WO1981003694A1

WO1981003694A1 - Improvements in and relating to solar ponds

Info

Publication number: WO1981003694A1
Application number: PCT/AU1981/000075
Authority: WO
Inventors: R Collins; D Fredricksen
Original assignee: R Collins; D Fredricksen
Priority date: 1980-06-19
Filing date: 1981-06-18
Publication date: 1981-12-24

Abstract

A solar pond (1) having a reconcentrating unit. The unit withdraws saline water from an upper level of the pond (17), through pipe (6), flash evaporates the water in a low pressure evaporator (10) and then passes the concentrated saline solution into the lower portion of the pond (9) through diffuser (13) the distilled water resulting from the evaporator (10) being added to the surface of the pond (1) via pump (12). Also disclosed are attenuators (2) suspended across a solar pond (1) to reduce wind induced waves on the surface of the pond (1).

Description

"IMPROVEMENTS IN AND RELATING TO SOLAR PONDS"

This invention relates to improvements in and relating to solar ponds, more particularly to improvements overcoming problems that have existed in the past.

BACKGROUND OF THE INVENTION

Solar ponds which comprise a body of water whose salt concentration increases with depth are known. The surface of the solar pond is ideally fresh water or very low salinity water, and these 0 low salinity upper layers allow solar radiation to reach and heat the salt saturated storage layers and then act as insulators to keep most of the heat from re-radiating to the surface. Except for minor leakage of heat, the heat keeps accumulating ₁₅ in the lower layers until some is drawn out of of the storage layer, and this heat can accumulate to an extent that the storage layer would tend to boil.

The unequal distribution of the salt in the

20. solar pond, that is the salinity gradient, effectively stops convection currents for each layer of water has greater density, because it is saltier than the one above it and so resists convection currents moving upwardly after it has been heated.

In Australian Patent Application No. 25965/77

25, entitled "Method and apparatus for producing energy" there is disclosed a solar pond the upper surface of which is covered with a thin layer of material which is substantially transparent with respect to solar radiation. Preferably the material is of low vapour pressure such as a diluted light oil. The layers in the pond can be formed of separate solid films or tubes.

Australian Patent Application No. 35883/78 entitled "Heat storage pond" is directed to an insulating layer that floats on the liquid and

10. is thick enough to thermally insulate the liquid against significant conductive heat loss to the ambient medium above the insulating layer. This insulating layer may be a gel either aqueous or non-aqueous .

-, c Australian Patent Application No. 46458/79 entitled "Heat storage in a pond containing a saturated aqueous saline solution" is directed to the extraction of hot solution at a low level region to cool the solution and bring about the precipitation of solute

2_Q therefrom, the cooled solute and the precipitated^" solute being returned to the appropriate layers in the pond.

With solar ponds it is highly desirable that any convection currents or wind induced currents ₂c be kept to an absolute minimum, and thus it is an object of the invention to provide means whereby

O wind induced currents and disturbances in the upper layer are kept to a minimum.

Also with solar ponds, there is a tendency for the salt from the concentrated layers to diffuse 5 upwardly to the less concentrated layers, and it is an object of this invention to provide means whereby the correct gradient is maintained.

Also as noted above if heat is not extracted from the solar pond the temperature in the. storage ■_{| Q} layer can reach 100 degrees Centigrade or more and thus it is an object of this invention to provide means to provide high temperature protection to the pond in the event that sufficient heat is not being extracted from the pond.

c Also it is highly desirable to maintain the correct gradients of the salt concentration in the pond, and means are also provided to assist in maintaining these gradients.

BRIEF DESCRIPTION OF THE INVENTION

0, In accordance with one aspect of the present invention there are provided on the solar pond - aerodynamic wind attenuators to significantly reduce the wind induced waves on the surface of the pond. In accordance with another aspect of the invention a portion of the less saline liquid is removed from the pond, evaporated to produce non-saline water and more saline water, the non-saline water being returned to the upper surface of the pond, and the more saline water is returned to a lower portion of the pond to maintain the saline gradient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in cross- section one form of 2__Q__ pond according to the invention,

FIG. 2 shows a plan view of the diffuser,

FIG. 3 shows a diffuser in detail and

FIG. 4 shows the wind attenuators.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in Fig. 4 the pond 1 has^~attenuators

15, 2 suspended across the pond to significantly reduce the wind induced waves on the surface of the pond.

This reduction would help to limit the size of the upper convective zone and increase efficiency 20. of the pond, and also provide good protection from destructive winds.

^υi- ' The attenuators 2 preferably comprise a plurality of planar members 3 suspended across the pond, these being spaced in a grid like pattern for example ten to twenty metres apart. Each attenuator 5_# could preferably comprise a sheet of plastics material extending across the width of the pond, and be suspended from wires or cables 4 anchored on posts 5 on opposite sides of the pond.

