SOLVATION ENTROPY ENGINE

Abstract

A power generation process is disclosed, the process comprises dissolving a solute (10) into an unsaturated stream (140) to produce a high concentration stream (130) and converting latent mixing energy present in a high concentration input stream (130) into power by passage through a power unit (20) in which the concentration of the high concentration input stream (130) is reduced. The process comprises using a reduced concentration output stream (140) derived from the high concentration input stream (130) following passage through the power unit (20) as the unsaturated stream (140). A first fraction of the high concentration stream (130) is passed to the power unit (20) for use as the high concentration input stream (130) and a second fraction of the high concentration stream (130) is output from the process.

Claims

1-25. (canceled)

26. A power generation process comprising the steps of: dissolving a solute into an unsaturated stream to produce a high concentration stream; converting latent mixing energy present in a high concentration input stream into power by passage through a power unit in which a concentration of the high concentration input stream is reduced; using a reduced concentration output stream derived from the high concentration input stream following passage through the power unit as the unsaturated stream; and wherein a first fraction of the high concentration stream is passed to the power unit for use as the high concentration input stream and a second fraction of the high concentration stream is output from the process.

27. A process according to claim 26, wherein latent mixing energy is converted into electricity by passage through the power unit.

28. A process according to claim 27, wherein the power unit is a salinity gradient energy (SGE) power unit in which electricity is produced using a difference in concentration between the high concentration input stream and a low concentration input stream.

29. A process according to claim 26, wherein an amount of the high concentration stream output as the second fraction is varied so as to keep a volume of fluid within the process substantially constant.

30. A process according to claim 26, wherein the power unit is an osmotic power unit comprising a semi-permeable membrane which permits a passage of water but not a passage of salts, said high concentration input stream is passed over one side of the semi-permeable membrane, and a low concentration input stream being passed over another side of said semi-permeable membrane.

31. A process according to claim 30, wherein a flow of the second fraction of the high concentration stream output from the process equals a flow through the semi-permeable membrane from the low concentration input stream.

32. A process according to claim 30, wherein the high concentration input stream is pressurized by passage through a pressure exchanger, a pump or other suitable energy recovery device (ERD) before being passed over said semi-permeable membrane.

33. A process according to claim 28, wherein the power unit is an electrodialysis power unit comprising a plurality of cation exchange membranes and a plurality of anion exchange membranes, and in which the high concentration input stream is passed over one side of a cation exchange membrane and one side of an anion exchange membrane, the low concentration input stream being passed over another side of the cation exchange membrane and another side of the anion exchange membrane.

34. A process according to claim 27, wherein a first part of said high concentration input stream is converted into electricity by passage through the power unit, a second part of said high concentration input stream being used as an input to a second power unit.

35. A process according to claim 34, wherein the reduced concentration output stream derived from the first part of said high concentration input stream after passage through the power unit and/or the second part of said high concentration input stream after passage through the second power unit, is used as the unsaturated stream.

36. A process according to claim 26, wherein at least part of the reduced concentration output stream derived from the high concentration input stream following passage through the power unit is used as a high concentration input stream for a second power unit.

37. A process according to claim 36, wherein the reduced concentration output stream derived from the high concentration input stream after passage through the power unit and/or the second power unit is used as the unsaturated stream.

38. A process according to claim 26, wherein the process comprises transporting the solute to a location in which the process is carried out and, optionally, extracting the solute from a salt dome or other suitable an underground formation before transporting the solute to the location at which the process is carried out.

39. A power generation system comprising: a solute; a power unit configured to generate power using a difference in concentration between a high concentration input stream and a low concentration input stream thereby producing a reduced concentration output stream derived from the high concentration input stream; a high concentration outlet; and wherein the system is arranged such that the reduced concentration output stream is used to dissolve the solute, and a first fraction of the resulting high concentration stream is passed to the power unit for use as the high concentration input stream and a second fraction of the resulting high concentration stream is passed to the high concentration outlet.

40. A power generation system according to claim 39, wherein the power unit is configured to generate electricity using the difference in concentration between the high concentration input stream and a low concentration input stream.

