CHAPTER – 1
INTRODUCTION SOLAR energy is a very large, inexhaustible source of energy. The power from the sun intercepted by the earth is approximately .! "# $% which is much more larger than the present consumption rate on the earth of all commercial energy sources. Thus, in principle, solar energy could supply all the present and future energy needs of the world on the continuing basis. This ma&es it one of the most promising of the unconventional energy sources. 'n addition to its si(e, solar energy has two other factors in its favour. )nergy is the primary and the most universal &ind of wor& by human beings and nature. $ost people use the word energy for input to their bodies or to the machines and thus thin& about crude fuels and electric power. po wer. The fossil fuels are used to produce thermal power* and according to the prediction they will be exhausted in the near future. Therefore there is a need to use non+conventional and renewable source of energy. And these forms of energy are being used by several countries. These energy are solar, wind, sea, geothermal and bio mass which are available in plenty. Also these energy are cheap and eco friendly too. Solar energy can be used as the maor source of power. 't has the greatest potential among among all the other sources of energy. And also it could give the greatest potential is a small amount of it is used. 't is an energy which would become the main supplier of energy when the other forms of energy get depleted.
-irst unli&e fossil fuels and nuclear power, it is an environmental clean source of energy. Second, it is free and available in adeuate uantities in almost all parts of the world where people live. /owever, there are many problems associated with its use. The main problem is that it is a dilute source of energy. )ven in the hottest regions on earth, the solar radiation flux flux rare rarely ly exce exceed edss &%h &%h0m 0m1 1 and and the the tota totall radi radiat atio ion n over over a day day is best best abou aboutt 2 &%h0 &%h0m1 m1.T .Thes hesee are are low low value valuess from from the the point point of view view of tech technol nologi ogica call util utili( i(at atio ion. n. 3onseuently, large collecting areas are reuired in many applications and this result in excessive costs. A second problem associated with the use of solar energy is that its availability varies widely with time. The variation in availability occurs daily because of the day+night cycle and also 1
seasonally because of the earth4s orbit around the sun. 'n addition, variation occurs at a specific location because of local weather conditions. 3onseuently, the energy collected when the sun is shining must be stored for use during periods when it is not available. The need for storage significantly adds to the cost of the system. Thus, the real challenge in utili(ing solar energy as an energy alternative is to address these challenges. One has to strive for the development of cheaper methods of collection and storage so that the large initial investments reuired at present in most applications are reduced. 5arabolic dish collectors is an insulated weather proofed dish containing about !## pieces of convex mirror .
2
1.1 OVE VER RVIE IEW W 6owadays, most of the world4s energy 7!#89 is produced from fossil fuels. $assive exploitation is leading to the exhaustion of these resources and imposes a real threat to the environment, apparent mainly through global warming and acidification of the water cycle. The distribution of fossil fuels around the world is eually uneven. $iddle )ast possesses morethan half of the &nown oil reserves. This This fact fact leads leads to economi economical cal instab instabili ilitie tiess around around the world which which affect affect the whole whole geopolitical system.The present system as it is cannot be maintained for more than two generations. The impact it has on the environment as well to the humans cannot be disputed. -irstly there is the greenhouse effect. This effect is the capacity of the atmosphere to retain heat. Seen from space,the earth radiates energy at wavelengths characteristic of a body at +!:3. /owever, the average surface is some ;;:3 higher, due to the presence of gases that are relatively transparent to solar radiation but opaue to the infrared radiation given off by the earth. These gases effectively trap the heat between the surface and mid atmosphere. 3arbon dioxide 3O1 is particularly important in this respect. The burning of fossil fuels, coal in particular inevitably produces atmospheric emissions of 3O1. 't should be said here that a doubling of 3O1 concentration 7expected by 1#;<+1#<<9 will cause an average temperature rise of ; to <:3. This euals the rise between the coldest period of the last ice age, !### years ago and the presence moment. Such heating is going to have disastrous conseuences for humanity. $aor parts of polar ice caps will melt and the sea level will increase covering big areas of the earth. $any ecosystems will be destroyed, unable to adapt to the change.
3
-igure .= 5redicted temperature change under several emissions scenarios according to the '533 report Renewable energy sources eliminate the wea&nesses of conventional sources. >ut because of less &nowledge about these sources and high initial cost of the conversion systems limits the use of these resources. -rom the renewable energy resources, solar energy has a huge potential for the fulfillment of today energy needs. The total solar radiation energy falling on earth atmosphere is #? watts . Amount of solar radiations reaches earth is #2 watts, this is ### times more than the world energy need. So if <8 of this energy is utili(ed, this is <# times of world energy demand.
4
1.2 GLOBAL MARKET OVERVIEW Renewable energy supplies @ percent of global final energy consumption, counting traditional biomass, large hydropower, and renewable 7small hydro, modern biomass, wind, solar, geothermal, and bio fuels9. Of this @ percent, traditional biomass, used primarily for coo&ing and heating, accounts for approximately ; percent and is growing slowly or even declining in some regions as biomass is used more efficiently or is replaced by more modern energy forms. /ydropower represents ;.1 percent and is growing modestly but from a large base. Other renewable account for 1.2 percent and are growing very rapidly in developed countries and in some developing countries.
-ig.1.+ )nergy consumption+ 'n context of 'ndia Renewable energy replaces conventional fuels in four distinct mar&ets= power generation, hot water and space heating, transport fuels, and rural 7off+grid9 energy services. This section provides an overview of recent developments in the first three mar&ets. lobal renewable energy capacity grew at rates of #B2# percent annually for many technologies during the five year period from the end of 1##C through 1##@. -or many renewable technologies, such as wind power, growth accelerated in 1##@ relative to the previous four years. $ore wind power capacity was added during 1##@ than any other renewable technology.
rid connected solar
photovoltaic 75D9, however, increased the fastest of all renewable technologies, with a 2#+ percent annual average growth rate for the five+year period. >io fuels also grew rapidly, at a 1#+ percent annual average rate for ethanol and a <+percent annual average for
5
biodiesel 7reflecting its lower production levels9, although growth rates began declining later in the period. Other technologiesEincluding hydropower, biomass power and heat, and geothermal powerEare growing at more ordinary rates of ;B2 percent, ma&ing them comparable with global growth rates for fossil fuels 7;B< percent, although
higher in some developing
countries9. 'n several countries, however, the growth in these other renewable technologies far exceeds the global average.
