04-01-2013, 12:04 PM
EFFECT OF INTERNAL AND EXTERNAL HEAT TRANSFER ON THE PERFORMANCE OF THE SOLAR DISTILLATION SYSTEM
[attachment=46433]
ABSTRACT
In this communication, an attempt has been made to find out the effect of water capacity on internal and external heat transfer for a solar distillation system. It is a well known fact that the distillate output decreases significantly with the increase of water depth in the basin of the solar still. A single slope basin type solar still is fabricated with inner dimensions of 1000mm x 500mm (effective area 0.5 m2) and the glass cover is tilted at 10° with respect to the horizontal. The objectives of the present paper are (i) To study the effect of water capacity in the basin on the internal, external heat transfer and performance of the system, (ii) To study the effect of water capacity on the cumulative energy balance of the distillation system. It is inferred that the internal and external heat transfer depends significantly on the water capacity in the basin. It is also observed that the evaporative, radiative and convective heat transfer from water to glass are plays an important role on the performance of the distillation system.
INTRODUCTION
Where the demand for fresh water exceeds the amount that fresh water sources can meet, desalination of lower quality water provides a reasonable new fresh water source. A diversity of desalination technologies are being used to separate fresh water from saline water; including multi stage flash (MSF), multiple effect (ME), vapor compression (VC), reverse osmosis (RO), ion exchange, electro dialysis, phase change and solvent extraction. These technologies are expensive, however, for the production of small amount of fresh water. On the other hand, the use of conventional energy sources (hydrocarbon fuels) to drive these technologies has a negative impact on the environment.
In any conventional simple horizontal solar desalination still , solar energy heats a mass of water at the basin, which acts as an evaporating zone, and evaporates its surface layer. By means of natural convection, the water vapour reaches the transparent glass inner surface, which is at relatively lower temperature than the water vapour and inclined to the horizontal at an angle equal to the latitude of the place, and so condenses on it. The condensed drops of water slide on the inner glass surface and are collected in the collecting channel, see Fig. 1. These stills suffer from some drawbacks that make them uneconomical for use for domestic purposes.
History of Solar Distillation
Distillation has long been considered a way of making salt water drinkable and purifying water in remote locations. As early as the fourth century B.C., Aristotle described a method to evaporate impure water and then condense it for potable use. Arabian alchemists were the earliest known people to use solar distillation to produce potable water in the sixteenth century. However, the first documented reference for a device was made in 1742 by Nicolo Ghezzi of Italy, although it is not known whether he went beyond the conceptual stage and actually built it. The first modern solar still was built in Las Salinas, Chile, in 1872, by Charles Wilson. It consisted of 64 water basins (a total of 4,459 square meters) made of blackened wood with sloping glass covers. This installation was used to supply water (20,000 liters per day) to animals working mining operations. After this area was opened to the outside by railroad, the installation was allowed to deteriorate but was still in operation as late as 1912--40 years after its initial construction. This design has formed the basis for the majority of stills built since that time.
Developments of solar distillation systems
Solar distillation systems (Solar Stills) are classified broadly into two categories: passive and active solar still. Passive system in which solar energy collected by structure elements (basin liner) itself for evaporation of saline water. In the case of active solar still, an additional thermal energy by external mode is required for faster evaporation. The extra energy for active system may be obtained from a flat plate solar collector with circulating pump, additional condenser etc. A classification of published literature on solar distillation is given in Table 1. The details of some of the designs along with the performance are discussed in subsequent sections.
Simple passive solar stills (Single and double slope solar still)
The simple single and double slope solar stills are shown in fig 3 and fig 4. The sun’s energy in the form of short electromagnetic waves passes through a clear glazing surface such as glass. Upon striking a darkened surface, this light changes wavelength, becoming long waves of heat, which is added to the water in a shallow basin below the glazing. As the water heats up, it begins to evaporate. The warmed vapor rises to a cooler area. Almost all impurities are left behind on the basin. The vapor condenses onto the underside of the cooler glazing and accumulates in to water droplets or sheets of water. The combination of gravity and the tilted glazing surfaces allows the water to run down the cover and into a collection trough, where it is channeled in to storage.