41. A power generation system according to claim 39, wherein the power unit is an osmotic power unit arranged to generate electricity through Pressure Retarded Osmosis (PRO) or an electrodialysis power unit arranged to generate electricity through Reversed ElectroDialysis (RED).

42. A power generation system according to claim 41, wherein the osmotic power unit comprises a semi-permeable membrane which permits a passage of water but prevents a passage of salts and the system further comprise an energy recovery device (ERD), wherein the system is arranged such that the high concentration input stream is pressurized by passage through the ERD before being passed to the semi-permeable membrane, and wherein the pressure of the reduced concentration output stream is reduced by passage through the ERD before being used to dissolve the solute, the ERD being configured to transfer pressure from the reduced concentration output stream to the high concentration stream.

43. A power generation system according to claim 39, wherein the system is arranged such that the high concentration stream is used as a high concentration input stream of a second and/or further power unit, or the reduced concentration output stream from the power unit is used as a high concentration input stream to the second and/or further power unit.

44. A power generation system according to claim 43, wherein the reduced concentration output stream from the power unit, the second power unit and/or any further power unit is used to dissolve the solute.

45. A power generation system according to claim 39, comprising a compartment in which the reduced concentration output stream is used to dissolve the solute.

Description

DESCRIPTION OF THE DRAWINGS

[0073] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

[0074] FIG. 1 shows an example solvation entropy engine in accordance with the present invention;

[0075] FIG. 2 shows a variant of the process of FIG. 1 where the produced high concentration solution is used in a further power unit;

[0076] FIG. 3 shows a variant of the process of FIG. 1 in which multiple power units are connected in series;

[0077] FIG. 4 shows a variant of the process of FIG. 1 in which multiple power units are connected in series within the solvation entropy engine;

[0078] FIG. 5 shows a system with no recirculation of the high concentration solution to the mixing system;

[0079] FIG. 6 shows an example osmotic power unit;

[0080] FIG. 7 shows an example process in accordance with the invention in which the osmotic power unit of FIG. 6 is used to harvest mixing energy;

[0081] FIG. 8 shows a variant of the process of FIG. 7;

[0082] FIG. 9 shows a plot of the energy produced per ton of NaCl (sodium chloride) as this goes into solution as a function of applied pressure when a PRO process is used to extract the energy;

[0083] FIG. 10 shows more details of an osmotic power unit; and

[0084] FIG. 11 shows more details of a reverse electrodyialysis power unit.

DETAILED DESCRIPTION

[0085] In many cases it is necessary to dissolve solids into solution. In this process, atoms or molecules locked in the solid will interact with the solvent and move out into the solution as dissolved species. The change from a system consisting of two relatively pure phases, a solid (s) and a solvent (aq), to a mixed solution will give an increase in entropy (S).

ΔS=S.sub.A(aq).sup.θ−S.sub.A(s)θ

[0086] Where S is entropy, solid, solvent-solvent and solid-solvent interactions, enthalpy may either increase, decrease or stay unchanged, which can lead to further changes in entropy as heat is either added or removed from the system. As long as the net change in entropy is positive, the dissolution process will be spontaneous.

[0087] The entropy generation is available as Gibbs free energy, in this case called mixing energy as described by the following equation:

−ΔG.sub.mix=RT([Σx.sub.i ln(α.sub.i)].sub.M−ϕ.sub.A[Σx.sub.i ln(α.sub.i)].sub.A−ϕ.sub.B[Σx.sub.i ln(α.sub.i)].sub.B)

where G.sub.mix is the Gibbs free energy of mixing, R is the gas constant, T the absolute temperature, x.sub.i the mole fraction of species “i”, a, the activity of species “i” and ϕ.sub.A and ϕ.sub.B are the ratios of moles in solutions A and B respectively to the total moles in the system, solutions A and B being mixed to give solution M.