1.3 SOLAR ENERGY Almost all the renewable energy sources originate entirely from the sun. 5hotovoltaic solar energy conversion is the direct conversion of sunlight into electricity. This can be done by flat plate and concentrator systems. An essential component of these systems is the solar cell, in which the photovoltaic effect the generation of free electrons using the energy of light particles ta&es place. These electrons are used to generate electricity. Solar radiation is available at any location on the surface of the )arth. The maximum irradiance of sunlight on )arth is about ,### watts a suare meter, irrespective of location. 't is common to describe the solar source in terms of insolation the energy available per unit of area and per unit of time 7such as &ilo+watt+hours per suare meter a year9. The ratio of diffuse to total annual insolation can range from # percent for bright sunny areas to 2# percent or more for areas with a moderate climate, such as %estern )urope. The actual ratio largely determines the type of solar energy technology that can be used. The sun4s rays that reach the outer atmosphere are subected to absorption, reflection, and transmission processes through the atmosphere before reaching the earth4s surface. Solar radiation is the world4s most abundant and permanent energy source. The amount of solar energy received by the surface of the earth per minute is greater than the energy utili(ation by the entire population in one year. Solar energy is referred to as renewable and0or sustainable energy because it will be available as long as the sun continues to shine. )stimates for the life of the main stage of the sun are another C B < billion years. The energy from the sunshine, electromagnetic radiation, is referred to as insulation. There are two ways in which solar energy can be converted into electrical energy= . SOLAR T/)R$AL 5LA6T 1. SOLAR 5/OTODOLTA'3 6
1.4 SOLAR CONCENTRATORS 1.4.1 INTRODUCTION: Solar concentrators are the collecting devices, which increases the flux on the absorber surface as compared to the flux existing on the entrance aperture. Optical concentration is achieved by the use of reflecting or the refracting elements to concentrate the incident flux to onto suitable absorber. Fue to the apparent diurnal motion of the sun, the concentrating surface, weather reflecting or refracting will not be in a position to redirect the solar radiation on the absorber throughout the day if both the concentrator surface and absorber are stationary. 'deally the total system consisting the mirror0 lens should follow the sun4s apparent motion so that the absorber always captures the sun4s rays. 'n general, therefore, a solar concentrator consists of . A focussing device
1. A blac&ened metallic absorber with a transparent cover
;. A trac&ing device for the continuous following of the sun
4. Temperature as high as ;### degree centigrade can be achieved with solar concentrators and
the application in both photo thermal and photo voltaic conversion of solar energy.
A solar collector is a device used for collecting solar radiation and transfers the energy to a fluid passing in contact with it. Gtili(ation of solar energy reuires solar collectors. These are general are of two type= •
•
6on+concentrating 3oncentrating type
1.4.2 SOLAR COLLECTOR
1.4.2.1 Non on!n"#$"%n& o''!"o# ($) *'$" +'$"! o''!"o#,
-lat plate collectors are typically used for water or space heating in domestic or commercial building. The operating temperature is typically between C# :3+!# :3. 7
This technology is the most mature of all the solar collector type and is of a generally simple configuration consisting of an absorber plates, tubes welded to the absorber plate, a frame with bac&ing insulation and transparent gla(ing. The absorber plate is sometimes coated with a selective coating with high solar absorbance and low thermal emittance to maximi(e solar gain and minimi(e radiation losses. Trac&ing is not reuired. The tubes welded to the absorber plate either transport the medium to be heated, or a heat transfer fluid which carries the heat to an exchanger where heat is transferred to the intended medium. 'ndirect heating avoids problems li&e scaling and bloc&age of tubes. -lat plate collectors have the advantage of absorbing both direct and diffuse solar radiation. 'n other words the collector will wor& even in cloudy weather where almost all visible light is due to diffuse radiation.
-ig. .;= Typical -lat plate collector.
Table= . 3ollector overview and operating temperature ranges Co''!"o# "-+!
A++#o%/$"! M$%/0/
Co//!n",
*'$" P'$"!
O+!#$"%n& T!/+!#$"0#! ( C) C#+!#
>est
&nown
developed Non E$0$"! Co/+o0n P$#$o'% Con!n"#$"o# E$0$"! T0! CPC
of
and all
most
collector
!#+1#
types. 6on trac&ing 6on trac&ing0
##+1#
adustments. 6on+Trac&ing0 Seasonal
<#+C<#
Adustments 5roven for $% scale solar 8
Seasonal
thermal power. Reuires
P$#$o'% "#o0&5
continuous, accurate +axis C!n"#$' R!!%!#
<##7possibly
trac&ing 1+axis trac&ing
P$#$o'% %,5 o# +o%n" 6o0,
mare9 <##7possibly
1+Axis trac&ing
*#!,n!' '!n,
mare9
1.4.2.2 7G'$8! 6'$"7+'$"! o''!"o#,
la(ed flat+plate collectors are very common and are available as liuid+based and air+ based collectors. These collectors are better suited for moderate temperature applications where the demand temperature is ;#+?#3 and0or for applications that reuire heat during the winter months. The liuid+based collectors are most commonly used for the heating of domestic and commercial hot water, buildings, and indoor swimming pools. The air+ based collectors are used for the heating of buildings, ventilation air and crop+drying.
