US 4,283, 913 comprises a saturated non-convective water reservoir which captures solar energy and which is used as a separation unit in combination with reverse electro dialysis or pressure retarded osmosis for energy production. From the water reservoir which partly can separate a solution, a higher concentrated stream and a less concentrated stream is passed into two chambers separated with a semi-permeable membrane. Parts of the energy which is created by permeation of the stream with lower concentration through the membrane and the subsequent mixing of the two mentioned streams are transformed into energy before the streams are returned to the water reservoir.

From US 4,193,267 it is known a procedure and an apparatus for the production of power by pressure retarded osmosis, wherein a concentrated solution with high hydraulic pressure is passed along a semi-permeable membrane, and where a diluted solution is passed along the opposite side of said membrane. A portion of the diluted solution is transported through the membrane and creates a pressurized mixed solution. The potential energy stored in this pressurized mixture is converted into applicable energy by pressure release and pressurizing of the diluted solution.

In US 3,978, 344 a procedure is described for producing energy by pressure retarded osmosis by the use of a turbine and a semi-permeable membrane.

Further it is known from US 3,906, 250 production of energy by pressure retarded osmosis by hydraulic pressurizing of a first liquid which is introduced on one side of a membrane, after which another liquid with lower hydraulic pressure and a lower osmotic pressure is introduced on the other side of a membrane. Pressure retarded osmosis will lead to transport of parts of the other liquid through the semi-permeable membrane and thereby a pressurized mixed solution is formed with a larger volume than the first liquid alone. The stored energy is then

transformed in a turbine into useable energy such as electrical or mechanical power.

For centuries it has been known that when salt water and fresh water are partitioned in two different chambers by a semi-permeable membrane, made for example of a biological membrane, e. g. of hog's bladder, fresh water will press itself through the membrane. The driving force is capable of elevating the salt water level above the level of the fresh water, whereby a potential energy is obtained in the form of a static water height. The phenomenon is called osmosis and belongs to the so-called colligative properties of a solution of a substance in another substance. This phenomenon can be thermodynamically described and the amount of potential energy is therefore known. In a system of fresh water and ordinary sea water the theoretical potential expressed as pressure is approximately 26 bars. The energy potential can in principle be utilized by several technical methods where the energy can be recovered as i. e. steam pressure and stretching of polymers. Two of the technical methods are using semi-permeable membranes, and these are reverse electro dialysis (energy potential as electrical DC voltage) and pressure retarded osmosis, PRO, (energy potential as water pressure).

Calculations have been made to find the costs of energy production at PRO plants. The uncertainty of such calculations is illustrated by the fact that reported values for the energy costs fluctuate over more than a magnitude. Wimmerstedt (1977) indicated a little more than 1 NOK/kWh, whereas Lee et. al. (1981) indicated prohibitive costs. Jellinek and Masuda (1981) indicated costs of less than 0.13 NOK/kWh. Thorsen (1996) made a cost estimate which stated 0.25- 0.50 NOK/kWh based on an evaluation of recent data for membrane properties and prices. All of these evaluations are based on the use of fresh water and sea water. Earlier conclusions also indicated costs of energy produced by PRO that varied very much. A comprehensive elucidation of methods for energy production today and in the future is included in the book"Renewable Energy" (ed. L.

Burnham, 1993), prior to the Rio conference regarding environment and development. Here salt power is only mentioned very briefly, and it is maintained that the costs are prohibitive.

When fresh water is mixed with salt water there is an energy potential (mixing energy) for PRO corresponding to a downfall of 260 meters for fresh water, and the locations of most interest are rivers flowing into the ocean. In the present invention it has been found that 35-40 % of this energy can be recovered by PRO. In a practical power plant the energy will be liberated as water pressure by approximately 10 bars in the stream of brackish water which develops after the fresh and salt water have been mixed together. This pressure can be used for operating conventional turbines. The effective amount of energy will then be between 50 and 100% of the naturally occurring downfall energy in fresh water on world basis.

