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CHAPTER 1
INTRODUCTION
1.1Solar Energy
Solar energy is radiant energy that is produced by the sun. Every day
the sun radiates, or sends out, an enormous amount of energy. The sun radiates
more energy in one second than people have used since the beginning of time!
Where does the energy come from that constantly radiates from the sun? It comes
from within the sun itself. Like other stars, the sun is a big ball of gasesmostly
hydrogen and helium atoms. The hydrogen atoms in the suns core combine to
form helium and generate energy in a process called nuclear fusion.
During nuclear fusion, the suns extremely high pressure and
temperature cause hydrogen atoms to come apart and their nuclei (the central cores
of the atoms) to fuse, or combine. Four hydrogen nuclei fuse to become one helium
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atom. But the helium atom contains less mass than the four hydrogen atoms that
fused. Some matter is lost during nuclear fusion. The lost matter is emitted into
space as radiant energy.
It takes millions of years for the energy in the suns core to make its
way to the solar surface, and then just a little over eight minutes to travel the 93
million miles to Earth. The solar energy travels to the Earth at a speed of 186,000
miles per second, the speed of light. Only a small portion of the energy radiated by
the sun into space strikes the Earth, one part in two billion. Yet this amount of
energy is enormous. Every day enough energy strikes the United States to supply
the nations energy needs for one and a half years!
Where does all this energy go? About 15 percent of the suns energy
that hits the Earth is reflected back into space. Another 30 percent is used to
evaporate water, which, lifted into the atmosphere, produces rainfall. Solar energy
is also absorbed by plants, the land, and the oceans. The rest could be used to
supply our energy needs.
1.2 Solar ponds
A solar pond 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 air
through convection, or evaporates, taking heat with it. The colder water, which isheavier, 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.
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SYNOPSIS
SUN is largest renewable energy source in the world .This energy is
collected and stored by various methods. But our case this energy is collected and
stored by another one important renewable energy of WATER. This method is
called SOLAR POND. Here the collecting medium is water and storage medium is
salt.
The collection of solar energy is different according to the sunshine.
And the storage level also different according to the salt content, density, pH value,
and concentration and heat loss.
This method should not affect the environment and cheapest method. In
this method two renewable energy are combine to produce power.
In this solar pond various types are available but we are chosen method
is Salinity-Gradient Solar Pond .We are doing to analyze the temperature level andstorage level. In analyze is carried out by various water. That is ordinary water
with salt, ground water and sea water.
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CHAPTER 2
TYPES OF SOLAR POND
2. 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.
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2.1 Convecting Pond
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
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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.
2.2 Non-Convecting Ponds
There are two main types of non-convecting 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 suns 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 200o F
(93.3oC). 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. Another type of non
convecting 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.
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2.2.1 Natural Solar Lakes
Naturally occurring salinity gradient solar lakes are known as
heliotherma lakes, the salinity gradient is the halocline, and the temperature
gradient is the thermolcline. These lakes are either a chloride or sulfate brine.
Saline lakes with a density gradient are referred to as meromictic and the density
gradient is called the pycnocline. Natural solar lakes have importance for solar
ponds because many of the early advances in the understanding of double-
diffusive convection were made by the scientists who were studying the natural
lakes and similar phenomena in the oceans. A good example of natural solar lake is
Lake Medve in Transylvania, then in Hungary (Hull, 1979).
2.2.2 Artificial Solar Ponds
The studies on artificial solar ponds were initiated in Israel in
1948 by Dr Rudolph Bloch and carried out by a group led by Tabor until 1966
(Boegli et al., 1982; Collins et al., 1985). The first artificial solar pond was built in
Israel in the beginning of 1958. Artificial solar ponds offer low cost heat and are
also known as man-made solar ponds. These solar ponds demonstrated
advancement of solar pond technology in Israel including electricity generation on
a relatively large scale. On the basis of convection, artificial solar ponds can be
classified into two groups known as convective and non-convective solar ponds.
2.2.3 Unsaturated Solar Ponds
The salt gradient solar pond also referred to as the unsaturated
solar pond is thermodynamically unstable. The concentration gradient decays
gradually, as the result of salt diffusion potential between the concentrated solution
at the bottom and the more dilute upper layers. This tendency towards the
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destruction of the layered configuration can be counteracted only by a proper
maintenance of the pond (Vitner., et al., 1990).
