Salinity-Gradient Solar Technology Page

  • What are Salinity-Gradient Solar Technologies
  • Potential Applications
  • Energy Capture Potential and Economics
  • El Paso Solar Pond Project
  • Salinity-Gradient Solar Technology Consortium
  • More Available Information

    What are Salinity-Gradient Solar Technologies?

    During the winter of 1987 this person (right) stands on the
    frozen surface of the El Paso Solar Pond, while 7 feet below
    the ice the temperature remains at 154 F- hot enough to
    generate electricity.

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

    Salinity-gradient solar applications were not invented, they were discovered. Naturally occurring salinity-gradient solar lakes are found many places on the earth. The phenomenon was first observed in Transylvania in the early 1900's. Natural salinity-gradient lakes form when fresh water flows onto salt brine and mixes to create a salinity gradient. Salinity-gradient solar applicationes include using the salinity gradient to protect fish from "cold kill" in aquaculture applications, to control crystallization in certain mining operations, and to attain higher temperatures for water desalination or electricity production.

    How it works:

    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 sissolved 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 degrees C (212 F)--and is very salty.

    Visual of how solar pond works

    Typical Salt Gradient Solar Application

    Separating these two layers is the important gradient zone. Here salt content increases with depth as shown above. Water in the gradient can't rise, because 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.


    At Right: Generating electricity at night.
    Solar ponds are capable of delivering
    power on demand even at night or after
    long periods of cloudy weather.

    The capability of salinity gradient solar
    technologies to capture and store solar
    thermal energy isunique. One of their
    many advantages over other solar tech-
    nologies is that this energy is available
    on demand, decoupled from short-term
    variations in solar input. Salinty-gradient
    solar technologies can deliver energy at
    night or during periods of cloudy weather
    with no short-term degradation in quality.
    The ability to deliver energy on demand is
    an important factor in examining potential
    applications for this technology.

    At Right: Winter 1987. Solar pond operation
    continues during extreme weather conditions.

    Another advantage is that these technologies can utilize what is often considered a waste product, namely reject brine, as a basis to build the salinity gradient. This is an important point, when considering using solar ponds for inland desalting and fresh-water production, or for brine concentration in salinity control and environmental cleanup applications.

    Other advantages include: the use of readily available materials, such as salt and brackish water, the ease of expanding projects to larger areas, they are site built, they produce thermal energy without pollution or waste materials, they have potentially long-life spans, and they have significant economies of scale, improving the profitably of larger projects.


    When considering applications, one should consider also the constraints that are inherent in the technology. Of course, the basic requirements of land, salt and water must be available. Also, a liner may be necessary to contain the salt brine, salinity-gradient solar ponds cannot be built above moving ground water colse to the surface, they have a low-conversion effiiciency--approximately 15 to 20 percent solar to thermal energy efficiency, they are temperature limited, temperatures follow an annual cycle based on solar input, they require significant solar input for high temperature operation, and applications are typically very site specific.

    Potential Salinity-Gradient Solar Pond Technology Applications

  • Energy to drive desalting units, providing fresh water production for municipal water systems and an energy-producing receptacle for waste brines.
  • Supplemental energy source for peaking electrical producgion or 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 opearions.
  • Energy to drive desalting units for brine concentration.
  • Receptacles for brine disposal using waste brines from crude oil production.
  • Heat for greenhouses, livestock buildings and other low-temperature applications.
  • Space heating and absorpton cooling systems.
  • Low-temperature aquaculture applications.
  • Surface water cleanup, especially for irrigation return flows, saline waste waters, and river desalination. (For example, the Red River in Texas.)
  • Thermal energy storage systems in areas where brine is available to create the ponds and waste thermal energy is available. (For example, power plant cooling tower blowdown systems and cogeneration systems, in which brine disposal is a problem.

    Energy Capture Potential and Economics

    Energy capture potential:

    Salinity-gradient, solar energy capture potential is substantial, especially for an area in which there is a unique combination of insolation (solar energy), salt, and brackish water to make this technology a viable energy source, such as the southwestern area of the United States. For example, in West Texas a 1000-acre, salinity-gradient solar lake (the size of a large saline lake) could deliver the energy equivalent of approximately six billion cubic feet of natural gas per year. Worldwide estimates show that if the eight largest natural saline lakes were converted to salinity gradient solar lakes for electric power production, the output would be approximately 100,000 Megawatts of installed, base-load capacity.

    Obviously, the technology is not ready ofr implementation on this scale. The point to be made is that the potential for salt-gradient solar technology is immense, and for those area such as the Southwest United States, the abundance of underground salt resources, brackish water, and natural salt lakes represents a potentially significant, untapped resource.


    Salinity-gradient solar technologies must compete with conventional energy supplies and storage technologies based on the cost of energy delivered as derived from first costs and operating costs. There is, of course, no fuel cost.

    Economic analysis has demonstrated a significant economy of scale associated with salinity-gradient solar technology, as shown in figure. The difference between the high- and low-cost solar pond options depends on lining and salt costs. Also, note that the vertical axis is the total cost of delivered heat, including boilers, pipes, etc. Low-cost ponds of 2.5 acres and larger are estimated to produce medium-grade, thermal energy (120 to 200 degrees F) at costs competitive with the current price of delivered heat from natural gas and significantly below that for oil.