According to the present invention the actual potential for amounts of power seems to be 25-50% of the water power which today has been developed in Norway. Power plants based on the present invention do not lead to significant emissions into air or water. Further this form of energy is fully renewable, and is only using natural water as driving force in the same manner as conventional water power plants. The object of the present invention is to allow the commercial utilization of salt power in a bigger scale.

Assumed area expenditure for an intended salt power plant will be relatively small and in the same magnitude as for a gas power plant, and substantially smaller than for wind power. The method is therefore especially friendly to the environment. Briefly the method with regard to the environmental effects and the use properties can be characterized as follows : - no C02 emissions or other large quantities of emissions other than water - renewable, like conventional power from water - stable production, unlike the wind and wave power - small areas are required, and small impacts on the landscape areas - flexible operation - suitable for small as well as large plants Known art is not dealing with effective semi-permeable membranes with reduced loss of energy where the largest part of the salt gradient in the membrane is present in the same layer as the flow resistance if the membrane is used for PRO.

Therefore an effective and optimized membrane/membrane module has to be developed where the requirements as to the salt gradient in the membrane and the flow resistance as mentioned above are satisfied. This can not satisfactorily be achieved in existing membranes designed for filtering (reverse osmosis).

An important feature of the present invention is that most of the salt gradient in the membrane is localized in the same layer-the diffusion skin-as the flow resistance. Further the present patent application also consists of a porous carrier material for the diffusion skin with no resistance worth mentioning against water transport and salt diffusion. In the present invention salt therefore does not appear in unfavourably high concentrations in parts of the membrane other than the diffusion skin. According to the present invention membranes with particular inner structures are important. Further the concentration polarization of salt at the fresh water side of the membrane skin is reduced compared to conventional membranes.

In a PRO plant the pressure energy in the brackish water is in a direct manner recovered hydraulic for the pressurizing of incoming sea water. Thereby a part of the loss which ordinarily would occur in an ordinary water pump for this purpose is avoided. By avoiding this loss the PRO plant according to the present invention can be built on ground level instead of below ground level and nevertheless achieve acceptable efficiency.

Recovery of pressure energy by direct hydraulic pressurizing of incoming sea water takes place in a device where the turbine pressure in half of the device is pushing sea water directly into the membrane module. In the other half the brackish water is pushed back and out of the PRO plant as the sea water is pumped in. The mentioned processes which take place in the respective halves of the device for hydraulic pressurizing of sea water alternate by rotation of the water containing part or by a controlled valve system in the mentioned device. The mentioned direct hydraulic pressure transfer leads to that the use of sea water pumps with limited efficiency is considerably reduced.

Production of energy by pressure retarded osmosis (PRO) takes place by osmosis through a semi-permeable membrane. The membrane should have as high permeability for water as possible and as low permeability for salt as possible. The flux of water, Jv, and salt, Js, through the membrane is determined by the equations: Water flux = Jv = A (z p) (1) Salt flux = Js = B-dcs (2) Here A is the water permeability and B is the salt permeability. Both of these are determined by the characteristics of the membrane. Ap is the pressure difference and Acs is the salt concentration difference over the membrane skin (the diffusion skin/the membrane skin). is the natural osmotic pressure, which is proportional to the concentration difference over the skin of the membrane: r=nR TACs (3) Here n is the number of particles, e. g. ions, to which the salt dissociates when it is dissolved in water. R is the gas constant and T is the absolute temperature.

The membrane skin (the diffusion skin) is a thin layer, usually on one membrane surface, which is much denser than the rest of the cross section of the membrane.

The characteristics of the skin are also vital for the performance of the membrane in PRO. The rest of the cross section of the membrane consists of porous structures of different geometrical forms. These structures serve to support the skin and give all of the membrane the necessary mechanical strength and carry water to the skin.