2.2.4 Saturated Solar Ponds
Another form of non-convective solar pond is a saturated solar
pond. It uses salts like KNO3, NH4NO3, and MgCl2, whose solubility increases
rapidly with temperature in the pond and saturation of salt concentration is
maintained at all depths. The pond is hottest at the bottom region and the
temperature progressively decreases from the bottom to the top. Consequently an
increasing concentration of salt is found towards the bottom. Because there is
saturation at each level, the vertical diffusion of salt is checked and the density
gradient is stable. This provides the possibility of a maintenance free solar pond.
Because of the solute cycle, saturated solar ponds have the possibility for self
maintenance. The diffusion from the concentrated bottom layer increases the
concentration in the upper layers. Then evaporation from the surface reinforced by
winds brings the upper layer to supper saturation, which is relieved under suitable
conditions by crystallization. The crystals, on attaining the proper size, sink to the
bottom and dissolve there, completing the solution cycle.
2.2.5 Gel Solar Pond
In gel or viscosity stabilized solar ponds, the non-convecting
layer is composed of a viscous polymer gel positioned by transparent films. The
gel is located in the upper part of the solar pond, is less dense than water and is
optically clear. It provides insulation against heat loss, and allows the sun to heat
the bulk of the water below the gel. The gel has low thermal conductivity and is
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viscous enough to avoid convection. It uses high viscosity rather than a salinity
gradient to suppress convection.
2.2.6 Salinity-Gradient Solar Pond
The Salinity or salt gradient solar pond is thermodynamically
unstable. The concentration gradient decays gradually as the result of the salt
diffusion potential between the concentrated solution at the bottom and the more
dilute upper layers. This tendency towards the destruction of the layered
configuration can be counteracted only by a proper maintenance of the pond.
CHAPTER 3
SALINITY-GRADIENT SOLAR POND
3.1 INTRODUCTION
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A salinity-gradient solar pond (SGSP) is a combined solar energy
collector and heat storage system reliant upon an aqueous solution of salt at
varying densities to suppress natural convection and store thermal energy. The
SGSP consists of three different zones; the upper convective zone (UCZ) with
uniform low salinity; a non-convective zone (NCZ) with a gradually increasing
density; and a lower convective zone (LCZ), called the storage zone.
Figure 3.1 Schematic of a salt gradient solar pond
A solar pond is a solar thermal collector and storage system which
is essentially a water pool with suppressed heat losses. It can supply heat up to a
temperature of approximately 95 c0 as shown in Figure 3.1. The Solar pond
involves simple technology and uses water as working material for collection of
solar radiant energy and its conversion to heat, storage of heat and transport of
thermal energy out of the system Solar pond technology inhibits heat loss
mechanisms by dissolving salt into the bottom layer of the pond, making it too
heavy to rise to the surface, even when hot.
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The salt concentration increases with depth, thereby forming a salinity
gradient. The solar energy which reaches the bottom of the pond remains entrapped
there. The useful thermal energy is then withdrawn from the solar pond in the form
of hot brine. The pre-requisites for 2 establishing solar ponds are: a large tract of
land, solar radiation, and cheaply available salt such as Sodium Chloride or bittern.
A salt gradient solar pond is an efficient, low cost solar energy collection
and long range storage system for low temperature heat. In a salinity gradient solar
pond, the concentration of salt dissolved in the water increases with depth. It is
important to maintain the clarity of a solar pond to allow as much solar radiation as
possible to reach the lower zone. The stability of its salinity gradient must also be
maintained for it to perform efficiently as a store of solar energy. Water clarity is
essential for high performance of solar ponds.
3.2 History
Naturally occurring solar ponds are found in many places on the earth (
Hull et al.,1988).Solar energy was first used approximately 2500 years ago when
great Roman baths were heated by the sun (Arumugam, 1997). The discovery of
solar ponds dates back to the early years of 1900s. The phenomenon of natural
solar ponds was first discovered by Von Kalecsinky in natural occurring lakes in a
salt region of Transylvania and Hungry (Hull et al, 1989 citing V. Kalecsinsky,
1902). One of the natural solar lakes was found near Elat, Israel. This lake wasmixed and salinity uniformly high during summer but, during the winter, the fresh
seawater forms a relatively low saline upper layer, giving rise to the solar pond
effect. Naturally occurring salinity gradient or solar lakes were found in many
places in Israel and Hungry.
