Salinity-Gradient Solar Technology Page
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.
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.
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as a transparent insulator, permitting sunlight to be trapped in the hot bottom layer from which useful heat may be withdrawn or stored for later use.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.
At Right: Winter 1987. Solar pond operation 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.
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.
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.
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.
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.
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; aswift@cs.utep.edu.
Salinity-Gradient Solar Technology Consortium