However, no membranes are completely salt tight. Therefore, salt will always find its way into the porous structure in the membrane, irrespective of on which side of the membrane the salt water is. The salt will diffuse from salt water to fresh water because diffusion always goes towards a place with lower concentration if this is not counteracted by means of external forces such as an electrical field or by other means. However, the fresh water flows in the opposite direction, towards the highest salt concentration. This is the principle of the natural osmotic flux. The speed of the salt diffusion increases with the area of free water which is available

for this. If the available water area for diffusion is reduced, the diffusion speed will decrease, and for the same reason the velocity of the water will increase. The diffusion speed will always be reduced by the velocity with which the water flows through the membrane against the diffusion speed, which is analogous to walking contrary to the wind. A limitation of the water area for flow and diffusion within the porous structures in the membrane will therefore with double effect reduce the possibility of the salt to get out of the structure. This will lead to accumulation of salt and high salt concentrations in the structure, also close to the skin. This will reduce the difference in salt concentration over the skin, Acs, and therefore the osmotic flux, because the osmotic pressure is reduced, see equation (3). Because both flux and pressure is reduced, the energy production will fall drastic because energy is the product of pressure and water flow, i. e. flux.

The object of the present invention is therefore to provide a semi-permeable membrane where the diffusion velocity of the water through the porous layer of the membrane increases.

The present invention therefore describes a semi-permeable membrane comprising a layer of a non-porous material (the diffusion skin/the membrane skin) and one or more layers of a porous material where the porous material comprises one or more hydrophilic materials which spontaneous are wetted with water.

One of the objects of the present invention is to provide a semi-permeable membrane comprising one layer of a non-porous material (the diffusion skin/the membrane skin), and one or more layers of a porous material. The porous material has a structure where water filled porosity ¢, the thickness of the membrane minus the skin thickness x (m), and tortuosity, stand in relation to each other as indicated by the equation x-s=ç-S (4) Because the skin thickness is very thin, X will in the following be called the thickness of the membrane.

In the equation (4) S is a structure parameter, which effective value is defined by the equation (4). The porous material further comprises one or more hydrophilic materials which are spontaneous wetted by water.

In the present semi-permeable membrane the pore volume in the porous part of the membrane is filled with more than 60% water when water flows through the membrane, preferably more than 90%. The porous layer of the membrane further comprises pores with a pore size which preferably is bigger than 10 nm. In the invention the wetting angle between water and the porous material is less than 60°. In the present invention wetted porosity in the porous part of the membrane is everywhere more than 5% within volume parts of the membrane of 10 x 10 x 10 , um (micrometers), and at least 30% in average for the whole of the porous part of the membrane.

The permeability for water, A, is higher than 5-10' m/sec Pa, and the permeability of salt, B, is less than 8-10-7 m/s, preferably less than 3 10-7 mus in the present invention. Further the diffusion skin/membrane skin of the non- porous material is less than 1, um. The thickness of the porous material is less than 200 Fm.

If the porous structure in the membrane is not fully wetted with water, this means that gas (normally air) is contained within the structure. Lack of wetting means that the gas is attracted more by the membrane material than the water and the gas therefore has a tendency to fasten on the walls inside the structure. The gas could occur as bubbles or more or less as a layer on the surface of the membrane material inside the structure. In both cases the gas will reduce the area of the water for flowing and diffusion. Bubbles can fill some pores in the sponge-like structure inside the membrane. The passages between these pores will often be few and more narrow than the diameters of the pores. Thereby larger parts of the structure could be blocked for water passage even if water is present within some of the pores in the structure.

Further described are semi-permeable membranes or membrane modules where the membranes comprise a thin diffusion skin with natural osmotic properties, and the rest of the membrane has an increased porosity so that salt is not collected here (the porous layer). The membranes can consist of organic polymers, for example polymers based on cellulose or based on nylon.