-ig. .C la(ed flat+plate collector 'n this type of collector a flat absorber efficiently transforms sunlight into heat. To minimi(e heat escaping, the plate is located between a gla(ing 7glass pane or transparent material9 and an insulating panel. The gla(ing is chosen so that a maximum amount of sunlight will pass though it and reach the absorber.
1.4.2.3 7Un&'$8! 6'$"7+'$"! ,o'$# o''!"o#, 9
'n 6orth America ungla(ed flat+plate collectors currently account for the most area installed per year of any solar collector. >ecause they are not insulated, these collectors are best suited for low temperature applications where the demand temperature is below ;#3. There is also a mar&et potential for these collectors for water heating at remote, seasonal locations such as summer camps.
-ig.+.< ungla(ed flat+plate collectors Gngla(ed flat+plate collectors are usually made of blac& plastic that has been stabili(ed to withstand ultraviolet light. Since these collectors have no gla(ing, a larger portion of the sun4s energy is absorbed. 1.4.2.47Un&'$8! +!#6o#$"! +'$"! o''!"o#,
The &ey to this type of collector is an industrial+grade siding0cladding that is perforated with many small holes at a pitch of 1+C cm. Air passes through the holes in the collector before it is drawn into the building to provide preheated fresh ventilation air. )fficiencies are typically high because the collector operates close to the outside air temperature. These systems can be very cost+effective, especially when they replace conventional cladding on the building, because only incremental costs need be compared to the energy savings. The most common application of this collector is for building v entilation air heating.
1.4.2.97B$7+$,, ,o'$# o''!"o#,
Air+based collectors use solar energy to heat air. Their design is simple and they often weigh less than liuid+based collectors because they do not have pressuri(ed piping. Air+ based collectors do not have free(ing or boiling problems. 'n these systems, a large solar absorber is used to
heat the air. The simplest designs are single+pass open
collectors. 3ollectors that are coated with a gla(e can also be used to heat air for space 10
heating. This type of collector may be integrated in the building and combined with thermal mass.
1.4.2.;7A%# $,! ,o'$# o''!"o#, +
The energy collected from air+based solar collectors can be used for ventilation air heating, space heating and crop drying. The most common application in 3anada is for ventilation air heating.
Three types of air+based collectors and their corresponding suitably for three
applications are+
Table= .1 3ollectors and their properties+
Fesigns for the first three collector types are simple. The collectors usually weigh less than liuid+based collectors because they do not have pressuri(ed piping. Another advantage of air+based collectors is that they do not have free(ing or boiling problems. All four of these air+based collectors can be integrated into buildings and form part of a building4s envelope.
1.4.2.<7B$"5 ,o'$# o''!"o#,7
One hundred years ago, water tan&s that were painted blac& were used as simple solar residential water heaters. Today their primary mar&et is for residential water heating in warm countries. Furing winter the tan&s must be protected from free(ing or they must be drained. $odern batch collectors have a gla(ing that is similar to the one used on flat plate collectors and0or a reflector to concentrate the solar energy on the tan& surface. >ecause the storage tan& and the solar absorber act as a single unit, there is no need for other components. 1.4.2.=7 L%>0%7$,! ,o'$# o''!"o#,
11
Liuid+based collectors use sunlight to heat a liuid that is circulating in a Hsolar loopH. The fluid in the solar loop can be water, an antifree(e mixture, thermal oil, etc. The solar loop transfers the thermal energy from the collectors to a thermal storage tan&. The type of collector you need depends on how hot the water must be and the local climate. The most common liuid+based solar collectors are= .la(ed flat+plate 1.Gngla(ed flat+plate ;.Dacuum tube C.>atch collector <.3oncentrating
1.4.2.?7 Con!n"#$"%n& ,o'$# o''!"o#, >y using reflectors to concentrate sunlight on the absorber of a solar collector, the si(e of the
absorber can be dramatically reduced, which reduces heat losses and increases efficiency at high temperatures. Stationary concentrating collectors may be liuid+based, air+based, or even an oven such as a solar coo&er. There are four basic types of concentrating collectors= . 5arabolic trough 1. 5arabolic dish ;. 5ower tower C. Stationary concentrating collectors 1.4.2.1@7 P$#$o'% %,5 ,-,"!/,
A parabolic dish collector is similar in appearance to a large satellite dish, but has mirror+ li&e reflectors and an absorber at the focal point. 't uses a dual axis sun trac&er. A parabolic dish system uses a computer to trac& the sun and concentrate the sunIs rays onto a receiver located at the focal point in front of the dish. 'n some systems, a heat engine, such as a Sterling engine, is lin&ed to the receiver to generate electricity. 5arabolic dish systems can reach ### :3 at the receiver, and achieve the highest efficiencies for converting solar energy to electricity in the small+power capacity range.
12
-igure=+.2 5arabolic dish
-igure= +.?
5arabolic dish collector
with a
mirror+li&e reflectors
and an
absorber at the focal
point
1.4.2.117 P$#$o'% "#o0&5 ,-,"!/
5arabolic troughs are devices that are shaped li&e the letter JuK. The troughs concentrate sunlight onto a receiver tube that is positioned along the focal line of the trough. Sometimes a transparent glass tube envelops the receiver tube to reduce heat loss. 5arabolic troughs often use single+axis or dual+axis trac&ing. 'n rare instances, they may be stationary. Temperatures at the receiver can reach C## :3 and produce steam for generating electricity.
13
-ig=+.! 5arabolic troughs
-ig=+.@ 5arabolic trough system
1.4.2.127 Po!# To!# S-,"!/7
A heliostat uses a field of dual axis sun trac&ers that direct solar energy to a large absorber located on a tower. To date the only application for the heliostat collector is power generation in a system called the power tower. A power tower has a field of large mirrors that follow the sunIs path across the s&y. The mirrors concentrate sunlight onto a receiver on top of a high tower. A computer &eeps the mirrors aligned so the reflected rays of the sun are 14
always aimed at the receiver, where temperatures well above ###:3 can be reached. /igh+ pressure steam is generated to produce electricity.
