Salt Gradient Solar Ponds
Solar Ponds

A solar pond is a pool of saltwater which acts as a large-scale solar thermal energy collector with integral heat storage for supplying thermal energy. A solar pond can be used for various applications, such as process heating, desalination, refrigeration, drying and solar power generation.


A solar pond is simply a pool of saltwater which collects and stores solar thermal energy. The saltwater naturally forms a vertical salinity gradient also known as a "halocline", in which low-salinity water floats on top of high-salinity water. The layers of salt solutions increase in concentration (and therefore density) with depth. Below a certain depth, the solution has a uniformly high salt concentration.

There are 3 distinct layers of water in the pond:

The top layer, which has a low salt content.

An intermediate insulating layer with a salt gradient, which establishes a density gradient that prevents heat exchange by natural convection.

The bottom layer, which has a high salt content.

If the water is relatively translucent, and the pond's bottom has high optical absorption, then nearly all of the incident solar radiation (sunlight) will go into heating the bottom layer.

When solar energy is absorbed in the water, its temperature increases, causing thermal expansion and reduced density. If the water were fresh, the low-density warm water would float to the surface, causing a convection current. The temperature gradient alone causes a density gradient that decreases with depth. However the salinity gradient forms a density gradient that increases with depth, and this counteracts the temperature gradient, thus preventing heat in the lower layers from moving upwards by convection and leaving the pond. This means that the temperature at the bottom of the pond will rise to over 90 °C while the temperature at the top of the pond is usually around 30 °C. A natural example of these effects in a saline water body is Solar Lake in the Sinai Peninsula of Egypt.

The heat trapped in the salty bottom layer can be used for many different purposes, such as the heating of buildings or industrial hot water or to drive an organic Rankine cycle turbine or Stirling engine for generating electricity.

Advantages and disadvantages

The approach is particularly attractive for rural areas in developing countries. Very large area collectors can be set up for just the cost of the clay or plastic pond liner.
The evaporated surface water needs to be constantly replenished.
The accumulating salt crystals have to be removed and can be both a valuable by-product and a maintenance expense.
No need of a separate collector for this thermal storage system.


The energy obtained is in the form of low-grade heat of 70 to 80 °C compared to an assumed 20 °C ambient temperature. According to the second law of thermodynamics (see Carnot-cycle), the maximum theoretical efficiency of a cycle that uses heat from a high temperature reservoir at 80 °C and has a lower temperature of 20°C is 1-(273+20)/(273+80)=17%. By comparison, a power plant's heat engine delivering high-grade heat at 800 °C would have a maximum theoretical limit of 73% for converting heat into useful work (and thus would be forced to divest as little as 27% in waste heat to the cold temperature reservoir at 20 °C). The low efficiency of solar ponds is usually justified with the argument that the 'collector', being just a plastic-lined pond, might potentially result in a large-scale system that is of lower overall levelised energy cost than a solar concentrating system.


Further research is aimed at addressing the problems, such as the development of membrane ponds. These use a thin permeable membrane to separate the layers without allowing salt to pass through.


The largest operating solar pond for electricity generation was the Beit HaArava pond built in Israel and operated up until 1988. It had an area of 210,000 m² and gave an electrical output of 5 MW.[1]

The first solar pond in India (6000 sq. metres) was built at Bhuj. The project was sanctioned under the National Solar Pond Programme by the Ministry of Non-conventional Energy Sources in 1987 and completed in 1993 after a sustained collaborative effort by TERI, the Gujarat Energy Development Agency, and the GDDC (Gujarat Dairy Development Corporation Ltd). The solar pond successfully demonstrated the expediency of the technology by supplying 80,000 litres of hot water daily to the plant. The Energy and Resources Institute provided all technical inputs and took up the complete execution of research, development, and demonstration. TERI operated and maintained this facility until 1996 before handing it over to the GDDC. The solar pond functioned effortlessly till the year 2000 when severe financial losses crippled GDDC. Subsequently, the Bhuj earthquake left the Kutch Dairy non-functional.[2]

The 0.8-acre (3,200 m2) solar pond powering 20% of Bruce Foods Corporation's operations in El Paso, Texas is the second largest in the U.S. It is also the first ever salt-gradient solar pond in the U.S.[3]


C, Nielsen; A, Akbarzadeh; J, Andrews; HRL, Becerra; P, Golding (2005), "The History of Solar Pond Science and Technology", Proceedings of the 2005 Solar World Conference, Orlando, FL

Solar Gradient Solar Ponds, accessed on 28 November 2009,

MacInnis, Roberta. "Solar pond producing power for Texas cannery." Energy User News 12 (March 30, 1987): 8(1). General OneFile. Gale. BENTLEY UPPER SCHOOL LIBRARY (BAISL). 8 Oct. 2009

Salt of the Earth

Here is a pond that contains heat in layers of varied temperature—the deeper you delve into it, the hotter it gets, seemingly defying a simple heat theory that makes hot-air balloons fly. But this is no wonder pond, nor does it fall outside any scientific premise. The pond is a large saline water body in which solar energy is trapped in the salt. The salt solution gets denser with depth, making it possible to maintain a concentrated layer of hot brine at the bottom. T ERI researchers built a salt-gradient solar pond near Bhuj in Gujarat and put it to a unique use— supply process heat to an enduser, for the first time in India.

The heat factory

Coming back to the heat theory, air or water, when heated, rise as they lose weight in the process. In an ordinary water body, when sunlight heats up the water, it rises to the surface and loses its heat to the atmosphere, keeping the water at nearly atmospheric temperature. The solar pond technology inhibits this phenomenon. With depth, the salt concentration increases, thereby creating a salinity or density gradient at the middle layer—the all-important NCZ (non-convective zone). This stable ‘gradient zone’ does not allow the less concentrated salt water from the upper convective zone at the top to move down and the densely concentrated salt water from the lower convective zone at the bottom to move up. The NCZ acts as a transparent insulator that lets sunlight reach the bottom where it remains entrapped, creating a storehouse of thermal energy in the form of hot brine.

The Bhuj experiment

The salt-gradient solar pond near Bhuj

In wake of the looming threat from global warming and also the rising scarcity of fossil fuels, there has been an accentuated global call for maximizing the use of renewable energy. TERI researchers foresaw promising results in the indigenously developed salt-gradient solar ponds. The result was the construction of the Bhuj solar pond—an idea mooted by a group of scientists in 1983.

The project was sanctioned under the National Solar Pond Programme by the Ministry of Non-conventional Energy Sources in 1987 and completed in 1993 after a sustained collaborative effort by TERI, the Gujarat Energy Development Agency, and the GDDC (Gujarat Dairy Development Corporation Ltd). TERI provided all technical inputs and took up the complete execution of research, development, and demonstration. TERI operated and maintained this facility until 1996 before handing it over to the GDDC. The solar pond functioned effortlessly till the year 2000 when severe financial losses crippled GDDC. Subsequently, the Bhuj earthquake left the Kutch Dairy non-functional.

Scaling new heights

The Bhuj solar pond covered an area of 6000 aquare metres. The Bhuj solar pond project stood out in many regards. The first-ever solar pond in India to have connected itself to an end-user – supplying industrial process heat to the Kutch Dairy – this pond, covering an area of 6000 square metres, was, at that time, the largest operating solar pond in the world. Avoiding use of imported membrane lining, the project developed a cost-effective, indigenous lining scheme, using locally mined clay and plastics. While the pond attained a record 99.8 °C under stagnation, stability of the salinity gradient was maintained even at such elevated temperatures. With only one injection diffuser on one side of the pond, the desired salinity profile was achieved even at the farthest end. More important, laboratory scale testing and the success in catering to actual user-demand have paved way for the commercial exploitation of the technology in India.

Carving a niche

The hot brine – extracted from the bottom of the pond – is pumped through a shell-and-tube heat exchanger where it heats water up to a temperature of 70 °C. Further, this hot water was delivered to the Kutch Dairy plant to be used as pre-heated boiler feed water as well as for cleaning and washing. The entire exercise at the Bhuj solar pond successfully demonstrated the expediency of the technology by supplying 80 000 litres of hot water daily to the plant.


The Bhuj experiment significantly placed the solar pond option as a comparable alternative to technologies that are in use — fossil-fuel-fired process heating and solar flat plate collectors for water heating (Box 1).

Opening more windows

Other than process heating, thermal energy collected in a solar pond can be used in many more applications:

Process heat can produce hot air for industrial and space heating applications.
Solar pond-based desalination system offers cost-effective solution for production of potable water from brackish/sea water.
Using the vapour absorption refrigeration system, the heat contained in a solar pond can be used for cold storage of food products and also for air-conditioning.

