SIDDAGANGA INSTITUTE OFTECHNOLOGYTUMKUR-572103, KARNATAKADepartment of Chemical EngineeringA TECHNICAL SEMINARONSOLAR POND AND ITS APPLICATIONSBYKALHAN MOZA1SI09CH017VIII SEMB.E. (CHEM. ENGG.)
OUTLINEINTRODUCTIONWHAT IS A SOLAR POND-WORKING PRINCIPLETYPES OF SOLAR PONDS-NON-CONVECTING SOLAR PONDS-CONVECTING SOLAR PONDSAPPLICATIONSADVANTAGESCONCLUSIONREFERENCES
INTRODUCTION The sun is the largest source of renewable energy and this energy is abundantlyavailable in all parts of the earth. It is in fact one of the best alternatives to the non-renewable 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 toexploit solar energy more efficiently, but most of this work remained mainlyacademic. One way to tap solar energy is through the use of solar ponds. Solar ponds arelarge-scale energy collectors with integral heat storage for supplying thermalenergy. It can be use for various applications, such as process heating, waterdesalination, refrigeration, drying and power generation .
WHAT IS A SOLAR POND A solar pond is a body of water that collects and stores solar energy. Solarenergy will warm a body of water (that is exposed to the sun), but the waterloses its heat unless some method is used to trap it. Water warmed by the sunexpands and rises as it becomes less dense. Once it reaches the surface, thewater loses its heat to the air through convection, or evaporates, taking heatwith it. The colder water, which is heavier, moves down to replace the warmwater, creating a natural convective circulation that mixes the water anddissipates the heat. The design of solar ponds reduces either convection orevaporation in order to store the heat collected by the pond. They can operatein almost any climate . A solar pond can store solar heat much more efficiently than a body of waterof the same size because the salinity gradient prevents convection currents.Solar radiation entering the pond penetrates through to the lower layer, whichcontains concentrated salt solution. The temperature in this layer rises sincethe heat it absorbs from the sunlight is unable to move upwards to the surfaceby 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 wateror air is heated they become lighter and rise upward. Similarly, in an ordinarypond, the sun’s rays heat the water and the heated water from within thepond rises and reaches the top but loses the heat into the atmosphere. Thenet result is that the pond water remains at the atmospheric temperature.The solar pond restricts this tendency by dissolving salt in the bottom layer ofthe pond making it too heavy to rise. A solar pond is an artificially constructed water pond in which significanttemperature rises are caused in the lower regions by preventing theoccurrence of convection currents. The more specific terms salt-gradientsolar 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 usesa pond between one to four metres deep as a working material.
The solar pond possesses a thermal storage capacity spanning the seasons.The surface area of the pond affects the amount of solar energy it cancollect.The dark surface at the bottom of the pond increases the absorptionof solar radiation. Salts like magnesium chloride, sodium chloride or sodiumnitrate are dissolved in the water, the concentration being densest at thebottom (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 solarcollector/receiver and which is relatively the most shallow in depth and isgenerally close to ambient temperature. A gradient which serves as the non-convective zone which is much thickerand occupies more than half the depth of the pond. Salt concentration andtemperature increase with depth. A lower convective zone with the densest salt concentration, serving as theheat storage zone. Almost as thick as the middle non-convective zone, saltconcentration and temperatures are nearly constant in this zone.
When solar radiation strikes the pond, most of it is absorbed by the surface atthe bottom of the pond. The temperature of the dense salt layer thereforeincreases.But the salt density difference keeps the ‘layers’ of the solar pondseparate.The denser salt water at the bottom prevents the heat being transferredto the top layer of fresh water by natural convection, due to which thetemperature of the lower layer may rise to as much as 95°C.
Diagram of the different layers of a Solar Pond
TYPES OF SOLAR PONDS• CONVECTING SOLAR PONDSA well-researched example of a convecting pond is the shallow solar pond.This pond consists of pure water enclosed in a large bag that allowsconvection but hinders evaporation. The bag has a blackened bottom, hasfoam 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 hotwater is pumped into a large heat storage tank to minimize heat loss.Excessive heat loss when pumping the hot water to the storage tank haslimited the development of shallow solar ponds.
• NON-CONVECTING SOLAR PONDS The main types of nonconvecting ponds is salt gradient ponds. A salt gradientpond has three distinct layers of brine (a mixture of salt and water) of varyingconcentrations. Because the density of the brine increases with saltconcentration, the most concentrated layer forms at the bottom. The leastconcentrated layer is at the surface. The salts commonly used are sodiumchloride and magnesium chloride. A dark-colored material usually butyl rubberlines the pond. As sunlight enters the pond, the water and the lining absorb the solarradiation. As a result, the water near the bottom of the pond becomes warmup to 93.3°C. Even when it becomes warm, the bottom layer remains denserthan the upper layers, thus inhibiting convection. Pumping the brine throughan external heat exchanger or an evaporator removes the heat from thisbottom layer. Another method of heat removal is to extract heat with a heattransfer fluid as it is pumped through a heat exchanger placed on the bottomof the pond.
APPLICATIONS Process heatStudies have indicated that there is excellent scope for process heatapplications (i.e. water heated to 80 to 90° C.), when a large quantity of hotwater is required, such as textile processing and dairy industries. Hot air forindustrial uses such as drying agricultural produce, timber, fish andchemicals and space heating are other possible applicationsA visual Demonstration of how a Solar Pond is used to Generate Electricity
DesalinationDrinking water is a chronic problem for many villages in India. In remotecoastal villages where seawater is available, solar ponds can provide a cost-effective solution to the potable drinking water problem. Desalination costs inthese places work out to be 7.5paise per litre, which compares favourablywith the current costs incurred in the reverse osmosis orelectrodialysis/desalination process. RefrigerationRefrigeration applications have a tremendous scope in a tropical country likeIndia. Perishable products like agricultural produce and life saving drugs likevaccines can be preserved for long stretches of time in cold storage usingsolar pond technology in conjunction with ammonia based absorptionrefrigeration system.
ADVANTAGES 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 ispossible. Expensive cleaning of large collector surfaces in dusty areas is avoided .
CONCLUSIONSolar ponds can be effectively used as replacements in industries that usefossil fuel to generate thermal energy. Solar ponds can be used for processheating, refrigeration, water desalination, production of magnesium chloride,bromine recovery from bittern, enhancement of salt yield in salt farms. It willbe the future energy source.
REFERENCES C, Nielsen; A, Akbarzadeh; J, Andrews; HRL, Becerra; P, Golding (2005), "The History of Solar Pond Science and Technology", Proceedings of the 2005 Solar World Conference, Orlando, FL. Solar Gradient Solar Ponds, accessed on 28 November 2009, http://www.teriin.org/tech_solarponds.php MacInnis, Roberta. "Solar pond producing power for Texas cannery." Energy User News 12 (March 30, 1987): 8(1).