AU2003283126B2

AU2003283126B2 - Enhancing water evaporation

Info

Publication number: AU2003283126B2
Application number: AU2003283126A
Authority: AU
Inventors: John Finnigan
Original assignee: Commonwealth Scientific and Industrial Research Organization CSIRO; Energy Resources of Australia Ltd
Current assignee: Commonwealth Scientific and Industrial Research Organization CSIRO; Energy Resources of Australia Ltd
Priority date: 2002-11-28
Filing date: 2003-11-28
Publication date: 2009-09-10
Anticipated expiration: 2023-11-28
Also published as: AU2003283126A1

Description

WO 2004/048272 PCT/AU2003/001587 ENHANCING WATER EVAPORATION FIELD OF THE INVENTION 5 The present invention relates to a method of enhancing water evaporation as well as to evaporation enhancing structures. The invention has particular, but not exclusive, application in enhancing the evaporation of waters from water retaining ponds, dams and from tailings 10 storage facilities including mined-out pits at a uranium mine where other evaporation techniques are not suitable. BACKGROUND OF THE INVENTION 15 Mines use tailings storage facilities including above-grade structures and mined-out pits to contain solid and liquid residues from the mineral or metal extraction processes. Tailings storage facilities are designed to contain such by-products. Best practice management of 20 these facilities sets out to minimise the volume of liquid (process water) residue in order to maximise the settlement and consolidation of solid (tailings) residues which accumulate in the bottom of the tailings storage facility. 25 To increase the rate of evaporation of process water residue from the tailings storage facility, one option is to increase the size of the storage facility However, this not always practical as the area which can 30 accommodate an above grade tailings storage facility may be limited or the mined-out pit which is used for storage of mineral processing residues is fixed in size by the mining activity. Further, increasing the area of an above grade storage facility increases the area of land 35 which is potentially contaminated by the residues of the mineral or metal extraction process., The rate of evaporation of process water (liquid residue)from a WO 2004/048272 PCT/AU2003/001587 -2 tailings storage facility must match the rate of inflow (including rainfall) into the storage facility or the water level will rise. Physical constraints or regulatory controls and approvals may limit the height to which the 5 process water (liquid residue) can be permitted to rise. Finally, in the tropics, the net annual evaporation is the difference between evaporation and wet season precipitation. If this is small or zero, increasing the area of an above-grade storage facility may be an 10 inefficient way to increase net evaporation. As a result, there have been efforts to increase the rate of evaporation of water from tailings storage facilities. Previous attempts to improve evaporation have 15 included the use of snow-making machines to pump water from tailings storage facilities and blow it into the air as a mist or spray. However, these techniques are not suitable for use in humid atmosphere where the air is already heavily saturated with water. Further, it is 20 possible for contaminants in the water to be sprayed into the air with the water and spread over a wide area by the wind. This is a particular problem where the contaminants pose a potential health risk to humans or the environment. For example, where the tailings storage facility is that 25 of a uranium mine and the contaminants are radioactive. Furthermore, in many tropical landscapes, where retention of wastewaters by mines or industrial operations is required by legislation so that downstream environments 30 and ecosystems are protected from the potential for contamination by wastewaters. Accordingly, there is a need for an alternative technique for enhancing water evaporation from a technique 35 for enhancing evaporation from any body of water such as a tailings storage facility.

