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Solar ponds

The Principle

In a mass of water of low depth solar radiation falling on the surface will penetrate and be absorbed at the bottom, raising the water temperature. But the buoyancy will immediately cause this water to the surface and the heat will be rapidly dissipated to the surroundings. If the water in the lower region of the pond could be made heavier than that at the top then it could stay at the bottom and retain the absorbed heat and thereby yield greater temperature difference between the bottom and the surface layer of the water. Without the difference in concentration of the lower and upper layers of water the natural convection currents set into motion prevent a rise in temperature of more than a few degrees.
In a solar pond the bottom layer of water is made more saline than the top layer at the surface. The solar pond is thus a unique energy trap with the added advantage of built-in long-term heat storage capacity. Its cost per square metre of solar collector area is considerably lower, almost one-fifth that of flat plate solar collectors where low grade heat (i.e. below 100deg. C) is collected. In addition, a kilogram of salt as ‘salt- water concentrate’ can supply as much energy and three times more heat than the same amount of coal burnt in a combustion chamber, with hardly any loss of salt. Solar pond applications include process heat, drying, desalination, refrigeration and power generation. The cost benefit factor of solar ponds has led to a number of experimental projects around the world as well as in India.

The experimental solar pond at El Paso, Texas, which supplied hot water for a food plant has recently also generated electricity using a 100kW power system and has also been used for a multi-stage flash evaporation unit to desalt water. The El Paso pond has been very successful in demonstrating the three most important applications of the solar pond; industrial process heat, grid power generation and fresh water supply. Solar ponds are especially suitable for developing countries where enough land area, local skills and materials are available.

The Technology
A solar pond is an artificially constructed water pond in which significant temperature rises are caused in the lower regions by preventing the occurrence of convection currents. The more specific terms salt-gradient solar pond or non-convecting solar pond are also used.
The solar pond, which is actually a large area solar collector is a simple technology that uses water- a pond between one to four metres deep as a working material for three main functions;

The solar pond possesses a thermal storage capacity spanning the seasons. The surface area of the pond affects the amount of solar energy it can collect. The bottom of the pond is generally lined with a durable plastic liner made from material such as black polythene and hypalon reinforced with nylon mesh. This dark surface at the bottom of the pond increases the absorption of solar radiation. Salts like magnesium chloride, sodium chloride or sodium nitrate are dissolved in the water, the concentration being densest at the bottom (20% to 30%) and gradually decreasing to almost zero at the top.
Typically, a salt gradient solar pond consists of three zones;

The solar pond is filled in stages. To obtain the difference in salt density through this stepped gradient, the pond is filled with at least three successive layers of salt solution (10 to 20cms. thick), one on top of the other, each less dense than the layer below, so that the top layer is fresh water or nearly so, while the bottom layer contains the most salt. Naturally this kind of stepped concentration cannot remain stable for long, and would eventually disappear due to diffusion and evaporation. In order to maintain the stability, concentrated brine is introduced at the bottom while the top is frequently ‘washed’ with fresh water.

The Mechanism
When solar radiation strikes the pond, most of it is absorbed by the surface at the bottom of the pond. The temperature of the dense salt layer therefore increases. If the pond contained no salt, the bottom layer would be less dense than the top layer as the heated water expands. The less dense layer would then rise up and the layers would mix. But the salt density difference keeps the ‘layers’ of the solar pond separate. The denser salt water at the bottom prevents the heat being transferred to the top layer of fresh water by natural convection, due to which the temperature of the lower layer may rise to as much as 95deg. C.

In order to extract the energy stored in the bottom layer, hot water is removed continuously from the bottom, passed through a heat exchanger and then returned to the bottom. To generate electricity, heat stored in hot water is piped to an evaporator. Liquid freon in the evaporator is heated and converted into gas. The pressure generated by the gas spins a turbine and electricity is produced by the generator. Freon gas is then cooled and recycled and used again.

Solar ponds are environmentally benign since they produce energy without creating air or water pollution, or emitting gases that exacerbate the greenhouse effect. However, unless sealed properly, a salt gradient pond can lead to salt pollution of land and ground water, which could be particularly harmful effect on agriculture in the region. Salt gradient ponds can actually have a short-term beneficial effect on the environment by the utilisation of unwanted salt. The requirement of large tracts of land is, however, a disadvantage. As the collection efficiency of solar ponds is low, they require larger area of land compared to other renewable energy technologies. Inexpensive land, water and salt are essential for economic viability. Research is now concentrated on reducing the costs and improving the performance of commercial solar ponds.

Maintenance
A variety of problems are encountered in the operation and maintenance of solar ponds. These include growth of algae in the upper convective layer, maintenance of pond transparency, gradient zone stability and gradient zone erosion. Salt pollution can also develop into a major problem if proper precautions are not taken.

The increase in the depth of the upper convective layer is a distinct disadvantage, as it grows at the expense of the gradient zone. The increase is mainly a result of convective currents due to large solar radiation absorption in the top few centimetres and the action of wind effects on the water surface causing mixing of zones.

A reduction in the depth of the upper layer would result in substantial increase in the efficiency of the solar pond. A floating transparent plastic at or near the surface would reduce the effect of wind. But its main disadvantage is that debris settling on the surface can drastically reduce the amount of solar radiation transmitted to the convective zone below. A method of control using an enhanced rate of surface washing has the advantage of countering both convective overturn and wind stirring.

Good transparency is essential for pond performance. The transparency of the pond is generally sullied by the growth of algae and bacteria and the dirt falling in (leaves, organic matter, industrial particulate, sand and fine dust). These severely decrease the transmissivity of heat. The growth of algae and bacteria can be controlled by chemical treatment. Skimming the surface regularly and continuously filtering the upper convective layer can control dirt falling in.

The existence of a stable density gradient is crucial for the effective operation of solar ponds. The slowly diffusing salt acts as a stabiliser against the faster diffusing heat. Thus gradient zone instability will mix the layers in the solar pond and reduce the temperature at he bottom of the pond. It is standard practice to maintain a substantial safety factor over the marginal stability criterion to minimise the risk of internal convective zone formation. In addition to maintain the proper density gradient concentrated brine is continuously added at the bottom while the surface layer is periodically washed with fresh water.

Applications

Process heat
Studies have indicated that there is excellent scope for process heat applications (i.e. water heated to 80 to 90deg. C.), when a large quantity of hot water is required, such as textile processing and dairy industries. Hot air for industrial uses such as drying agricultural produce, timber, fish and chemicals and space heating are other possible applications.

Desalination
Drinking water is a chronic problem for many villages in India. In remote coastal villages where seawater is available, solar ponds can provide a cost-effective solution to the potable drinking water problem. Desalination costs in these places work out to be 7.5paise per litre, which compares favourably with the current costs incurred in the reverse osmosis or electrodialysis/desalination process.

Refrigeration
Refrigeration applications have a tremendous scope in a tropical country like India. Perishable products like agricultural produce and life saving drugs like vaccines can be preserved for long stretches of time in cold storage using solar pond technology in conjunction with ammonia based absorption refrigeration system.