-ig.+.# 5owertowersystem
1.4.2.137 S"$"%on$#- on!n"#$"%n& ,o'$# o''!"o#,
Stationary concentrating collectors use compound parabolic reflectors and flat reflectors for directing solar energy to an accompanying absorber or aperture through a wide acceptance angle. The wide acceptance angle for these reflectors eliminates the need for a sun trac&er. This class of collector includes parabolic trough flat plate collectors, flat plate collectors with parabolic boosting reflectors, and solar coo&ers. Solar coo&ers are used in the developing countries.
-ig=+. Stationary concentrating solar collectors 1.4.2.147 V$00/ "0! ,o'$# o''!"o#,
Dacuum 7also JevacuatedK9 tube solar collectors are amongst the most efficient and most costly types of solar collectors. These collectors are best suited for moderate temperature applications where the demand temperature is <#+@<3 and0or for very cold climates such as in 3anada4s far north. Fue to their ability to deliver high temperatures efficiently another potential application is for the cooling of buildings by regenerating refrigeration cycles. 15
1.4.2.197 C!n"#$' R!!%!# S-,"!/
A central receiver system consist of a central receiver surrounded by a field heliostats 7independently movable 1+ axis flat mirrors9 that focus onto the receiver see figure 1. Temperature of up to <## 3 can be reached. The receiver will typically contain a molten salt to store the energy as latent heat and comprise bundled tubes with a heat transfer fluid .
-ig=+ .1 A 3entral Receiver solar power plant
() E$0$"! "0! ,o'$# o''!"o#
)vacuated heat pipe tubes 7)/5Ts9 are composed of multiple evacuated glass tubes each containing an absorber plate fused to a heat pipe. The heat from the hot end of the heat pipes is transferred to the transfer fluid 7water or an antifree(e mixEtypically propylene glycol9 of a domestic hot water or hydronic space heating system in a heat exchanger called a HmanifoldH. The manifold is wrapped in insulation and covered by a sheet metal or plastic case to protect it from the elements. The vacuum that surrounds the outside of the tube greatly reduces convection and conduction heat loss to the outside, therefore achieving greater efficiency than flat+plate collectors, especially in colder conditions. This advantage is largely lost in warmer climates, except in those cases where very hot water is desirable, for example commercial process water. The high temperatures that can occur may reuire special system design to prevent overheating.
16
-ig=+.; )vacuated tube solar collector Some evacuated tubes 7glass+metal9 are made with one layer of glass that fuses to the heat pipe at the upper end and encloses the heat pipe and absorber in the vacuum. Others 7glass+ glass9 are made with a double layer of glass fused together at one or both ends with a vacuum between the layers 7li&e a vacuum bottle or flas&9, with the absorber and heat pipe contained at normal atmospheric pressure. lass+glass tubes have a highly reliable vacuum seal, but the two layers of glass reduce the light that reaches the absorber. $oisture may enter the non+evacuated area of the tube and cause absorber corrosion. lass+metal tubes allow more light to reach the absorber, and protect the absorber and heat pipe from corrosion even if they are made from dissimilar materials. The gaps between the tubes may allow for snow to fall through the collector, minimi(ing the loss of production in some snowy conditions, though the lac& of radiated heat from the tubes can also prevent effective shedding of accumulated snow.
Con!+" o6 So'$# A##$-7
17
The solar array consists of C C parallel connected solar panels with the sections of @2 x @2 mm in si(e made on the basis of silicon solar cells. Six single+side panels are mounted on the facets of the sub satellite at the distance of # mm from metallic surface, 1 panels are to be deployed in space. After deploying their axes have ## deg angle with respect to sub satellite axis directed toward Sun. The current of the panel being orthogonal to the Sun direction is about #.1 A at the operation voltage of C Dolt. The maximum total power of the solar array at the nominal solar orientation is ;2 %.
1.9 PHOTOVOLTIC CELL 18
Solar 5D cells are electronic devices that use 5+6 unctions to directly convert sunlight into electrical power. A complex relationship between voltage and current is exhibited by the 5+6 unction in the solar cell. The voltage and current both being a function of the light falling on the cell, there exists a complex relationship between insolation 7sunlight9 and output power. Solar cells capture slow+moving low energy electrons. These effects are saturated and cause a fixed energy loss under bright light condition. /owever, on an overcast day i.e. at lower insolation levels these mechanisms show an increasing percentage of the total power being generated. Too much insolation causes saturation of cells, and the number of free electrons or their mobility decreases greatly. -or an example in case of silicon the holes left by the photoelectrons neutrali(es ta&ing some time, and in this time these absorb a photoelectron from another atom inside the cell. This causes maximum as well as minimum production rates.The cooling of photovoltaic 75D9 cells is a problem of great practical significance. The usable energy produced from solar energy displaces energy produced from fossil fuels, and thereby contributes to reducing global warming. /owever, the high cost of solar cells is an obstacle to expansion of their use. 5D cooling has the potential to reduce the cost of solar energy in three ways. -irst, the electrical efficency of 5D cells decreases with temperature increase. 3ooling can improve the electrical production of standard flat panel 5D modules. Second, cooling ma&es possible the use of concentrating 5D systems. 3ooling &eeps the 5D cells from reaching temperatures at which irreversible damage occurs, even under the irradiance of multiple suns. This ma&es it possible to replace 5D cells with potentially less expensive concentrators. -inally, the heat removed by the 5D cooling system can be used for building heating or cooling, or in industrial applications.