The semi-permeable membrane comprised by the invention can more specific be described as a semi-permeable membrane consisting of one thin layer of a non- porous material (the diffusion skin), and one or more layers of one porous material (the porous layer), where an amount of salt containing water is in contact with one side of the diffusion skin, characterized by the porous layer having properties where a parameter B (salt permeability in the diffusion skin) fulfils the relation: B = (##D#(dc/dx)/#-J#c)#1/#cs (5) wherein: B is the salt permeability (m/s), Acs is the difference in salt concentration over the diffusion skin (moles/cm3), is the porosity, x is the thickness of the membrane (m), J is the water flux (m/s), c is the salt concentration (moles/cm3), D is the diffusion coefficient of the salt (m2/s), is tortuosity, where the efficiency of the membrane in pressure retarded osmosis for a given value of a water permeability, A (m/s/Pa), can be expressed by an integration of (5), and simplification so that: #cs/cb = exp(-ds#J/D)/{1 + B#[(exp(df#J/D + S J/D)-exp (-ds J/D)]/J} (6) wherein: cb is the concentration of salt water minus the concentration of salt in the fresh water (moles/cm3), df and ds are the thickness of the diffusion films (concentration polarization) at the fresh water side and salt water side, respectively, of the membrane (AIm),

ZIC5/cb expresses the efficiency of the membranes in pressure retarded osmosis for a given value of the water permeability, where a precise expression for the structure in the porous part of the membrane can be stated by the structure parameter S (m), where S=x"c/. (7) The value of the structure parameter S and thereby the inner structure of the membrane is decisive for its efficiency in pressure retarded osmosis. The structure should have only one thin and non-porous layer wherein salt has considerably lower diffusion velocity than water. All of the other layers must be porous so that salt and water can be transported with as little resistance as possible. Several porous layers can be present to give the membrane the correct mechanical properties and/or as a result of the production method. In those cases where the diffusion skin lies between two or more porous layers, or the membrane is laterally reversed in relation to fresh water and salt water, the expressions will be more complicated, but the following discussion will be valid in the same manner.

Ac, can be expressed as a function of the"bulk"concentration of salt in the water solution outside the membrane, cb, see equation (6). When cb is known, Jv and Js from equation (1) and (2) can be calculated. We then have A, B, S, df and ds as unknown. df and ds are the thickness of the water films outside the membrane where the salt concentration is changed in relation to the"bulk"concentrations further away. These film thickness are determined by the movement of the water in relation to the membrane and can be measured or estimated separately. In addition, they are less than S and will therefore not influence an osmosis trial as much as the parameter S.

The structure parameter S should have a value of 1. 5 10-3 or lower. Such a requirement as to S value makes probable that x cannot have a high value (x less than 200, um) ; X cannot be too high (lower than 2.5) and (p cannot have a value that is too low, i. e. higher than 50%.

The permeability for salt, B, is less than 8 10-7 m/s, preferably less than 3 10-3 m/s, and the water permeability, A, is higher than 5-10' m/s/Pa. The thickness of the diffusion film on the side containing lesser salt and the side containing more salt is less than 60 lim, preferably less than 30 p-m.

Membrane modules comprise flow breakers consisting of threads of polymers which are forming a net with a square or rhombic pattern. Further, several membranes are packed together to modules (rolled up to spiral membranes) where the distance between adjacent membranes are from 0. 4 to 0.8 mm.

In the spiral modules the channels for the salt containing feed stream are 10-50% filled with one ore more flow breaking devices consisting of threads of polymer which form a net with square or rhombic pattern.

The pressure in the salt containing feed stream on the membrane/membrane module is in the area from 6 to 16 bars. The best efficiency in a salt power process is obtained when the pressure difference over the membrane is approximately half of the osmotic pressure.

As an alternative to spiral membranes parallel hollow fibers can be placed in layers so that salt water flows on the outside and fresh water on the inside of the fibers, or vice versa. The above mentioned will then be a little altered, but the pressure will be the same.

The skin of the membrane can possibly be located either against the sea water or the fresh water. Locating the diffusion skin against the fresh water side will have the advantage that contamination in the fresh water being more readily rejected on the membrane surface because the diffusion skin has far smaller pores compared to the porous layer. Since there is a net volume stream moving in- towards the membrane at the fresh water side, this volume stream will be able to transport different types of impurities which can lead to fouling of the membrane.

