Solar Ponds h384e

SOLAR PONDS

CONTENTS • 1. INTRODUCTION • 2. WHAT A SOLAR POND IS –

2.1 WORKING PRINCIPLE

• 3. TYPES OF SOLAR PONDS – –

3.1 NONCONVECTING 3.2 CONVECTING

• 4. APPLICATIONS

• 5. ADVANTAGES and DISADVANTAGES • 6. CONCLUSION • 7. REFERENCES

INTRODUCTION • The sun is the largest source of renewable

energy and this energy is abundantly available in all parts of the earth. It is in fact one of the best alternatives to the nonrenewable sources of energy. • Solar energy has been used since prehistoric times, but in a most primitive manner. Before 1970, some research and development was carried out in a few countries to exploit solar energy more efficiently, but most of this work remained mainly academic .

• After the dramatic rise in oil prices in the

•

1970s, several countries began to formulate extensive research and development programmes to exploit solar energy . One way to tap solar energy is through the use of solar ponds. Solar ponds are largescale energy collectors with integral heat storage for supplying thermal energy. It can be use for various applications, such as process heating, water desalination, refrigeration, drying and power generation.

WHAT A SOLAR POND IS • A solar pond is a body of water that collects and

stores solar energy. Solar energy will warm a body of water (that is exposed to the sun), but the water loses its heat unless some method is used to trap it. Water warmed by the sun expands and rises as it becomes less dense. Once it reaches the surface, the water loses its heat to the airthrough convection, or evaporates, taking heat with it. The colder water, which is heavier, moves down to replace the warm water, creating a natural convective circulation that mixes the water and dissipates the heat. The design of solar ponds reduces either convection or evaporation in order to store the heat collected by the pond. They can operate in almost any climate .

• A solar pond can store solar heat much more

efficiently than a body of water of the same size because the salinity gradient prevents convection currents. Solar radiation entering the pond penetrates through to the lower layer, which contains concentrated salt solution. The temperature in this layer rises since the heat it absorbs from the sunlight is unable to move upwards to the surface by convection. Solar heat is thus stored in the lower layer of the pond .

WORKING PRINCIPLE • The solar pond works on a very simple

principle. It is well-known that water or air is heated they become lighter and rise upward. Similarly, in an ordinary pond, the sun’s rays heat the water and the heated water from within the pond rises and reaches the top but loses the heat into the atmosphere. The net result is that the pond water remains at the atmospheric temperature. The solar pond restricts this tendency by dissolving salt in the bottom layer of the pond making it too heavy to rise .

• 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 saltgradient 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 .

• Collection of radiant energy and its conversion into heat (upto 95° C)

• Storage of heat • Transport of thermal energy out of the system.

• 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 .

• An upper convective zone of clear fresh water •

•

that acts as solar collector/receiver and which is relatively the most shallow in depth and is generally close to ambient temperature, A gradient which serves as the non-convective zone which is much thicker and occupies more than half the depth of the pond. Salt concentration and temperature increase with depth, A lower convective zone with the densest salt concentration, serving as the heat storage zone. Almost as thick as the middle nonconvective zone, salt concentration and temperatures are nearly constant in this zone .

• 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 95 °C .

TYPES OF SOLAR PONDS • There are two main categories of solar ponds: nonconvecting ponds, which reduce heat loss by preventing convection from occurring within the pond; and convecting ponds, which reduce heat loss by hindering evaporation with a cover over the surface of the pond .

CONVECTING SOLAR PONDS • A well-researched example of a convecting pond is the shallow solar pond. This pond consists of pure water enclosed in a large bag that allows convection but hinders evaporation. The bag has a blackened bottom, has foam insulation below, and two types of glazing (sheets of plastic or glass) on top. The sun heats the water in the bag during the day. At night the hot water is pumped into a large heat storage tank to minimize heat loss. Excessive heat loss when pumping the hot water to the storage tank has limited the development of shallow solar ponds .

• Another type of convecting pond is the deep, saltless pond. This convecting pond differs from shallow solar ponds only in that the water need not be pumped in and out of storage. Double-glazing covers deep saltless ponds. At night, or when solar energy is not available, placing insulation on top of the glazing reduces heat loss .

NONCONVECTING SOLAR PONDS • There are two main types of nonconvecting

ponds: salt gradient ponds and membrane ponds. A salt gradient pond has three distinct layers of brine (a mixture of salt and water) of varying concentrations. Because the density of the brine increases with salt concentration, the most concentrated layer forms at the bottom. The least concentrated layer is at the surface. The salts commonly used are sodium chloride and magnesium chloride. A dark-colored material usually butyl rubber lines the pond. The dark lining enhances absorption of the sun's radiation and prevents the salt from contaminating the surrounding soil and groundwater .

• As sunlight enters the pond, the water and the

lining absorb the solar radiation. As a result, the water near the bottom of the pond becomes warm up to 93.3°C. Although all of the layers store some heat, the bottom layer stores the most. Even when it becomes warm, the bottom layer remains denser than the upper layers, thus inhibiting convection. Pumping the brine through an external heat exchanger or an evaporator removes the heat from this bottom layer. Another method of heat removal is to extract heat with a heat transfer fluid as it is pumped through a heat exchanger placed on the bottom of the pond [3].

• Another type of nonconvecting pond, the

membrane pond, inhibits convection by physically separating the layers with thin transparent membranes. As with salt gradient ponds, heat is removed from the bottom layer . In figure 2 you can see an example of salt gradient solar pond.

Fig. 2 Salt Gradient Solar Pond [7].

APPLICATIONS • Salt production (for enhanced evaporation • • • • • •

or purification of salt, that is production of ‘vacuum quality’ salt) Aquaculture, using saline or fresh water (to grow, for example, fish or brine shrimp) Dairy industry (for example, to preheat feed water to boilers) Fruit and vegetable canning industry Fruit and vegetable drying (for example, vine fruit drying) Grain industry (for grain drying) Water supply (for desalination) .

 Process heat Studies have indicated that there is excellent scope for process heat applications (i.e. water heated to 80 to 90 ° 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 [6].

 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 [6].

 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 .

EXAMPLES OF SOLAR PONDS  5.1 BHUJ SOLAR POND  5.2 El PASO SOLAR POND  5.3 PYRAMID HILL SOLAR POND

ADVANTAGES AND DISADVANTAGES • Low investment costs per installed • • • •

collection area. Thermal storage is incorporated into the collector and is of very low cost. Diffuse radiation (cloudy days) is fully used. Very large surfaces can be built thus large scale energy generation is possible. Expensive cleaning of large collector surfaces in dusty areas is avoided [15].

• Solar ponds can only be economically

constructed if there is an abundance of inexpensive salt, flat land, and easy access to water. Environmental factors are also important. An example is preventing soil contamination from the brine in a solar pond. For these reasons, and because of the current availability of cheap fossil fuels, solar pond development has been limited .

CONCLUSION • Solar ponds can be effectively used as

replacements in industries that use fossil fuel to generate thermal energy. Solar ponds can be used for process heating, refrigeration, water desalination, production of magnesium chloride, bromine recovery from bittern, enhancement of salt yield in salt farms. It will be the future energy source.

REFERENCES • http://edugreen.teri.res.in/explore/renew/po • • • •

nd.htm http://edugreen.teri.res.in/explore/renew/so lar.html http://www.eere.energy.gov/consumerinfo/f actsheets/aa8.html http://www.solarpond.utep.edu www.google.co.in

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