WO2012084960A1

WO2012084960A1 - Membrane system for pressure retarded osmosis (pro)

Info

Publication number: WO2012084960A1
Application number: PCT/EP2011/073425
Authority: WO
Inventors: Eugen Reinhardt; Mirko REINHARDT
Original assignee: Statkraft Development As
Priority date: 2010-12-21
Filing date: 2011-12-20
Publication date: 2012-06-28

Abstract

The present invention relates to semi-permeable membranes and in particular to a semi-permeable membrane sheet, modules and cassettes. A particular use is in pressure retarded osmosis (PRO) driven power plants. Various aspect of the invention relates to different structures, comprising semi-permeable membrane sheets, typically being made from a foil of an impermeable material comprising a first opening, a second opening and a membrane opening. The membrane opening being covered by a semi-permeable membrane attached to the foil along the rim of the membrane opening.

Description

Membrane system for pressure retarded osmosis (PRO)

FIELD OF THE INVENTION

The present invention relates to semi-permeable membranes and in particular to a semi-permeable membrane sheet. A particular use is in pressure retarded osmosis, such as in pressure retarded osmosis (PRO) driven power plants. Various aspect of the invention relates to different structures, comprising semi-permeable membrane sheets, typically being made from a foil of an impermeable material comprising a first opening, a second opening and a membrane opening. The membrane opening being covered by a semi-permeable membrane welded to the foil along the rim of the membrane opening. The first and the second openings being adapted to allow fluid transport, and the membrane opening being adapted to allow fluid to flow through the semi-permeable membrane and the membrane opening.

BACKGROUND OF THE INVENTION

For centuries it has been known that when salt water and fresh water are partitioned in two different chambers separated 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 can occur when two solutions with different solute concentrations are separated by a semi-permeable membrane. The energy potential can be utilized by several technical methods where the energy can be recovered. Osmotic power plants have proven to have an enormous potential as to generation of power. Such plants comprise a membrane module wherein fresh water (feed fluid) and salt water (draw fluid) is separated by a semi-permeable membrane allowing feed to flow into the draw solution.

It has been demonstrated that pressure retarded osmosis in praxis beneficially can be used to generate power although there still remain some issues as to production of among others the membrane unit. For production of power by PRO the membrane area should be in the order of several hundred square meters to achieve an economical and effective process. Additionally the membranes need support to allow fixation in the membrane unit. As of today no such large sized membrane modules are available. Furthermore, such large membranes will be expensive and should be utilized to the highest degree possible. In addition the two fluids involved (the feed fluid and draw fluid) need to be efficiently distributed along the surface of the membrane requiring some kind of fluid distribution in the membrane unit.

Another issue regarding PRO power plants is malfunctioning of the membranes. Often malfunction is due to clogging of the membrane, breakage or the like, and remedy of such malfunction may be a tedious and labour intensive procedure which eventually may lead to disadvantageous down time for the plant.

The problems relating to use and production of semi-permeable membranes may be encountered in other technical areas than power production by use of pressure retarded osmosis.

Membranes are e.g. used in filtration such as ultra and nano filtration, forward and reverse osmosis. Such processes involves two fluids a 1st fluid and a 2nd fluid. In the filtration use, one fluid (e.g. the 1st fluid) represents a part removed from the other fluid and the process is often characterized in the one fluid enters a membrane unit and two fluids leaves the membrane unit. In osmosis and PRO both fluids are fed into a membrane unit and both fluids leaves the unit, wherein one fluid is increasing in volume while the other is decreasing in volume, in a two fluids in - two fluids out configuration.

A further difference between PRO and filtration as well as forward osmosis relates to mass transport. In PRO the mass transport will be from the low pressure side to the high pressure side while the mass transport during filtration will be from the high pressure side to the low pressure side. In other words in PRO the mass is transported against the pressure. In forward osmosis the pressure is low on both sides and the difference in concentration of the fluids will be the driving force of the process.

WO 89/05181 discloses an apparatus for separating a feed liquid into a

concentrate and a permeate fraction by membrane filtration. The apparatus comprises an array of cassette frames separated by intermediate plates. The array is adapted to be clamped together in its longitudinal direction and loosened, as required, to remove any selected cassette frame. Each cassette frame contains a filtration unit comprising a stack of membranes, in which a first flow passage system is provided which connects two free zones within the cassette frame, said free zones being so connected as to establish a series or parallel or combined series and parallel flow from a feed liquid inlet to a concentrate outlet. In each stack of membranes a second system of flow passages is also provided, which is isolated from said first system and serves to conduct permeate penetrated through the membranes to at least one separate outlet from each filtration unit. In the method of making a filtration unit, envelope-shaped membrane units are placed in a stack, a curable liquid binder is introduced between the membrane units from at least one surface of the filtration unit to a predetermined depth, and upon curing of the binder a layer of the binder together with the marginal portions of the edges of the membrane units embedded therein is removed.

However, the solution suggested by WO 89/05181 still suffers from the drawback of being difficult to produce, high price, prone to leakage, etc. and is in particular found unsuited for PRO and PRO power plants. The system according to WO 89/05181 is a typical filtration system configured for one fluid in - two fluids out and does not envisage the presence of a second feed port which allows a second fluid to get in contact with the first fluid across the membrane, and therefore being unsuitable for PRO. US 2005/0126981 discloses a disposable filtration device and more specifically a tangential or cross flow filtration cassette. The filtration cassette disclosed is characterized by a plurality of stacked subassemblies comprising porous membranes encapsulated in an overmolded frame having one retentate port, one feed port and one filtrate port. In this tangential flow cassette a fluid is pumped through the feed port into one of the many membranes channels and forced through the membrane into the filtrate channel. The filtrate fluid then exits the cassettes through the filtrate port, while the retentate fluid exits the cassette trough the retentate port. The filtration cassette as disclosed in US 2005/0126981 is found unsuited for being used in PRO. In PRO power plants two fluids are necessary, e.g. fresh water and salted water, which will be in contact across a semi-permeable membrane. The cassette disclosed in US 2005/0126981 does not envisage the presence of a fluid port which allows a second fluid to get in contact with a first fluid across the membrane, preventing therefore its suitability for PRO application.