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Following the natural solar lakes concept, further investigations were
conducted in the 1950s and 1960s in Israel in relation to artificial solar ponds. The
cumulative research experience provided some insight into how future solar pond
facilities can be designed and operated to extract solar heat. Artificial solar ponds
offer the promise of low cost heat. However, to the present day, most man-made
solar ponds have been constructed and operated primarily as research and
development facilities.
The first artificial salinity gradient solar pond was established at Sdom,
Israel in 1960. The pond was of size 25 x 25 m and 0.8 m depth. The purpose of
this pond was to study the physics of the solar pond and its economic viability. The
pond operated between September 1959 and April 1960 and attained a maximum
heat storage zone temperature of 92o C. The second solar pond at Sdom was
operated between June and December 1962.
In addition to the Sdom solar pond in Israel, two more experimental solar
ponds were constructed. One of these ponds was very close to the first pond
located at Sdom while the second pond was contructed at Atlith Salt works near
Haifa, Israel (Tabor, H., et al 1965). The solar pond constructed at En Boqeq in
Israel was started in 1979 and it was the first solar pond to generate and supply
commercial electricity at a peak output of 150KW.Another solar pond was built in
1984 at Bet Ha Arava in Israel and this solar pond supplies a 5 MW power station.
Solar pond technology was introduced in the United States in 1973 with the
work of Rabl and Nielson at Ohio State University and the experimental work was
mostly related to heat extraction. The solar pond constructed at Chattanooga,
Tennessee, together with two auxiliary ponds, was used for brine control and heat
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extraction until 1986. In Iowa, New Jersey, Ohio and EI Paso, Texas, commercial
solar ponds are being used for heat extraction for private use.
In Australia, Solar ponds were initially investigated in a research project
undertaken between 1964 and 1966 (Tabor, H.Z., 1966). The research work on
solar ponds was begun again in Australia in 1979. Solar ponds in Australia were
constructed for research and development but some of these solar ponds have also
generated commercial electricity including a solar pond at Pyramid Hill, Victoria.
A solar pond was built in 1984 at Alice Springs, Northern Territory.
In Melbourne, the University of Melbourne in conjunction with Cheetham
Salt Ltd is operating two solar ponds at salt works for research purposes. The
Energy Conservation and Renewable Energy group at RMIT University, Bundoora
East has initiated a research & development project in conjunction with Pyramid
Salt Pty Ltd at Pyramid Hill in northern Victoria. The solar pond at Pyramid Hill
was officially opened on 14 August 2001 and began supplying heat for commercial
salt production in June 2001. Geo-Eng Australia and Pyramid Salt, in conjunction
with RMIT University, were working together on this project. The current research
indicates that there is little evidence of publication in the field of clarity
monitoring, maintenance and stability control of solar pond operation (Tsilingiris,
Panayopas, T; 1988, 41-48). Several different clarification techniques have been
used in the past including acidification, polymerization, filtration and saturation,
but all these techniques were found to have some disadvantages, such as chemical
handling and corrosion of diffusers and pipes.
In another study, the spectral transmission of halo bacteria and selected
chemicals in de-ionized water at several concentrations levels were determined for
their effect on solar radiation transmission in salt water (Wang, J; et al., 1993). The
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chemicals that were used included copper sulphate, hydrochloride acid and bleach.
However, there is still little 16 literature on the effects of water turbidity and salt
concentration levels on solar penetration (Kishore, V.V.N; et al, 1996).
In another study, a major problem with bittern and seawater solar ponds has
been found to be the growth of algae and bacteria for example in the Margherita Di
Savoia solar pond project (Folchitto, S, 1993). In another investigation, three
significant aspects of solar pond operation using bittern were discussed including
monitoring of thermal and salinity profile data, reduction of wind and filtration
techniques for water clarity (Macdonald, R.W.G.; et al., 1991).
Most of the previous research work on maintaining clarity in solar ponds
was carried out using chemical techniques. These techniques are expensive and
have disadvantages like hazards in chemical handling and potential corrosion of
metal components including diffusers and pipes for heat extraction and brine
transfer (Lu, Huanmian, et al., 1993). Brine shrimps have been used previously in
biological management of solar salt works (Tackaert, W; et al., 1993). However,
study indicates that the use of brine shrimps to maintain water clarity in bittern
solar ponds has not been investigated before. Generally, there have been few
investigations of the methods for the removal of algae and bacteria in bittern
ponds.