The hot brine can also generate electricity, using an organic Rankine cycle engine.

The capital and operating costs of the technology are very much site- and application-specific. However, these are expected to be lower than the competing technologies. For example, the cost of heat energy from a solar pond is about 60% of the cost of energy from a flat plate solar water-heating system. Process innovation like clay– plastic–clay lining reduces the construction cost. With suitable government incentives, the technology may soon become one of the most viable solar energy options.

Solar Pond @ Bhuj

Sep 27, 2008
Uploaded by Pramod Mathur

The largest operative solar pond project in the world is located in Bhuj, in the state of Gujarat. By maintaining ...

Heat extraction methods from salinity-gradient solar ponds...

Jimmy Leblanca, et al.

  Energy Conservation and Renewable Energy Group, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, P.O. Box 71, Bundoora, Victoria 3083, Australia


Heat has generally been successfully extracted from the lower convective zone (LCZ) of solar ponds by two main methods. In the first, hot brine from the LCZ is circulated through an external heat exchanger, as tested and demonstrated in El Paso and elsewhere. In the second method, a heat transfer fluid circulates in a closed cycle through an in-pond heat exchanger, as used in the Pyramid Hill solar pond, in Victoria, Australia. Based on the experiences at the El Paso and Pyramid Hill solar ponds, the technical specifications, material selection, stability control, clarity maintenance, salt management and operating strategies are presented. A novel method of extracting heat from a solar pond is to draw the heat from the gradient layer. This method is analysed theoretically and results of an experimental investigation at Bundoora East, RMIT, are presented. An in-pond heat exchanger made of polyethylene pipe has been used to extract heat for over 2 months. Results indicate that heat extraction from the gradient layer increases the overall energy efficiency of the solar pond by up to 55%, compared with conventional method of heat extraction solely from the LCZ. The experimental results are compared with the theoretical analysis. A close agreement has been found. From this small-scale experimental study, convection currents were found to be localised only and the density profiles were unaffected. An experimental study using an external heat exchanger for brine extraction and re-injection at different levels within the gradient layer still needs to be conducted to determine the effect of the heat extraction from the non-convective zone (NCZ) on the stability of the salinity gradient (both vertically and horizontally) and an economic analysis needs to be conducted to determine the economic gains from increased thermal efficiency.
El Paso Solar Pond

An aerial photograph of the El Paso solar pond.The El Paso Solar Pond project is a research, development, and demonstration project initiated by the University of Texas at El Paso in 1983. It has operated since May 1986 and has successfully shown that process heat, electricity, and fresh water can be produced in the southwestern United States using solar pond technology. An organic Rankine-cycle engine generator was installed on site in 1986, making it the first in the U.S. to generate grid connected power, producing up to 70kW. Most of this power has been delivered to Bruce Foods Corporation for peak power shaving. This demonstrates one of the primary benefits of solar ponds: power on demand -- even at night or after long periods of cloudy weather.
May/June 1980

 Israel's 150kw Solar Pond

By the MOTHER EARTH NEWS editors

In 1980, one of MOTHER EARTH NEWS' technical editors and her tour director returned from a wonderfully informative ten-day solar tour of Israel jointly sponsored by Jordan College, Solar Age magazine, and THE MOTHER EARTH NEWS. The trip was intended to both entertain and educate ... and judging from the enthusiastic response of the tour group, it fully succeeded on both counts!

One of the most impressive sights of the entire excursion was a small power-generating station located at the southern end of the Dead Sea... where a considerable amount of electricity is being produced using solar energy. You might well assume that the plant operates on photovoltaics, but — if that's what you're thinking — you'll have to guess again. The new power-generating facility is a solar pond ... inexpensive, often naturally occurring phenomena that may just hold the secret to a safe and reliable middle-technology form of energy production!

Needless to say, MOTHER EARTH NEWS is very interested in any power system that delivers a lot for a little. So — in addition to reporting on what we saw in Israel — we fully intend to research, first hand, the possibility of scaling down both the size and technology of the operation ... in order to put it on the "little guy's" level, and maybe open up a whole new world of energy independence for us all!

As anyone who's worked with solar energy knows, one of the major stumbling blocks to a successful "sun" system is the difficulty of obtaining effective heat storage, regardless of whether the collection medium is air or liquid. (With a fluid system, the problem is further compounded by the fact that leakage may be disastrous, and equipment costs can be prohibitive.) What's more, solar setups of any kind require collectors ... which can often mean additional expense and headaches.

So it makes sense that any solar energy system which does away with collectors and storage tanks has an obvious advantage over the more common techniques. The question is, how can a solar power operation get by without what are usually thought of as the two most important components? And the answer is ... by using a saline pond. Yes, believe it or not, technicians from Ormat Turbines, Ltd. — an Israel-based manufacturer of Rankine-cycle drive turbines — have developed a system which uses heat that's gathered and stored in a pool of salt water to generate up to 150 kilowatts of electrical power!

Ormat's entire system is actually quite straightforward. The firm's pilot project — located at En Boqeq on the Dead Sea — utilizes a man-made pond measuring 75,347 square feet and extending to a depth of just over eight feet. This pool is lined with a reinforced rubber "skin" that prevents seepage of the briny liquid into the ground and into the fresh water table below. (The En Boqeq installation uses no insulative layer between the earth and the water ... however, under certain conditions, such thermal protection may be desirable — in order to maintain optimum water temperature — in addition to the brine barrier.)

The pond itself is filled with a dense salt solution, which naturally separates into gradient levels: In other words, a weaker salt mix exists near the water's surface, and the higher — and thus denser — saline concentrations settle to the bottom. The topmost layer is, of course, relatively fresh water. As the sun beats down on the brine reservoir, its thermal energy passes through the "lighter" surface levels and heats the heavy saline mass below. Because of the intense density of the solution at the bottom of the pond, the mixture resists the convection process which would cause mixing, and thus uniform warming, in a body of fresh water . . . and the result is that the dense salt solution — held at the bottom of the pool and thermally protected by the surface layer — can reach temperatures as high as 194°F (year-round working temperatures generally fluctuate between 158 and 176°F). The water at the surface of the pond, of course, stays at a warmth that's "normal" for the desert region: between 68 and 90°F, depending on the season.

In order to take advantage of the heat stored in the salt gradient pool, Ormat engineers utilize a three-cycle (evaporation, drive, and condensation) system. Here's how it works: The hot salt water is pumped through a heat exchanger, which is surrounded by a vessel filled with a substance similar to freon. This, in turn, is connected to a turbine that's specially designed to be driven by a much lower-temperature propellant than that used in a conventional steam turbine. Since the medium changes from liquid to gas at a relatively low heat, the sun warmed water instantly flashes the fluid into a pressurized vapor ... which drives the turbine and its 150-kilowatt AC generator.

After the vapor has done its work, it passes on to yet another chamber where it's condensed to the liquid stage again by cooler water — taken from the surface of the pond — that's pumped into a second set of heat exchange tubes within the vessel. The drive fluid is then ready to be recycled. In this manner, the "refrigerant" fluid — and both the cool and hot water reserves — are used repeatedly, within a closed system, to eliminate waste. The only liquid that has to be replenished now and then is — because of evaporation losses — the upper layer of fresh water.

The beauty of the En Boqeq installation — and of any solar pond — is that it will function day and night, winter and summer, regardless of whether or not the sun is shining steadily ... since the pool provides such a massive heat sink. The Ormat pilot project has been so successful, in fact, that the Israeli government is working with the firm to build a 5,000kw power station that should be completed within two years, and the nation eventually hopes to use a 154-square mile portion of the Dead Sea to generate enough electricity to supply all of Israel's power requirements for the future! (Of course, in a body of water this size—and even in a pool as relatively small as the En Boqeq test site — winds can cause undesirable disturbances. The problem is minimized by the use of plastic nets strung across the surface of the pond.)

It's not difficult to imagine that solar ponds might just be the "new wave" of alternative energy ... especially since costs (calculated by Ormat to be in the $2,000-per-kilowatt range) are even now competitive with conventional utility-supplied power. But if the technology can be reduced to a "backyard tinkerer's" level—and the environmental danger of brine leakage into the water table can be eliminated — there's no reason why anyone with even a few acres of land can't be totally energy self-sufficient ... or, at the very least, enjoy the benefit of reliable solar heat at a minimum of cost.