Further, for production of power by PRO the membrane surface should have an area in the order of several hundred square meters to achieve an economical and effective process. The cassette disclosed in US 2005/0126981 is designed for filtering small volume of fluids and its membranes and structures are not adapted to be used in a PRO system, i.e. are not design to withstand the pressures necessary for PRO, are not suitable in size and may collapse during usage preventing fluid to flow across the membranes.

Furthermore, in the filtration cassette disclosed in US 2005/0126981, during normal filtration, the fluid mass transport is driven by a pressure gradient, i.e. the flow of fluid across the membrane is driven by a higher pressure on one side of the membrane towards a lower pressure on the other side of the membrane. As mentioned, this is not the case for PRO where mass transport occurs against the pressure, i.e. mass transport occurs from the feed side, having a low pressure, to the draw side, having a high pressure. This further shows the unsuitability of this filtration cassette when applied to PRO. US 5,429,742 discloses a plastic frame filter unit for a stack assembly defining three apertures and having a sheet of filter medium inserted by molding.

This filter unit is not suitable for being used in a PRO system. Firstly, the unit disclosed in US 5,429,742 is a rigid construction having a low degree of flexibility. The unit frame is plastic molded and when encapsulated in a stack of filter units, the frame has the function of bearing the load of the construction of the stack of filter units. The frames are not freely moveable within the stack of filter units as the frames are arranged so to support each other by having their surfaces in complete contact with each other in a compact solid carrying construction.

More importantly, in this filter unit three apertures, for feed, filtrate and retentate, are defined, thus hindering the use in PRO. Accordingly this filter unit does not envisage the presence of a fluid port that allows a second fluid to get in contact with a first fluid across the membrane as such a port would be incompatible with its use as a filter. Hence, an improved membrane device adapted to be used in a PRO power plant would be advantageous, and in particular a more efficient and/or reliable membrane device would be advantageous.

OBJECT OF THE INVENTION

It is a further object of the present invention to provide an alternative to the prior art. In particular, it may be seen as an object of the present invention to provide a device that solves the above mentioned problems of the prior art with respect to inter alia size, easy production, low price, flexible construction and non-leakage. SUMMARY OF THE INVENTION

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a semi-permeable membrane sheet

- an interim component comprising two semi-permeable membrane sheets and a spacer

an element comprising two interim components and a further spacer a stack of elements

a cassette comprising a stack of elements having a defined size and structure elements

a membrane module comprising one or more cassettes as defined in the accompanying claims. In the present context a number of terms are used in a manner being ordinary to a skilled person. However, elaborations on some of the terms used are presented below.

1st and 2nd fluid refers to the two fluids present on each side of the semi- permeable membrane. The actual compositions of the 1st and the 2nd fluid typically change during their presence on each side of the semi-permeable membrane. In many applications, the 1st fluid may be termed feed fluid or feed solution and the 2nd fluid may be termed draw fluid or draw solution. In embodiments pertaining to PRO, the feed fluid is typically fresh water and the draw fluid is typically salt water typically being sea water. Hence, the difference between the feed fluid and the draw fluid is the difference in concentration of the solute(s) (in this case the salt) wherein the concentration of solute is lower in the feed fluid than the draw fluid. Fluid refers to substances being in liquid phase.

The invention is particularly, but not exclusively, advantageous for obtaining PRO. The present invention provides easy production of module parts and the membrane module itself in which the osmotic process takes place.

In the present description joining of semi-permeable membrane sheets, foils, interim components, elements, etc. are disclosed with focus on assembling by welding. However, it is envisaged that other joining techniques such as gluing may be applicable and advantageous in connection with the present invention. Accordingly, the invention relates inter alia also to the herein disclosed semipermeable membrane sheets, elements and stacks being glued together.

Similarly, the membrane e.g. is attached to the foil and focus is placed on welding although gluing is considered part of the invention. Preferably, the joining and attaching results also in a seal between components in question being obtained.

Thus, the invention relates in a first aspect to semi-permeable membrane sheets comprising a foil and a semi-permeable membrane. The foil may advantageously be made from an impermeable material comprising a first opening, a second opening and a membrane opening, all openings being through going openings in the plane of the foil whereof the membrane opening is covered by the semipermeable membrane being attached to the foil along the rim of membrane opening so as to attach the semi-permeable membrane and provide a seal between the rim of the membrane and the foil, and the other openings are adapted to allow fluid to flow through the membrane sheet. The function of this semi-permeable membrane sheet is related to the separation of the fluids allowing mass transport only across the semi-permeable membrane while the foil is impermeable. In that the function of the semi-permeable membrane sheet is not related to bearing the load of the construction of the stack of semi-permeable membrane sheets and will have a high degree of flexibility. Accordingly, the element/stack of the present invention may be seen as a floating construction inside the cassette, where flexible elements and stacks are able to move, i.e. float, upon flow and pressure variations as they are joined to the cassette self-carrying construction only at their ends by means of supporting elements.

This floating construction of elements and stacks originates also from the high degree of flexibility of the membrane sheets, due to their small thicknesses and large area, combined with the presence of spacers in between semi-permeable membrane sheets.

One of the advantages of this floating and flexible construction is that a low pressure drop across the semi-permeable membrane is achievable. In turn, this low pressure drop implies that a reduced loss of energy will occoure in a PRO power plant and thus an increase in energy conversion efficiency of the plant is obtained.

In preferred embodiments, the membrane opening may be centrally placed in the foil and the first and second openings may be placed on opposite sides of the membrane opening.

Preferably, foils according to the invention may be octagonal, rectangular, stretch diamond, stretch hexagonal, circular or ellipsoid shaped, the membrane opening being centrally placed in the foil and the first and second openings being placed on opposite sides of the membrane opening along an axis going through the centre of the membrane opening and the centres of the first and the second opening.

A fast way of producing the semi-permeable membrane sheet according to the present invention is obtained by welding. Accordingly, the semi-permeable membrane may preferably be attached to the foil by welding. Alternatively, the semi-permeable membrane may be attached to the foil by gluing.