-igure=+.C pv array wor&ing
19
I7V C5$#$"!#%,"%,:
'+D 3haracteristics is a curve between current and voltage. The curve shows a inverse relation. The area under the '+D curve is the maximum power that a panel would produce operating at maximum current and maximum voltage. The area decreases with increase in solar cell voltage due to its increase in temperature. Fue to fluctuations in environmental conditions, temperature change and irradiance level the 'D curve will change and thus maximum power point will also change. Thus the $55T algorithm &eeps on trac&ing the &nee point. The above figure shows two characteristics i.e. Far& and 'rradiated characteristics. %hen the 56 unction is illuminated the characteristics get modified in shape and shift downwards as the photon generated component gets added with the reverse lea&age current. The maximum power point can be obtained by plotting the hyperbola defined by DM'N constant such that it is tangential to the '+D characteristics. The voltage and current corresponding to this point are pea& point voltage and pea& point current. There is one point on the curve that will produce maximum electrical power under incident illumination level. Operating at any other point other then maximum power point will mean that cell will produce maximum thermal power and less electrical power.
-ig=.< 'D+curve of a solar cell both under irradiated and dar& conditions. The yellow area shows the maximum power operating region.
1.; CLASSI*ICATION O* PHOTOVOLTAIC CELL 20
The four general types of silicon photovoltaic c ells are= 7i9 S%n&'!7#-,"$' ,%'%on . 7ii9 Po'-#-,"$''%n! ,%'%on 7also &nown as multicrystal silicon9. 7iii9 R%on ,%'%on . 7iv9 A/o#+5o0, ,%'%on 7abbreviated as HaSi,H also &nown as thin film silicon9.
S%n&'!7#-,"$' ,%'%on
$ost photovoltaic cells are single+crystal types. To ma&e them, silicon is purified, melted, and crystalli(ed into ingots. The ingots are sliced into thin wafers to ma&e individual cells. The cells have a uniform color, usually blue or blac&.
-igure=+.2 $ono+3rystalline Silicon 5D 3ell
Po'-#-,"$''%n! ,%'%on
5olycrystalline cells are manufactured and operate in a similar manner. The difference is that alower cost silicon is used. This usually results in slightly lower efficiency, but polycrystalline cell manufacturers assert that the cost benefits outweigh the efficiency losses. The surface of polycrystalline e cells has a random pattern of crystal borders instead of the solid color of single crystal cells.
21
-igure=+.? 5oly+3rystalline Silicon 5D 3ell
R%on ,%'%on
Ribbon+type photovoltaic cells are made by growing a ribbon from the molten silicon instead of an ingot. These cells operate the same as single and polycrystalline cells. The anti+reflective coating used on most ribbon silicon ce lls gives them a prismatic rainbow appearance.
-igure=+.! Amorphous or thin film silicon The previous three types of silicon used for photovoltaic cells have a distinct crystal structure.Amorphous silicon has no such structure. Amorphous silicon is sometimes abbreviated HaSiHand is also called thin film silicon. Amorphous silicon units are made by depositing very thin layers of vapori(ed silicon in a vacuum onto a support of glass, plastic, or metal. Since they can be made in si(es up to several suare yards, they are made up in
22
long rectangularHstrip cells.H These are connected in series to ma&e up Hmodules.
-igure=+.@ Amorphous or thin film silicon plate collector
23
1.< IMPORTANT TERMS
'n designing the optimal tilt angle and orientation of a fixed solar panel for maximi(ing its energy collection is to acuire the maximum solar radiation availability at the reuired location, a number of studies have been conducted by various researchers to determine the optimum location for solar radiation collection using different empirical models H$n& T%$n P$01@ Orientation of solar collector in space is the main factor influencing the uantity of
absorbed solar radiation energy. 'n the case with optimal angles of a solar collector, we will have the maximum of solar radiant energy. 1.<.17A%# M$,, (/): The ratio of the mass of atmosphere through which beam radiation
passes to the mass it would pass through if the sun were at the (enith. Thus at sea level, mN when the sun is at the (enith, and mN1 for a (enith angle
of 2# . -or (enith angles from #
to ?# at sea level, to a close approximation, 1
mN
79
cos Ѳ
-or higher (enith angles, the effect of the earth4s curvature becomes significant and must be ta&en into account.
-ig=+ .1# The path length in units of Air mass, changes with the enith angle
24
1.<.27 So'$# R$%$"%on7 Solar radiation describes the visible and near+visible 7ultraviolet
and near+infrared9 radiation emitted from the sun. The different regions are described by their wavelength range within the broad band range of #.1# to C.# Pm 7microns9. Terrestrial radiation is a term used to describe infrared radiation emitted from the atmosphere. The following is a list of the components of solar and terrestrial radiation and their approximate wavelength ranges= •
Gltraviolet=#.1#+#.;@Pm
•
Disible=#.;@+#.?!Pm
•
6ear+'nfrared=#.?!+C.##Pm
•
'nfrared= C.## + ##.## Pm
Approximately @@8 of solar, or short+wave, radiation at the earthIs surface is contained in the region from #.; to ;.# Pm while most of terrestrial, or long+wave, radiation is contained in the region from ;.< to <# Pm. outside the earthIs atmosphere, solar radiation has an intensity of approximately ;?# watts0meter 1. On the surface of the earth on a clear day, at noon, the direct beam radiation will be approximately ### watts0meter1 for many locations. $. B!$/ R$%$"%on: The solar radiation received from the sun without having been
scattered by the atmosphere. . D%660,! R$%$"%on: The solar radiation received from the sun after its direction has
been changed by scattering by the atmosphere.