On the other hand, a continuous water stream from the membrane at the sea water side will contribute to keeping the surface of the membrane clean.

Because almost all of the pressure difference in the present process lies over the non-porous material (the diffusion skin), it can be an advantage that the diffusion skin lies at the sea water side since the overpressure will press the diffusion skin against the porous layer. With the diffusion skin at the fresh water side there can be a risk that the diffusion skin loosens from the porous layer, and the membrane can be destroyed.

Further, the parameters for the water permeability, A, and the salt permeability, B, are of great importance as to the performance of the membrane.

For a membrane which is totally without salt leakage, the thickness, porosity and tortuosity of the porous layer will not be of great importance for the energy production.

It seems to be a considerable dependence on film thickness due to concentration polarization at the sea water side alone, as concentration polarization at the fresh water side is low for a membrane with a small salt leakage.

The thickness of this diffusion film is a critical size for the energy production by pressure retarded osmosis. This size has to be determined experimentally from transport trials where flux data are adapted to the actual model. Theoretical calculations with a more complex transport model indicate a thickness of the diffusion film of approximately 25 10-6 m _ 50 10-6 m.

The thickness of the diffusion film on the surface of the membrane facing the sea water side can be reduced by increasing the flow velocity at the sea water side, and by using devices which increase the stirring of the flowing sea water (turbulence promoters). Such efforts will increase the loss by friction during the flow of the sea water, and there will be an optimum point with regard to the sea water rate through a membrane module and the shaping of the membrane module.

As mentioned above, the concentration polarization of salt will be a small problem at the fresh water side in a good membrane module. This is a great advantage

since the fresh water rate has to be low in parts of a good device because most of the fresh water is to be transported through the membrane and over to the sea water.

By pressure retarded osmosis the most important members of loss will be in connection with pressurizing sea water, pumping of water through the membrane module and loss by conversion of pressure energy in water into electric energy by means of turbine and generator.

Due to loss of friction a drop in pressure will develop over the membrane module. The water must be pumped through a narrow channel which is provided with a distance net to keep the required width of the channel, and which at the same time can promote mixing of the water phase. Thus the thickness of the diffusion film can be reduced, and the efficiency in the PRO process can be improved.

In PRO processes with a good membrane module concentration polarization will only be a considerable problem at the sea water side, since the salt concentration at the fresh water side only shows a low increase. Further, the rate at the sea water side will be higher than at the fresh water side, because fresh water is transported over to the sea water, and because there exists a desire to maintain the highest possible salt concentration in the sea water. The latter is achieved by having a high through flow of salt water, but the gain of a high salt water rate has to be considered versus the expenses. The rate of the salt water can be increased by recycling of salt water.

In a membrane process sea water is pressurized before it flows through the membrane module. Then the sea water together with the fresh water which has been transported through the membrane, will expand through a turbine. The pump as well as the turbine will have an efficiency of less than 1, and energy will consequently be lost in these unit operations.

To reduce the loss when large quantities of sea water first have to be compressed, and then expanded through a turbine, pressure exchange can be

used. In pressure exchange the pressure in outgoing diluted sea water (brackish water) is used to compress incoming sea water. Only a quantity of water corresponding to the fresh water which flows through the membrane will pass through the turbine, and a far smaller turbine can therefore be used. The high pressure pump for pressurizing the sea water is completely eliminated.

Figure 1 describes a PRO plant wherein both fresh water and sea water are fed into separate water filters prior to the streams passing each other on each side of a semi-permeable membrane. A portion of the mixture of permeate and salt water with elevated pressure is passed to a turbine which is connected to a generator for the production of electric power. The rest of the permeate stream is passed to a pressure exchanger where incoming sea water is pressurized. The pressurized sea water is then fed into the membrane module. The plant further comprises water filters for purification of a salt containing and a less salt containing feed stream, one or more semi-permeable membranes or membrane modules.