Method for extracting lithium carbonate from carbonate brine

The invention discloses a method for extracting lithium carbonate from carbonate brine. A salt gradient heat-preserving solar pond is built by clay; black ethylene-propylene-diene monomer waterproof coiled material, geomembrane or high density polyethylene spraying canvas and the like are laid at the bottom surface of the pond as the mat; a heat exchanging pipeline or an electric heating device is arranged 0.5-1m above the bottom of the pond; lithium-rich brine which is prepared by freezing and solarization is directly poured into the salt gradient heat-preserving solar pond, a layer of fresh water is laid at the surface of the brine, the brine is heated by the double action of solar irradiation and heat exchanging or electric heating and lithium carbonate is separated out concentratedly; and finally, lithium mixed salt obtained in the salt gradient heat-preserving solar pond is scrubbed with fresh water at proper temperature. The combination of the salt gradient heat-preserving solar pond, the heat exchanging or electric heating device and scrubbing by fresh water at proper temperature is applied to the extraction of lithium carbonate from carbonate brine, therefore, the production efficiency and the product quality are greatly improved, the production cycle is shortened, the production cost is lowered and lithium carbonate concentrate is obtained directly.


Device for maintaining stability of salt gradient solar pond
The utility model provides a device for maintaining the stability of a salt gradient solar pond, wherein the bottom of the salt gradient solar pond is provided with a salt water supplement pipe. The salt water supplement pipe is uniformly provided with salt supplement holes. The salt water supplement pipe is communicated with a premixing pond through a salt inlet pipe. From the salt gradient solar pond to the premixing pond, the salt inlet pipe is successively provided with a salt supplement valve and a salt supplement pump. The middle and lower part of the salt gradient solar pond is communicated with the premixing pond through a salt water pumping pipe. From the salt gradient solar pond to the premixing pond, the salt water pumping pipe is successively provided with a salt water pumping pump and a salt water pumping valve. The upper part of the salt gradient solar pond is provided with a water supplement pipe. The water supplement pipe is uniformly provided with water supplement holes. The water supplement pipe is connected with a clear water pond through a water inlet pipe. From the salt gradient solar pond to the clear water pond, the water inlet pipe is successively provided with a water supplement valve and a water supplement pump. According to the utility model, the upper troposphere is supplemented with clear water through the clean water pond, while the lower troposphere is supplemented with salt water through the premixing pond. Therefore, after the salt gradient solar pond is operated for a long time, the salt concentration distribution and the clarity of the salt gradient solar pond can be quickly recovered.

Solar pond composite drying system

The invention relates to a solar pond composite drying system. The system comprises a salt gradient solar pond, wherein an upper layer of the salt gradient solar pond is an upper convection layer which consists of clear water, a lower layer of the salt gradient solar pond is a lower convection layer, namely a heat storage layer, which consists of saturated salt solution, and a middle layer of the salt gradient solar pond is a non-convection layer, namely a gradient layer. A porous heat absorption dielectric layer is arranged in the heat storage layer of the salt gradient solar pond. Waste boiler slag is used as a heat absorption dielectric in the porous heat absorption dielectric layer. In order to enhance a heat absorption effect, the waste boiler slag is laid at the bottom of the solar pond to serve as the heat absorption dielectric. The solar pond has functions of a heat collector and a heat storage device; the system has a simple structure and is low in manufacturing cost; heat can be stored for a long time (extraseasonally); and a low-temperature heat source with stable performance can be provided all year around. Through the system, the problem of serious influence of changes of weather on a heat collector-greenhouse type solar dryer is solved, solar energy is fully utilized, and drying efficiency is increased.


An apparatus and method for the desalinization of salt water utilizing a humidity chamber under partial vacuum and a water collection structure to collect fresh water product. Saltwater having a first temperature and cooling water contained in a condenser having a second temperature lower than the first temperature are introduced into the humidity chamber via a solar powered vacuum pump. A temperature gradient created by a difference in temperature between the saltwater and cooling water in combination with a partial vacuum (e.g., 10-20%) created by a solar powered vacuum pump is used to distill salt-free water from the saltwater with high efficiency. The temperature gradient is created in part by the use of a salinity gradient solar pond.; The salt-free water is obtained by condensation of the water on a collection surface cooled by the cooling water followed by collection of the water in a storage apparatus.






To provide a solar pond having no deterioration in performance by a method wherein a sheet material at a side wall other than a non-convection flow layer of a solar pond is of a sheet material having a high rate of reflection. CONSTITUTION:Although a bottom surface and a side wall of a thermal accumulation layer 4 of a solar pond 1 are covered with a black sheet 5 in order to improve a rate of absorption of solar light, a side wall and a ground part other than a non-convection layer 3 are covered with, for example, a white sheet material 6 having a low optical absorption and a high rate of reflection. Due to this fact, the solar light is not absorbed by the sheet material 6, but most of it are reflected by the sheet material and reach a thermal accumulation layer 4 and effectively accumulated there. Thus, an abnormal heating is salt water at the sheet material and a part near the sheet material is not produced, and a gradient in concentration can always be kept constant and a thermal loss can be prevented from being increased upwardly. This may be helpful for increasing the temperature of the sheet material through a direct radiation of solar light and preventing the deterioration of the material.


To provide a solar pond having a simple constitution in which the number of peripheral devices is reduced by a method wherein an evaporation pond is arranged in a solar pond system, the evaporation pond is divided into some sections to add an adjustment of concentration of solar pond or a formation of initial concentration gradient to a function of evaporation. CONSTITUTION:Under a normal operation, a partition plate 18 in an evaporation pond 9 is removed and an entire evaporation pond 9 may act as an evaporation pond. When a gradient of concentration of a solar pond 4 is to be adjusted, the partition plate 18 is fixed to divide the evaporation pond 9 into two sections so long as a concentration of an entire evaporation pond 9 does not show a desired concentration value. When a condensed salt water is required at a concentration adjustment part 14a, salt is fed to make an adjustment and in turn when a thin salt water is required, fresh water is fed to make desired salt water having a desired concentration, a valve 19 is closed, a valve 20 is opened and then the water is fed into the solar pond 4 through a return port 16 under an operation of a salt water returning pump 10. When an initial concentration gradient is to be made, the evaporation pond 9 need not be divided so that the partition plate 18 is removed, salt and fresh water are fed into the entire evaporation pond 9 and agitated to make salt water having a desired concentration and then the salt water is fed into the solar pond 4 in sequence under an operation of the salt water returning pump 10. With this arrangement, it is possible to eliminate a concentration adjustment device.


To escape gas, produced from earth in the vicinity of the bottom part or the side wall of solar pond, without staying therein and to prevent breakdown of the solar pond, by a method wherein a sheet being impermeable to liquid but permeable to gas or steam is laid to a side wall part and a bottom part. CONSTITUTION:A waterproof sheet, e.g. polytetraphloroethylene porous material, being impermeable to liquid but permeable to gas, is used between a solar pond 1 and earth 2. When a gas pressure increases, gas permeates the sheet into liquid in the solar pond situated above the sheet. Since bubbles, incoming through the sheet to the interior of liquid, gradually raise in such a state as to form small bubbles thanks to very fine meshes of the sheet, a concentration gradient in the solar pond, produced by salt, is prevented from breaking down.

To increase the heat insulating properties and the thermal efficiency of a solar pond, by certainly forming a linear concentration gradient layer by forecasting the change with time in the distribution of concentration by logic calculation, and by intermittently raising a nozzle to the position which is determined by the difference between the forecast value and a target value. CONSTITUTION:A solar pond is filled with salt water of uniform concentration up to the half of the thickness (=2L) of a concentration gradient layer 3 to be formed, and a nozzle 4 to feed clear water is located at the lowermost part (height Z6) of a concentration gradient layer. The height of a nozzle is held constant and clear water is fed. A method of providing the distribution of concentration which is nearly linear is as follows.; The distribution of concentration is logically computed per predetermined time interval DELTAt. The concentration lowers as the time elapses. When the curve of concentration distribution 'Dr' crosses with the target line of concentration distribution 'Do' and the upper and the lower areas A and B in the parts surrounded by two distribution lines become equal, the nozzle is moved to the upper crosspoint 'Z2' of two distribution lines. The above-mentioned procedure is repeated and the nozzle is raised intermittently. The nozzle is also moved to the crosspoint 'Z2' of two distribution lines when part of the curve of concentration distribution 'Dr' comes lower than the target line of concentration distribution 'Do'.