In this study, brine shrimps were used in salinity gradient solar ponds to
achieve good water clarity and thermal efficiency. As a result of successful
application of a biological method (brine shrimps), good brine transparency in a
solar pond has been achieved and water clarity was improved. Research work also
established that the biological method is economical and cost effective. The main
objective of this research project was to investigate chemical and biological
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techniques for maintaining solar pond water clarity and salinity gradient, and hence
maximize the thermal efficiency. Solar ponds constructed both using common salt
(sodium chloride) and bittern (a waste-product of common salt production
comprising mainly magnesium chloride) are studied. This research has
demonstrated that solar pond technology is economical and feasible for storing
significant amounts of energy for several days. The solar pond approach is now
adopted in almost all parts of the world and interest in the technology continues to
increase. A number of projects in numerous countries are 17 being undertaken, and
these activities indicate the interest in using solar pond technology for a wide
variety of purposes.
3.3 ZONES
The SGSP consists of three different zones; the upper convective zone
(UCZ) with uniform low salinity; a non-convective zone (NCZ) with a gradually
increasing density; and a lower convective zone (LCZ), called the storage zone.
Fig: 3.2 zones of solar pond
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3.3.1 The Upper Convective Zone
An upper convective zone consists of clear fresh water whichacts as a solar collector or receiver and has relatively small depth and is generally
close to ambient temperature. The upper convective zone is the primary site where
external environmental influences impinge upon the pond. The upper convective
zone is influenced by wind agitation and convective mixing. Its thickness is
between 0.2-0.5 m and its salinity ranges from 2% to 80%. The upper convective
zone is a zone of absorption and transmission.
3.3.2 The Non-Convective Zone
The non-convective zone has a salt gradient, is much
thicker than the upper convective zone and occupies more than half the depth of
the pond. Salt concentration and temperature increase with depth. The non-
convective zone separates the upper and lower convective zones and that possess
several different salt concentration layers, constituting a salinity gradient. The
main focus of concern for the gradient zone is on its internal stability. A solar pond
cannot operate without an internally stable salinity gradient and, as part of a
minimum requirement; density must either be uniform or increase downwards to
prevent any gravitational overturn. This means that the salt concentration must
increase downwards as well. Instability of a solar pond is usually linked to a weak
salt gradient, commonly because of a gradient breakdown. Due to its unique
makeup, the gradient zone acts primarily as insulation so that little energy is lost
when solar radiation is transmitted through the surface zone and the gradient zone
and stored in the lower convective zone.
3.3.3 The Lower Convective Zone
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The lower convective zone has high salt concentration and
serves as the heat storage 3zone. Almost as thick as the middle non-convective
zone, salt concentration and temperatures are nearly constant in this zone. The
lower convective zone is normally the region in which useful heat is stored and
from which it is extracted. Most of the heat removed derives from heat provided by
solar energy either absorbed in the volume of pond water or the floor of the pond.
There may also be heat transfer to or from the gradient zone and heat transfer to or
from the earth underneath the lower zone and the floor of the pond. Heat removal
can be accomplished by extracting brine or usually by passing it through an
external heat exchanger. The lower convective zone is a homogeneous,
concentrated salt solution that can be either convecting or temperature stratified.
Above it the non-convective gradient zone constitutes a thermal insulation layer
that contains a salinity gradient. Unlike the surface zone, transparency in the lower
convective zone does not have as much influence on the thermal performance of
the solar pond, poor transparency resulting mostly from dirt from the bottom of the
pond stirred up by circulation in the lower region. The simplicity of the solar pond
and its capability of generating sustainable heat above 60 0 C makes it attractive for
a lot of applications. The energy stored and collected in a solar pond is low grade
heat at temperatures limited by the boiling point of the bottom zone brine.
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CHAPTER 4
TEMERATURE MEASUREMENT
4. 1 Temperature Measurement
Temperature can be measured via a diverse array of sensors.
All of them infer temperature by sensing some change in a physical characteristic.
Six types with which the engineer is likely to come into contact are:
1. Thermocouples
2. Resistive Temperature Devices (RTDs and thermistors)3. Infrared Radiators
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4. Bimetallic Devices5. Liquid Expansion Devices6. Change-Of-State Devices
4.1.1 Thermocouple Temperature Measurement
Sensors
Thermocouples consist essentially of two
strips or wires made of different metals and joined at
one end. Changes in the temperature at that juncture
induce a change in electromotive force (emf) between
the other ends. As temperature goes up, this output
emf of the thermocouple rises, though not necessarily
linearly.