Semi-permeable membrane sheets according to the present invention may be given a large area and in a preferred embodiment being advantageous for use in PRO the membrane opening may be rectangular preferably having side lengths of lm, the first and the second opening preferably being circular preferably with a diameter of 80-120mm. Preferably, the semi-permeable membrane sheet being attached to the foil by a welding and the welding being preferably applied along the rim of the membrane opening, typically in a distance of 0.25 to 2cm from the edge of membrane opening. Semi-permeable membrane sheet according to the present invention may preferably be from a plastic material. Alternatively or in combination thereto the semi-permeable membrane may preferably be adapted for pressure retarded osmosis. Semi-permeable membrane sheets according to the present invention are used in PRO driven power plants. These membrane sheets may consist of a thin layer of a non-porous material, also referred to as the diffusion skin, and one or more layers of a porous material, also referred to as the porous layer. The porous layer of the PRO semi-permeable membrane sheet may be characterized by a structure where porosity φ, thickness x (m), and tortuosity τ, stand in relation to one another as given by the equation below

X · τ = φ · S Equation (1) where S is a structure parameter. Such semi-permeable membrane sheet suitable for PRO may have a S value of 0.0015 meter or lower.

In a second aspect, the invention relates to an element comprising four semipermeable membrane sheets according to the first aspect of the invention.

Preferably, the semi-permeable membrane sheets being arranged adjacent to each other and the element comprising two interim components each comprising two semi-permeable membrane sheets being joined to each other by joining the rims of the first

respectively the second openings of each of the two sheets, so as to fasten the two semi-permeable membrane sheets to each other and provide a sealing,

wherein

said two interim components being arranged upon each other and the foils being adjacent to each other being joined along their outer rims, so as to fasten the two foils to each other and provide a sealing.

Preferably, elements according to the second aspect may comprise first spacers being interposed between the semi-permeable membrane sheets of the interim components. Alternatively or in combination thereto, the element may preferably comprise a second spacer arranged between said two interim components being arranged upon each other.

Advantageously, the first and the second spacers are preferably adapted to during use keep the membrane sheets spaced apart, allowing fluid to pass through them while preventing permanent indentations in the semi-permeable membranes. Alternatively or in combination the first spacer and/or the second spacer may preferably acts as support for the membrane and may preferably be made from a fluid permeable material.

When in use in a PRO system the presence of spacers in the element provides support to the flexible membrane sheet and ensures avoiding collapse of the membranes on each other, which would prevent fluid flow across the membranes.

The elements are therefore adapted to allow for inflow of two fluids, i.e. the feed and the draw fluid, separated by the semi-permeable membrane and for outflow of two fluids, i.e. the feed fluid and the draw fluid including part of the feed fluid.

The four membrane sheets being preferably joined by welding; the welding along the rims being preferably applied in a distance of 0.25 to 2cm from the edges of the first and the second openings and from the edges of the outer rims of the foils. Alternatively, the four membrane sheets are preferably joined by gluing; the gluing along the rims being preferably applied in a distance of 0.25 to 2cm from the edges of the first and the second openings and from the edges of the outer rims of the foils.

In a third aspect, the invention relates to a stack of semi-permeable membrane sheets. Such stacks may preferably comprise a plurality, such as more than 50, preferably more than 100, even more preferably more than 150 elements according to the second aspect of the invention. The plurality of elements is preferably stacked and the elements are joined by joining the neighboring foils of adjacent elements along their outer rims so as to fasten the two foils to each other and provide a sealing. Preferably, the plurality of elements may be stacked with second spacers interposed between adjacent foils of adjacent elements. The second spacers may preferably be adapted to during use keep the membrane sheets spaced apart, allowing fluid to pass through them while preventing permanent indentations in the semi-permeable membranes. The joining may preferably be provided by welding preferably being applied in a distance of 0.25 to 2cm from the edges of the outer rims of the foils. Alternatively, the joining may preferably be provided by gluing preferably being applied in a distance of 0.25 to 2cm from the edges of the outer rims of the foils. In a fourth aspect the invention relates to a cassette comprising

a stack of semi-permeable membrane sheets to the third aspect of the invention,

support elements attached in a sealed manner, preferable by welding or gluing, to the two outermost semi-permeable membrane sheets of the stack, the support elements comprising openings for allowing fluid to flow into the interior volume defined by the stack, and

structure elements adapted to spread out the stack of elements.

The structure elements comprises preferable a casing confining the stack and the support elements in a fluid tight manner, the casing comprising inlets and outlets for the fluids to enter into and out of the stack. Alternatively, the structure elements may preferably comprise supporting poles, preferably being rod-shaped elements, extending between the support elements. In a fifth aspect, the present invention relates to a membrane module comprising a module house with fluid connections to allow fluid to flow into and out of the interior of the house, wherein one or more cassettes according to the fourth aspect of the invention being arranged inside the house and having fluid connections extending from one or more of the support elements to the outside of the house or to a next cassette so as to allow fluid to flow into and out of the interior volume defined by the stack of semi-permeable membranes, as well as having fluid connections between the support elements of the different cassettes to allow fluid to flow into and out of each cassette. Preferably, the fluid connections may be arranged so as to provide a cross flow between fluid flowing in the interior of the house and fluid flowing in the interior volume. Alternatively or in combination thereto, the fluid connections are preferably arranged so as to provide a counter current between fluid flowing in the interior of the house and fluid flowing in the interior volume.

In a sixth aspect, the invention relates to a pressure retarded osmosis power plant comprising one or more membrane modules according the fifth aspect of the invention. In a seventh aspect, the invention relates to use of the various other aspects of the invention for pressure retarded osmosis.

The different aspects of the present invention may each be combined with any of the other aspects. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

Various aspects of the invention will now be described in more detail with regard to the accompanying figures. The figures show different ways of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the present invention.

Figure 1 shows schematically a pressure retarded osmosis power plant;

Figures 2a-d show schematically assembly of a semi-permeable membrane sheet, an interim component and an element;

Figures 3a-d show schematically a cassette comprising a stack of elements having a defined size and structure elements;

Figures 4a-d show schematically a module comprising three cassettes and a module house; and

Figures 5a-b show a semi-permeable membrane unit comprising several membrane modules. DETAILED DESCRIPTION OF AN EMBODIMENT

Figure 1 shows schematically a PRO power plant. The power plant comprising one or more membrane modules 1 in a unit receiving freshwater through a filter 2. The membrane modules 1 comprising semi-permeable membranes 3 allowing freshwater to flow into salt water through the semi-permeable membranes 3. Salt water is fed into the membrane modules 1 of the unit via a filter 5 and a pressure exchanger 4 maintaining the pressure. Salt water from the outlet of the membrane modules 1 is partly fed into a turbine 6 for producing e.g. electrical energy, and partly fed into the pressure exchanger 4. Following the above notation fresh water is the feed fluid or 1st fluid and sea water is the draw fluid or 2nd fluid.