Aquaculture in nonconvective solar ponds

Apparatus is presented for cultivating aquaculture and mariculture crops predominantly in the warm storage zone (SZ) of a durable, salt gradient, solar pond. This SZ would be maintained near the optimum salinity and temperature for the particular crop and especially guarded against overheating. The nonconvective zone (NCZ) of this pond would insulate the SZ and buffer diurnal temperature oscillations in this SZ. Variations of the basic invention include using a partition to separate the SZ and NCZ, not using a pond liner, and adding heat from an external source to the SZ, such as geothermal or power plant waste heat. Because temperature elevations will usually be only 10 DEG to 25 DEG C., it will commonly be possible to insure stable stratification with modest salinity changes and to supply sufficient heat from directly absorbed solar energy alone. These solar ponds could economically provide optimum growing conditions fall through spring in temperate latitudes. Three variations are worth particularly noting. In solar ponds in dry climates the SZ could have a salinity near 12% while the UCZ was near 2%. High salinity crops would be cultivated in the SZ such as Artemia, the brine shrimp, or Dunaliella, an algae. An attractive variation for locations near the shore would use a fresh water UCZ, and cultivate marine crops in a seawater SZ. Fresh water crops could be cultivated in a fresh water SZ separated by a partition form a stable, saline NCZ. All of these ponds could be very inexpensive, located outdoors at a wide variety of sites, many of which are now not useful, and provide optimum growing conditions and high productivity yearround. Moreover cultivating crops in a solar pond will require less sophisticated engineering and management of the pond than extracting energy from it.

Salt gradient solar pond
A pipeline system is arranged at the bottom of a circular solar pond filled with salt water of varying concentration. Upon insolation, the energy is stored in the lowermost layer with the highest specific gravity. A medium which is passed through the pipeline system absorbs the energy from the storage layer without coming into contact with the storage layer, the heat exchange thus taking place without turbulence of the layers in the solar pond.

To enable a large-sized solar pond and make a stable operation of the pond by a method wherein a water inlet port and a water suction port are made variable in their directions in the solar pond having salt therein. CONSTITUTION:In case that a gradient of initial concentration is to be formed, each of water inlet port 1 and water return port 2 is placed near the bottom part of a solar pond 10 with each of the position adjusting devices 6a and 6b being adjusted. Water is filled in such a degree as they are immersed with water, the pump 7 is operated and the water is circulated among the heat exchanger 11, tank 8a and solar pond 10 while salt being supplied to the tank 8a.; When the desired concentration is reached, the operation is repeated while the water inlet port 1 and the water return port 2 being pulled up by the position adjusting devices 6a and 6b and then the concentration is gradually decreased. During this operation, the pump 7 continues to operate and the amount of feeding of salt, a pulling-up speed of the water inlet port 1 and the water return port 2 are merely adjusted. Therefore, it is possible to make a continuous gradient in concentration is a short period of time and to make a stable operation.


Separation and purification of salts in a non-convective solar pond


Brine containing at least two salts, one or more of the salts having a higher hydrated form and a lower hydrated or anhydrous form, is fed to a non-convective solar pond and one of the salts having a higher hydrated form and a lower hydrated or anhydrous form is crystallized in a higher hydrated form, dehydrated to a lower hydrated form, and recovered from the bottom of the pond in solid, pure form essentially free from the other salts in the brine. To effect separation, the salt having a higher hydrated form and a lower hydrated or anhydrous form, which is to be recovered in pure form must be present in the pond in an essentially saturated concentration. The concentrations of any of the other salts must generally not exceed saturation concentration at the temperature in the top layer of the pond, and must not exceed saturation concentration at the conditions in the bottom layer of the pond. Concentrations of other salts in the pond must be controlled such that the required density gradient is maintained.

To permit to form, maintain and repair the concentration gradient of salt content continuously and easily by a simple device by a mothed wherein the solar pond is divided into a plurality of layers in the depthwise direction of flow fluid horizontally substantially and to control the concentration of the salt content of the fluid flowed through said plurality of layer. CONSTITUTION:The solar pond 1 is filled with fresh water or salt water of some concentration at first in a degree that the water intake pipe 2 and the return pipe 3 of the lowest stage are submerged perfectly or more to a depth corresponding to the thickness of a heat accumulating layer. A water intake valve 4 and a return vavle 5 are opened and a pump 6 is operated to circulate the fresh water or the salt water in the solar battery 1. Salt is thrown into a concentration gradient maintaining device 9 to increase the concentration of the salt gradually under mixing it. The concentration of salt is measured in the solar pond 1 and when it has become a specified concentration finally, the water intake vavle 4 and the return valve 5 at the position of heat accumulating layer are closed, the same valves at the position of a non-convection layer immediately above the heat accumulating layer are opened and above-mentioned operations are repeated to control the concentration of salt so that the upper the laye the lower the concentration of the salt in the solar pond 1.

To obtain the solar pond facilitating to maintain a concentration gradient in spite of the demand of heat at the side of heat load by a method wherein an auxiliary load device is operated based on a signal from a monitoring device when solar energy, reserved in the solar pond, has exceeded a predetermined value. CONSTITUTION:The solar energy is reserved in the bottom layer of the solar pond 1 and salt water, whose temperature has become high, is pumped out by a pump 2 to circulate again into the bottom layer of the solar pond 1 after effecting heat exchange between fluid to be used in the side of heat load in the outdoor type heat exchanger 3. On the other hand, the heat load side fluid, received the heat in the heat exchanger 3 from the primary side, supplies the heat to the heat load device 5 and is recirculated into the heat exchanger 3 after reducing the temperature thereof. The auxiliary heat load device 4a and/or the same device 4b are operated by the commanding signal of the heat load monitoring device 6, monitoring the amount of solar energy absorbed and reserved in the solar pond 1, to limit the amount of energy in the solar pond 1 or the upper limit of salt water temperature in the bottom layer of the pond, for example, and prevent boiling or convection in the solar bond 1.


PURPOSE:To contrive to maintain easily the concentration gradient in the solar pond and prevent the pond from flowing out of expensive salt by a method wherein a water is extracted from the intermediate layer of a salt-containing solar pond, a higher concentraion salt water is supplied into a bottom layer of the solar pond, a lower concentration salt water is supplied into a surface layer of the solar pond. CONSTITUTION:An intermediate salt concentration water extracted from an intermediate layer of a solar pond 1 is supplied into a concentrating device 2 through an intermediate water extracting line M. In the concentrating device 2, the intermediate salt concentration is concentrated up to the required high concentration, then the highly concentrated salt water is supplied into the bottom layer in the solar pond 1 by a bottom layer water supply line B, the remaining low concentrated salt water is circulated to a surface layer in the solar pond through a surface layer water supply line V. Therefore, the concentration gradient in the solar pond can be maintained easily, accordingly, a new salt and fresh water supply is not required.


Method for utilization of oil field waste brine to develop a salt gradient solar pond
A process and method is disclosed for utilizing oil field waste brine to develop and maintain a salt gradient solar pond which in turn provides thermal energy for doing work, including improved separation of oil/brine emulsions into waste brine, crude oil, and natural gas; hot brine from the storage layer of the developed solar pond provides heat to a process heat exchanger which is intended to elevate the temperature of a working fluid such as an emulsion of crude oil and brine coming from producing oil wells prior to a separation process within a conventional heater treater. Less fuel is required to operate the heater treater. Waste brine from the crude oil process is utilized to develop and maintain the solar pond rather than simply being disposed.



To obtain a solar pond whose absorbing efficiency is high and having no salt water loss from an unconvention layer and effective use of a pool of a transparent fluid as a fish breeding tank or a hot water pool is contrived, by a method wherein unconvection layer is provided above a convection layer of the bottom part, on which the pool of the transparent fluid is arranged through a transparent partition layer. CONSTITUTION:Energy of insolation S is absorbed partly by a pool 5 of a transparent fluid, a transparent partition layer 4 and unconvection layer 3 and the rest of the same arrives at a convection layer 2 of a solar pond 1. As concentration C of salt of the unconvection layer 3 increases according as a depth becomes deeper and possesses distribution of the concentration which becomes the maximum concentration C1 at a boundary between the convection layer 2, which is the deepest part of the unconvection layer 3, and the unconvection layer 3, a convection is not generated. The convection layer 2, therefore, is turned into a state as if it is covered with a heat insulating layer and solar energy is accumulated as heat. As the unconvection layer 3 does not touch the open air, a disturbance of a concentration gradient of salt close to the surface of the unconvection layer 3 through wind is prevented from occurring and seaweed in both the layers is prevented from growing.

Saltless solar pond

A solar pond (16) adapted for efficiently trapping and storing radiant solar energy without the use of a salt concentration gradient in the pond is disclosed. A body of water (20) which may be fresh, saline, relatively clear or turbid, is substantially covered by a plurality of floating honeycomb panels (18). The honeycomb panels (18) are made of a material such as glass which is pervious to short wave solar radiation but impervious to infrared radiation. Each honeycomb panel (18) includes a multitude of honeycomb cells (42) having a height-to-width aspect ratio of at least approximately 14 to 1. The honeycomb panels (18) are divided into the elongated honeycomb cells (42) by a multitude of intermediate plates (44) disposed between a bottom plate (34) and top plate (36) of the panel (18). The solar pond (16) of the invention is well suited for providing hot water of approximately 85 DEG -90 DEG C. temperature for direct heating applications, and for electrical power generation.




