Gourt Home::Gourt :: Solar pond
- For the fictional character by August Derleth, see Solar Pons
A Solar pond is large-scale solar energy collector with integral heat storage for supplying thermal energy. A solar pond can be used for various applications, such as process heating, water desalination, refrigeration, drying and Solar power generation.
Description
A solar pond is simply a pool of water which collects and stores solar energy. It contains layers of salt solutions with increasing concentration (and therefore density) to a certain depth, below which the solution has a uniform high salt concentration.
When solar radiation (sunlight) is absorbed, the density gradient prevents 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 90ºC while the temperature at the top of the pond is usually around 30ºC. The heat trapped in the salty bottom layer can be used for many different purposes, such as the heating of buildings, industrial hot water or to drive a turbine for generating electricity. There are 3 distinct layers of water in the pond:
- The top layer which has a low salt content
- The bottom layer which has a high salt content.
- An intermediate insulating layer with a salt gradient, which establishes a density gradient that prevents heat exchange by natural convection.
Advantages
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.
Disadvantages
The system is however relatively difficult to maintain.
- The salt gradient is very delicate and is easily upset by wind.
- The pond can foul up with algea
- The liner can spring a leak
- The evaporated surface water needs to be constantly replenished,
- The accumulating salt crystals have to be removed and can be both a valuable byproduct and a problem.
Efficiency
The energy obtained is in the form of low grade heat of 70-80°C compared to a 20°C ambient temperature, which has an upper Carnot-cycle extractable efficiency of 1-(273.15+20)/(273.15+80)=15%. By comparison a solar concentrator system with molten salt delivering high grade heat at 800°C would be able to convert 73% of absorbed solar heat into useful work, and be forced to divest only 27% as waste heat to the cold temperature reservoir (ambient air.)
Development
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.
See also
External links
"Solar pond"