[0088] One example embodiment of the invention is illustrated schematically in FIG. 1. The inputs to a power unit (or EEED) 20 are a draw solution 130 and a suitable feed solvent solution 100. The power unit 20 produces power 120 and a diluted draw solution 140, which is a mix of the inputted draw solution 130 and feed solvent solution 100. The diluted draw solution 140 is sent to a mixing system 10 together with solids 110. The draw solution 130 is the output of the mixing system 10 when the solid 110 is mixed with and thus dissolved by the outputted diluted draw solution 140 from the power unit 20. The diluted draw solution 140 is the inputted draw solution 130 outputted with a lowered solution concentration. The boundary of the resulting theoretical device 1 for the extraction of solvation entropy is denoted by a dashed line in FIG. 1.

[0089] To have a steady state process with constant volume, part of the output draw solution 130 is sent out of the device 1, said output draw solution 130 being of a relatively high concentration. The overall device 1 thus operates with inputs of solids 110 and a solution of relatively low concentration (feed solvent solution 100) and outputs of power/energy 120, an output draw solution 130 of relatively high concentration. Said device 1 is called a solvation entropy engine FIG. 1 further shows a possible departure of a residual solvent 200 being a left over solvent 100 after passage through the power unit 20, the flow rate of which may be controlled to adjust the fluids within the device 1. In some embodiments all of feed solvent solution 100 is used within the device 1.

[0090] In the present context, the solvent solution 100 is a fluid, for example a liquid or gas, with a low (or zero) concentration of solutes (e.g. sugar or salts like Sodium Chloride, Potassium Chloride, Calcium Chloride etc.) and, optionally, fresh water or other solvent. The solid 110 is to be understood as the solute in a solid state. In other embodiments the solute may be in a non-solid state. The solvent solution 100 is in a low concentration state compared to the solid 110. The diluted draw solution 140 and solids 110 in the mixing system 10 mix and create a solution of relatively high concentration (the draw solution 130) as the solids 110 dissolve in the diluted draw solution 140.

[0091] The output draw solution 130 of relatively high concentration can be used for secondary purposes. FIG. 2 illustrates an embodiment where an additional power unit 30 (of the same type as power unit 20, within the device 1) is added to the output draw solution 130 stream of the system of FIG. 1. In this embodiment the draw solution 130 is diluted by a suitable solvent feed solvent 100 in the additional power unit 30 to produce a diluted draw solution 140 and, optionally, a residual solvent solution 200. The additional power unit 30 is positioned outside the device 1 and may consist of several additional power sub-units 30 operating on different parameters. The power unit 20 placed in the device 1 may also consist of several sub-units within the device 1, such as illustrated in FIG. 3.

[0092] FIG. 3 illustrates an embodiment of such a series of power sub-units 20a, 20b, 20c, each sub units having the same input and output streams as the power unit 20 described above. A first power sub-unit 20a feeds a diluted draw solution 140 to the second power sub-unit 20b that feeds diluted draw solution 140 the a third power sub-unit 20c. Though the figure illustrates three such power sub-units 20a, 20b, 20c, the device 1 could include any number of power sub-units in succession.

[0093] FIG. 3 describes a way of connecting power sub-units in series, where each subunit can operate at similar or different conditions. If a series of power sub-units 20a, 20b, 20c are used for energy extraction, recirculation may follow from any (or all) of them as shown in FIG. 4. In FIG. 4, a portion of the diluted draw solution 140 from each power sub-unit 20a, 20b, 20c is passed to the mixing system 10. Any such power sub-unit from which recirculation occurs will be within the (theoretical) boundary of the solvation entropy engine device 1, while others (from which recirculation does not occur) will be outside.

[0094] In FIG. 5 a system with no recirculation is shown. The system of FIG. 5 is a comparative example falling outside the scope of the present invention. Here feed solvent 100 is sent to the mixing system directly to create a draw solution 130 for the power unit 20. The mixing of the feed solvent 100 with the solids 110 in the mixing system 10 leads to an increase in entropy, but because this mixing is not mediated via a power unit (in contrast to the system of FIG. 1) it is uncontrolled, and therefore, the increase in entropy cannot be used for energy generation. From FIG. 5 it can also be seen that feed solvent 100 is sent to both the mixing system 10 and the power unit 20. More feed solvent 100 is thus required to run the process. Furthermore, the outlet of the process in FIG. 5 is a dilute draw solution 140 flow of lower concentration than the outlet draw solution 130 in FIG. 1.