. To"$' So'$# R$%$"%on: The sum of the beam and diffuse solar radiation on a
surface. . I##$%$n!: The rate at which radiant energy is incident on a surface, per unit area of
surface. The symbol is used for solar irradiance. !. In,o'$"%on7 The incident energy per unit area on a surface, found by integration of
irradiance over a specified time, usually an hour or a day. 'nsolation is a term 25
applying specifically to solar energy irradiation. The symbol / is used for insolation for a day. The symbol ' is used for insolation for an hour 7or other period if specified9. 1.<.37 So'$# T%/!: Solar time in minutes is+
Solar timeN standard timeC %here
)
719
is the standard meridian for the local time (one,
is the longitude of the
location in uestion 7in degrees west9 and ) is the euation of time 7in minutes9. ) is calculated using below euation D066%! . A. $n B!/$n W. A. 13: )N#.####?<#.##!2!cosQ+#.#;1#??sinQ+#.#C2
, n is the day of the year and can be obtained using+
-ig=+.1 Solar Time Ds day 1.<.47 Ho0# An&'!( ω):
Angular displacement of the sun east or west of the local meridian due to rotation of the earth on its axis at < per hour. The hour angle is variable within the day, negative for morning, positive for afternoon and (ero at solar noon as shown. 't can be expressed by N<7 U19
7C9
VN solar time+noon time "<
7<9
-ig=+.11 /our angle+solar time relationship
26
%here solar time and noon times are in hours 7noon time is 1=##9 and V is the hour angle in degrees and is the solar time in hours.
-ig=+.1; Dariation of declination angle with months of the year
1.<.97 D!'%n$"%on (F): The angular position of the sun with respect to the euatorial plane at
solar noon. Feclination is in the range +1;.C< WXW1;.C< 76orth is the positive direction.9 and below euation is used to calculate it+ XN
23.45 sin
[
(
360 284 + n 365
)
]
729
-ig=+.1C Angle of Solar Feclination Ds Fay of the Year
27
-ig=+.1< Dariation of Feclination angle with euinox 1.<.;7 L$"%"0!( ): The angular distance from euator plane. Latitude is given in the range
76orth is the positive direction.9 1.<.<7 Lon&%"0! (L): The angular distance from prime meridian. Longitude ranges from #:
to !#:, either east or west.
-ig=+.12 )arth+Sun Angles+ Latitude 79, Feclination Angle 7 9 and /our Angle 7 9
1.<.=7 S'o+! o# T%'"( ): The angle between the plane of the surface in uestion and the
hori(ontal. 't is in the range # Z Q Z
7Q [@# means that the surface has a downward
facing component9.
28
1.<.?7 S0#6$! $8%/0"5 $n&'!( γ): The angle between south and the proection of normal of
the surface on the hori(ontal ground plane. Firection of an angular displacement from south to west of south is positive. 't is in the range +!# Z \ Z!# .
1.<.1@7 An&'! o6 %n%!n! ( ): The angle between the beam radiation on a surface and the
normal to that surface. )uations relating the angle of incidence of beam radiation on a surface to the other angles are+ cos]N sinX sin^ cosQ+sinX cos^ sinQ cos\cosX cos^ cosQ cosV cosX sin^ sinQ cos\ cosVcosX sinQ sin\ sinV
1.<.11 !n%"5 $n&'!(
7?9
) : The angle between normal of the ground plane and the line to the
sun, i.e., the angle of incidence of beam radiation on a hori(ontal surface. )uation for (enith angle+ cos Ѳz =cosф.cosδ cosω+ sinф.sinδ
7!9
29
1.= SOLAR TERMINOLOGY BASICS SOLAR INSOLATION
The sun is a large sphere of very hot gases, the heat being generated by various &inds of fusion reactions. 'ts diameter is .;@_#2 &m. %hile that of earth is .1?_#C &m. The mean distance between the two is .<_#! &m. Although the sun is large, it subtends an angle of only ;1 minutes at earth surface. This is because of its very large distance. Thus the beam radiation received from the sun on earth is almost parallel. The brightness of the sin varies from its centre to its edge. /owever for engineering calculation, it is customary to assume that the brightness all over the disc uniform as viewed from the earth, the radiation coming from the sun appears to be essentially euivalent to that coming from a blac& surface ecause of sun4s distance and the activity vary throughout the year* the rate of arrival of solar radiation varies accordingly. The so called solar constant is thus an average from which the actual values vary up to ; 8 in either direction. This variation is not important however, for most practical purposes. The national aeronautics and space administration4s 76ASA9 standards the value for the solar constant, being expressed in the three most common units, is as follows= +;<; &ilowatts per suare meter
2.< Langley 7calories per suare centimetre9 per hour
[email protected] >tu per suare feet per hour.
30
1.? APPLICATIONS The applications of solar energy are=+ . /eating and cooling of residential buildings.
1. Solar water heating
;. Solar drying of agriculture and animal products
C. Solar distillation
<. Solar coo&er
2. Solar engines for water pumping
?. -ood refrigeration
!. Solar furnace
-resh water is a necessity for the sustenance of life and also the &ey to match prosperity. -resh water sources are rapidly becoming insufficient to meet the needs of increasing population, both for domestic and agricultural uses and for continuously fast developing industries. The problem of getting water in arid and semi arid areas and at some of the coastal areas is acute. Saline or brac&ish is defined as any water with less dissolved salts than in sea water.
31
'n village, it is common for the people travelling long distance to get portable water for drin&ing purpose. Solar energy which is available in abundance and at the site can be used for converting the ground water available which is saline into desalinated water.
Fesalination means conversion of saline water into suitable for human consumption. This separation needs energy. Solar energy is basically radiation energy so that it can be used for water distillation. Fue to limited population and isolation in the conventional schemes of transporting portable water involves high investments were developed. Fistillation of salty or brac&ish water solves to some extent the diversified and innumerable water problems. A number of existing desalination plants use fossil fuels as source of energy. Although few techniues such as multi effect evaporation, multistage flash evaporation, and thin film distillation are employed, the process is energy intensive and the cost is high. /ence, the application of solar powered or solar augmented distils, can replace need for a large proportion of oil or other desalination plants.
