Solar Ponds
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Introduction
A solar pond is a body of water that collects and stores solar energy. Solar energy will warm a body of water (that is exposed to the sun), but the water loses its heat unless some method is used to trap it. Water warmed by the sun expands and rises as it becomes less dense. Once it reaches the surface, the water loses its heat to the air through convection, or evaporates, taking heat with it. The colder water, which is heavier, moves down to replace the warm water, creating a natural convective circulation that mixes the water and dissipates the heat. The design of solar ponds reduces either convection or evaporation in order to store the heat collected by the pond. They can operate in almost any climate.
Types of Solar Ponds
There are two main categories of solar ponds: nonconvecting ponds, which reduce heat loss by preventing convection from occurring within the pond; and convecting ponds, which reduce heat loss by hindering evaporation with a cover over the surface of the pond.
Nonconvecting Ponds
There are two main types of nonconvecting ponds: salt gradient ponds and membrane ponds. A salt gradient pond has three distinct layers of brine (a mixture of salt and water) of varying concentrations. Because the density of the brine increases with salt concentration, the most concentrated layer forms at the bottom. The least concentrated layer is at the surface. The salts commonly used are sodium chloride and magnesium chloride. A dark-colored material—usually butyl rubber—lines the pond. The dark lining enhances absorption of the sun's radiation and prevents the salt from contaminating the surrounding soil and groundwater.
As sunlight enters the pond, the water and the lining absorb the solar radiation. As a result, the water near the bottom of the pond becomes warm—up to 200°F (93.3°C). Although all of the layers store some heat, the bottom layer stores the most. Even when it becomes warm, the bottom layer remains denser than the upper layers, thus inhibiting convection. Pumping the brine through an external heat exchanger or an evaporator removes the heat from this bottom layer. Another method of heat removal is to extract heat with a heat transfer fluid as it is pumped through a heat exchanger placed on the bottom of the pond.
Another type of nonconvecting pond, the membrane pond, inhibits convection by physically separating the layers with thin transparent membranes. As with salt gradient ponds, heat is removed from the bottom layer.
Convecting Pond
A well-researched example of a convecting pond is the shallow solar pond. This pond consists of pure water enclosed in a large bag that allows convection but hinders evaporation. The bag has a blackened bottom, has foam insulation below, and two types of glazing (sheets of plastic or glass) on top. The sun heats the water in the bag during the day. At night the hot water is pumped into a large heat storage tank to minimize heat loss. Excessive heat loss when pumping the hot water to the storage tank has limited the development of shallow solar ponds.
Another type of convecting pond is the deep, saltless pond. This convecting pond differs from shallow solar ponds only in that the water need not be pumped in and out of storage. Double-glazing covers deep saltless ponds. At night, or when solar energy is not available, placing insulation on top of the glazing reduces heat loss.
Applications
Applications for solar ponds include community, residential and commercial heating; low-temperature industrial and agricultural process heat; preheating for higher-temperature industrial process applications; and electricity generation. Heat extracted from ponds can also run absorption chillers.
Several U.S. organizations, in consultation with the Israelis—the leaders in solar pond technology—built a 0.8 acre (0.32 hectare) salt gradient solar pond on the grounds of a food cannery in El Paso, Texas. The first application of the pond was to produce heat for the canning operation. The pond has been producing heat in this manner since the summer of 1986. The system operates at about 185°F (86°C) and delivering about 300 kilowatts (kW) of thermal energy. In July 1986, the operators added a Rankine Cycle heat engine to the system. In September, it became the first in the United States to generate electricity, producing up to 70 kW. In May 1987, the operators added a 24 stage, falling-film, low temperature desalting unit. In June, it began producing about 4,600 gallons of desalinated water per day (16,000 liters/day). In 1992, the facility was shut down due to a failure of its original XR-5 liner. The pond was reconstructed with a geosynthetic clay liner system and operations resumed in the spring of 1995.
There are several other demonstration projects in the United States. A Miamisburg, Ohio operation uses a salt gradient pond to heat a recreational building and swimming pool. The Tennessee Valley Authority built several shallow solar ponds for various purposes, and has assisted others with similar projects. There are also installations in other countries including Israel and India. Information on a demonstration facility at a dairy in Bhuj, India, is available.
Feasibility
Solar ponds can only be economically constructed if there is an abundance of inexpensive salt, flat land, and easy access to water. Environmental factors are also important. An example is preventing soil contamination from the brine in a solar pond. For these reasons, and because of the current availability of cheap fossil fuels, solar pond development has been limited in the United States. The greatest potential market for solar ponds in this country could be in the industrial process heat sectors.