[0095] In one example, the power unit 20 is an osmotic power unit as illustrated in FIG. 6 which generates electricity through pressure retarded osmosis (PRO). In PRO two liquid solutions of different concentration; draw solution 130 and feed solvent solution 100, being high concentration and low concentration solutions respectively, pass either side of a semi-permeable membrane 450 which permits the passage of water but not of salts. Due to the net difference in osmotic pressure, solvent permeates the membrane 450 and moves from low to high concentration to try and equalize the osmotic difference. This leads to an expansion of draw solution 130, which can be sent to an energy generating device 500 such as a turbine. The draw solution 130 is pressurized at a pressure below the osmotic pressure difference between the draw solution 130 and feed solvent solution 100 before being sent to the semi-permeable membrane 450 by passage through a pressure exchanger 400. In the same or yet further embodiments, a pump may be used to pressurise the draw solution 130 before it is passed to the semi-permeable membrane. A fraction of diluted draw solution 140 is recirculated to the pressure exchanger 400 as the high pressure stream and pressure is transferred from the diluted draw solution 140 to the draw solution 130 in the pressure exchanger 400. In yet further embodiments, the pressure exchanger or pump may be absent. A feed pump 600 pressurises the feed solvent solution 100 before it is sent to the semi-permeable membrane 450.

[0096] FIG. 7 shows a solvation entropy engine device 1 based on PRO. Here solids 110 (for example salt) are sent to a mixing system 10 where they mix with the dilute draw solution 140 coming from the power unit 20 and create a draw solution 130 of relatively high concentration. This draw solution 130 is split into two fractions; one is directed to the pressure exchanger 400 in the power unit 20 and the other (130a) is directed out of the solvation entropy engine device 1. In this embodiment, the flow of draw solution (130a) directed out of the solvation entropy engine device 1 equals the permeate flow of feed solution 100 through the membrane 450, that is to say V_130a=V_100−V_200. After passage through the pressure exchanger 140 and energy generating device 500 the two streams of diluted draw solution 140 are recombined before re-entering the mixing system 10.

[0097] In order to utilize the full amount of the feed solution 100, the residual solvent 200 stream can be mixed together with the diluted draw solution 140 from the energy generating device 500 and the pressure exchanger 400, such as in the embodiment seen in FIG. 8. In this system, the flow of the draw solution (130a) directed out of the solvation entropy engine device 1 equals the inflow of the feed solution to the membrane 450, that is to say, V_130a=V_100.

[0098] In some cases, it is not necessary to transport the solids 110 to the mixing system 10. If the solids 110 themselves form a structure that can be used as a mixing system 10, dilute draw solution 140 can be recirculated directly to this. This may be the case where the solids are in an underground formation, for example a salt dome or rock salt layer.

[0099] In order to maximize energy generation from solvation entropy, it may be beneficial to operate the PRO process at the highest possible pressure, as the gross energy of the process is the product of the flow of permeate across the semi-permeable membrane 450 in the PRO process and the applied pressure.

E.sub.gross=V.sub.solvent,permeate.Math.P.sub.applied

[0100] In the PRO process the limiting maximum pressure will be the osmotic pressure of the draw solution. When the applied pressure is increased, less feed solution 100 can permeate the membrane 450 relative to the draw solution 130 flow rate. To maintain the permeate flow, it will therefore be necessary to increase the draw solution 130 flow rate. This increase in draw solution 130 flow rate may be handled by the pressure exchanger 400, and efficiency losses in this process will mean there is an optimum applied pressure where net energy is maximized. Net energy in this context is defined as

E.sub.net=E.sub.gross−EDG.sub.loss−ERD.sub.loss−FP.sub.loss

[0101] Where EGDloss, ERDloss and FPloss are the losses incurred in the energy generation device 500, the energy recovery device (for example pressure exchanger 400) and the feed pump 600 respectively.

