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What is a Solar Pond?

ca02_02.jpg (12534 bytes)A salinity gradient solar pond is an integral collection and storage device of solar energy. By virtue of having built-in thermal energy storage, it can be used irrespective of time and season. In an ordinary pond or lake, when the sun's rays heat up the water this heated water, being lighter, rises to the surface and loses its heat to the atmosphere. The net result is that the pond water remains at nearly atmospheric temperature. The solar pond technology inhibits this phenomena by dissolving salt into the bottom layer of this pond, making it too heavy to rise to the surface, even when hot. The salt concentration increases with depth, thereby forming a salinity gradient. The sunlight which reaches the bottom of the pond remains entrapped there. The useful thermal energy is then withdrawn from the solar pond in the form of hot brine. The pre-requisites for establishing solar ponds are: a large tract of land (it could be barren), a lot of sun shine, and cheaply available salt (such as Sodium Chloride) or bittern.

What are Salinity-Gradient Solar Technologies?

Salinity-gradient solar technologies is a generic name given to the application of a salinity gradient in a body of water for the purpose of collecting and storing solar energy. One type of salinity-gradient technology is called the salinity-gradient solar pond. Solar ponds generally utilize a one- to two meter salinity gradient and operate at moderately high temperatures.

How was this technology invented?

Salinity-gradient solar applications were not invented; they were discovered. The phenomenon was first observed in Transylvania in the early 1900s.

Naturally occurring salinity-gradient solar lakes are found in many places on the earth. Natural salinity-gradient lakes form when fresh water flows onto salt brine and mixes to create a salinity gradient.

How does a solar pond work?

Most people know that fluids such as water and air rise when heated. The salinity gradient stops this process when large quantities of salt are dissolved in the hot bottom layer of the body of water, making it too dense to rise to the surface and cool.

Generally, there are three main layers. The top layer is cold and has relatively little salt content. The bottom layer is hot -- up to 100°C (212°F) -- and is very salty. Separating these two layers is the important gradient zone. Here salt content increases with depth. Water in the gradient cannot rise because the water above it has less salt content and is therefore lighter. The water below it has a higher salt content and is heavier. Thus, the stable gradient zone suppresses convection and acts as a transparent insulator, permitting sunlight to be trapped in the hot bottom layer from which useful heat may be withdrawn or stored for later use.

This is a simplified description. No attempt is made here to describe the hydrodynamic phenomena which influence zone and interface stability, salt and heat transport, and other complex behavior.

For an expanded description with diagram, click here.

Do solar ponds work in winter?

Yes! Even when covered with a sheet of ice and surrounded by drifts of snow, the El Paso Solar Pond's lower zones produced temperatures of 154°F -- hot enough to generate electricity.

What can a solar pond be used for?

  • generating heat
  • generating electricity
  • water desalination
  • thermal energy storage

Why consider a solar pond?

Solar ponds have several advantages. They have a low cost per unit area of collection and an inherent storage capacity. Also, they can be easily constructed over large areas, enabling the diffuse solar resource to be concentrated on a grand scale.

What are the environmental advantages of a solar pond?

Solar ponds address three environmental issues arising from the use of conventional fuels. First, heat energy is provided without burning fuel, thus reducing pollution. Second, conventional energy resources are conserved. Third, solar ponds coupled with desalting units can be used to purify contaminated or minerally-impaired water, and the pond itself can become the receptacle for the waste products.

Specifically, what salinity-gradient solar pond technology applications are possible?

Energy to drive desalting units

  • fresh water production for municipal water systems
  • energy producing receptacle for waste brines
  • brine concentration

Supplemental energy source

  • peaking electrical power production
  • baseload power for remote locations

Process heat for production of chemicals, foods, textiles, and other industrial products

Heat for separation of crude oil from brine in oil recovery operations

Receptacle for brine disposal using waste brines from crude oil production

Heat for:

  • greenhouses
  • livestock buildings
  • other low-temperature agricultural applications

Space heating and absorption cooling systems

Low-temperature aquaculture applications

Surface water clean-up

  • irrigation return flows
  • saline waste waters
  • river desalination

Thermal energy storage systems in areas where brine is available to create the ponds and waste thermal energy is available

  • power plant cooling tower blowdown systems
  • cogeneration systems

Control of crystallization in certain mining operations


The Water Desalination Research and Development (DesalR&D) Program was authorized by Congress under the Water Desalination Act (Act) of 1996. The Act authorized program funding beginning October 1997 for a six year period. To start the program, funding was appropriated at $3.7 million for fiscal year 1998. Current fiscal year 1999 funding was appropriated at only $2.5 million. The Act is based on the fundamental need in the US and world-wide for additional sources of potable water. Funding for the DesalR&D program is provided through Reclamation's Office of Research, Director, Dr. Stanley Ponce.

The design, construction and operation of a small-scale salt-gradient solar pond (below ground level) having dimensions of 1,65 m diameter and 0,94 m depth is presented. It was covered with a transparent interlaced nylon cloth and its surfaces were painted black. The pond was lined with a composite material (concrete blocks and cement plaster) and this liner material selection was made on the basis of cost, availability and thermal conductivity. The optimum thickness of the density gradient layer, to achieve the desired temperature in the storage zone, was estimated. The pond was filled with several layers of different salt concentrations. A programme was developed using the Turbo C++ software which allows the thickness and salinity of each layer and the amount of salt and water needed for all the layers to be determined. The tests started in January 1997. Measurements have been carried out at room temperature for the vertical temperature and salinity distributions. The pond thermal heat losses from the side and bottom walls and from the top surface were determined. The pond performance was evaluated by a collection efficiency based on the maximum energy gain and the average annual global solar irradiation.

[To the Mecheng Homepage]A Closed-Cycle Salt-Gradient Solar Pond...

On the rooftop of RMIT’s Building 3, the Energy CARE Group has constructed a 5 metre diameter salt-gradient solar pond, along with a rectangular evaporation pond of the same area. Together they comprise a closed-cycle salt-gradient solar pond (CCSGSP) in a salt-gradient solar pond, the salinity of the water increases with depth, resulting in a corresponding increase in density. The density gradient inhibits convective heat loss to the surface, thereby enabling the pond to store solar energy. A salt-gradient solar pond has three distinct layers. The top layer, which is convective, is a few centimetres thick and is maintained at low salinity (approximately that of seawater). The central gradient layer is stable and non-convective. Most ponds have gradient layer thicknesses of about 1 metre. The bottom layer, where the collected energy is stored, has as high a salinity as possible and is convective. Its thickness differs for different ponds, depending on factors such as the maximum pond temperature, the amount of energy to be stored and the availability of salt.The RMIT pond has a 10 centimetre top layer, 60 centimetre gradient layer and a 15 cm storage layer. Gradient construction was completed in September 1991 and the storage layer reached 70°C in November 1991. The evaporation pond became operational in April 1992.

Solar Pond Power PlantsSchema des SPPP


Ein großes Wasserreservoir mit schwarzem Boden dient als Kollektor/Absorber und Speicher der eingestrahlten Solarenergie. Ein Salzkonzentrationsgradient innerhalb der Becken dient dazu die Schichtung der unterschiedlich warmen Wassermassen aufrecht zu erhalten.






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