Solar ponds

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 90 °C while the temperature at the top of the pond is usually around 30 °C. 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 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 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 problem.

Efficiency

The energy obtained is in the form of low grade heat of 70 to 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.