4.1.2 Resistance Temperature Devices (RTD)
Resistive temperature devices capitalize on
the fact that the electrical resistance of a material
changes as its temperature changes. Two key types are
the metallic devices (commonly referred to as RTDs),
and thermistors. As their name indicates, RTDs rely on
resistance change in a metal, with the resistance rising
more or less linearly with temperature. Thermistors are
based on resistance change in a ceramic
semiconductor; the resistance drops nonlinearly with
temperature rise.
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4.1.3 Infrared Temperature Measurement Devices
Infrared sensors are non-contacting devices.
They infer temperature by measuring the thermalradiation emitted by a material.
4.1.4 Bimetallic Temperature Measurement
Devices
Bimetallic devices take advantage of the
difference in rate of thermal expansion between
different metals. Strips of two metals are bonded
together. When heated, one side will expand more than
the other, and the resulting bending is translated into a
temperature reading by mechanical linkage to a
pointer. These devices are portable and they do not
require a power supply, but they are usually not as
accurate as thermocouples or RTDs and they do not
readily lend themselves to temperature recording.
4.1.5 Fluid-Expansion Temperature Measurement
Devices
Fluid-expansion devices, typified by the
household thermometer, generally come in two main
classifications: the mercury type and the organic-liquid
type. Versions employing gas instead of liquid are also
available. Mercury is considered an environmental
hazard, so there are regulations governing the
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shipment of devices that contain it. Fluid-expansion
sensors do not require electric power, do not pose
explosion hazards, and are stable even after repeated
cycling. On the other hand, they do not generate data
that is easily recorded or transmitted, and they cannot
make spot or point measurements.
4.1.6 Change-of-State Temperature Measurement
Devices
Change-of-state temperature sensors consist
of labels, pellets, crayons, lacquers or liquid crystals
whose appearance changes once a certain temperature
is reached. They are used, for instance, with steam
traps - when a trap exceeds a certain temperature, a
white dot on a sensor label attached to the trap will
turn black. Response time typically takes minutes, so
these devices often do not respond to transienttemperature changes. And accuracy is lower than with
other types of sensors. Furthermore, the change in
state is irreversible, except in the case of liquid-crystal
displays. Even so, change-of-state sensors can be
handy when one needs confirmation that the
temperature of a piece of equipment or a material has
not exceeded a certain level, for instance for technical
or legal reasons during product shipment.
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4.2 Types of Thermocouple
Certain combinations of alloys have become popular as industry
standards. Selection of the combination is driven by cost, availability, convenience,
melting point, chemical properties, stability, and output. Different types are best
suited for different applications. They are usually selected based on the
temperature range and sensitivity needed. Thermocouples with low sensitivities (B,
R, and S types) have correspondingly lower resolutions. Other selection criteria
include the inertness of the thermocouple material, and whether it is magnetic or
not. Standard thermocouple types are listed below with the positive electrode first,
followed by the negative electrode. They are
1. K
2. E
3. J
4. N
5. Platinum types B,R and S
5.1
B
5.2R
5.3S
6. T
7. C
8. M
9.
Chromel-gold/iron
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4.2.1 K
Type K (chromel{90 percent nickel and 10 percent chromium}
alumel)(Alumel consisting of 95% nickel, 2% manganese, 2% aluminium and 1%
silicon) is the most common general purpose thermocouple with a sensitivity of
approximately 41 V/C, chromel positive relative to alumel. It is inexpensive, and
a wide variety of probes are available in its 200 C to +1350 C / -328 F to
+2462 F range. Type K was specified at a time when metallurgy was less
advanced than it is today, and consequently characteristics may vary considerably
between samples. One of the constituent metals, nickel, is magnetic; a
characteristic of thermocouples made with magnetic material is that they undergo a
deviation in output when the material reaches its Curie point; this occurs for type K
thermocouples at around 350 C .
4.2.2 E
Type E (chromelconstantan)has a high output (68 V/C) which makes
it well suited to cryogenic use. Additionally, it is non-magnetic.
4.2.3 J
Type J (ironconstantan) has a more restricted range than type K (40 to
+750 C), but higher sensitivity of about 55 V/C. The Curie point of the iron
(770 C)causes an abrupt change in the characteristic, which determines the upper
temperature limit.
