Stable density stratification solar pond

A stable density-stratification solar pond 10 for use in the collection and storage of solar thermal energy including a container 12 having a first section 14 characterized by an internal wall of a substantially cylindrical configuration and a second section 16 having an internal wall of a substantially truncated conical configuration surmounting the first section in coaxial alignment therewith, the second section of said container being characterized by a base of a diameter substantially equal to the diameter of the first section and a truncated apex defining a solar energy acceptance opening 20. A body 26 of immiscible liquids is disposed within the container and comprises a lower portion 28 substantially filling the first section of the container and an upper portion 30 substantially filling the second section of the container, said lower portion being an aqueous based liquid of a darker color than the upper portion and of a greater density. A protective cover plate 36 is removably provided for covering the acceptance opening.

3. Background Discussion 
The invention generally relates to solar energy collectors and more 
particularly to an optimized "natural" solar pond particularly suited for 
use in capturing and storing solar thermal energy for residential and 
industrial usage, and providing minimum environmental problems. 
DESCRIPTION OF THE PRIOR ART 
Solar ponds of the types generally known, utilize "natural" or artificial 
density stratification facilitated by salty solutions of different 
concentrations. When exposed to the sun, rays of solar energy 
progressively penetrate the pond's depth with attendant increase in 
thermal energy absorption. 
This heat-transfer mechanism normally results in a relatively hot, dense 
bottom solution disposed beneath a relatively cool, dilute upper solution. 
Density gradients in the pond's depth preferably are designed large enough 
to stabilize density currents resulting from temperature differences and 
buoyancy forces. Diffusion at the thermocline boundary layer is considered 
to be inevitable, particularly during long periods of operation. 
Additionally, use of salty solar ponds tends to be limited to those areas 
where there can be found near a point of utilization, high-salinity lakes, 
seas, swamps, marshes, and so forth, of little environmental use. In such 
instances, a portion of the salty water beds can be made to serve as a 
solar pond, particularly where proper concentration levels are maintained. 
Utilization of man-made salty brine solar ponds, of course, tends to be 
limited because of the overwhelming operational and maintenance costs 
involved in such usage, particularly when compared to the low overall 
efficiency ultimately achieved and environmental problems associated with 
brine ponds. 
The prior art is replete with disclosures of solar-pond collector systems. 
For example, during the course of a preliminary search conducted with 
respect to the instant invention, the following patents were discovered 
U.S. Pat. No. 3,077,190, P. S. Allen; U.S. Pat. No. 3,372,691, S. Shachar; 
U.S. Pat. No. 4,063,419, Garrett; U.S. Pat. No. 4,066,062, Houston; U.S. 
Pat. No. 4,086,958, Lindner et al.; U.S. Pat. No. 4,099,558, Bricard et 
al.; U.S. Pat. No. 4,121,567, Carson; U.S. Pat. No. 4,159,736, Denis et 
al. None of the references discovered during the course of the search 
disclose the invention hereinafter described and claimed. However, it is 
noted that the patent to Garrett U.S. Pat. No. 4,063,419, discloses a 
method for obtaining solar energy utilizing solar ponds which includes a 
broad concept of using a dense liquid, such as a brine for solar energy 
collection and a covering agent, such as a floating oil, or other 
immiscible fluid or surface-active reagent, as well as an 
evaporation-inhibiting film in order to minimize or prevent evaporation of 
the pond liquid. Moreover, the pond liquid may include a dye for enhancing 
solar thermal energy absorption. It also is noted, however, that the prior 
art, including the patent to Garrett, clearly fails to disclose a stable, 
density-stratification, non-brine, solar pond of simplified economic and 
practical construction, which tends to render such a device suitable for 
residential and industrial usages. 
It is therefore the general purpose of the instant invention to provide a 
solar thermal energy collector which is particularly designed for enhanced 
operational efficiency in residential and industrial environments, and in 
particular has improved power output and may be used efficiently for 
cooling as well as heating. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the instant invention to provide a stable 
density-stratification solar pond and method for collecting solar thermal 
energy. 
It is another object to provide a solar pond, solar thermal energy 
collector designed for enhanced operational efficiency. 
It is another object to provide an improved solar pond having reduced 
requirements for continuous maintenance of fluid levels to compensate for 
evaporation and undesired interface diffusion. 
It is another object to provide an optimized, stable density solar pond for 
use in collecting and storing solar thermal energy for usage in 
low-temperature Rankine cycle power systems. 
It is another object to provide a natural, completely stable, 
density-stratification, non-brine, solar pond employing two immiscible 
fluids of different densities combined in a container simulating a cavity 
radiometer for enhancing the operational efficiency thereof. 