-3 SUMMARY OF THE INVENTION Accordingly, the invention provides a method of enhancing water evaporation from a body of water 5 comprising providing a plurality of evaporation enhancing structures at spaced apart locations on said body of water, each evaporation enhancing structure comprises a vortex generator for increasing the supply of dry air to the surface of the body of water, and floating support 10 means for supporting the vortex generator when it is floating on the body of water.. The method preferably involves anchoring the evaporation enhancing structures relative to the body of 15 water so that they may move to align themselves with the prevailing wind. Typically, this comprises attaching each evaporation enhancing structure to a buoy anchored to floor of the body of water and providing each evaporation enhancing structure with wind alignment means - for 20 example, in the form of a fin. The method preferably comprises spacing the evaporation enhancing structures so they cannot interfere with one another when they move to align themselves with 25 the prevailing wind. Accordingly, the evaporation enhancing structures are preferably spaced from one another in a staggered array. 30 The vortex generator is preferably a solid triangular wing with one side close and parallel to the water surface and one point elevated. The wing shape is approximately an equilateral triangle. The wing is 35 preferably supported by the support means at an angle of between 15 degrees and 35 degrees to the surface of the body of water, most preferably at an angle of 25 degrees. 1512208.2 (GHMatter) 06.08.09 Where the evaporation enhancing structures include vortex generators it is preferred that the vortex generators are spaced so that the trailing vortices of the 5 vortex generators do not interfere with the generation of vortices by other vortex generators. Typically, the spacing between the apex of each wing is at least 3 chord widths in both the streamwise and cross-stream directions. 10 The invention also extends to an evaporation enhancing structure comprising a vortex generator for increasing the supply of less humid air to the surface of the body of air, and floating support means for supporting the vortex generator when it is floating on the body of 15 water. Further features of the invention will become apparent from the following description of the preferred embodiments of the invention. 1512208_2 (GHMagers) 08.08.09 - 5 BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows schematically how a plurality of 5 evaporation enhancing structures can be deployed at spaced apart locations on a body of water; Figure 2 shows one example of an evaporation 10 15122082 (GHMtors) 06.08.09 WO 2004/048272 PCT/AU2003/001587 -6 enhancing structure; Figure 3 shows a "delta wing" vortex generator used in wind tunnel tests; 5 Figure 4 is a temperature pattern relating to the vortex generator of Figure 3; Figure 5 shows an array of "delta wing" vortex 10 generators used in a wind tunnel test; Figure 6 is a surface temperature pattern of the array of Figure 5; 15 Figure 7 shows a staggered array of evaporation enhancing structures where the evaporation enhancing structures are porous mesh fences; Figure 8 shows the surface temperature of the 20 pattern for Figure 7; and Figure 9 shows an alternative evaporation enhancing structure. 25 DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method of the first preferred embodiment of the invention a plurality of evaporation enhancing structures having vortex generators are located at spaced 30 apart locations on a body of water in the form of a tailings storage facility. The tailings storage facility contains contaminants or by-products of a mineral or metal extraction process. 35 The arrangement of the evaporation enhancing structures incorporating vortex generators is illustrated schematically in Figure 1. The evaporation enhancing WO 2004/048272 PCT/AU2003/001587 -7 structures 1 are anchored to respective anchor points 2 so that their movement is limited to being within circles 3. The evaporation enhancing structures 1 are anchored in such a manner that they may align themselves with the 5 direction of the prevailing wind 4. Each evaporation enhancing structure is attached to a buoy by a rope or chain and each evaporation enhancing structure is provided with wind alignment means - for example, in the form of a fin. The prevailing wind will then cause the evaporation 10 enhancing structures to pivot around anchor point 2 in order to align themselves with the prevailing wind. As is shown in Figure 1, the circles which define the limit of movement of each of the evaporation enhancing structures are arranged so that they do not overlap so that the 15 evaporation enhancing structures 1 cannot interfere with one another. An evaporation enhancing structure 1 incorporating a vortex generator is shown in Figure 2. 20 The evaporation enhancing structure 1 includes a vortex generator in the form of a "delta wing" 5. The "delta wing" 5 is mounted on a floating support consisting of base member 7 which is formed from a plurality of hollow plastic tubing elements and struts 8 which support the 25 wing 5 at the desired angle of attack. The preferred wing shape is an equilateral triangle with the point 6 of the wing 5 being elevated and oriented towards the prevailing wind. The wing is supported by struts 8 at an angle of 25 degrees to the surface of the water 10. This provides an 30 ideal vortex generator. Persons skilled in the art will appreciate that less than perfect vortex generators can also be used. For example, a delta wing can be deployed at angles between 15 degrees and 35 degrees. 35 The evaporation enhancing structure 1 also includes additional float means 9 for improving the buoyancy of the structure, and rope 11 for anchoring the WO 2004/048272 PCT/AU2003/001587 -8 vortex generator to a buoy. As shown in Figure 1, the optimum arrangement of the vortex generators is a sparsely staggered array that 5 ensures that the trailing vortices of one wing do not interfere with the development of vortices from the next row of wings downstream. The vortices disturb a boundary layer of moist 10 air which exists immediately above the surface of the water by drawing additional dry air down to the surface of water, thereby enhancing the rate of evaporation from the water. The ideal spacing for equilateral triangle vortex generators occurs when the apices 6 of the wings are 15 arranged so that they are at least three cord widths apart in both the streamwise (ie., the wind direction), and the cross-stream directions, successive rows displaced by one and a half cord widths relative to each other in the cross-stream direction (ie., they are offset). More 20 distant spacings are acceptable with only minor loss of effectiveness until the spacings are six cord widths in both the streamwise and cross-stream directions, in that successive rows displaced by one and a half cord widths. 25 The evaporation enhancement structure of the preferred embodiment were trialed in a wind tunnel. In a first test, an evaporation enhancement structure 1 having a "delta wing" was placed on an electrically heated black surface. Figure 4 shows a temperature pattern behind an 30 isolated evaporation enhancement structure incorporating a vortex generator. As in Figure 1, arrow 4 indicates the prevailing direction of the wind in the wind tunnel. Cooler region 16 directly behind the evaporation enhancing structure 1 show the development of strong trailing 35 vortices caused by the evaporation enhancement structure. Figure 5 shows an optimum array of evaporation WO 2004/048272 PCT/AU2003/001587 -9 enhancing structures also prepared for a wind tunnel test. The temperature pattern of Figure 6 shows that this arrangement leads to cooler regions 16a and 16b immediately behind vortex generators la and lb as well as 5 a cooler band 17 between regions 16a and 16b. Figure 7 shows a test rig for a wind tunnel test of the method of a second preferred embodiment which uses an alternative evaporation enhancement structure. Each 10 evaporation enhancement structure has a wettable portion which extends in a plane which is substantially transverse to the surface of the body of water. In the second preferred embodiment, the wettable portion is provided by a rectangular frame covered by wettable material. The 15 wettable material would typically be porous and caused to come into contact with the body of water in order to draw water from the body of water by capillary reaction. Accordingly, it is preferred that the wettable material has high capillarity. Thus, the evaporation enhancing 20 structures of this embodiment work by a combination of .drawing water from the body of water so as to wet a vertical surface that is exposed to the wind, hence increasing the area undergoing evaporation, and reducing the aerodynamic resistance of the water surface. 25 The evaporation enhancement structures are arranged in parallel rows. A row consists of a series of evaporation enhancement structures aligned normally to the long axis of the row like rungs on a ladder. The long 30 axis of each row is aligned with the prevailing wind. In any one row the distance between evaporation enhancement structures in the direction of the wind may be greater but not less than 10H with an optimum spacing of 12H. Adjacent rows of devices are staggered by half the spacing between 35 devices in any one row. The distance between the centrelines of adjacent rows should be approximately 10% less than the cross-wind width of the devices so that WO 2004/048272 PCT/AU2003/001587 - 10 devices in adjacent rows overlap when viewed in the direction of the wind. This produces an overall arrangement of devices consisting of a staggered array so that the projected area of fence normal to the wind 5 divided by the plan area is 1 divided by 20. For such a staggered array, the change in aerodynamic resistance is indicated in Figure 8, where there are cooler patches 21 proximate the edges of the 10 evaporation enhancing structures 20. The aerodynamic resistance is reduced by 18% by these evaporation enhancing structures compared to a reduction of 29% by the evaporation enhancing structures 1 15 incorporating vortex generators. However, there is the additional effect of the increased area for evaporation due to water being drawn from the surface by capillary reaction. 20 The height to width ratio of the wettable portion is 1:20, although other height to width ratios can also be used. For example, height to width ratios in the range of 1:10 to 1:30. 25 The height (H) of the wettable portion is typically in the range of 1-5 metres. Table 1 sunmarises the predicted changes in evaporation for various different wind speeds. 30 Table 1.