[0102] FIG. 9 shows an example of such an optimum applied pressure. Here the solid 110 is sodium chloride (NaCl) and the solution 100 is water. A turbine is used as the energy generating device 500, and a pressure exchanger 400 with a booster pump (not illustrated) to overcome pressure losses is used to pressurise the draw solution 130.

[0103] The turbine 500 operates at an efficiency of 90%, the pressure exchanger at an efficiency of 95%, with the booster pump operating at an efficiency of 90%. The feed pump 600 operates at an efficiency of 80%, delivering feed water to the PRO system at a pressure of 10 bar, and 90% of the feed water is utilized in the PRO process. The entropy solvation engine device 1 follows the design shown in FIG. 7. Also, the mixing ratio between solid 110 and feed solution 100 is fixed by defining the osmotic pressure of the dilute draw solution 130 coming from the outlet of the PRO process. It is fixed to 10 bar above the applied pressure (P.sub.applied+10 bar). In this case, the increasing flow rate in the recirculation loop leads to increasing losses in the pressure exchanger, which results in an optimum applied pressure of 270 bar.

[0104] FIG. 10 shows more details of an osmotic power unit 20, for example of the type used in FIG. 7. A saline stream 31 (which may for example be feed stream 130 is passed to an osmosis unit 29 containing a semi-permeable membrane 30 which permits passage of water but not of salts, and flows at one side of membrane 30. An aqueous stream 33 which is of lower salinity than stream 31 (for example feed stream 100) enters osmosis unit 29 and flows at the other side of membrane 30. Arrows show the direction of water transport by osmosis across membrane 30. An output stream 35 (for example stream 200) derived from original input stream 33 and now containing a higher concentration of salt, leaves osmosis unit 29. An output stream 36 consisting of original input stream 31 now containing a lower concentration of salt (for example diluted feed stream 140), leaves osmosis unit 29 via a turbine 37 which drives a generator 38 thus producing electricity.

[0105] In one example, a reverse electrodialysis (RED) power unit 20 is used instead of an osmotic power unit 20 in the process of FIG. 7. An example reverse electrodialysis (RED) power unit is illustrated in FIG. 11. The RED power unit 20 comprises a stack 70 of cation exchange membranes 75 alternating with an anion exchange membranes 76. The stack 70 is located between a cathode 79 (on the left of FIG. 11) and an anode 80 (on the right of FIG. 11). A saline stream 71 (which may for example be feed stream 130) flows between each cation exchange membrane 75 (on the left of stream 71 in FIG. 11 which permits the passage of cations (e.g. sodium) but not anions (e.g. chlorine) and an anion exchange membrane 76 (on the right of stream 71 in FIG. 11). An aqueous stream 73 which is of lower salinity than stream 71 (for example feed stream 100 in FIG. 7) flows on the other side of each cation exchange membrane 75 and the anion exchange membrane 76. Thus, there is an alternating series of saline streams 71 and aqueous streams 73 flowing through the stack 70. For the sake of clarity only four membranes are shown in FIG. 11, but the stack may include many more membranes. Arrows show the direction of sodium transport across cation exchange membrane 75 and chloride transport across anion exchange membrane 76. This movement of cations and anions across the membranes generates an electric current. An output stream 77 (for example residual solvent 200) derived from original input stream 73 (feed stream 100) and now containing a higher concentration of salt, leaves the power unit 20. An output stream 78 (for example diluted draw stream 140 in FIG. 7) consisting of original input stream 71 (draw stream 130 now containing a lower concentration of salt, leaves the power unit 20.

[0106] As for PRO, the salinity of saline stream 71 (i.e. draw stream 130) is reduced and the salinity of aqueous stream 73 (feed stream 100) is increased by passage through the RED unit. However, with a RED process this is because positive and negatively charged ions (for example sodium ions and chlorine ions) have passed from the saline stream 71 to the aqueous stream 73. This movement across the cation exchange membrane 75 and anion exchange membrane 76 generates an electric potential which can be used to drive an electric current.

[0107] Both PRO and RED are examples of SGE technologies. It will be appreciated that other SGE technologies may be used in the example described above.

[0108] Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

[0109] Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.