32
1.1@ ORIGIN O* PROPOSAL The prices of energy have been increasing exponentially worldwide. 'ndustrial Refrigeration is one of the most energy consuming sector. %hat if a refrigeration system is designed which uses no energy or minimal amount of energy The solution lies in absorption refrigeration system. >y producing an adsorption refrigeration system we are not only cutting down the energy costs but also preserving our environment. This refrigeration system doesn4t use any of the 3-3s so our o(one layer is safe. reenhouse gases and their damaging effects on the atmosphere have received increased attention following the release of scientific data by Gnited 6ations )nvironment 5rogramme and %orld $eteorological Organi(ation that show carbon dioxide to be the main contributor to increased global warming7G6)5, @@9. The domestic refrigerator+free(ers operating on alternative refrigerants such as /-3+;Ca, contribute indirectly to global warming by the amount of carbon dioxide produced by the power plant in generating electricity to operate over a unit over its lifetime. This contribution is nearly ## times greater than the direct contribution of the refrigerant alone. $oreover, approximately 21 million mew units are being manufactured worldwide every year, and hundreds of millions are currently in. use.7G6)5,@@<9 it is anticipated that the production of refrigerator+free(ers will substantially increase in the near future as a result of the increased demand, especially in the developing countries. Therefore, in response to global concerns over greenhouse resorts are being made to produce refrigerator+free(ers with low energy consumption. 'n most of the developing third world, adeuate supplies of drin&ing water and water for irrigation are a scarce commodity. 'n many places in Africa, 'ndia and 3entral and South America, adeuate supplies of water are found only at considerable depth below the surface. These locations generally do not have the infrastructure to provide an electrical grid to pump the water with electricity nor do they have the infrastructure to provide roads to bring in electrical generators or even the fuel for those generators. Therefore without an electrical grid or without generators to generate electricity, isolated areas do not have potable water nor do they have the refrigeration to &eep medicine or foodstuffs from spoiling. )ven in the Gntied States, there are communities such as the Amish communities where electricity is 33
banned. /ere the lac& of cooling capabilities severely limits the production of various products. >ecause of the lac& of cooling, mil& production is limited to rade >. Referred to as advanced adsorption chillers they represent one of the new technology options that are under development. Advanced adsorption cooling technology offers the possibility of chillers with greater 3O5s and reduce cost of the system. The invention can improve refrigerating unit, raise coefficient of performance, reduce energy cost of refrigerating unit and has notably social and economic benefit. 3ompared with the existing compressor refrigeration system, the system reali(es simplified structure, low energy consumption and reduction of discharge and environmental pollution by ha(ardous substance4.
34
1.11 BENE*ITS . 'ndia is among the world leaders in agricultural production however much of our produce goes waste due to absence of proper storage facilities. Refrigeration is thus vitally important for our country. 1. $il& produce is also adversely affected due to lac& of refrigeration. ;. 3ool drin&ing water is unavailable to the people in non electrified villages. C. $edical facilities are also adversely affected due to brea& in the cold chain as the medicines move from the production (one to the rural areas. <. Gsage of 3-3s affect the environment adversely. The current proect can result in development of a system which can be a decisive step in bringing refrigeration to the far off rural areas. 'n urban areas a huge chun& of households consume more refrigeration energy than is reuired due to inefficient usage. This proect also holds promise to reduce if not eliminate this hug e portion of energy consumption pie.
35
1.12 PROBLEM DE*INITION Though absorption refrigeration system is an ideal way to combat energy consumption of refrigeration sector, it suffers some serious faults li&e= 1.Lo COP 'deally spea&ing 3O5 of an absorption refrigeration system is about 1.#. >ut in reality it is less than 7about#.?9.
2.L$#&! S%8! 50&! !%&5" They are much more complex than a normal refrigerator and occupy a huge space. They reuire much larger cooling towers to reect the waste heat owing to their low 3O5s, and thus servicing them is not less than a nightmare.
3. H%&5 o," The absorption refrigeration systems are much more expensive than the vapor+compression refrigeration systems which are uite obvious as their cost of production is high because of complex and large parts. This also ma&es them difficult to service.
36
1.13 OBECTIVE . 3O5 %e aim to improve the 3O5 of the adsorption0absorption refrigerator to ma&e it more attractive for usage. 1. Si(e %e aim to reduce the si(e of the assembly by ma&ing it more compact. ;. %eight The absorption0adsorption refrigeration system is too bul&y. 'ts weight reduction is also one of the aims. 't can be reduced by using polymers. C. 3ost 3ost is the biggest barrier in implementation of Adsorption0absorption refrigeration. %e aim to minimi(e it as far as possible. <. )xtended Gsability Till date absorption refrigeration is limited for industrial purposes. %e aim to ma&e it available for mass rural use as stated above in small capacities by using solar adsorption0absorption.
37
1.14 ADVANTAGES AND DISADVANTAGES O* CONCENTRATING COLLECTORS OVER *LAT PLATE COLLECTORS:
ADVANTAGES
. Reflecting surface reuire less material and are structurally simpler than flat plate collectors. -or a concentrating system the costs per unit area of solar collecting surface is therefore potentially less than that for a flat plate collector. 1. The absorber area of a concentrator is smaller than that of a flat plate collector system for the same solar energy collection and therefore the isolation intensity is greater. ;. >ecause of the area from which heat is lost to the surroundings per unit of the solar collecting area is less than that for a flat plate collector and because the insulation on the absorber is more concentrated, the wor&ing fluid can attain a higher temperature in a concentrating system of the same solar energy collecting surface. C. -ocusing or concentrating systems can be used for electric power generation when not used for cooling or heating purpose. <. >ecause of the higher temperature attainable with concentrating collector system is higher, the amount of heat, which can be stored per unit volume, is larger and conseuently the heat storage costs are less for air conditioning with concentrators system.
DISADVANTAGES
. Out of beam and diffuse solar radiation component, only beam component is collected in case of focussing collectors because diffuse component cannot reflect and thus it is lost.