WO 2004/048272 PCT/AU2003/001587 + CN ~-~ ~ -+ o 4J-W4J0 0 4--00 -, 4j a) Da) -H 4j )WWQ) 0 0 4J a)(d , N m r-m -W Q) ) Wm m 0 H q)~ 0+ 0n 0 0+ 0) I nc mD It C I )I 0+ IQ 0 0n + c -Hrn 1 a)U D H ) ( 41J -C0HOW 4J~ 4HHW r 4 (N 0 w1 U 4J5 qm mr-I 0 4J 1 -H H '0a n n 41 C; C r 0 m~C-.m m 00 c' w w r- w N N 0: 0 L M .4j 0\OX 4J m (D 0) \0 0 0\0 0\0 0\0 .H W4J i4>Q P4OO H i-IoCDo m H- 0 zd ml H- +1 H H H- -H m~lr1~ H4wH :r mN>nr Cf) OH 0 H WO 2004/048272 PCT/AU2003/001587 CD ~ L CD '!) 1'-~ *LOT~ D ~-l 4 H o- 0I)) 0 4-4 0 0)O 0 41WOW 0H4)W W 0 rOW dfd4 00O4rd -H 4 a D Q)- D a) ) 0 0 4J ,q'0 ~ cN~ o a) zv n 0) kfl o S 4J W w N w ,1' 41 0) CN 0 CD tNO O oI O H OH CD 0 + M0 0 + ca0 0HWUW -H0 4W W CD 4 U a Hci (D U a) 4J1 -W~ -W H J 4J 4 ~W CH0H rq W H W) N (n cor-I WDCOV -W0L LH iim 4 0 WH O rd a ( . k rdW. W IQ Wn u)L 4-CH 0 0 0 HH HH HHH N C co ) m(N r-(N ( ;:v w 0 0 > ;Ilw 0 4J r- 0 0w omwo 1r- Wn ~ w w 4)H 0 m Ln, 4)H 0N 0 H rd M 4) W) 0\0 4) ( ~(d W) 0\0 0 0\0 0\0 0\0 H m H 0(dco H + 1-1H H 4JW d--, ,0 t m 0 0 (d HH a)0 > = 4) H pH(qO - Cl ) I OH ' __ __ _ ______ L_ __ _ -- I I IE WO 2004/048272 PCT/AU2003/001587 - 13 In Table 1, the evaporation enhancing structures 1 which include vortex generators are referred to as vortex generators, whereas the evaporation enhancing structures which include a wettable first portion are 5 referred to as fences. It will be seen that in both cases the fences and the vortex generators provide improved evaporation compared to water alone, with the vortex generators providing the greater improvement in evaporation. 10 It will be apparent to persons skilled in the art that various modifications may be made to the invention without departing from the scope and spirit of the invention. For example, where the evaporation enhancing 15 structure is a wettable portion (ie., a floating fence), the wettable portion 25 need not be rectangular, as shown in Figure 9 which utilises a triangular wettable portion 25. Again, the frame 26 of the structure can be formed by tubes of plastic piping and a fin 27 can be provided in 20 order to allow the structure to align with wind direction 4. In Figure 9, Figure 9a is a plan view, Figure 9b is a side view, and Figure 9c is a front view, with Figure 25 9d being a perspective view. These and other modifications will be apparent to persons skilled in the art.