An air conditioning system and/or a heating system is described in combination with a solar pond, especially a pond which is of the gradient type, wherein it is important to maintain a concentration of salt which increases with the depth of the pond. The pond is regenerated, that is, the salt concentration gradient is maintained, by components of the air conditioning system, or by special concentrator towers wherein moisture is removed from brine that is circulated to the towers from the pond.



Solar pond power plant

Method of operation and apparatus for a salt gradient solar pond employing a novel barge carrying a plurality of two axis stabilized high temperature concentrator solar cell arrays including means to control the flow rate of the concentrator solar cell array cooling fluid to optimize power station characteristics.



PURPOSE: To lower the running cost of a solar pond, by employing such an arrangement that a required temperature gradient is formed spontaneously in the solar pond. CONSTITUTION:  A solar pond of this invention is constructed by blackening the bottom surface 5 of a water tank by use of asphalt or the like, extending a longitudinal finned tube 6 horizontally above the bottom surface 5, accumulating crystals 7 of salts blackened by way of a special treatment over the surface of the bottom surface 5, filling a saturated salt solution 8 above the crystals 7 of salts, forming a layer of fresh water 9 above the solution 8. With such an arrangement, drastic change of concentration is formed at the interface between the solution 8 and the layer 9 of fresh water, and the layer 9 of fresh water functions to insulate heat and also to cause the above concentration change continuously.; The solar heat is absorbed at the bottom surface 5 and raise the temperature of ambient salt solution 8, which causes further eluation of crystals 7, so that a certain concentration gradient is formed spontaneously in the vicinity of the bottom surface 5. Thus, low-temperature water A is heated by the tube 6 and high-temperature water B thus obtained is supplied via a heat exchanger 3 to a hot-water utilizing system.

Method for maintaining a correct density gradient in a non-convecting solar pond

The present invention resides in a method and apparatus for maintaining a substantially constant salt density gradient in a non-convecting salt gradient pond. The apparatus for carrying out the method of the present invention enables one to maintain a substantially constant salt density gradient automatically in a highly efficient, simple and economic manner.


The present invention resides in a falling pond method for maintaining a salt density gradient in a non-convecting salt gradient pond, i.e., falling solar pond, and means for carrying out said method.

A non-convecting solar pond is an efficient and relatively inexpensive energy collection and storage system. The design of a solar pond is such that it takes advantage of several important properties of water, namely, high heat capacity, transparency to visible and ultraviolet light, opacity to infrared radiation and poor heat conductivity.

The general principles involved in designing a solar pond are relatively simple. A body of water collects large amounts of heat from the sun. Ordinarily the water temperature remains close to the ambient air temperature because the heating of the water produces a convection circulation which brings the absorbed heat to the surface where it is dissipated into the air, largely by evaporation. It has been found that by establishing a salt density gradient which increases with depth, convection circulation can be inhibited thereby greatly reducing loss of heat at the surface of the pond.

For most ordinary uses, such as space heating or industrial process heating, a solar pond should be between two and three meters deep and at least a few hundred square meters in size. The top layer of the pond has little or no salt dissolved therein and the concentration of salt increases with depth until a density gradient layer is established which is between one and one and a half meters deep. Below this gradient region is the heat storage region which is generally of constant density equal to the density of the lowermost region of the salt gradient region. The storage region may be in direct contact with the gradient region or separated therefrom by means of transparent membrane such as plastic or the like in which case the storage region could be salt free. The choice between a direct contact or a separate storage region depends on the relative cost of the salt and plastic. In either case, convection is permitted and in fact is desirable in the heat storage region.

While salt gradient solar ponds are the most cost effective solar thermal system and can provide useful heat at a cost that is less than most conventional methods, the concept has remained largely undeveloped. The principal reason for this lack of development resides in the problems encountered in attempting to maintain the salt density gradient which, if left alone, tends to diffuse away leaving a pond of uniform salinity thereby resulting in convection circulation and a corresponding loss of heat.

Heretofore, one method used to maintain the salt gradient has been simply to add salt to the bottom layers of the pond while flushing the top layers of brine away with fresh water. This method, while simple, suffers from a number of disadvantages the most important of which are added cost for the additional salt and lack of automatic means for determining when salt must be added. While the movement of salt upward from the bottom layers is rather slow, on the order of 0.3 mm per day, a large amount of salt, about 18,000 kg per year, is still needed in order to maintain the proper density gradient in a quarter acre pond. The added expense for the salt coupled with the need for a continual personal surveillance in order to predict when salt must be added has prohibited this method for becoming commercially feasible.

A second method previously employed to maintain the salt gradient in a solar pond comprises removing the top layer of water from the pond which has become salty due to diffusion and transporting it to a holding evaporation pond where the water is allowed to evaporate after which the more concentrated salt solution is returned to the bottom layer of the pond. While this method overcomes the disadvantage of requiring extra salt to maintain the gradient, it requires a large amount of space for the evaporation pond and still requires constant surveillance on the part of personnel in order to determine when the salt water must be removed and returned.

A third method which has been suggested by Dr. Harry Tabor uses a flash evaporator to perform the same function as the evaporation pond discussed above. This particular method has been untried to date due to the considerable expense of the flash evaporator and the complicated system required to employ same. Again, as with the previously discussed methods constant surveillance is required to determine when salt should be added. In addition to the foregoing drawbacks, all of the above-noted methods correct the salt gradient only after a significant amount of salt has reached the surface, i.e. when the salinity gradient has already decayed substantially thereby increasing the likelihood of convection.

Naturally, it would be highly desirable to provide a method for maintaining a substantially constant salt gradient in a solar pond which eliminates the extra cost of adding additional salt and at the same time automatically controls the salt gradient thereby eliminating any convective heat loss.

Accordingly, it is the principal object of the present invention to provide a method for maintaining a substantially constant salt density gradient in a non-convecting solar pond.

It is a further object of the present invention to provide a method as outlined above which is fully automatic.

It is a still further object of the present invention to provide a method as outlined above which is of simple and inexpensive construction.

Further objects and advantages of the present invention will appear hereinbelow.


In accordance with the present invention it has now been found that the foregoing objects and advantages may be readily achieved.

The present invention resides in a method and apparatus for maintaining a substantially constant salt density gradient in a falling non-convecting salt gradient pond. The apparatus for carrying out the method of the present invention enables one to maintain a substantially constant salt density gradient automatically in a highly efficient, simple and economic manner.

In accordance with the present invention, a solar pond is provided with a spillway which acts as an evaporator. Pumps are automatically actuated in response to various predetermined conditions for feeding salty water to the spillway where the water trickles down the spillway toward a catch basin where it is collected. The exposure of the salt water to the sun and air results in evaporation of the water as it progresses toward the catch basin. The concentrated brine received in the catch basin is then pumped back into the pond preferably at the base of the salt density gradient region, that is, at the interface of gradient region and the heat storage region of the solar pond. Automatic means are provided for adding fresh water to the top of the pond to replace that water lost as a result of the evaporation process.

Thus, is can be seen that the method and apparatus of the present invention provides a simple, inexpensive and automatic arrangement for maintaining a substantially constant salt density gradient in a non-convecting salt gradient pond. By employing the method and apparatus of the present invention disadvantages associated with prior art solar ponds are overcome thereby increasing the commercial feasibility of solar ponds as an energy source.


FIGS. 1 and 2 show a schematic illustration of a salt gradient pond employing the method and apparatus of the present invention.


FIGS. 3A, 3B and 3C illustrate a hydrometer employed in the method of the present invention.


Referring to FIG. 1, a solar pond 10 is illustrated which is constructed by excavating a pit and employing the removed dirt to form raised banks around the pond proper. The pit is lined with plastic and is filled with water, and a salt density gradient is thereafter established. The top region 12 of the pond is substantially salt free. The salt density gradient 14 is established beneath the fresh water region 12. As noted previously the density in region 14 increases with the depth of the pond. The heat storage region 16 is located beneath the gradient region 14 and may be, as noted earlier, either in direct contact with or separated from the gradient region by a transparent membrane. For purposes of illustration and for describing the method of the present invention the salt gradient region is shown in direct contact with the heat storage region. If the heat storage region is in direct contact with the gradient region, the concentration of the heat storage region should be approximately equal to the density of salt at the bottom of the salt gradient region. If on the other hand the regions are separated by a membrane, the storage region may be salt free water. On at least one of the raised banks, preferably the bank having southern exposure, the trickle evaporator 18 of the present invention is constructed.