These and other objects and advantages are achieved through the use of a 
container characterized by a generally cylindrical upright interior the 
uppermost wall of said container having an upper conical segment within 
which there is defined a solar energy acceptance opening, and a body of 
liquid disposed within the container consisting essentially of two layers 
of immiscible, non-brine liquids of mutually differing densities having a 
boundary layer defined there-between. Some of the unique features of this 
invention are: (1) that the area of the opening to the surface area of the 
internal walls is in the range of 1/10 to 1/50, and the opening area is no 
greater than 50% of the projected area of the container; (2) that the 
upper liquid is substantially less dense than the lower liquid which is of 
a color which absorbs solar energy. The "projected area" of the container 
means the cross sectional area of a plane through the container which 
results in the maximum area. For example, if the container was a 
cylindrical sphere, the plane would be through the center of the sphere at 
the maximum diameter. Typically the upper liquid is at least 20% less 
dense than the lower liquid. Because of these combined features the pond 
of this invention is able to reach higher temperatures than conventional 
systems (for example, 207.degree. F. for the pond of this invention 
compared to 170.degree. F. for conventional ponds). Moreover, because the 
liquid is essentially salt free (non-brine), it does not present a 
pollution problem and is easy to maintain.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, wherein like reference characters designate 
like or corresponding parts throughout the several views, there is shown 
in FIG. 1 a solar pond, generally designated 10, embodying the principles 
of the instant invention. 
As shown in FIG. 1, the solar pond 10 includes an upright container 12 
having a base segment 14 and a shroud segment 16. As best illustrated in 
FIG. 4, the internal surface of the base segment 14 is of a generally 
cylindrical configuration while the internal surface of the shroud segment 
is of a substantially frusto-conical configuration disposed in coaxial 
juxtaposition with the base segment. As a practical matter, the container 
12 includes a planar bottom segment 18, FIG. 4. Of course, the base 
segment 14, the shroud segment 16, and the bottom segment 18 are 
integrally related to form an integral structure. Moreover, within the 
plane of truncation of the shroud segment 16, there is defined an opening, 
herein referred to as a solar energy acceptance aperture 20. This aperture 
is closed by a transparent cover 22 formed of a suitable material, such as 
thermoplastic polymers and the like. The cover 22 serves as a 
fluid-evaporation barrier, as well as a barrier to wind, trash, and the 
like, and is supported by an annular structure, herein referred to as a 
protective ring 24. The ring 24 circumscribes the opening 20 and is, in 
turn, supported in a suitable manner. 
The ratio of the area of the opening to the internal surface area of the 
base, shroud and bottom segments is within a range of 1/10-1/50. As the 
ratio approaches a ratio of 1/50, the pond 10 approaches a black-body 
configuration of a relatively large volume. Accordingly, the ratio of the 
areas aforementioned serve to dictate the efficiency of the pond, and the 
angles of the shroud segment need only to accommodate the establishment of 
a desired ratio. In this example, the projected area corresponds to the 
area of base of the container. 
Within the container 12, there is disposed a body 26 of immiscible liquids, 
generally designated 26, FIG. 4. This body of liquids 26 includes a lower 
liquid 28 which substantially fills the base segment 14 of the container 
12, while there is deposited thereon, in a layered relationship therewith, 
therefore an upper liquid 30 of the body 26. The upper liquid 30 
substantially fills the shroud segment 16 with an optimum height between 
10 to 50% of the lower liquid. As a practical matter, a boundary layer 32 
is defined between the upper and lower liquids 28 and 30, respectively. 
It is important here to note that the lower liquid 28 of the body 26 of 
liquids is of greater density than the upper liquid 30 thereof. To 
exemplify, the body 26 comprises two immiscible liquids which may, where 
so desired, include water, glycerol-water or ethyleneglycol-water mixtures 
or the like as the lower liquid, and oil of paraffin, mixed, and naphthene 
base, or synthetic fluids such as polyolefins, polyalkylene glycols, 
silicones, halogenated hydrocarbons or vegetable oils or the like as the 
upper liquid. Because of their relative immiscibility, relatively 
widespread availability, low cost, and non-toxicity, oil and water are 
particularly suited for usage in solar ponds. Brines of differing 
densities could be used to establish a thermal gradient, but they present 
serious pollution and maintenance problems and, therefore, are 
unacceptable. 
Additionally, it is preferred that the lower liquid 28 of the body 26 of 
immiscible liquid, comprises a water or water of a dark color. Such a 
color may be imparted thereto through the use of a black dye or any other 
soluable dye with high absorptivity to the solar energy spectrum. The 
second portion 30 of the body 26 should be substantially transparent to 
rays of solar energy with a low extinction coefficient to solar radiation 
wave lengths. Thus, the upper liquid will act as low absorber and high 
transmitter to short wavelengths of the solar spectrum band. 