Figure 2a shows a semi-permeable membrane sheet 7 comprising a foil 8 and a semi-permeable membrane 9. The foil 8 is made from an impermeable material (typically being plastic) having a thickness of less than 280μηι and comprising a first opening 10, a second opening 11 and a membrane opening 12. The openings 10, 11 and 12 are through going openings in the plane of the foil 8. The semipermeable membrane 9 is arranged to cover the membrane opening 12, and the semi-permeable membrane 9 is welded 13 to the foil along the rim of membrane opening 12. The welding 13 is provided so that it establishes a sealing.

Employing thin foils has the advantage that more semi-impermeable membrane sheets can be contained per volume of cassette. Due to its small thickness, i.e. less than 280μΓη, and to the large dimension of the semi-permeable membrane sheets, i.e. in the square meter size, the foil has a very high degree of flexibility. The flexibility of the foil may also be due to the intrinsic flexibility properties of the material used to produce the foils, e.g. plastic materials. In PRO, as the foil, element and stack of elements are not a load bearing construction, the use of flexible foils may be of great advantage, e.g. allowing for a floating construction. Figure 2b shows two of the semi-permeable membrane sheets 7 (shown in fig. 2a) combined with a first spacer 14a having the same outer contour as the foil 8 although being smaller than the foil but larger that the membrane 9. The spacer 14a comprises openings 15 with centres coinciding with the centres of the openings 10, 11.

The spacer 14a is made from a fluid permeable material being non-compressible to an extent where two semi-permeable membrane sheets 7 arranged with a spacer in between does not collapse during use (which would prevent fluid flow across the membranes). The selection of suitable spacers often depends on the use, pressure levels and desired mass flow. Typically, a spacer should be selected so as to allow fluid to flow across and in contact with the surface of the

membranes and the surface of the spacer should be sufficiently even to avoid generation of indentations in the membrane; the latter suggest considering selection of membrane and spacer dependently on each other. Thus, a spacer is considered to be a component which during use allows transport of fluid across the membranes and at the same time supports the membranes without destruction of the membranes. In addition, the spacer 14a should advantageously create a turbulent or turbulent like flow in the fluid flowing by the semi-permeable membrane 9. The openings 15 being slightly larger than the openings 10, 11 so as to allow the two foils 7 to be welded together along the rim of the openings 10 and 11 as indicated in fig. 2b with numerals 16. The weldings 16 are provided so that they establish sealings. Thereby an interim component 17 comprising two foils 8 and one spacer 14a for an element of the semi-permeable membrane stack or cassette is produced.

Figure 2c shows the assembly of an element 18. Two of the interim components 17 (shown in fig. 2b) are arranged upon each other with a second spacer 14b (similar to the use of spacer 14a in figure 2b) in between. The foils adjacent to (neighbouring foils) each other 7' and 7" are welded together along their outer rims as indicated by numerals 19. The welding 19 is provided so that it

establishes a sealing. Such element may - depending on the actual production process - be considered as building blocks for a stack of elements.

It is noted, that although the first spacers 14a of the interim components 17 and the second spacers 14b arranged between the interim components 17 are disclosed to be similar or even identical they may differ in certain embodiments. As will become clear from the description of use of the elements, the draw fluid is in contact with the spacers 14a of the interim components 17 and the feed fluid is in contact with the spacers 14b arranged between the interim components 17 (or vice versa). Depending of the characteristics e.g. viscosities of the feed and the draw fluid the spacers 14 involved may be selected differently e.g. to provide little flow resistance and good contact between the fluid and the membrane.

Figure 2d shows a cross section (along A-A in figure 2c) of the element 18 of fig. 2c in expanded form (reference is also made to figure 3), that is expanded due to the presence of spacers (not shown for clarity reasons) and/or due to a pressure difference between the interior and exterior of the stack. The element 18 comprises four semi-permeable membrane sheets 7 being arranged adjacent to each other.

Two of the semi-permeable membrane sheets 7, one sheet 7' from the first interim component and one sheet 7" from the second interim component, being joined to each other by a weld 19 along the outer rim of the foils 8; the spacers are not shown in figure 2d.

The element 18 has interiorly a second spacer 14b being located in between the two neighbouring foils (of the semi-permeable sheets 7' and 7") being welded along the outer rim, and two exterior first spacers 14a being located between the neighbouring foils of the interim components welded together along the openings 10 and 11 (spacers are not shown).

As disclosed above, the various foils are welded along rims and the weldings are provided so that they establish sealings. Typically, along the rims means a welding being applied in a distance of 0.25 to 2cm from the edges of the rims in question. Accordingly, welding seams are located inside of the rims in question.

Figures 3a and 3b show different embodiments of a cassette 27. In general a cassette 27 comprises a stack of elements 18 having a defined size and structure elements 21. In all of these embodiments the outermost semi-permeable membrane sheets are fastened to support elements 22a and 22b along their outer rims or as shown in figure 3a and 3b along the openings 10 and 11. The function of the structure elements is to spread out the stack of elements in a

predetermined manner to ensure a certain inner volume of the stack to allow easy flow of the fluids. Contemplated structure elements are, e.g., a casing 21a as shown in figure 3a, supporting poles 21b as shown in figure 3b and a frame (not shown). Figure 3a shows a cassette 27 comprising a stack of elements arranged inside a compartment 20 defined by a casing 21a. The dimensions of the compartment 20 are only illustrative. In many applications the compartment will have an elongated shape as the number of semi-permeable membrane sheets 7 may be in the order of 50, such as 100 or even higher.

Figure 3b shows a cassette 27 comprising a stack of elements and supporting poles 21b. The poles are fastened to the support elements 22a and 22b arranged close to the rim of the support element. Thereby the sides of the cassettes are open for fluid to flow into the cassette. In figures 3a and 3b, numeral 9 indicates as in figure 1 the semi-permeable membranes. It is noted, that although not shown in figures 3a and b, spacers are provided in the stack of semi-permeable membrane sheets. The stack of membrane sheets is produced by repeating the process forming elements 18 and stacking such elements upon each other and welding the elements together by welding the rims of adjacent foils of two elements 18 together, similarly to welding two interim components 17 together to form an element 18.