In accordance with the present invention, as can best be seen in FIGS. 1 and 2, the trickle evaporator 18 comprises a sloped graded spillway 20 which is sloped at an angle equal to the geographic latitude plus or minus up to 15 DEG. The slope is covered with any material which will withstand exposure to heat, brine and oxygen and at the same time allow for substantially evan flow of the salt water down the spillway. A preferred material for covering spillway 20 would be plastic sheeting, preferably plastic sheeting which is dark in color so as to maximize the effort of the sun in promoting evaporation. Other suitable covering materials include masonry and metal products.

At the top of the spillway 20 is a trough 22 which receives the brine from a water distributing perforated pipe 24 which is in fluid communication with the heat storage region 16 in the case of direct contact or with the bottom of salt gradient region 14 in the case of separated regions by means of fluid line 26. A fluid pump 28 is provided in fluid line 26 for removing the brine from the heat storage region 16 or bottom of salt gradient region 14 and delivering same to the distributing perforated pipe 24 where the water is distributed over the length of the spillway 20. The fluid pump 28 is responsive to and actuated by a pump drive motor which senses environmental conditions and actuates the fluid pump upon sensing certain predetermined values of the monitored environmental conditions. In accordance with a preferred embodiment of the present invention, pump 28 is responsive to either ambient temperature or the degree of sunlight or both. The values of the light intensity, I, and ambient temperature, T, which activate fluid pump 28 are selected so that the pump 28 is activated at a time when evaporation would be efficiently carried out. Thus, if the ambient temperature (T) is too low and/or the amount of sunshine (I) too little, the pump system will not be activated. The system will be activated in two cases, i.e., where I>Io and T>Tmin or when I>Imin and T>To where Io >Imin. The particular sensing means employed may be selected from any of a number of commercial sensing means which are readily available such as thermostats, light sensitive eyes, etc., and the detailed structure of said sensing means forms no part of the instant invention. Alternatively, one could sense the relative humidity rather than temperature and light intensity to activate the control sequences. This could be done by employing dry and wet bulb sensing devices in conjunction with a microprocessor to compute relative humidity.

As noted above, the brine delivered to pipe 24 by pump 28 is distributed over the entire length of the spillway 20 and trickles down the spillway where a substantial amount of evaporation of water takes place. A receiving trough 30 is provided at the bottom of the spillway to collect the concentrated brine. The trough has sloping side walls which feed the collected brine to a catch basin 32 where the brine, under appropriate conditions, is fed by pump 34 to one or more locations in the pond below the gradient region 14, preferably just below the interface between regions 14 and 16 in a direct contact pond or at the bottom of region 14 in a separated region pond, via feed line 36. It is preferred that pump 34 be activated automatically upon actuation of fluid pump 28. In addition, pump 34 is preferably provided with an automatic override control which prohibits pump operation upon sensing that the catch basin 32 is empty so as to prohibit damage to the pump 34. It is a critical feature of the present invention that the catch basin 32 be small in size so that it does not collect a substantial amount of fresh rain water which would tend to dilute the brine. Preferably, the catch basin is sized so that excess rain water will overflow and run away from the pond. It should be appreciated that, depending on the size of the pond, a plurality of feed lines and pumps may be employed for removing brine and returning concentrated brine to the regions of the solar pond so as to maintain a homogeneous concentration.

As water is lost through the evaporation process, the height of the solar pond falls. In accordance with the falling pond method, fresh water is automatically added to the top region 12 of pond 10 by means of pump 38 which is responsive to the level of water in the pond 10. Pump 38 may be controlled by float means, electrodes, or any other suitable means. As the water level in the pond decreases pump 38 is activated to reestablish the water level. It can be seen that the continual addition of fresh water to the top of region 12 of solar pond 10 in combination with the lowering of the gradient region 14 due to the extraction of heated brine from the region 16 results in the maintenance of a tri-layered pond whereby the salt gradient density is controlled and maintained by recirculating the collected brine to the interface between the heat region 16 and the bottom of the gradient region 14 in the case of direct contact regions or to the bottom of gradient region 14 in the case of separated gradient regions. The pond is provided with an overflow lip 40 for removing excess water from the pond which may occur due to excessive rainfall.

In accordance with the present invention a hydrometer is employed to monitor the density gradient of the solar pond to assure that the evaporation system of the present invention is operating properly. It is highly desirable that the hydrometer be observed without the necessity of removing it from the pond or of removing water samples from the pond as the act of extracting is likely to stir the water and thereby affect the density gradient. In a preferred embodiment as illustrated in FIGS. 3A-C the hydrometer 50 consists of a transparent cylinder 52 made of glass, plastic or the like which is provided with a plurality of perforations 54 over the entire surface thereof which allows the water in the gradient region to readily pass into the interior of the cylinder 52. A graduated scale 56 is ruled on cylinder 52. It should be appreciated that the length of the hydrometer 50 should be sufficient to monitor the entire depth of the gradient region. The interior of cylinder 52 is filled with a plurality of differently colored floating objects 58 such as spheres. Each of the objects has a different specific gravity chosen so that the order and vertical separation form an easily recognizable pattern which may be seen from the surface of the pond and compared to a key to determine if the gradient is correct. The color coding allows the observer to determine whether the objects 58 are floating in the correct order and thus determine if there are any reversed density levels. The separation between the objects 58 indicates the density gradient. Thus, the hydrometer 50 of the present invention differs from known hydrometers in that it shows not only the specific gravity at any location but also the relative density gradient. The specific size of the floating objects 58 is critical. Naturally, the objects must be large enough so as to be readily observed from the surface of the pond. Likewise, it is critical that the objects be small enough to be affected by small scale density variations. Furthermore, the size of the objects should be such that they can readily pass over each other in the cylinder 52 in the event of any reversed density levels. In the case of spheres, it has been found that a diameter of from 0.25 cm to 2.00 cm is useful.

Alternatively, the hydrometer may be replaced by an immersed array of electrodes which would measure the electrical conductivity of the salt solution and thereby the gradient concentration.

It should be appreciated that the system of the present invention may be used in conjunction with a concentrating solar collector such as parabolic mirrors or evaporators having external heat sources in the event of extended cold or cloudy periods. This supplemental system would fit into the pump system in parallel with the trickle evaporator and generally would be used sparingly if at all.

It is difficult to predict the exact water evaporation rate for a certain location and time because the rate depends on a combination of factors, including water temperature, air temperature, relative humidity, wind speed, and insolation energy. The following calculations will show, however, that the designed system of the present invention will concentrate the brine sufficiently for reasonable evaporator areas and flow rates. In the system of the present invention the pump controls are set to provide the correct flow rate for the particular characteristics of the solar pond.

The exact upward salt transport rate depends on various factors such as the overall temperature gradient and concentration gradient in the pond. Measurements taken in existing solar ponds show an average transport rate of about 0.06 kg/m@2 /day for a typical pond (see for example, Carl E. Nielsen "Control of Gradient Zone Boundaries" in International Solar Energy Society Annual Meeting Proceedings, Atlanta, May, 1979). To maintain a 20% salt solution in the storage region, it is necessary to remove 5 times that much fresh water from the convecting storage region. In other words, the pond must fall 0.30 mm/day. Thus, one must evaporate about 100 kg/m@2 /year (0.08 gal/day). For example, in an 850 m@2 pond in the middle Atlantic states, if one were to correct for the upward motion of the salt by adding new salt to the bottom and flushing the top with fresh water, as is the case with known prior art, one would need to add approximately 20.0 tons of salt. If on the other hand, one were to use the system of the present invention, it would be necessary to evaporate about 68 gallons of water per day by means of the trickel evaporator. For standing water with a free surface (such as a lake) the rate of evaporation is given by the empirical formula (found in a number of sources; for example, J. T. Czarnecki, Swimming Pool Heating--TR19, Highett, Victoria, Australia 1978).
Mevap =(7.2.times.10@-3) (3.1+4.1V) (Pw -Pa)
M=evaporated water in (grams/M@2)/second
V=wind velocity in meters/second
Pw =vapor pressure of water at temperature Tw in kilopascals (kPa)
Pa =partial water vapor pressure of air at Ta and relative humidity RH
Pa =Psaturated .times.RH.div.100 (kilopascals)

For solar pond trickle evaporation assume
Tw =52 DEG C.
Pw =100 mm Hg=13.60 kPa
Ta =16 DEG C.; RH=60%
Pa =8.0 mm Hg=1.10 kPa
V=2 m/second
Mevap =1 gram/meter@2 /second=0.95 gal/hr./m@2

To evaporate 68 gallons one needs 70.0 (m@2) (hours) of exposure, or in other words 30 m@2 of surface of water at that temperature exposed for 2.3 hours or an equivalent combination of surface area and exposure time, such that Area.times.Time=70 m@2 hr. This estimate of evaporation rate is based only on stagnant air contact. However, because the water is flowing down the spillway in the present invention evaporation is enhanced somewhat. Solar insolation (which is generally greater than 300 cal/cm@2 /day on an annual average in central latitudes) will increase this evaporation rate by about 30%. Furthermore, higher water temperature, higher wind velocity, or lower relative humidity, all of which are not unlikely for a typical pond, will give faster evaporation rates. The flow rate necessary to accomplish the evaporation of 68 gallons per day would be at least 300 gallons per day and usually about 600 gallons per day, flowing across the trickle evaporator, i.e. a few gallons per minute.