4.2.4 N
Type N (NicrosilNisil) (Nickel-Chromium-Silicon/Nickel-Silicon)
thermocouples are suitable for use at high temperatures, exceeding 1200 C, due to
their stability and ability to resist high temperature oxidation. Sensitivity is about
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39 V/C at 900 C, slightly lower than type K. Designed to be an improved type
K due to increased stability at higher temperatures, it is becoming more popular,
though the differences may or may not be substantial enough to warrant a change.
4.2.5 Platinum types B,R and S
Types B, R, and S thermocouples use platinum or a platinum
rhodium alloy for each conductor. These are among the most stable thermocouples,
but have lower sensitivity than other types, approximately 10 V/C. Type B, R,
and S thermocouples are usually used only for high temperature measurements due
to their high cost and low sensitivity.
4.2.5.1 B
Type B thermocouples use a platinumrhodium alloy for each
conductor. One conductor contains 30% rhodium while the other conductor
contains 6% rhodium. These thermocouples are suited for use at up to 1800 C.
Type B thermocouples produce the same output at 0 C and 42 C, limiting their
use below about 50 C.
4.2.5.2 R
Type R thermocouples use a platinumrhodium alloy containing 13%
rhodium for one conductor and pure platinum for the other conductor. Type R
thermocouples are used up to 1600 C.
4.2.5.3 S
Type S thermocouples are constructed using one wire of 90%
Platinum and 10% Rhodium (the positive or "+" wire) and a second wire of
100% platinum (the negative or "-" wire). Like type R, type S thermocouples
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are used up to 1600 C. In particular, type S is used as the standard of
calibration for the melting point ofgold (1064.43 C).
4.2.6 T
Type T (copperconstantan) thermocouples are suited for measurements in
the 200 to 350 C range. Often used as a differential measurement since only
copper wire touches the probes. Since both conductors are non-magnetic, there is
no Curie point and thus no abrupt change in characteristics. Type T thermocouples
have a sensitivity of about 43 V/C.
4.2.7 C
Type C (tungsten 5% rhenium tungsten 26% rhenium) thermocouples are
suited for measurements in the 0 C to 2320 C range. This thermocouple is well-
suited for vacuum furnaces at extremely high temperatures. It must never be used
in the presence ofoxygen at temperatures above 260 C.
4.2.8 M
Type M thermocouples use a nickel alloy for each wire. The positive wire (20
Alloy) contains 18% molybdenum while the negative wire (19 Alloy) contains
0.8% cobalt. These thermocouples are used in vacuum furnaces for the same
reasons as with type C. Upper temperature is limited to 1400 C. It is less
commonly used than other types.
4.2.9 Chromel-gold/iron
In chromel-gold/iron thermocouples, the positive wire is chromel and the
negative wire is gold with a small fraction (0.030.15 atom percent) of iron. It can
be used for cryogenic applications (1.2300 K and even up to 600 K). Both the
sensitivity and the temperature range depends on the iron concentration. The
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sensitivity is typically around 15 V/K at low temperatures and the lowest usable
temperature varies between 1.2 and 4.2 K.
4.3 Multichannel Temperature Digital Controller
Multi-channel digital temperature indicator is an ideal instrument
virtually for any industry or application, where multi-channel temperature have to
be measured and monitored from a convenient and centralized place. This compact
and highly reliable instrument can accept any one type of RTD or thermocouple
sensors. All thermocouple inputs are compensated for cold junction error. The
scanned channel number is indicated on a 2-digit display and the scanned
temperature is indicated on a 31/2 digit display.
Fig 4.1 Multichannel Digital Controller
Open sensor indication is a standard feature. The temperature
indication resolution is 10c. This instrument can be used either in auto-scan or
manual mode. In manual mode the measured temperature can be viewed one
channel after another by pressing the Advance switch. In Auto scan mode, all
the channel are scanned and displayed automatically one channel after and if the
user desires to view any particular channel for a long time, by pressing the
HOLD.
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CHAPTER 5
SALT
5.1 Introduction
Salt, also known as table salt, or rock salt, is a mineral that is
composed primarily ofsodium chloride (NaCl), a chemical compound belonging to
the larger class ofionic salts. It is essential for animal life in small quantities, but is
harmful to animals and plants in excess. Salt is one of the oldest, most ubiquitous
food seasonings and salting is an important method offood preservation.