Figure 2 shows the stream pattern for cross-stream in a spiral module.

Figure 3 shows stream lines in a spiral module.

Figure 4 shows the build-up of the interior structure of a membrane, a non-porous layer, called diffusion skin, and one or more porous layers.

Figure 5 shows the relation between pressure on the one side of the membrane which is in contact with a quantity of salt containing water (the sea water side), and osmotic flux. When A is 10-11 m/s/Pa and B is 3 10-8 mus and both are constant, it can be seen that a lower value of S gives better perfomance of the membrane. Technical economic calculations show that S must have a value of 1, 5 10-3 m or lower. Laboratory measurements have shown that the membranes intended for reverse osmosis, which gives the best performance in pressure retarded osmosis, have S-values around 3 10-3 m. This means that S has to be improved with a factor of 2 or more in relation to these membranes. Lower values of B will modify the requirement for S.

Figure 6 shows effect as function of pressure at the sea water side for a process with conditions as given in table 2.

Figure 7 shows concentration relations along membrane for PRO with conditions as given in table 2 (the salt concentrations at the fresh water side are hardly visible).

Figure 8 shows volume flux of water through the membrane for a process with conditions given in table 2.

Figure 9 shows that the osmotic flux is increasing with a factor of 8 after a pressure increase from 1 to 4 bar absolute. The gas volume in the membrane, Vgas, is then reduced to one fourth because the volume of a gas is in inverse ratio with the pressure.

Figure 10: The degree of hydrophobic character can be characterized by the so- called contact angle, 0, between the boundary surface for water and gas where this meets the solid surface of the membrane material inside the structure. This is general knowledge which is described in textbooks of surface chemistry. The angle faces the water phase. This is illustrated in figure 10.

Figure 11 shows the principle and a sketch of an apparatus for osmosis trials and measurement of Jv and Js. Since there are three parameters which have to be determined, at least three different measurements have to be carried out of salt and/or water flux.

Figures 12 and 13 show measured and modelled flux with different salt concentrations for membrane V93/5S.

The necessary values for the salt permeability, B, the water permeability, A, the structure parameter, S, and the thickness of the diffusion films will also apply to possible fiber membranes. A principal drawing for fiber membranes will be as for spirals with exception of that which concerns the use of flow breaking distance nets.

Examples of energy production: The mixing zone for salt water and fresh water can be considered as adiabatic, i. e. there is no heat exchange (dq=0) with the surroundings. Since the mixing enthalpy is approximately zero, and work (dw), but not heat, is extracted from the mixture, it is obtained from the energy preservation law : dE = cpdT= dq + dw = dw (g) wherein dE is the change in the inner energy of the total system and cp is the heat capacity of the system.

Extraction of work will according to equation (9) lead to a certain cooling. If one mole of fresh water with 52,5 J/mole is reversibly mixed with three moles of salt water, the diluted salt water will be cooled down with 0. 17°C. In a real process optimized for energy production per mixing unit, half of the reversible work will be taken out. This leads to a cooling of the mixture of less than 0.1 °C.

As mentioned above, only 50% of the possible mixing free energy will be utilized in a practical device to maximize the energy production. Further, energy will be lost by operation of the process. With the assumption that 20% of the energy which is produced in the mixing unit is lost in the process (loss because of friction, operation of pumps, turbines, etc. ) about 20 J per mole of fresh water which passes through the process could be produced. This causes an energy production for some locations based on mean rate of water flow according to the present invention as illustrated in table 1.

Table 1-Examples of possible power plants based on average water flow Example of rivers Water flow (m3/s) Power production (MW) Small local river 10 10 Namsen (Norway) 290 300 Glomma (Norway) 720 750 Rhine (Germany) 2 200 2 400 Mississippi (USA) 18000 19000 Examples of operating variables: For calculation of water and salt transport through the membrane as well as energy production per area unit of membrane, it is necessary to have real values of the different parameters which describe the actual membrane, the shape of the membrane module, parameters describing the process conditions, as well as some physical data. Necessary parameters for the calculations are summarized in table 2.