The principle disadvantage with the system of the present invention is that the evaporation process drains heat from the pond, but the amount of heat removed is tolerable. Furthermore, the heat loss is somewhat mitigated by the fact the pond is more stable as well as more efficient when kept at a lower temperature. In spite of the heat loss this system is economically preferable to known alternatives.

It is to be understood that the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible of modification of form, size, arrangement of parts and details of operation. The invention rather is intended to encompass all such modifications which are within its spirit and scope as defined by the claims.

Solar collection system

The concentration of the solute in solution in a saturated non-convecting solar pond is greatest at a lower hotter level decreases continuously toward a higher cooler level and is saturated at all levels, thus preventing solute diffusion. In the customary operation of such saturated non-convecting solar ponds, the heat generated by incident radiation is absorbed in a bottom or lower layer. Because of the continuously decreasing density of solute which is near saturation from the lower to the upper layers, there is little salt convection and the pool tends to be further stabilized. Further, by heating due to absorption of the radiation largely at the lower level, a temperature gradient is maintained in which the upper layers remain cool, evaporation and consequent loss of heat is inhibited and the efficiency of the solar pond is improved. It has been discovered that a brine consisting essentially of an appropriate mixture of sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) as a solute in water is an especially desirable solution for use in a saturated non-convecting solar pond. A cover material may be used as a barrier material over the solution to impede carbon dioxide transfer across the solution surface.


One means for collecting solar energy is a solar pond. A solar pond may be defined as "a shallow artificial black bottom pond or lake in which the incident solar insolation is converted into a local temperature rise in the water near the bottom." (Mehta, G. D., "Non-Convecting Solar Ponds", Technical Report ETG-4, Hydronautics, Inc., Oct., 1975). A successful solar pond should develop simultaneously a high temperature at the bottom of the pond as a result of the radiation and a low temperature at the top. The low temperature is desirable to minimize excessive energy losses through evaporation, conduction and radiation.

The objective of high temperature at the bottom and low temperature at the top cannot be readily achieved, if at all, by using a pond containing water alone. With water alone, the higher temperature at the bottom of the pond decreases the density of the liquid relative to that at the surface, thus causing convection currents which quickly equalize the water temperatures. These density convection currents can be eliminated by introducing a density gradient maintained by a suitable salt concentration gradient. Such ponds are referred to as non-convecting solar ponds. They have been tested and studied rather extensively.

One of the advantages of the non-convecting solar pond is that it has a substantial storage capacity. It has been estimated that the solar pond could have a storage capacity of as much as 31 days, i.e. deliver substantial thermal energy for this period without incident sunlight.

The advantage of the storage capacity plus other advantages, such as low cost, make the non-convecting solar pond a promising candidate for the collection of unfocussed solar energy. On the other hand, there are certain disadvantages at present, the major one of which is maintaining the salt concentration gradient necessary for pond stability. The very existence of a salt concentration gradient causes salt diffusion which tends to destroy the gradient. The salt diffusion occurs because the brine is unsaturated at all levels with the salts usually used, such as NaCl or MgCl2, for which solubility is relatively insensitive to temperature. With such salts the saturation concentration is relatively constant over the substantial temperature range which usually prevails from the top to the bottom of the pond. Therefore, the brine is at a lower concentration relative to saturation at the upper, cooler level than at the lower, warmer levels, and diffusion of salt can proceed unimpeded. It has been suggested (Styris, D. L., et al, "The Non-Convecting Solar Pond--an Overview of Technological Status and Possible Applications", Battelle Pacific Northwest Laboratories, Report BNWL-1891-UC-13, Jan., 1975), that the problem of diffusion could be largely solved if the pond were substantially saturated with a salt having a solubility which is a direct function of temperature. In such a saturated non-convecting solar pond (saturated pond for short), one way of viewing the resultant action is that the salt could no longer diffuse successfully to a less concentrated (cooler) region because it would move to an already-saturated region, causing it to precipitate and sink to the hotter, now unsaturated region where it would redissolve. The saturated pond should be self-generating (assuming some temperature gradient always exists from the extra solar radiation absorbed near the bottom), self-maintaining, and self-repairing, all qualities which unsaturated ponds do not possess. Thus the saturated pond should be simpler in construction and operation than the unsaturated pond. Unfortunately, no saturated solar pond has been built because of the apparent lack of a temperature-sensitive solute that is cheap, stable, nontoxic, transparent, available in large quantities and the average solubility of which, over the pond temperature range of 20 DEG to 100 DEG C. is not too high. For example, ammonium nitrate (NH4 NO3) and potassium nitrate (KNO3), have been considered for a saturated solar pond. The curves of FIG. 1 for these compounds are derived from data from Perry Chemical Engineer's Handbook, 4th Edition. With KNO3, for example, the high cost and high average solubility over the desired temperature range are such that the KNO3 for a saturated solar pond would cost in the order of 100 dollars per square meter of pond surface for a typical pond depth of one meter. Such a cost is prohibitively expensive, when it is realized that capital cost in one of the chief contributions to the cost of operating a solar pond. In comparison, with an unsaturated solar pond using Mg Cl2 or NaCl, the salt would cost in the order of 10 dollars per square meter of pond surface.


Other prior publications besides the Styris, et al paper noted above, relating to solar ponds which may have a bearing on the present invention are: Mehta, G. D., "Non-Convecting Solar Ponds", Technical Report ETG-4; "Hydronautics, Inc.", Oct., 1975; Tabor, H., "Large Area Solar Collectors for Power Production", Solar Energy, 7, p. 189, 1963; Jain, G. C., "Heating of Solar Pond", "The Paris Congress on Solar Energy", July, 1973; Saulnier, B., et al, "Field Testing of a Solar Pond", presented at the International Solar Energy Meeting at UCLA, July 28-Aug. 1, 1975; Rabl, A., et al, "Solar Ponds for Space Heating", Solar Energy, Vol. 17, pp. 1-12 (1975); Dickinson, W. C., et al, "The Shallow Solar Pond Energy Conversion System," Solar Energy, Vol. 18, No. 1, pp. 3-10 (1976); Styris, D. L., et al, "The Nonconvecting Solar Pond Applied to Building and Process Heating," Solar Energy, Vol. 18, No. 3, pp. 245-252 (1976); Mehta, G. D., et al, "Engineering and Economics of a Solar Pond System," 16th Annual ASME Symposium on Energy Alternatives, Albuquerque, New Mexico, Feb. 26-27, (1976); and Nielsen, C. E., " Experience with a Prototype Solar Pond for Space Heating", Proceedings of the International Solar Energy Society meeting at Winnipeg, Canada, Vol. 5, pp. 169-182, 1976.

The following article is of general interest: McCoy, H., "Equilibrium in the System Composed of Sodium Carbonate, Sodium Bicarbonate, Carbon Dioxide, and Water," American Chemical Journal, XXIX, January-June, 1903.

In addition, the following patents may be of interest:

US 3,372,691 Shachar, March 12, 1968
US 2,388,009 Pike, October 30, 1945
US 3,277,883 Rowelsamp, October 11, 1966
US 3,314,414 Rowelsamp, April 18, 1967
US 3,314,415 Rowelsamp, April 18, 1967
US 3,161,193 Rowelsamp December 15, 1964
US 3,667,980 Neitzel, et al, June 6, 1972
US 3,910,253 Thomason, et al, October 7, 1975
US 4,026,270 Ramey, May 31, 1977

Also, Australian Pat. No. 236,337 to Tabor, et al, accepted Nov. 7, 1961.


According to the invention, a saturated non-convecting solar pond uses a saturated brine consisting preferably of a mixture of sodium carbonate (Na2 CO3) and sodium bicarbonate (NaHCO3) in water together with a transparent cover on the pond surface to impede transfer of carbon dioxide. The utility of mixtures of Na2 CO3 and NaHCO3 is an unexpected and unforeseen result in view of the fact that neither Na2 CO3 by itself nor NaHCO3 by itself would be satisfactory, as discussed in the following detailed description.