Wick type solar stills
The design of a multiple wick solar still is shown in fig , in which blackened wet jute cloth forms the liquid surface which can be oriented to intercept maximum solar radiation and attain high temperatures on account of low thermal capacity. The wet surface is created by a series of jute cloth pieces of increasing length separated by thin polythene sheets; these pieces are arranged along an incline and the upper edges are dipped in a saline water tank. Suction by the capillary action of the cloth fibre, provides a surface of the liquid and the arrangement ensures that all the surface, irradiated by the sun is wet at all times
Single slope FRP still.
A novel feature in this type of still is the absence of insulation on the sides and bottom. The still is south oriented.To set up the still, a hole was drilled to make an outlet for the drainage of distilled water on the side (OD), and a plastic pipe was fixed in it. An inlet pipe (I) was also fixed at the back of the still. The edges of the still were covered with flat rubber gaskets which were fixed with rubber solution. The edges of the glass were covered with a U-shaped rubber gasket. Next, the glass sheet was placed on the still top, and holes were drilled in the edges through the aluminium angles ‘L’ which covered the edges of the still, and held the glass in place. The gaskets make the system air tight so that the water vapour does not escape. The advantages of the single slope FRP still are: long life expectancy of at least 10 years, easy to handle and set up, and absence of any kind of insulation.
CONCLUSION
In this present study, several conclusions can be obtained as follows,
(a) In higher water levels, the maximum temperature of the basin water, vapour and water is recorded in the late afternoon hour between 15hrs and 18hrs where as lower levels are attained from the middle of afternoon.
(b) In lower levels gives more yields from 9.00hrs to 13.00hrs, then productivity is decreases till evening where as higher water levels are started its productivity slowly from 9.00hrs – 13.00 hrs, and then steadily increasing till the end of the day’s experiment. Between 18.00hrs and 9.00hrs (next day), the productivity is more in higher water levels than the lower water levels. As the water depth decreases from 60mm to 10mm the productivity increased by 12%.
© The largest temperature (83.9°C) of the solar still is recorded at inner wall surfaces and is almost constant for all water levels and the next largest temperature is recorded at vapour side (78.8°C). The lowest temperature is recorded at the bottom side of the still (32.2°C).
[attachment=46433]
ABSTRACT
In this communication, an attempt has been made to find out the effect of water capacity on internal and external heat transfer for a solar distillation system. It is a well known fact that the distillate output decreases significantly with the increase of water depth in the basin of the solar still. A single slope basin type solar still is fabricated with inner dimensions of 1000mm x 500mm (effective area 0.5 m2) and the glass cover is tilted at 10° with respect to the horizontal. The objectives of the present paper are (i) To study the effect of water capacity in the basin on the internal, external heat transfer and performance of the system, (ii) To study the effect of water capacity on the cumulative energy balance of the distillation system. It is inferred that the internal and external heat transfer depends significantly on the water capacity in the basin. It is also observed that the evaporative, radiative and convective heat transfer from water to glass are plays an important role on the performance of the distillation system.
INTRODUCTION
Where the demand for fresh water exceeds the amount that fresh water sources can meet, desalination of lower quality water provides a reasonable new fresh water source. A diversity of desalination technologies are being used to separate fresh water from saline water; including multi stage flash (MSF), multiple effect (ME), vapor compression (VC), reverse osmosis (RO), ion exchange, electro dialysis, phase change and solvent extraction. These technologies are expensive, however, for the production of small amount of fresh water. On the other hand, the use of conventional energy sources (hydrocarbon fuels) to drive these technologies has a negative impact on the environment.
In any conventional simple horizontal solar desalination still , solar energy heats a mass of water at the basin, which acts as an evaporating zone, and evaporates its surface layer. By means of natural convection, the water vapour reaches the transparent glass inner surface, which is at relatively lower temperature than the water vapour and inclined to the horizontal at an angle equal to the latitude of the place, and so condenses on it. The condensed drops of water slide on the inner glass surface and are collected in the collecting channel, see Fig. 1. These stills suffer from some drawbacks that make them uneconomical for use for domestic purposes.