    Salinity-gradient solar technologies gain the competitive edge when there is an economic and environmental synergism between application and technology. Desalting is one example. Since concentrated brine disposal is always a cost associated with inland desalination, the economic benefit of using a salinity-gradient solar pond to convert what is otherwise waste into useful energy makes these projects considerably more attractive. This fact led the Bureau of Reclamation toconclude that large scale desalination using solar ponds in West Texas and Southwestern New Mexico's Tularosa Basin could produce water for approximately two dollars per thousand gallons, making such a project attractive.

    In reviewing the economics and the list of applications, it appears that there is significant market potential for salinity-gradient solar technologies, not only in the U.S., but also worldwide. Unfortunately, impediments to implementation of the technology remain. These are:

  • demonstration of reliable and easy-to-use solar pond lining and operating systems;
  • finding appropriate loads at suitable sites;
  • society's low priority on energy production that doesn't damage the environment or produce waste materials; and
  • low prices and abundant supplies of oil and gas available at this time.

    Salinity-Gradient Solar Technology Consortium

    The El Paso Solar Pond is a research, development and demonstration project operated by the University of Texas at El Paso and funded by the U.S. Bureau of Reclamation and the State of Texas. The project, which is located on the property of Bruce Foods, Inc., a food canning company, was initiated in 1983 in cooperation with the U.S. Bureau of Reclamation. Since 1985, the El Paso Solar Pond had been continuously operated for seven years. The El Paso solar pond becam the first in the world to deliver industrial process heat to a commercial manufacturer in 1985, the first solar pond electric power generating facility in the United States in 1986, and the nation's first experimental solar pond powered water desalting facility in 1987. The El Paso Solar Pond sustained record breaking, near-boiling temeratures, developed and tested the new methods of gradient establishment and management, and successfully demonstrated the feasibility of the periodic pond concept. Also, new clarity and stability control strategies have been developed that help identify an optimum stability margin for maintaining a high performance solar pond.

    The El Paso Solar Pond has been reconstructed with a geosynthetic clay liner (GCL) system and operations resumed in Spring of 1995, after experiencing a failure of its original XR-5 membrane liner in 1992. The GCL liner features several advantages over membrane liners. GCL is self-healing and puncture-proof with easy installation and predictable permeability.

    Current projects being undertaken at the El Paso Solar Pond include a bimass waste-to-energy project using heat from the pond, use of solar ponds for desalination and brine management, and an industrial application for sodium sulfate mining.

    Salinity-Gradient Solar Technology Consortium

    In January 1989, a Consortium of three universities- the University of Texas at El paso, the University of Houston, and Texas A&M University-was formed to develop salinity-gradient solar technologies for use in commercial applications in the Southwest. Although research at the three universities is diverse, the underlying theme of developing this technology for commercial applications remains the same.

    A focal point of the Consortium, the El Paso Solar Pond is maintained as a research center, demonstration laboratory, and training facility. An informal advisory board, representing goverment, industry, and the solar pond research community helps guide the activities of the Consortium.

    The State of Texas, the Governor's Energy Office, the U.S. Bureau of Reclamation's Analysis and Water Treatment Group, and the University of Texas at El Paso have provided major funding for this project.

    If your organization is active in this area and wishes to join the Consortium to help advance salinity-gradient solar technology research and applications, we encourage you to do so by contacting the Department of Mechanical and Industrial Engineering, University of Texas at El Paso, El Paso TX 79968. Telephone: 915-747-5450; Fax: 915-747-5019; email;

    More Available Information


    The following publications on Salinity-Gradient Solar Technologies and Solar Ponds are available from the Department of Mechanical and Industrial Engineering, University of Texas at El Paso, El Paso TX 79968-0521

    El Paso Solar Pond Bibliography, 69 entries, 1993
    El Paso Solar Pond Brochure.
    Proceedings of the 3rd International Conference: Progress in Solar Ponds, Golding, P., Sandoval, J., and York, T., editors. May 23-27, 1993. 379pp.
    Salinity-Gradient, Solar Ponds-A Practial Manual, Vol 1,2, Solar Pond Design & Construction. Xu H., editor. 1993
    Swift, A.H.P., Project Director, Texas Solar Pond Consortium Final Report 1989-1993, August 1993. Submitted to Texas Higher Education Coordinating Board, Energy Research in Applications Program.

    The following are available from the sources listed.

    Cler, G. and Newell, T. PONDFEAS: A Feasibility Study and Design Tool for Salt Gradient Solar Ponds-User's Manual. U.S. Army Construction Engineering Research Laboratory, September 1990.
    Solar Pond Bibliography. Available from National Renewable Energy Laboratory. 1617 Cole Blvd., Golden, CO 80401.
    Hull, J>, Nielsen, C., and Golding, P. Salinity-Gradient, Solar Ponds. CRC Press, 2000 Corporate Blvd., Boca Raton, FL 33431, 1989. 277pp.
    Solar Pond Newsletter, Available from American Solar Energy Society, 2400 Central Ave., Boulder, CO 80301.