The stack of semi-permeable membrane sheets defines a number of flow channels 25, in the interior of the stack of semi-permeable membrane sheets, being in fluid communication with each other. The flow channels 25 are by way of example defined as the region in between two adjacent semi-permeable membrane sheets 7 welded together along their outer rims and welded to adjacent foils along the rims of the openings 10 and 11. Internal fluid connections 26, connecting neighbouring channels 25, are provided by the openings 10 of two adjacent semipermeable membrane sheets 7 being welded together along the rims of the openings (similarly for openings 11 of two adjacent semi-permeable membrane sheets 7).

As shown in figure 3a two voids are thereby provided inside the compartment 20. One void VI is defined by the interior of the stack of semi-permeable membrane sheets and another void V2 is defined between the boundaries of the

compartment 20 and the exterior of the stack of semi-permeable membrane sheets.

In both figure 3a and 3b the outermost semi-permeable membrane sheets 7a and 7b are fastened to support elements 22a and 22b. The fastening is preferably provided by welding but other suitable fastening techniques may be applied. The fastening provides a fluid tight sealing between the semi-permeable membrane sheets 7a, 7b and the support elements 22a and 22b, respectively.

Figure 3c is a schematic drawing of the support elements 22a and 22b. The support elements 22a and 22b are shaped so as to allow fluid to be exchanged between void VI (the interior of the stack of semi-permeable membrane sheets) and the connections 23a and 23b, respectively. This is typically provided by the support elements 22a and 22b comprising bores for receiving the tubular shaped connections 23a and 23b in a fluid tight manner so as to allow fluid to flow through the connections 23a and 23b and into void VI without flowing into void V2. The support elements are thicker than the foils and sufficiently stiff to allowed the supporting function and at the same time be allowed for welding of the membrane sheets. In figure 3a the compartment 20 also comprises fluid connections 24a and 24b for fluid to be exchanged between the void V2 (defined between the boundaries of the compartment 20 and the exterior of the stack) and the exterior of the compartment 20. Typically, the connection 23a is connected to a feed source which with reference to figure 1 is the feed of fresh water. The feed flows into the void VI via the connection 23a and as adjacent semi-permeable membrane sheets 7 are welded together at the openings 10, 11 and along the outer rims (as disclosed above) the feed is distributed to the channels 25 formed inside the void VI by the internal fluid connections 26. Thereby, the feed flows across the surface of the membrane 9 towards the downstream located internal fluid connections 26. Accordingly, the feed flowing into the stack of semi-permeable membrane sheets is distributed into parallel streams in the various channels 25 where after the feed flows towards the downstream connections 26 and is collected into a single stream leaving the compartment 20 through the connection 23b.

The draw fluid flows into the void V2 of the compartment 20 through connection 24a. The draw fluid is distributed and flows across the surface of the semipermeable membrane sheets 7, typically in a direction being perpendicular to the direction of the flow of the feed in the channels 25. Thereby the feed flows on one side of the membranes 9 and the draw fluid flows on the other side of the membrane. This will result in a flux of feed through the membranes 9 into the draw fluid providing an increased volume and thereby an increased pressure in the draw fluid. The draw fluid with added feed fluid having an increased volume will leave the compartment 20 through connection 24b. For use in PRO power plants a connection 24a to allow the draw fluid to flow into the void V2 of the compartment 20 is needed. In this either the support elements 22a, 22b comprise the connection 24a or the compartment 20 may comprised the connection 24a.

The cassette shown in figure 3b comprises a stack of semi-permeable membranes and supporting poles 21b. The cassette is, during use, arranged into a

confinement (not shown) e.g. in the form of a module house or container with suitable fluid inlets and outlets. The cassette shown in figure 3b can be used as disclosed in relation to figures 4a-d. In such cases the void V2 will comprise the entire inner volume of the module house or container not being occupied by the void VI. In many advantageous embodiments of the cassette 27, the stack of semipermeable membranes sheets XI is embodied in a casing 21a comprising the stack XI and the support elements 22a and 22b (see figure 3a). The support elements 22a and 22b, e.g. as shown in figures 3c and 3d, often comprise two holes each. These holes are adapted to receive a tubular shaped connection 23a or 23b and a plug X3 closing the remaining hole of support element. By selecting the position of connection 23a or 23b and the position of the plug, the flow path internally in the cassette 27 may be selected. Additionally to the connections 23a, 23b the support elements may also comprise holes/connections 24a, 24b as shown in figure 3d for inlet/outlet of draw fluid into the void V2. In another aspect of the invention the flow direction within a module 1 can be selected similarly by plugging or connecting the holes in the support elements in the different cassettes in the module. Accordingly, the flow pattern through both cassettes and modules can be controlled by plugging or connecting the bores in the support elements. This selection of the flow directions by means of plugging or connecting the holes in the support elements is termed plug flow.

For the purpose of insertion and extraction of cassettes 27 the membrane module 1 comprises a closeable opening (not shown) allowing insertion into and extraction of cassettes 27 out of the membrane module. The cassette 27 of fig. 3a is equipped with tubular shaped connections 23a and 23b. The remaining two openings in the support elements 22a and 22b

respectively are closed by one or two plugs. Considering plug flow through the various channels is it often advantageous that a fluid element of the feed fluid and a fluid element of the draw fluid do not flow across the membranes 9 in a 1: 1 relation (that is a fluid element of the feed fluid and a fluid element of the draw fluid flows across the membranes 9 equal number of times) but may e.g. be 3: 1 (a fluid element of the feed fluid flows across the membranes 9 three times while a fluid element of the draw fluid flows across the membranes 9 one time). A reason for non-unity between numbers of passages of membranes 9 may be that utilization of e.g. the feed is not obtained in a 1 : 1 configuration. Such issues may advantageously be solved by configuring the system so that e.g. the feed fluid flows across the membrane area three times whereas the draw fluid flows across the membrane area one time.