Preferably they can be made of polycarbonate 10. (perspex) plastic material so that in this way the sunlight will either pass through them or be reflected into the pond from them so that they will not at all reduce the effective operative area of the pdnd.

15. The attenuators would be so suspended that they would be spaced with a small clearance above the surface of the pond and by being suspended from the cables to allow easy access to all parts of the pond by a rubber raft or other shallow

20. draft boat for maintenance purposes.

The attenuators can be completely planar, or can be formed with small ridges or baffles to reduce the air flow along the surface of the attenuators .

25. It is realised of course that the attenuators woμld be situated across the pond so that they would be transverse to the direction of the prevailing winds.

OMFI While the attenuators can be made of a poly¬ carbonate plastics- material, it is to be realised that other forms of material could also be used.

It is known that in any- solar pond the salt 5. from the concentrated layers will over a period of time diffuse upwardly to the less concentrated layers and although this diffusion is quite slow, it must be dealt with in all ponds using sodium chloride or other salts which have a non-temperature 10. dependent solubility.

According to one aspect of the invention the salt solution concentrator extracts the water from just above the storage layer, and by simple distillatio the reconcentrated salt solution is re-injected into th 3_₅ storage layer, while the distilled water obtained from the distillation plant is re-injected into the surface the pond. Thus this flow of fresh water balances the upward salt diffusion and maintains a stable salt gradient.

20. Referring to Fig. 1 there is shown in diagrammatic form one form of solar pond one provided with a distillation unit to maintain the saline layers. Surface water from convective layer 17 is drawn through intake pipe 6 through a pre-heater condenser 7 and heat

25, exchanger 8 in the heat storage layer 9 of the pond. T water has now been heated to a temperature just below t temperature of the storage layer 9, and on entering the fl-ash chamber 10 will immediately flash to steam, for the chamber 10 is maintained below atmospheric pressure.

The vapour is drawn off through pipe 11 and through the pre-heater condenser 7 where the vapour is condensed, the vapour being drawn off by vacuum pump 12 and distilled water is discharged by the pump 12.

The concentrated brine solution in the evaporator

10 flows back by gravity to the concentrated storage layer 9, it being added to this layer by a diffuser 13. The evaporator 10 has a sight glass 14 to indicate the fluid level, and a self cleaning vapour outlet 15 comprising an upwardly converging outlet portion 16 into which the end of the pipe

11 protrudes, so that any vapour which may condensate in this portion drains back into the evaporator so that this will tend to dissolve and wash any Salt which may collect there back into the evaporator. The distilled water can be returned back to the top layer to maintain this non-saline convecting layer 17.

The saline water is returned to diffuser 13 by ceasing operation of the pump, closing valve 18, 19, and opening valve 23 to allow the dense brine solution to drain slowly from the evaporator. Valve 24 is a test valve to sample and test the salinity of the solution in the evaporator to ensure that it is returned to the correct layer.

The same flash evaporator system is used to reconcentrate less concentrated brine solution at the top of the storage layer 9. The valves 18, 19 are adjusted so that the intake to the evaporator is through the diffuser 20. As shown in Figures 2 and 3, the diff sers 13 and 2 each comprise a series of diffuser elements 21 comprisin a pair of circular plates 22 separated from each other. The fluid thus flows into and out of these diffuser elements in a horizontal plane, and the flow from these 5. continues radially so that the fluid at the various levels can be controlled without influencing or disturbing adjacent layers.

These diffusers are also used to withdraw hot brine from the top of the storage layer where

10. the brine is hottest, make it available to an external heat exchanger and then return cooled brine to the bottom of the pond storage zone with a minimum of disturbance to the pond. The "Y" shape of the joints in the manifold minimises

15. hydraulic friction losses and also balances the flow to each of the diffuser elements.

In operation of the pond the depth of the lower layer 9 is determined by the designer depending on the storage requirements of the system. The 20. depth of the upper layer 17 has a very significant effect on the convers o-n—efficiehcy and this should be kept to a minimum.

The effect of the slow diffusion of salt from the more highly concentrated zone to the 25. less concentrated zone is to move the boundary layers on either side of the gradient close together. As the two boundary layers move closer together the performance of the solar pond is seriously impaired by two effects; the larger upper convective

^"$U

O zone, as mentioned above reduces the conversion efficiency, and the resulting smaller gradient which insulates the storage layer allows greater thermal losses. The purpose of the boundary layer control system is to prevent movement of the boundaries and maintain the desired thermal performance of the solar pond.

Thus the process controls the movement of the top boundary layer. It does this by transporting

10, salt from the upper convective zone back into the lower convective zone. The rate of this reconcentrating process can be said to match the salt diffusion rate into the upper convective zone and thus keep the boundary stationary. Controlling the reconcentration

15, rate can be done by a combination of varying the vacuum pump rate and adjusting the valve 18 which controls the fluid flow into the flash chamber.