Claims

1. A method of enhancing water evaporation from a body of water comprising providing a plurality of 5 evaporation enhancing structures at spaced apart locations on said body of water, each evaporation enhancing structures comprises: a vortex generator for increasing the supply of less humid air to the surface of the body of water; and 10 floating support means for supporting the vortex generator such that it floats on the body of water.

2. A method as claimed in claim 1, comprising anchoring the evaporation enhancing structures relative to 15 the body of water, in such a manner that they may move to align themselves with the prevailing wind.

3. A method as claimed in claim 2, comprising attaching each evaporation enhancing structure to a buoy 20 anchored to floor of the body of water; and providing each evaporation enhancing structure with wind alignment means.

4. A method as claimed in claim 3, wherein said wind 25 alignment means is a fin.

5. A method as claimed in any one of claims 1 to 4, further comprising spacing the evaporation enhancing structures so they cannot interfere with one another when 30 they move to align themselves with the prevailing wind.

6. A method as claimed in claim 5, comprising spacing the evaporation enhancing structures from one another in a staggered array. 35

7. A method as claimed in anyone of claims 1 to 6, wherein each vortex generator is a solid triangular wing 1512200_2 (GHWOates) 000.0.09 - 15 with one side close and parallel to the water surface and one point elevated.

8. A method as claimed in claim 7, wherein the wing 5 shape is an equilateral triangle.

9. A method as claimed in claim 7 or claim 8, wherein the wing is supported by the support means at an angle of between 15 degrees and 35 degrees to the surface 10 of the body of water.

10. A method as claimed in claim 7 or claim 8, wherein the wing is supported by the support means at an angle of 25 degrees to the surface of the body of water. 15

11. A method as claimed inanyone of claims 1 to 10, wherein the vortex generators are spaced so that the trailing vortices of the vortex generators do not interfere with the generation of vortices by other vortex 20 generators.

12. A method as claimed in claim 11, wherein the spacing between the apex of each wing is at least 3 chord widths in both the streamwise and cross-stream directions. 25

13. An evaporation enhancing structure comprising: a vortex generator for increasing the supply of less humid air to the surface of the body of water; and floating support means for supporting the vortex 30 generator such that it floats on the body of water.

14. An evaporation enhancing structure as claimed in claim 13 wherein the vortex generator is a solid triangular wing with one side close and parallel to the 35 water surface and one point elevated.

15. An evaporation enhancing structure as claimed in 15122O8_2 (GHMttrs) 06.08.09 - 16 claim 14 wherein the wing is an equilateral triangle.

16. An evaporation enhancing structure as claimed in claim 14 or claim 15 wherein the wing is supported by the 5 support means at an angle of between 15 degrees and 35 degrees to the surface of the body of water.

17. An evaporation enhancing structure as claimed in claim 14 or claim 15, wherein the wing is supported by the 10 support means at an angle of 20 degrees to the surface of the body of water.

18. An evaporation method substantially as described herein with reference to the accompanying drawings. 15

19. An evaporation enhancing structure substantially as described herein with reference to the accompanying drawings. 1512208_2 (GHMatters) 0.0809