The taste of salt (saltiness) is one of the basic human tastes.
Salt for human consumption is produced in different forms: unrefined
salt (such as sea salt), refined salt (table salt), and iodized salt. It is a crystalline
solid, white, pale pink or light gray in color, normally obtained from sea water or
rock deposits. Edible rock salts may be slightly grayish in color because of mineral
content.
Chloride and sodium ions, the two major components of salt, areneeded by all known living creatures in small quantities. Salt is involved in
regulating the water content (fluid balance) of the body. The sodium ion itself is
used for electrical signaling in the nervous system. Because of its importance to
survival, salt has often been considered a valuable commodity during human
history. However, as salt consumption has increased during modern times,
scientists have become aware of the health risks associated with too much salt
intake, including high blood pressure. Therefore health authorities have
recommended limitations of dietary sodium. The United States Department of
Health and Human Services recommends that individuals consume no more than
15002300 mg of sodium (37505750 mg of salt) per day depending on age.
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5.2 Salt Resources
The cost of salt is approximately 30 to 50 % of the total pond cost;
hence the solar pond should preferably be built near a salt producing area. The
coastal areas are economically advantageous zones as the sea is the natural source
of water and salt at very little cost. Also there is considerable advantage in locating
ponds in areas presently accepted as high salt regions. The basic criteria for salt
selection are transparency to solar radiation, low cost, minimal environmental
hazard, and adequate solubility.
5.3 Candidate Salts in solar pond
The essential components of a salinity gradient solar pond are
dissolved salt and water. To establish a salinity gradient requires only very
concentrated brine and a low- salinity brine or fresh water. The availability of low
cost brine or salts having suitable chemical and physical properties strongly affects
the overall economics of solar ponds.
The basic criteria for salt and brine selection are
Adequate solubility
Transparency to solar radiation.
Low cost
Minimal Environmental Hazard
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The solubility of the salt composition in water is of fundamental importance
because it determines the range of salinity (or salt concentration) and salinity
gradient profiles. Salt diffusivity and brine viscosity certainly influence gradient
stability and gradientboundary behavior.
Sodium chloride (NaCl) and magnesiun chloride (MgCl2) are the major
constituents of salts used in solar ponds. Sodium chloride (NaCl) brines have been
by far the most widely used salt solutions in solar ponds built around the world.
While there are many possible candidate salts, experience to date has been limited,
again with a few exceptions, to brine in which NaCl and or MgCl2 are the major
constituent salts. This can be correlated with the availability and cost.
The use of sodium chloride salt and water to produce solar pond brines has
the significant advantage of producing highly transparent solutions. During the
initial establishment of solar ponds, it has been demonstrated that the use of
sodium chloride and water provides a high degree of clarity because clean salt and
water produce clear brines in the ponds.
After sodium chloride (NaCl), magnesium chloride (MgCl2) is the second
largest salt constituent of sea water. Magnesium chloride (MgCl2) is a major
residue salt in the end brines of solar salt works. Compared to sodium chloride
(NaCl), magnesium chloride (MgCl2) provides much more concentrated brines and
greater density can be produced by dissolution of magnesium chloride (MgCl2)
salt in the water. As a practical result, solar ponds operated with magnesium
chloride (MgCl2) brines exhibit greater operational stability. It is established that
magnesium chloride (MgCl2) is more expensive than sodium chloride (NaCl) but
that in the salt processing plants, magnesium chloride (MgCl2) brine
concentrations are often cheaper. Bittern is concentrated seawater, a waste product
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in the manufacture of sodium chloride. It has sodium chloride (NaCl) and
magnesium chloride (MgCl2) as its main constituents. A chemical analysis of the
bittern used at the Laverton solar pond indicates the following major constituents
in mg/kg.
Table 5.1A Typical Bittern Chemical Composition (concentration mg/kg)
Chloride Sodium Magnesium Potassium Sulfate
180,000 51,000 36,000 12,000 6,000
5.3.1 Sodium Chloride
Sodium chloride has been used in many solar ponds around the
world because of its wide availability, relatively low cost and well known
properties. It is the major constituent of sea water and many other saline waters.
Sodium chloride based solar ponds are essentially an established technology for
low temperature applications. High- temperature applications of NaCl ponds
have also been successful but these applications require more careful operation and
maintenance procedures. Several ponds have operated at temperatures in excess of
80 0 C. The thermo physical properties of pure NaCl solutions have been widely
investigated.
