Solar Pond

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A salinity gradient solar pond shortly solar pond ( Engl. , German solar pond , solar pool or solar pool ), is a salt water ( brine -filled) pool (Engl. Pond ), which is covered with freshwater. The density of the water increases with increasing water depth (with a so-called density gradient , which is why such a pond is called Salinity Gradient Solar Pond ), near the bottom there is concentrated salt water and on the surface water with less salt, supplied fresh water or simply rainwater. A solar pond serves as a collector and store for solar heat .

The effect as a solar heat collector comes about because when reflected light rays or emitted heat rays pass from the optically denser medium (salt water) to the optically thinner medium (salt-free water), there is largely total reflection ; these rays remain “trapped” in the lower water layers and are ultimately absorbed. Total reflection does not only occur at one boundary surface, but also because of the gradient at numerous layers “stacked” one on top of the other.

The special storage effect is based on the high heat storage capacity of water, the lack of vertical thermal convection due to the largely persistent water stratification with fresh water above and salt water below and the prevention of radiation of heat from the lower heat reservoir triggered by total reflection. Losses due to thermal conduction (see also thermal conductivity ) must be compensated for by high levels of solar radiation during the day.

The effect of the solar ponds also occurs in natural salt lakes and was derived from this for technical use (cf. bionics ). The effect is "started" in a lagoon , for example, by rainfall or fresh water inflow and solar radiation, provided that lighter fresh water is layered over heavier salt water and there is no mixing by waves (i.e. when there is no wind). A natural solar pond on which the effect is particularly effective is the Solar Lake on the shores of the Red Sea in Egypt.

Physical mode of action

The density of water decreases with temperature above 4 ° C. This means that the warmer it is, the lighter the water. Because of the static buoyancy , heated water rises to the surface. On the surface, the water gives off thermal energy to the air through evaporation . The water on the surface cools down. Due to the reduced temperature, the cooled water has a higher density and sinks back down. In return, warm water flows in from below. A convective circulation flow is created which quickly cools the water.

The density of water increases with the salt content. Therefore, water with high salinity sinks . A stratification forms in which the salinity increases from top to bottom ( halocline ). Even with a moderate difference between the salt content on the surface and that on the bottom, the difference in the density of the water is greater than it can be due to different temperatures. A heating of the water at the bottom of the pond therefore does not lead to a circulation flow. A hot layer of water at the bottom of the pond can only give off thermal energy through thermal conduction to the water above. The heat conduction of water is comparatively low. The upper layers thus act as thermal insulation .

Since the pond slowly loses thermal energy despite the reduced losses, the temperature stratification in the pond can only be maintained in the long term if the brine is supplied with thermal energy. This supply of heat must take place directly in the lower layers of the pond. This is achieved by the fact that part of the naturally incident solar radiation penetrates the upper water layers and is then absorbed by the lower layers or the dark bottom of the pond .

Problems / technical improvements

If the water losses due to evaporation on the surface and seepage in the subsoil are not constantly compensated by adding fresh water or if the crystallized salt at the bottom of the pond is not drained, then the pond will dry out over the long term or at some point be completely filled with salt, such as this is the case with natural salt flats and salt pans in salt pans . This is undesirable in solar ponds.

Fresh water with a low salt concentration must be supplied close to the surface with little turbulence / mixing, because otherwise the salt concentration gradient necessary for the temperature stratification will be disrupted. The return of the brine after extraction and extraction of useful heat must, however, take place in the lower layers of the ponds.

One way of preventing evaporation and the associated water and heat losses is to cover the water surface with a floating foil . Such a film or suitable floating windbreakers also help to prevent wind-induced waves or currents that cause mixing and thus destruction of the salt concentration gradient.

A turbidity of the water through the salt crystals, the growth of algae and other impurities leads to a higher proportion of the solar radiation is absorbed near the surface or is reflected and penetrates less radiation in the heat storage layer brine. Such haze is therefore generally undesirable unless it is limited to the lower layers. In order to ensure that heat is absorbed from below, it is advisable to apply a heat-absorbing, dark color to the base of artificial solar ponds.

To get around the problems with maintaining the salt concentration gradient, there have been successful attempts, the top layer of a solar pond instead of fresh organic polymer - gel to form. The gel is lighter than the brine and therefore floats at the top, is more translucent than water, so that more energy reaches the brine layer and is highly viscous so that convection currents are prevented.

use

Due to the effect explained above, a solar pond can store a large amount of heat with relatively low losses even without complex and expensive thermal insulation , as is necessary with conventional hot water storage tanks . Unlike closed containers, solar ponds are cheap to manufacture using simple means, even with large dimensions. They are therefore considered simple technology (low-tech) with potential for use in sunny developing countries . The extraction of salt can be a desirable side effect.

The heat stored in the Solar Pond can be used in various ways:

Despite many years of development, the use of solar ponds to generate electricity is still in the development and testing stage. There are hardly any large commercial plants. Some existing or former research and pilot facilities are:

Web links

Individual evidence

  1. a b c S. P. Sukhatme: Solar Energy: Principles Of Thermal Collection And Storage , Verlag Tata McGraw-Hill, 2008, ISBN 9780070260641 , excerpts online on Google Books (English)
  2. Mother Earth News: ISRAEL'S 150-KW SOLAR POND (May / June 1980) at www.motherearthnews.com (in English)
  3. Salt-gradient solar ponds at www.teriin.org ( Memento of the original from May 10, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (English speaking) @1@ 2Template: Webachiv / IABot / www.teriin.org