All calculations in the following are carried out on the basis of 1 m2 membrane area. Because the water and salt fluxes through the membrane in most cases are considerable in relation to the incoming rates of salt water and fresh water, the concentrations, and therefore also the fluxes through the membrane, will vary along the membrane. Taking this into consideration, the membrane is by the calculations divided into 20 cells of equal size. The concentrations and rates of salt water and fresh water, respectively, to the first cell, and the sea water pressure on the membrane, are given by the input conditions, see table 2. The fluxes of water and salt for these conditions are then calculated iteratively from cell to cell by means of the necessary equations.

The salt water rate, Q, out from the last cell, defines the rate out of the process.

The difference between out-rate and in-rate for salt water and the pressure at the salt water side indicates the produced work. The exploitation ratio of fresh water is indicated by the difference between fresh water rate in and out in relation to fresh water rate in.

Table 2-Necessary parameters for model calculations of pressure retarded osmosis. Symbol Unit Example value Parameter A m/Pa/s 10-Watertransport coefficient through the membrane B m/s 10-7 Salt leakage through the membrane x m 0, 15#10-3 Thickness of the membrane # 0.5 Porosity in porous layer 1. 5 Tortuosity in porous layer dsea m 25-10-6 Thickness of diffusion film at the sea water side df m 33#10-6 Thickness of diffusion film at the fresh water side T °C 3 Process (water) temperature Psea Pa 13 105 Pressure at the sea water side c'"sea mol/m3 549 Incoming concentration of salt in salt water c'"f moi/m 0 Incoming concentration of salt in fresh water Qin m3/s 9#10-6 Volume rate of fresh water in on the membrane F 2 Feed ratio between salt water and fresh water Ds M2/S 7. 5.10-'° Coefficient of diffusion for salt (NaCI)

For each single set of parameters fluxes and rates are calculated as stated above.

For determination of optimal sea water pressure the sea water pressure is always varied keeping other conditions constant.

In the calculations pressure loss through the membrane module because of the flow resistance has not been taken into account. Neither has the efficiency of the pump which pressurizes the sea water and the turbine which produces energy from the process been considered. Produced work as presented is therefore related to the energy production during the mixing process, and is not equal to the real work that can be extracted from a real process. Such dimensions have to be estimated for the plants in question.

The coefficient of diffusion for salt is increasing with approximately 80% when the temperature increases from 3 to 20°C, but it does not change much with the concentration of salt. The coefficient of diffusion at 0.1 moles/l is therefore used in all calculations. As an example of calculations a basis point has been taken in the conservative parameter values stated in table 2. At these conditions the membrane produces 3.4 W/m2, and 23% of the fresh water which is supplied to the membrane is transported over to the sea water side. Figure 1 shows the effect per area unit of membrane as a function of the pressure at the sea water side. As shown in the figure the effect has a relatively flat optimum area between 13 and 18 bars. By selecting a little more favorable values for the membrane thickness, film thickness and temperature, the energy production can easily be higher than 5 Wem2.

The concentrations over the membrane from the inlet side and to the outlet are shown in figure 7 for a sea water pressure of 13 bars. Because the salt leakage through this membrane is small in this example, the increase of the salt concentration at the fresh water side is hardly noticeable, and reaches a discharge concentration of 0.5 moles/m3. Correspondingly the concentration polarization at the fresh water side can be fully neglected.

On the other hand, the concentration polarization at the sea water side is considerable, and gives a concentration drop just below 100 moles/m3.

Correspondingly there is a concentration drop of almost 100 moles/m3 over the porous layers. The driving concentration difference over the skin of the membrane corresponds to the concentration difference between the surface of the skin against the sea water and the side of the adjacent porous layer which faces the sea water, see figure 7, and amounts to approximately 320 moles/m3, or barely 60% of the concentration difference between sea water and fresh water. This illustrates the importance of reducing the polarization effects. This is achieved by minimizing the thickness of the diffusion film at the sea water side (high flow velocity and good stirring), and the thickness of the porous layers.