Where so desired, a layer of insulation foam, such as may be employed as a 
blanket 34 about the periphery of the external surface of the container 
12. Such is provided as desired in order to minimize thermal energy losses 
to ambient air, the ground, and the like. Also, where so desired, a 
thermally insulated cover 36 is provided for preventing loss of thermal 
energy through the solar energy acceptance opening 20. As a practical 
matter, the cover 36 is, where desired, formed of a flexible insulative 
material and stored on a reel 38 in a manner such that the cover 36 may be 
drawn across the opening 20 in the absence of impinging solar radiation, 
such as occurs during the night season. A cover typifying that which may 
be employed is illustrated in FIG. 5. As shown, the cover 36 is connected 
with spring-loaded take-up reels 40, via cables 42, which are so 
spring-biased as to draw the cover 36 toward the reels 40. However, as 
illustrated, a motor 44 is provided for driving a suitable shaft of the 
reel 38 in order to effect a winding of the cover 36 thereabout. Where so 
desired, the motor 44 is connected to a solar panel 46, via suitable leads 
not designated. Where so desired, the solar panel 46 may comprise a simple 
photocell for closing a circuit switch in order to complete an electrical 
circuit through the motor 44 to ensure operation of the pond as a solar 
absorber collector during the sunny hours, and actuate the motion of the 
cover 36 at night hours, for a complete unattended operation. 
It should be appreciated, however, that the panel 46, upon being energized 
by incident solar radiation, serves to energize the motor 44 for winding 
the cover 36 about the reel 38, against the forces applied by the reels 40 
via the cables 42. In the event the solar panel 46 is shaded from incident 
solar energy, the motor 44 de-energizes for thus permitting the reel 40 to 
draw the cover 36 across the solar energy acceptance opening 20. Thus the 
solar pond 10 is maintained in a suitable condition for receiving solar 
energy, and yet is protectively covered for inhibiting loss of heat when 
shaded from incident solar radiation. 
In view of the foregoing, it should now be apparent to those familiar with 
solar ponds, their construction and usages and the like that the relative 
thicknesses of the first and second portions 28 and 30 of the body 26 of 
liquids, the index of refraction and transmissivity coefficients of the 
upper and lower density liquids, as well as the thermal and physical 
properties of the liquids and the color of the bottom, higher density 
fluid can be varied in a manner well within the skill of the art. Such 
variations, of course, tend to reduce convection currents at the boundary 
layer 32 while the thermal storage characteristics of the pond are 
enhanced. Moreover, through the use of the transparent cover 22, the 
immiscible liquids forming the first and second portions of the body 26 of 
liquid can be maintained in a calm condition for thus further reducing the 
chances of developing vertical convection currents. 
Finally, the pond is provided with a heat transfer system 50. As shown, an 
inlet 52 and an outlet 54 for a coil 56 circumscribing the lower portion 
of the pond is provided. Suitable fluids such as organic fluids known as 
R-11 and R-113 serve quite satisfactorily as both heat transfer fluid and 
as a working fluid for a low-temperature Rankine cycle system. 
OPERATION 
It is believed that in view of the foregoing description, the operation of 
the invention herein disclosed and claimed readily is apparent, however, 
in the interest of completeness, the operation of the invention herein 
disclosed and claimed is, at this point, briefly reviewed. 
With the solar pond 10 having deposited within the container 12 a body 26 
of immiscible liquid, as hereinbefore described, the device is ready for 
operation simply by removing the cover 36, manually or mechanically. 
As rays of solar energy are accepted at the solar energy acceptance opening 
20, solar rays characterized by relatively short wave lengths, 0.3-3 
microns, penetrate the transparent upper liquid 30 of the liquid body 26, 
passing through the boundary layer 32. The solar thermal energy now is 
absorbed in the lower liquid 28 of the body 26 of immiscible liquids. Thus 
the temperature of the lower liquid 28 rises. As the temperature of the 
lower liquid 28 rises, it begins to radiate thermal energy, a small 
quantity of the radiation being of a wave length to which the upper liquid 
30 of the body 26 is transparent, while a much larger quantity of the 
radiation is of a wave length to which the upper liquid 30 of the body 26 
is opaque. Thus a major portion of the radiation is trapped in the lower 
liquid of the pond. 
Due to the differing densities of the first and second portions 28 and 30, 
respectively, of the body 26 of immiscible liquids, the upper and lower 
liquids 28 and 30 respectively tend to remain separated along the boundary 
layer 32. Of course, as the thermal energy of short wave lengths is 
radiated from the upper liquid 30 of the body 26 of immiscible liquids, 
the radiation strikes the internal surface of the shroud segment 16 and is 
re-reflected back toward the lower liquid 28 of the body 26 of liquids. 
Thus the thermal energy is, in effect, trapped due to the opacity of the 
body 26 as well as the geometry of the internal surfaces of the container 
12, in much the same manner as radiation is trapped within the cavity of a 
black body. Consequently, the lower liquid 28 of the body 26 of immiscible 
liquids is, in operation, maintained as a "hot" fluid relative to the 
upper liquid 30 of the body. Thus solar thermal energy is trapped within 
the pond 10 and the energy thus stored is usable as thermal energy for 
heating, cooling, or power generation in residential and industrial 
usages. 
In order to utilize the thermal energy thus collected and stored, a 
suitable heat transfer fluid is circulated through the heat transfer 
system 50 for conveying thermal energy from the pond 10 to a system, such 
as a low-temperature Rankine cycle power system, not shown. 
In view of the foregoing, it is believed to be readily apparent that the 
solar pond 10 provides a practical solution to the problems heretofore 
encountered by those engaged in the design of devices and systems intended 
to function as solar thermal energy collectors.