Fig. 4a shows a configuration with a flow through a module 1 being 3: 1. The module 1 comprises three cassettes X2a, X2b, X2c in a module house 29. By arranging plugs X3 in the openings of the support elements 22a and 22b of the cassettes as indicated in fig. 4a a zig-zag flow of the feed fluid is obtained while the draw fluid flows straight and parallelly through the cassettes. The combination of a zig-zag flow for the feed fluid and a parallel flow of the draw fluid provides the effect that the feed fluid flows across a membrane three times while a draw fluid flows across a membrane one time. Thus, in the configuration of fig. 4a the feed fluid flows in serial and the draw fluid flows in parallel through the

configuration.

It is noted, that the above considerations as to flow configurations being 1 : 3 resides in pure geometric considerations and the assumption of plug flow. In many practical implementations, other effects and means make contributions to whether e.g. a draw fluid is saturated during its passage through the module 1. Such means and effects are pressure drop over the module 1, viscosity of the fluids, and the like. Fig. 4b shows a configuration with a flow through a module 1 being 1 : 1. As in fig. 4a, the module comprising three cassettes X2a, X2b, X2c in a module house 29. Only plugs X3 are arranged in the upper and lower most support elements 22a and 22b whereby the feed fluid and the draw fluid are distributed evenly in parallel flow pattern through the module 1 as indicated in the figure. Thus, in the configuration of fig. 4b both the feed fluid and the draw fluid flow in parallel through the configuration.

The configuration of fig. 4c is similar to the configuration shown in fig. 4a except that the inlet and outlet for the draw fluid are located at the lower right and upper left corner of the figure. The flow pattern obtained by the configuration of figure 4c is as in figure 4a 1 : 3.

The configuration of fig. 4d is similar to the configuration shown in fig. 4b except that the inlet and outlet for the draw fluid are located at the lower right and upper left corner of the figure. The flow pattern obtained by the configuration of figure 4d is as in figure 4b 1: 1.

It is noted that the modules disclosed in fig. 4a-d all have an extension normal to the plane of the figures shown. The inlets and outlets are typically centrally arranged relatively to the dimension normal to the plane of the figures. However, the inlets and outlets may be arranged differently.

In many practical implementations of power plants a number of such modules 1 are arranged in parallel as indicated on figures 5a and b (four are shown in figure 5a and b). The modules may also be arranged in serial or in combinations of parallel and serial.

The modules 1 are exemplified with reference to figure 4 and the outer geometry is defined by the module house 29. It is noted that although the modules 1 are disclosed as box-shaped this should not be interpreted as limiting. Other shapes such as cylindrical or even non-elongate shaped may be selected. Materials

A semi-permeable membrane sheet is typically manufactured from the following materials:

Foil: plastic materials, e.g. Misubishi Polyester Film Hostaphan RN 125 - Membranes: any type of existing semi-permeable membranes e.g. GE C series membrane

Spacer: polypropylene diamond spacer e.g.

Support elements: polyamide plate e.g. Dimensions

The foil is to some extend dictating the outer contour of the stack of semipermeable membranes 7. In the above examples, the outer contour of the foils 8 is octagonal and the membrane opening 12 is rectangular. However, other shapes may be used for both the foil and the membrane opening 12.

Typically, by way of example the dimensions of the foil are:

Dopening io=Dopening liin the area of 80 to 120mm

Shape of membrane opening = rectangular with an area of around lxlm² Thus, for a set-up with a module having a membrane area of 2000m², 2000 semipermeable membrane sheets 7 are used. These sheets are assembled into cassettes each having 500 elements 18. Accordingly, this module is divided into 4 cassettes. The membrane dimension is crucial for use in PRO power plants and therefore for producing power by PRO the membrane area should be in the order of several hundred square meters to achieve an economical and effective process. Producing membranes having such a large active area and at the same time achieving the properties needed for application in a PRO system is not straightforward. Large size membranes have for example the drawback of collapsing and thereby preventing fluid flow across the membrane. Other problems with large area membrane is that the sealing along large perimeter may cause leakage problems in particularly upon use under the pressures involved in a PRO driven power plant. The element of the invention, comprising large area membrane, having a structure, e.g. the presence of spacer, and an optimal sealing of the membrane to the foil, e.g. by welding, complies with the requirements for PRO application overcoming the potential problems deriving from the use of large area membrane. Generally, the pressure involved in PRO is in the range between 5 and 30bar depending on the solute concentration of the fluids used. For example, the pressure involved in PRO for the draw side, e.g. seawater, of a semi-permeable membrane may be between 5 to 26bar. If fluids with higher concentration of salts are used the pressure may exceed this range. A typical pressure range for PRO is in the range between 5 to 20bar. Welding

Welding is performed in the following manner with the following characteristics. The foil to foil welding comprises at least two distinct weldings. The weldings along the rims of the openings 10, 11 are circular performed in a single step and the weldings along the outer rims of the foils are linear performed along one rim section at a time. The circular weldings are performed by a circular high frequency welding device and the linear weldings are performed by a linear high frequency welding device. Applied welding time was 0.2s, followed by a 10s long cooling period. The welding pressure applied was 5.2bar. The foil to membrane weldings are linear welding done along one or more rim sections of the membrane opening at a time. The welding is performed for example by a linear high frequency device having a temperature of 185°C, with a welding time of 15s, following by 30s long cooling period. The welding pressure applied was 2bar. However, these parameters might change with use of different membranes, foils, spacers and the size of the welding, and accordingly suitable welding parameters may be selected during tests.

Welding has the advantage of providing tight sealing, where other sealing methods may give rise to leakage problem in use in PRO power plants.

Further, welding allows for minimization of the area used for sealing. Due to the characteristics of the welding technique only a small area at the edge of the membrane/foil is needed to provide tight and efficient sealing. In this way membrane surface area is efficiently used as a minor part of it is involved in the sealing in comparison to other sealing methods where large parts of the

membrane edge are necessary enclosed in the sealing to provide a tight sealing. Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

ITEMS

The invention will now be described in further details in the following non-limiting items. 1. A semi-permeable membrane sheet (7) comprising a foil (8) and a semipermeable membrane (9), the foil being made from an impermeable material comprising a first opening (10), a second opening (11) and a membrane opening

(12) , all openings being through going openings in the plane of the foil whereof the membrane opening is covered by the semi-permeable membrane being attached to the foil along the rim of membrane opening so as to attach the semipermeable membrane and provide a seal between the rim of the membrane and the foil, and the other openings are adapted to allow fluid to flow through the membrane sheet. 2. A semi-permeable membrane sheet according to item 1, wherein the

membrane opening is centrally placed in the foil and the first and second openings are placed on opposite sides of the membrane opening.