Since movement of the boundary layers is quite slow, usually requiring several days, the

20. flash evaporator system can be time shared between the two different intakes, the one from the upper surface layer and the other from the top of the lower storage layer. Of course in an alternative there can be two completely separated systems

25, which can operate one for each boundary layer.

It is to be noted that the heat contained in the storage zone of the solar pond provides the majority of the energy required in the re¬ concentration process and thus the only power 0, required is that to operate the vacuum pump.

O FI /l m V/IFO The flash chamber is designed so that the unit is self cleaning at both the inlet and outlets therefrom and by the process the salt in the system is completely recycled and gradient maintenance 5. is accomplished without the continual addition of^" extra salt.

The maximum performance of the solar pond is. achieved by the optimum control of the boundaries and thus the system can be used to shape the gradien 10. near the boundaries for better^' thermal performance as desired, the diffusers being adjustable verticall so that the desired boundary gradients are achieved.

As noted above the flow of fluids through the entire reconcentrating system is driven by

15. a single vacuum pump, and if desired the output from the pump can be used as a distilled ater output for separate use, further water being added to the pond to make up the losses from this use and from evaporation. Thus the system can be

20. used for water desalinisation with the solar pond providing the heat for the process.

It has been found that by injecting the solutio at the desired rate that the diffuser had the desired position or layer in the pond, that on 25. injecting the solution, the solution would flow outwardly along this layer due to the fact that the solution being injected would be at the desired concentration for that layer and there would

OM be virtually no intermixing with the adjacent layers. A set of two diffusers with pump may be used to perform the maintenance on the salt gradient, the diffusers being adjustable vertically 5. so that they can be positioned to any layer in the gradient and the diffusers would thus be used periodically to re-adjust the layers as desired.

If no heat is extracted from the large non- convecting solar pond for .several days, the temperature 10. in the storage layer can reach the boiling point and if this happens mechanical action induced by boiling would destroy the gradient and thoroughly mix the layers.

In order to. protect against the possibility 15. of. this happening, for example the result of pump failure, maintenance or for other causes, this invention in another aspect includes means for significantly blocking or reducing the solar radiation from entering the pond.

20. This can simply be achieved by floating a large number of polystyrene beads onto the surface of the pond from a container which is attached to the pond. This could be done either manually or mechanically or automatically at some preset

25. storage layer temperature. When the pond is desired to be set in operation again, the beads will be skimmed from the surface and returned to their container. Thus it will be seen that according to the invention an improved solar pond is provided with means for increasing the efficiency thereof.

Although various forms have been described 5. in some detail it is to be realised that the invention is not to be limited thereto but can include various modifications falling within the spirit and scope of the invention.

OMPI

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A solar pond having succesive layers of saline water increasing in density from the top surface thereof, said solar pond having means to maintain the succesive layers of varying density comprising means to withdraw less saline water from the pond, means to evaporate said withdrawn less saline water to form vapour and more saline water, means to condense said vapour, and means to return said more saline water to the level below the level , at which the less saline water was withdrawn.

2. A solar pond as defined in claim 1 wherein the less saline water is withdrawn from or adjacent the surface of the pond and passes through a heat exchanger/condenser and heat exchange relation with said vapour to condense vapour and raise the temperature of said withdrawn less saline water.

3. A solar pond as defined in claim 1 or claim

2 wherein the withdrawn less saline water is passed through a heat exchanger in the more condensed layers of the pond to raise the temperature of the withdrawn less saline water.

4. A solar pond as defined in claim 1 wherein the means to evaporate the less saline water comprises an evaporator maintained at a low- pressure by a vacuum pump connected to a vapour outlet, said less saline water evaporating in said evaporator. 5. A solar pond as defined in claim 1 wherein said less saline water is withdrawn from the pond at a level having a greater density then the upper layers of the pond but less than a density of

5. the lower layers, the withdrawn water passing directly to the evaporator.

6. A solar pond as defined in Claim 1 and including wind attenuators suspended above the surface of the pond, said attenuators suppressing the formation of waves on the surface of the pond.

7. A solar pond as defined in claim 1 wherein the means to withdraw the water and return the water comprise diffusers within the pond, said diffusers each comprising a pair of horizontal

5. plates separated from each other, a fluid opening through the centre of one of said plates, wherein said fluid flows radially to and away from said plates without substantially disturbing adjacent fluid in the pond.

8. A solar pond having succesive layers of saline water increasing in density from the top surface thereof, said pond having wind attenuators suspended above the surface of the pond to suppress the

5. formation of waves on the surface of the pond.

9. A solar pond as defined in claim 8 wherein said attenuators are planar members suspended across but spaced from the surface of the pond, said planar members being transversed to the direction . 5. of the prevailing winds.

10. A solar pond as defined in claim 9 wherein

' said members are transparent to allow transmission of sun rays through plates, or be reflected from the surface of the plates onto the surface of the pond depending on the angle of instances of the suns rays.

11. A solar pond substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

OMPI

*&,** WIPO rN>