperature annealing to complete the process. This annealing improves the semipermeability of the membrane. The final membrane has a dense surface layer that is responsible for the salt rejection. The rest of the membrane film is spongy and porous and has high water permeability. For this type of membrane you can control the salt rejection and water flux by variation in temperature and duration of the annealing step.
Composite Polyamide Membrane
These membranes are manufactured in two distinct steps. The first step is to cast a polysulfone support layer onto a non-woven polyester fabric. This polysulfone layer is very porous and not semipermeable itself. In the second step the real semipermeable skin is made. It's formed on the polysulfone substrate (created in the first step) by interfacial polymerization of monomers containing amine and carboxylic acid chloride functional groups. This way of manufacturing makes it easier to optimize the properties of the different parts of the final membrane. Composite polyamide membranes are characterized by higher water flow and lower salt passage than cellulose acetate membranes. They're also stable over a wider pH range then the other type of membranes. Composite polyamide membranes have drawbacks though. One of them is the sensibility for free chlorine, while cellulose acetate membranes at least can tolerate some.
If you compare the two types of membranes you'll notice the smooth surface of the cellulose acetate membrane. It also is less charged then the surface of the composite polyamide membrane. This together with the tolerance for free chlorine gives the cellulose acetate membranes more stable performance in applications where the feed water has a high fouling potential, such as with sewage-treatment.
Filtration
There are different kinds of semipermeable membranes that will let different molecules to pass through. Which molecules that are allowed to passed is based on their charge. This way you can make the selection very precise.
Using the membranes
In most areas of usage the semi permeable membranes are under great pressure. Since the membranes themselves are extremely thin they can't handle the pressure by itself. To solve this problem you put the membrane on some kind of backing, like fabric or a grill.
List of reference
WWW
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CNG 97s schoolwork |
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Nigel D Purchon's website |
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Osmosis Lesson |
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World Wide Water |
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Thomas Keijzer's website |
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"What is Chemical Engineering?" |
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How stuff works |
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Mustang Engineering Inc. |
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Osmonics |
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Koch Membrane Systems Inc. |
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Ionics Inc. |
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Whatman Inc. |
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Hydronautics |
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Some alt. Energy page |