3. A semi-permeable membrane sheet according to any of the preceding items, wherein the foil is octagonal, rectangular, stretch diamond, stretch hexagonal circular or ellipsoid shaped, the membrane opening being centrally placed in the foil and the first and second openings being placed on opposite sides of the membrane opening along an axis going through the centre of the membrane opening and the centres of the first and the second opening.

4. A semi-permeable membrane sheet according to any of the preceding items, wherein the semi-permeable membrane being attached to the foil by welding

(13) . 5. A semi-permeable membrane sheet according to any of the preceding items, wherein the semi-permeable membrane being attached to the foil by gluing.

6. A semi-permeable membrane sheet according to any of the preceding items wherein the membrane opening is rectangular having side lengths of lm, the first and the second opening being circular with a diameter of 80-120mm. 7. A semi-permeable membrane sheet according to any of the preceding items, wherein the semi-permeable membrane sheet being attached to the foil by a welding, the welding being applied along the rim of the membrane opening, typically in a distance of 0.25 to 2cm from the edge of membrane opening.

8. A semi-permeable membrane sheet according to any of the preceding items, wherein the foil is made from a plastic material. 9. A semi-permeable membrane sheet accordingly to any of the preceding items, wherein the semi-permeable membrane is adapted for pressure retarded osmosis.

10. An element (18) comprising four semi-permeable membrane sheets (7) according to the any of the preceding items, the semi-permeable membrane sheets being arranged adjacent to each other, the element comprising

two interim components (17) each comprising two semi-permeable membrane sheets being joined to each other by joining the rims of the first respectively the second openings (10, 11) of each of the two sheets, so as to fasten the two semi-permeable membrane sheets to each other and provide a sealing (16),

wherein

said two interim components being arranged upon each other and the foils being adjacent to each other being joined along their outer rims, so as to fasten the two foils to each other and provide a sealing (19).

11. An element according to item 10 further comprising first spacers (14a) being interposed between the semi-permeable membrane sheets of the interim components. 12. An element according to item 10 or 11, further comprising a second spacer (14b) arranged between said two interim components being arranged upon each other.

13. An element according to any of the items 10-12, wherein the first and the second spacers are adapted to during use keep the membrane sheets (7) spaced apart, allowing fluid to pass through them while preventing permanent

indentations in the semi-permeable membranes.

14. An element according to any of the items 11-13 wherein the first spacer and/or the second spacer acts as support for the membrane and is made from a fluid permeable material.

15. An element according to any of the items 10-14 wherein the four membrane sheets being joined by welding; the welding along the rims being preferably applied in a distance of 0.25 to 2cm from the edges of the first and the second openings and from the edges of the outer rims of the foils.

16. An element according to any of the items 10-14, wherein the four membrane sheets being joined by gluing; the gluing along the rims being preferably applied in a distance of 0.25 to 2cm from the edges of the first and the second openings and from the edges of the outer rims of the foils.

17. A stack of semi-permeable membrane sheets, the stack comprising a plurality, such as more than 50, preferably more than 100, even more preferably more than 150 elements according to any of the items 10-16, wherein the plurality of elements are stacked and wherein the elements are joined by joining the neighbouring foils of adjacent elements along their outer rims so as to fasten the two foils to each other and provide a sealing. 18. A stack of semi-permeable membrane sheets according to item 17, wherein the plurality of elements are stacked with second spacers (14b) interposed between adjacent foils of adjacent elements.

19. A stack of semi-permeable membrane sheets according to item 18, wherein the second spacers are adapted to during use keep the membrane sheets spaced apart, allowing fluid to pass through them while preventing permanent

indentations in the semi-permeable membranes. 20. A stack of semi-permeable membrane sheets according to any of items 17-19, wherein the joining being provided by welding being preferably applied in a distance of 0.25 to 2cm from the edges of the outer rims of the foils. 21. A stack of semi-permeable membrane sheets according to any of items 17-20, wherein the joining being provided by gluing being preferably applied in a distance of 0.25 to 2cm from the edges of the outer rims of the foils.

22. A cassette (27) comprising

- a stack of semi-permeable membrane sheets according to any of the items 17-21,

support elements (22a, 22b) attached in a sealed manner, preferable by welding or gluing, to the two outermost semi-permeable membrane sheets of the stack (7a, 7b), the support elements comprising openings (23a, 23b) for allowing fluid to flow into the interior volume (VI) defined by the stack, and

structure elements (21) adapted to spread out the stack of elements.

23. A cassette according to item 22, wherein the structure elements comprising a casing (21a) confining the stack and the support elements in a fluid tight manner, the casing comprising inlets and outlets for the fluids to enter into and out of the stack.

24. A cassette according to item 22, wherein the structure elements comprising supporting poles (21b), preferably being rod-shaped elements, extending between the support elements (22a, 22b).

25. A membrane module (1) comprising a module house (29) with fluid

connections to allow fluid to flow into and out of the interior of the house (V2), wherein one or more cassettes (27) according to any of the items 22-24 being arranged inside the house and having fluid connections extending from one or more of the support elements (22a, 22b) to the outside of the house or being connected to a next cassette so as to allow fluid to flow into and out of the interior volume (VI) defined by the stack of semi-permeable membranes. 26. A membrane module according to item 25, wherein the fluid connections are arranged so as to provide a cross flow between fluid flowing in the interior of the house (V2) and fluid flowing in the interior volume (VI). 27. A membrane module according to item 25 or 26, wherein the fluid connections are arranged so as to provide a counter current between fluid flowing in the interior of the house (V2) and fluid flowing in the interior volume (VI).

28. A pressure retarded osmosis power plant comprising one or more membrane modules according to any of the item 25-27.

29. Use of a semi-permeable membrane sheet, a stack of semi-permeable membranes sheets, a cassette or a membrane module according to any of the items for pressure retarded osmosis.