5.3.2 Magnesium Chloride
Magnesium chloride (MgCl2), is the second largest constituent of
ocean water and it is also the principal evaporate of Dead Sea brine. Magnesium
chloride is a major residue salt in the end brines of solar salt works. Brines with
substantial MgCl2 concentration are often cheap and at a few selected sites they
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are freely available for the cost of pumping. Two experimental solar ponds in
Israel utilized MgCl2, as the only constituent salt. One of the largest solar ponds of
36,000 m 2 in the U.S uses magnesium brine. This pond uses a thin gradient zone
to keep the winter temperature of a deep brine storage pond above 130
C.
An experimental 850 m 2 pond using mixed MgCl2 and NaCl was operated at the
Great Salt Lake and magnesium ions are present in the brine used in the Los Banos
ponds. The Margherita di Savoia Solar, Pond 25,000 m 2 , 4 m deep, has been filled
with salt- work bittern and the gradient has been generated using seawater.
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CHAPTER 6
DESGIN AND CONSTRUCTION OF SOLAR POND
In this solar pond system having a number of components they are
1. Cylindrical tray
2. Glass plate(soda lime)
3. Insulation material
4. Thermocouple
5. Temperature digital controller
6.
Absorber medium (black coating, Nylon clothe)
7. Collecting & storage medium (Water and salt)
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1. Cylindrical Tray:It is made up of stainless steel sheet. The sizes of the sheet are
10075 cm and thickness is 5mm.the require size of the tray is 900580mm from
this required dimension the cylindrical shape is formed by the sheet metal
operation. Then the both end of the cylindrical sides are closed and its weld by the
Arc welding now the cylindrical tray is formed. The density and thermal
conductivity of the stainless steel are 7900kg/m3
and 15.1W/m K.
2.
Glass plate:The glass covers the top surface of the cylindrical tray box with
airtight. The glass dimension is 10075cm and thickness is 8mm.The transitivity
of ordinary glass is quite higher compared with other material and also hindrance
the Heat loss and evaporation. In this system three holes are drilled on the glass
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plates with equal diameter of 6mm. The first hole is drilled at the center of the
glass plate and another two holes are drilled on the left and right side from the
center hole with equal distance of 10cm. Then plastic tubes are inserted in three
holes with different height. The density and thermal conductivity of glass plate are
2500 kg/m3
and 7.44 W/m K.
3. Insulation material:Here, the thermocole is the insulation material. This material is
coated on the outside of cylindrical tray. It is used for minimize the heat
loss. The density and thermal conductivity are 1050kg/m3
and 0.036W/m K.
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ABSTRACT
A Solar pond may be used as a large area collection and forthe storage of solar energy. The salt gradient solar pond is a body saline is which
the concentration increases with depth. The density gradient inhibits thermal
convection with the result that solar radiation reaching the lower regions is trapped
and the temperature is raised. A method to estimate the storage temperature
radiation in the pond is described raise in .The storage temperature is estimated by
a numerical analysis using metrological observation data with a time interval of
one hour.
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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 suns 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 terms salt-gradient solar
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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 deep as a working material for three main functions.But in our sample
analyse project is small size solar pond is taken, small amount salt is used.
The some amount of salt is proceed in the pond it dissolved in bottem layer of the
pond. This salt to store the heat and with stand long time. The heat is tranfer to the
water throght the glass. The first layer to absorbe the
Most people know that fluids such as water and air rise when heated. Solar pond
stop this process when large quanties of salt are dissolved in the hot bottom layer
of the pond , making it too dense to the surface and cool.
Solar pond consist of three main layer. The top layer is cold and has little slat
content. The bottem layer is hot, 70-1000c (160-212
0F) , and very salty. Separating
these two layer is the important gradient zone. Here salt content increases with
depth as shown by the drawing . water in the gradient cant rise because the water
above it has less salt content and is therefore lighter. Similarly, water cant fall
because the water below it has a higher salt content and is heavier. Thus the stable
graient zone can act as a transparent insulator, permitting sunlight to be trapped in
the hot bottom layer, from which useful heat is withdrawn.
Inthis simplified description, no attempt is made to describe the
hydrodynamic phenomena which influence zone and interface stability, salt and
heat transport, and other complex behavior.