A solar pond is large-scale solar thermal energy collector with integral heat storage for
supplying thermal energy. A solar pond can be used for various applications, such as process
heating, desalination, refrigeration, drying and solar power generation.
A solar pond is simply a pool of saltwater which collects and stores solar thermal energy.
The saltwater naturally forms a vertical salinity gradient also known as a "halocline", in
which low-salinity water floats on top of high-salinity water. The layers of salt solutions
increase in concentration (and therefore density) with depth. Below a certain depth, the
solution has a uniformly high salt concentration.
There are 3 distinct layers of water in the pond:
The top layer, which has a low salt content.
An intermediate insulating layer with a salt gradient, which establishes a density gradient
that prevents heat exchange by natural convection.
The bottom layer, which has a high salt content.
If the water is relatively translucent, and the pond's bottom has high optical absorption,
then nearly all of the incident solar radiation (sunlight) will go into heating the bottom
When solar energy is absorbed in the water, its temperature increases, causing thermal
expansion and reduced density. If the water were fresh, the low-density warm water would
float to the surface, causing a convection current. The temperature gradient alone causes a
density gradient that decreases with depth. However the salinity gradient forms a density
gradient that increases with depth, and this counteracts the temperature gradient, thus
preventing 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 90 Ã‚Â°C while the
temperature at the top of the pond is usually around 30 Ã‚Â°C. A natural example of these
effects in a saline water body is Solar Lake, Sinai, Israel.
The heat trapped in the salty bottom layer can be used for many different purposes, such as
the heating of buildings or industrial hot water or to drive an organic Rankine cycle
turbine or Stirling engine for generating electricity.
The largest operating solar pond for electricity generation was the Bet Ha-Arava pond built
in Israel and operated up until 1988. It had an area of 210,000 mÃ‚Â² and gave an electrical
output of 5 MW.
Advantages and disadvantages
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.
The evaporated surface water needs to be constantly replenished.
The accumulating salt crystals have to be removed and can be both a valuable by-product and
a maintenance expense.
No need of a separate collector for this thermal storage system.
The energy obtained is in the form of low-grade heat of 70 to 80 Ã‚Â°C compared to an assumed
20 Ã‚Â°C ambient temperature. According to the second law of thermodynamics (see Carnot-cycle),
the maximum theoretical efficiency of a power plant's heat engine is:
1-(273+20)/(273+80)=17%. By comparison, a solar concentrator system with molten salt
delivering high-grade heat at 800 Ã‚Â°C would have a maximum theoretical limit of 73% for
converting absorbed solar heat into useful work (and thus would be forced to divest as
little as 27% in waste heat to the cold temperature reservoir at 20 Ã‚Â°C). The low efficiency
of solar ponds is usually justified with the argument that the 'collector', being just a
plastic-lined pond, might potentially result in a large-scale system that is of lower
overall levelised energy cost than a solar concentrating system.
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