The objects, advantages, and novel features of the invention will be more fully apparent from the following description when read in connection with the accompanying drawing in which:

FIG. 1 is a graph of solubility in water in grams per kilogram against temperature in DEGC. of some illustrative salts which have been proposed or are discussed herein for a saturated pond;


FIG. 2 is a graph of weight percent of anhydrous solute in saturated solution in water against temperatures in DEGC. of sodium carbonate, sodium bicarbonate, and two mixtures thereof;

FIG. 3 is a graph of the density of saturated solution at saturation temperature, in grams per cubic centimeter, against temperature in DEGC. for the two mixtures;

FIG. 4 is a schematic representation of a saturated non-convecting solar pond system embodying the invention.


Sodium bicarbonate by itself has a fairly desirable solubility curve extending from about 10 wt. % at 30 DEG C. to about 14% at 60 DEG C. (FIG. 2). Unfortunately, sodium bicarbonate tends to dissociate in water solution into sodium carbonate, water and carbon dioxide. Therefore, sodium bicarbonate is not a suitable material to form the desired brine. On the other hand, sodium carbonate in solution absorbs carbon dioxide, and its solubility is rather insensitive to temperature in the desired range, of say approximately 30 DEG to 100 DEG C., as shown in FIG. 2.

We have found that the density of a brine formed by an appropriate mixture of sodium bicarbonate and sodium carbonate increases with increase in temperature, the solubility at saturation of the mixture increases with temperature, and the average solubility is not too great. Furthermore, in such an appropriate mixture, the dissolution of sodium bicarbonate by CO2 loss is suppressed by the presence of the sodium carbonate and can be virtually eliminated by use of a transparent pond cover. The cover can be made of a material which acts as a barrier to CO2 transmission and may consist of a plastic film or alternatively a monomolecular liquid film. Some of the desirable qualities of the mixture are shown in FIG. 2 and FIG. 3, as exemplified by a 1.9/1 by weight (3:2 ratio in mols) mixture of sodium carbonate and sodium bicarbonate. However, it is not intended to imply that this is the only appropriate ratio, a 2/1 mixture and other ratios being adequate. Those skilled in the art will appreciate that the limits of the value of this ratio are imposed by saturated pond requirements and the characteristics of Na2 CO3 /NaHCO3 mixtures in solution.

We prefer a mixture of about 1.9 to 1 by weight of Na2 CO3 to NaHCO3 or about a three to two (3:2) molar ratio as exemplary for the purposes expressed herein. FIG. 2 shows the weight percent of an anhydrous solute in a saturated solution plotted against the temperature of the saturated solution in degrees centigrade for sodium bicarbonate (NaHCO3), a 1.9/1(3:2 mols) mixture of sodium carbonate (Na2 CO3) and sodium bicarbonate (NaHCO3), and sodium carbonate (Na2 CO3). The curve for sodium bicarbonate terminates at about 60 DEG C. at which point the sodium bicarbonate dissociates freely, liberating carbon dioxide. The curve for sodium carbonate shows that its solubility somewhat decreases and then becomes practically constant with temperature above about 30 DEG C. or a little over. This characteristic makes the use of sodium carbonate by itself in a saturated non-convecting solar pond impractical.

On the other hand, the 1.9/1 mixture is substantially stable at high temperatures, particularly if a transparent pond cover relatively impermeable to CO2 is used, and shows a regular increase in saturation concentration (and solution density) as the temperature increases. Conversely as the saturated 1.9/1 solution decreases in temperature, salt would tend to be precipitated. If the vertical decreases of temperature in a pond were from a lower level to a higher level in a saturated 1.9/1 solution, this characteristic would tend to prevent diffusional mixing and the layers would tend to remain intact, each at its own weight percent of the solute in saturation at the temperature of the particular layer under consideration. The curve for Trona (1:1 molar ratio) is shown for comparison. Trona, Na2 CO3.NaHCO3.2H2 O, is also known as sodium sesquicarbonate.

FIG. 3 shows the curve for the density of the saturated 1.9/1 (3:2 molar ratio) solution described above at saturation temperature in grams per cubic centimeter plotted against the temperature of the saturated solution in degrees centigrade. The density increases progressively from about 30 DEG C. to near 90 DEG C. The curve for Trona (1:1 molar ratio) is shown for comparison. The foregoing shows that an appropriate mixture of sodium carbonate with sodium bicarbonate is an outstanding solute for use in a saturated non-convecting solar pond system. The density of the saturated solution increases adequately with an increase in temperature. The average solubility is sufficiently low so that an inordinately large quantity of the salt is not required to achieve the saturated solution. Moreover, the solution is substantially transparent and nontoxic. Transparency in solution is essential in order that the pond may be insolated with some efficiency. The sodium bicarbonate in the solution is relatively stable against carbon dioxide liberation because of the presence of adequate sodium carbonate and is made more stable by a transparent pond cover relatively impermeable to CO2. Moreover, the dual salt mixture is inexpensive and is available in large quantities. It can be made from sodium carbonate and Trona, which is cheap and plentiful and available from several sources. One well known source is at Green River, Wyoming. The Trona from that source is about 95% pure by weight as mined and large quantities are secured with a relatively simple mining and crushing operation. Quite pure Trona is obtained from this Trona ore by a cheap, mass-production operation, as a prelude to sodium carbonate production. It is estimated that an appropriate 1.9/1 by weight mixture of Na2 CO3 and NaHCO3 in a saturated pond one meter deep would cost in the order of 10 dollars per square meter, a cost similar to that of salts (such as NaCl, MgCl2) presently used in unsaturated ponds.

Referring to FIG. 4, a solar pond 1 receives solar radiation 12. The bottom layer 14 is preferably black to absorb a large fraction of the solar radiation and convert it to heat. The top of the pond is preferably covered with a transparent plastic cover 10 such as "nitrile barrier" film (p. 32, Modern Plastics Encyclopedia, Vol. 52, No. 10A) which will impede the loss of generated CO2 and collect it at the brine surface at a gaseous pressure less than atmospheric pressure, but sufficient to achieve equilibrium with the partial pressure of CO2 in the liquid solution. The heat from the bottom layer may be converted to energy by any suitable heat exchange mechanism such as one or more coils 15 arranged in heat exchange relationship with the lowermost layer 18 of the solution in the pond. Alternatively the lowermost layer of the pond may be pumped outside of the pond to supply heat for heating purposes or to operate an engine producing mechanical or electrical energy (See for example, the Styris, et al Battelle Report 1891 cited above).

The concentration and density gradients are self-generating. It is merely necessary to place in the pond sufficient quantities of the appropriate Na2 CO3 /NaHCO3 mixture, such as the 1.9/1 by weight mixture, so that there will be a slight excess when final equilibrium is established. The incident solar energy on water added through supply pipe 20 will create an initial temperature difference. The 1.9/1 mixture will dissolve accordingly, setting up a slight concentration and hence density gradient. This density gradient will inhibit convection, thus encouraging a greater temperature difference, more dissolution, a greater concentration and density gradient, etc., until the pond is completely saturated. Alternatively, a cool, near saturation weak solution may be fed into the bottom zone, through inlet pipe 16, then slowly a warmer, more concentrated solution which will lift the weaker solution and so forth until a step-like density gradient has been produced.

Alternatively, a solution of different density may be added in the reverse order from the top either through a supply pipe 26 or by using the flexible hose 20. It may be desirable to add some water on the surface of the pond 10 through the supply pipe 26 in order to prevent or reduce loss of liquid by evaporation.

After a period of insolation the pond will have a temperature gradient substantially as desired with a substantial rise in temperature at the bottom and almost no rise in temperature at the surface which will approach the ambient temperature. There being no convection in the liquid, the transfer of heat from the bottom to the top will be solely by conduction, which is relatively small. The desired concentration, density, and temperature gradients should then be maintained. These limits on density and temperature can be determined from the probable temperature limits at the top of 25 DEG C. to 100 DEG C. at the bottom. The heat absorbed by such a pool may be used as described in this patent or any other suitable way suggested in the prior art, as in the Styris, et al report cited above.

In the on-going operation of the pond, there may be CO2 losses at a low rate. To make up for such losses NaHCO3 can be added at the same time that high alkalinity solution is removed.

It will be clear from the foregoing that there has been described a highly desirable non-convecting solar pond which uses a saturated brine solution of an appropriate mixture of sodium carbonate and sodium bicarbonate. Such a system has substantial advantages over unsaturated non-convecting solar ponds and over saturated non-convecting solar ponds utilizing presently contemplated solutes.

Although the present invention has been described with reference to a particular embodiment thereof, it should be understood that those skilled in the art may make many other modifications and embodiments thereof which will fall within the spirit and scope of the principles of this invention.