History of Solar Distillation
Distillation has long been considered a way of making salt water drinkable and purifying water in remote locations. As early as the fourth century B.C., Aristotle described a method to evaporate impure water and then condense it for potable use. Arabian alchemists were the earliest known people to use solar distillation to produce potable water in the sixteenth century. However, the first documented reference for a device was made in 1742 by Nicolo Ghezzi of Italy, although it is not known whether he went beyond the conceptual stage and actually built it. The first modern solar still was built in Las Salinas, Chile, in 1872, by Charles Wilson. It consisted of 64 water basins (a total of 4,459 square meters) made of blackened wood with sloping glass covers. This installation was used to supply water (20,000 liters per day) to animals working mining operations. After this area was opened to the outside by railroad, the installation was allowed to deteriorate but was still in operation as late as 1912--40 years after its initial construction. This design has formed the basis for the majority of stills built since that time.
Developments of solar distillation systems
Solar distillation systems (Solar Stills) are classified broadly into two categories: passive and active solar still. Passive system in which solar energy collected by structure elements (basin liner) itself for evaporation of saline water. In the case of active solar still, an additional thermal energy by external mode is required for faster evaporation. The extra energy for active system may be obtained from a flat plate solar collector with circulating pump, additional condenser etc. A classification of published literature on solar distillation is given in Table 1. The details of some of the designs along with the performance are discussed in subsequent sections.
Simple passive solar stills (Single and double slope solar still)
The simple single and double slope solar stills are shown in fig 3 and fig 4. The sun’s energy in the form of short electromagnetic waves passes through a clear glazing surface such as glass. Upon striking a darkened surface, this light changes wavelength, becoming long waves of heat, which is added to the water in a shallow basin below the glazing. As the water heats up, it begins to evaporate. The warmed vapor rises to a cooler area. Almost all impurities are left behind on the basin. The vapor condenses onto the underside of the cooler glazing and accumulates in to water droplets or sheets of water. The combination of gravity and the tilted glazing surfaces allows the water to run down the cover and into a collection trough, where it is channeled in to storage.
Wick type solar stills
The design of a multiple wick solar still is shown in fig , in which blackened wet jute cloth forms the liquid surface which can be oriented to intercept maximum solar radiation and attain high temperatures on account of low thermal capacity. The wet surface is created by a series of jute cloth pieces of increasing length separated by thin polythene sheets; these pieces are arranged along an incline and the upper edges are dipped in a saline water tank. Suction by the capillary action of the cloth fibre, provides a surface of the liquid and the arrangement ensures that all the surface, irradiated by the sun is wet at all times
Single slope FRP still.
A novel feature in this type of still is the absence of insulation on the sides and bottom. The still is south oriented.To set up the still, a hole was drilled to make an outlet for the drainage of distilled water on the side (OD), and a plastic pipe was fixed in it. An inlet pipe (I) was also fixed at the back of the still. The edges of the still were covered with flat rubber gaskets which were fixed with rubber solution. The edges of the glass were covered with a U-shaped rubber gasket. Next, the glass sheet was placed on the still top, and holes were drilled in the edges through the aluminium angles ‘L’ which covered the edges of the still, and held the glass in place. The gaskets make the system air tight so that the water vapour does not escape. The advantages of the single slope FRP still are: long life expectancy of at least 10 years, easy to handle and set up, and absence of any kind of insulation.
CONCLUSION
In this present study, several conclusions can be obtained as follows,
(a) In higher water levels, the maximum temperature of the basin water, vapour and water is recorded in the late afternoon hour between 15hrs and 18hrs where as lower levels are attained from the middle of afternoon.
(b) In lower levels gives more yields from 9.00hrs to 13.00hrs, then productivity is decreases till evening where as higher water levels are started its productivity slowly from 9.00hrs – 13.00 hrs, and then steadily increasing till the end of the day’s experiment. Between 18.00hrs and 9.00hrs (next day), the productivity is more in higher water levels than the lower water levels. As the water depth decreases from 60mm to 10mm the productivity increased by 12%.
© The largest temperature (83.9°C) of the solar still is recorded at inner wall surfaces and is almost constant for all water levels and the next largest temperature is recorded at vapour side (78.8°C). The lowest temperature is recorded at the bottom side of the still (32.2°C).