Claims

1. An element (18) for use in Pressure Retarded Osmosis (PRO), said element comprising four semi-permeable membrane sheets (7) each semi-permeable membrane sheet comprising a foil (8) and a semi-permeable membrane (9), the foil being made from an impermeable material comprising a first opening (10), a second opening (11) and a membrane opening (12), all openings being through going openings in the plane of the foil whereof the membrane opening is covered by the semi-permeable membrane being attached to the foil along the rim of membrane opening so as to attach the semi-permeable membrane and provide a seal between the rim of the membrane and the foil, and the other openings are adapted to allow fluid to flow through the membrane sheet; the semi-permeable membrane sheets being arranged adjacent to each other; the element comprising two interim components (17) each comprising two semi-permeable membrane sheets being joined to each other by joining the rims of the first respectively the second openings (10, 11) of each of the two sheets, so as to fasten the two semi-permeable membrane sheets to each other and provide a sealing (16),

wherein

- said two interim components being arranged upon each other and the foils being adjacent to each other being joined along their outer rims, so as to fasten the two foils to each other and provide a sealing (19), said element further comprising

first spacers (14a) being interposed between the semi-permeable membrane sheets of the interim components; and

a second spacer (14b) arranged between said two interim components (17) being arranged upon each other.

2. The element according to claim 1 wherein said first spacers (14a) are made from a fluid permeable material being non-compressible to an extent where two semi-permeable membrane sheets (7) arranged with a spacer in between does not collapse during use.

3. The element according to any of the preceding claims, wherein said first spacers (14a) during use in PRO, allows transport of fluid across said semi- permeable membrane (9) and supports said semi-permeable membrane (9) without destruction of said semi-permeable membrane (9).

4. The element according to any of the preceding claims, wherein the first and the second spacers are adapted to, during use in PRO, keep the membrane sheets (7) spaced apart, allowing fluid to pass through them while preventing permanent indentations in the semi-permeable membranes.

5. The element according to any of the preceding claims, wherein the first spacers and/or the second spacers act as support for the membrane and are made from a fluid permeable material.

6. The element according to any of the preceding claims wherein the four membrane sheets are joined by welding.

7. The element according to claim 6 wherein said welding along the rims is preferably applied in a distance of 0.25 to 2cm from the edges of the first and the second openings and from the edges of the outer rims of said foils.

8. The element according to any of the claims 1-5, wherein the four membrane sheets being joined by gluing; the gluing along the rims being preferably applied in a distance of 0.25 to 2cm from the edges of the first and the second openings and from the edges of the outer rims of the foils.

9. The element according to any of the preceding claims wherein when in use in PRO the first spacers (14a) of the interim components (17) are in contact with the draw fluid and the spacers (14b) arranged between the interim components (17) is in contact with the feed fluid.

10. A stack of semi-permeable membrane sheets, the stack comprising a plurality, such as more than 50, preferably more than 100, even more preferably more than 150 elements according to any of the preceding claims, wherein the plurality of elements are stacked and wherein the elements are joined by joining the neighboring foils of adjacent elements along their outer rims so as to fasten the two foils to each other and provide a sealing.

11. The stack of semi-permeable membrane sheets according to claim 10, wherein the plurality of elements are stacked with second spacers (14b) interposed between adjacent foils of adjacent elements.

12. The stack of semi-permeable membrane sheets according to claims 10-11, wherein the second spacers are adapted to, during use in PRO, keep the membrane sheets spaced apart, thereby allowing fluid to pass through them while preventing permanent indentations in the semi-permeable membranes.

13. The stack of semi-permeable membrane sheets according to claims 10-12, wherein the joining is provided by welding being preferably applied in a distance of 0.25 to 2cm from the edges of the outer rims of the foils.

14. The stack of semi-permeable membrane sheets according to claims 10-12, wherein the joining being provided by gluing being preferably applied in a distance of 0.25 to 2cm from the edges of the outer rims of the foils.

15. A cassette (27) comprising

- a stack of semi-permeable membrane sheets according to claims 10-14, support elements (22a, 22b) attached in a sealed manner, preferable by welding or gluing, to the two outermost semi-permeable membrane sheets of the stack (7a, 7b), the support elements comprising openings (23a, 23b) for allowing feed fluid to flow into the interior void (VI) defined by the stack, and the support elements optionally comprising fluid connections

(24a, 24b) for inlet/outlet of draw fluid into the interior void (V2) defined by the stack, and

structure elements (21) adapted to spread out the stack of elements.

16. The cassette according to claim 15, wherein the structure elements

comprising a casing (21a) confining the stack and the support elements in a fluid tight manner, the casing comprising inlets and outlets for the fluids to enter into and out of the stack.

17. The cassette according to claims 15-16, wherein the structure elements comprising supporting poles (21b), preferably being rod-shaped elements, extending between the support elements (22a, 22b).

5 18. The cassette according to claims 15-17, wherein said fluid connections (24a, 24b) allows for fluid to be exchanged between the void (V2), defined between the boundaries of the compartment (20) and the exterior of the stack, and the exterior of the compartment (20).

10 19. The cassette according to claims 15-18, wherein said fluid connections (24a, 24b) allows for draw fluid to flow across the surface of the semi-permeable membrane sheets (7) in a direction being perpendicular to the direction of the flow of the feed in the channels (25).

15 20. A membrane module (1) comprising a module house (29) with fluid

connections to allow fluid to flow into and out of the interior of the house (V2), wherein one or more cassettes (27) according to any of the claims 15-19 being arranged inside the house and having fluid connections extending from one or more of the support elements (22a, 22b) to the outside of the house or being

20 connected to a next cassette so as to allow fluid to flow into and out of the interior volume (VI) defined by the stack of semi-permeable membranes, wherein the fluid connections are arranged so as to provide a cross flow between fluid flowing in the interior of the house (V2) and fluid flowing in the interior volume (VI).

25 21. The membrane module according to claim 20, wherein the fluid connections are arranged so as to provide a counter current between fluid flowing in the interior of the house (V2) and fluid flowing in the interior volume (VI).

22. A PRO power plant comprising one or more elements according to any of the 30 claims 1-9.

23. Use of an element, a stack of semi-permeable membranes sheets, a cassette or a membrane module according to any of the preceding claims in a PRO power plant.

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