Figure 8 shows the volume flux of water through the membrane as a function of dimensionless position from the inlet side. As the figure shows, the water flux changes relatively little, and the reason for this is that the driving concentration difference also is relatively constant along the membrane, see figure 7.

Example of insufficient wetting of a porous structure In traditional reverse osmosis, e. g. desalting of seawater to potable water, insufficient wetting will be no problem. Semi-permeable membranes developed for this purpose will therefore not be optimized with regard to this. The problem will, however, considerably reduce the efficiency of a membrane for PRO. This can be illustrated by a mathematical analysis of net salt transport in a typical considered PRO membrane. A reduction in porosity from 20 to 5% in a layer with thickness 10 or 20 tm inside such a membrane will give that part, N, of the total salt concentration which falls over the skin (ducs). N is therefore a direct measure of the effectivity in PRO, see equation (1) and (3). The thickness of the membrane through the porous layers is typically twice the thickness of the layer because of twisting, i. e. a relative membrane thickness of 2.0. Then the effectiveness of the structure in a 10 lim layer will be reduced from 95 to 50%. In a 20 pm layer it will be reduced from 88 to 1 %.

Example of increasing osmotic flux If the water filled PRO apparatus is set under pressure the volume of the gas will be reduced and the volume which has been made free will be replaced by water.

This method can be used to control if insufficient wetting is a problem in a particular membrane. Figure 9 shows the result of such a trial. Here the osmotic flux is measured for a typical reverse osmosis membrane by zero pressure difference over the membrane. The membrane has a porous structure of polysulphone, which is an hydrophobic (water repellent) plastic. It is difficult to wet hydrophobic materials, in contrast to hydrophilic (water attracting) materials.

In figure 9 it is shown that the osmotic flux increases with a factor of 8 after a pressure increase from 1 to 4 bar absolute. The gas volume in the membrane, Vgas, is then reduced to one fourth because the volume of a gas is in inverse ratio with the pressure: Vgas =Po/P (10) Here po is the pressure of the environment, approximately 1 bar, and p is the system pressure.

It is immediately clear that a pressure increase wil lead to an increase in the available water area within the structure. According to the scientific literature the necessary pressure for effective wetting will increase in inverse ratio with the size of the pores. The pores are smallest close to the skin, typical diameter 0. 1 Vtm and at least 10 nm, and biggest most far away from the skin, typical 2 m. In a more detailed analysis of the membrane it is relevant to consider one layer inside the membrane at a time, but it is not done in this example.

That the effect of a pressure increase is maintained after a pressure release, see figure 9, indicates that a part of the gas disappears from the membrane. But the result shown in figure 9 is no complete solution, because the membrane performance still is substantially lower than for a membrane with hydrophilic structure. As mentioned, more pressure is necessary to wet the smallest pores.

Because the smallest pores are approximately 20 times smaller than the biggest,

a considerable pressure is necessary to wet the whole structure and it is an unpractical method. It is further dubiously if the wetting is durable, because the material in the porous structure still will be hydrophobic and will be able to take up gas again.

Example of measuring Jv and Js To measure A, 8 and S at the same time a membrane is placed as a wall between to water containers with stirring. The transport of water and salt throgh the membrane over time, Jv and JS, can then be measured. The apparatus which is used for such measurements is shown in the principal drawing in figure 11. The apparatus is partly made of glass (for p=0) and partly of steel (for p>0).

Table 3-Results of measurements and modelling with experimental membranes made for PRO (salt force) Membrane A (m/Pa/ss) B (m/s) S=X/(#T) (mm) V93/2 5. 7-1 0-12 2. 5#10-7 0,48 V93/5S1 6. 0#10-12 2. 3-10-7 0,57 V93/5S2 5. 6#10-12 1. 8-1 0-7 0,96