38
1. 'n some stationary reflecting system it is necessary to have a small absorber to rac& the sun image* in others the reflector may have to adust more than one position of year round operation is desired* in other words costly orienting system have to be used to trac& the sun. ;. Additional reuirement of maintenance particular to retain the uality of reflecting surface against dirt, weather, oxidation etc. C. 6on+uniform fluxes on the absorber where as in flat plate collector is uniform. <. Additional optical losses such as reflectance loss and intercept loss, so they introduce additional factors in energy balances. 2./igh initial cost.
39
CHAPTER 72
2.1 DESIGN AND CONSTRUCTIONAL DETAILS PARABOLIC DISH The number of panels were calculated and the reuired area for the absorber was calculated. Radius=2##mm 3ross sectional area=.C? m1 The depth of parabolic dish is ta&en as 12 centimeters. The focus of parabolic dish was calculated and was found out tobe at ;2 centimeters from the depth of parabolic dish. After this the frame of the parabolic dish was made as per the reuired dimensions of the parabolic dish and the fiber was cut into ! pieces. These pieces were then settled on the frame and the pieces were then fixed on the frame of the parabolic dish. A reuired trac&ing system has then incorporated with the parabolic dish to give the east west movement if the parabolic dish. The trac&ing system was made using simple mechanism.
40
-ig=+ 1. 5arabolic Fish
GENERATOR RECIVER $aterial of generator box is stainless steel. The dimensions we used for ma&ing generator are given below = Length of generator
N 1## mm
%idth of generator
N<# mm
/ight of generator
N <# mm
3apacity of generator
N #.1#M#.
41
-ig=+1.1 Fesign of enerator0 Reciver
-ig=+ 1.; enerator0 Reciver
CONDENSER 3ondenser is made up of mild steel. There are lot of fins provided on the condenser tube. Fimension of condenser is = 42
6o. of tubes
N@
Radius of circular edges
N# mm
Length of 3ircular edge
N 1r N 1M;.2M# N 2;.1 mm
Total no. of circuler edges
N!
Total circular lenth
N 2;.1M! NC!<.2 mm
Straight length of tube
N 1.<
Straight length of tube
N 1.
Total length of /eat exchanger 0 condenser N C!<.2 @; N 2C1.; mm
43
-ig=+ 1.CFesign of condenser
44
-ig=+1.< 3ondenser
CAPILLARY TUBE EPANSION VALVE The material used for ma&ing 3apillary tube is stainless steel. The length of ca pillary tube is mm. And the diameter of 3apillary tube is
mm.
-ig=+1.2 Fesign of 3apillary tube0 )xpansion valve
-ig=+ 1.? 3apillary tube0 )xpansion valve
EVAPORATOR 45
Length of evaporator tube is ;### mm and the diameter of evaporator tube is @.<< mm.
-ig=+ 1.! Fesign of )vaporator
-ig=+1.@ )vaporator 46
ABSORBER The materials used for ma&ing absorber are 5D3 5ipe, mild steel pipe and a 5ump for spraying water on vapours.
-ig=+ 1.# Fesign of absorber
-ig=+1. Absorber 47
2.2
TECHNICAL
DETAILS
AND
INTRODUCTION
TO
THE
TECHNOLOGY ABSORPTION
3omparing the absorption refrigeration cycle with the more familiar vapor compression refrigeration cycle is often an easy way to introduce it. The standard vapor compression refrigeration system is a condenser, evaporator, throttling valve, and a compressor. -igure below is a schematic of the components and flow arrangements for the vapor compression cycle.
48
-igure=1.1 Dapor 3ompression 3ycle 'n the vapor+compression refrigeration cycle, refrigerant enters the evaporator in the form of a cool, low+pressure mixture of liuid and vapor 7C9. /eat is transferred from the relatively warm air or water to the refrigerant, causing the liuid refrigerant to boil. The resulting vapor 79 is then pumped from the evaporator by the compressor, which increases the pressure and temperature of the refrigerant vapor. The hot, high+pressure refrigerant vapor 719 leaving the compressor enters the condenser where heat is transferred to ambient air or water at a lower temperature.
'nside the
condenser, the refrigerant vapor condenses into a liuid. This liuid refrigerant 7;9 then flows to the expansion device, which creates a pressure drop that reduces the pressure of the refrigerant to that of the evaporator. At this low pressure, a small portion of the refrigerant boils 7or flashes9, cooling the remaining liuid
refrigerant to the desired evaporator
temperature. The cool mixture of liuid and vapor refrigerant 7C9 travels to the evaporator to repeat the cycle.
49
-igure= 1.; Dapor Absorption 3ycle Absorption refrigeration systems replace the compressor with a generator and a absorber. Refrigerant enters the evaporator in the form of a cool, low+pressure mixture of liuid and vapor 7C9. /eat is transferred from the relatively warm water to the refrigerant, causing the liuid refrigerant to boil. Gsing an analogy of the vapor compression cycle, the absorber acts li&e the suction side of the compressorEit draws in the refrigerant vapor 79 to mix with the absorbent. The pump acts li&e the compression process itselfEit pushes the mixture of refrigerant and absorbent up to the high+pressure side of the system. The generator acts li&e the discharge of the compressorEit delivers the refrigerant vapor 719 to the rest of the system. The refrigerant vapor 719 leaving the generator enters the condenser, where heat is transferred to water at a lower temperature, causing the refrigerant vapor to condense into a liuid. This liuid refrigerant 7;9 then flows to the expansion device,
which creates a
pressure drop that reduces the pressure of the refrigerant to that of the evaporator. The resulting mixture of liuid and vapor refrigerant 7C9 travels to evaporator to repeat the cycle.
2.3 SIMILARITIES BETWEEN VAPOR COMPRESSION AND VAPOR ABSORPTION CYCLES
50
The basic absorption chiller cycle is similar to the traditional vapor compression chiller cycle in that . >oth cycles circulate refrigerant inside the chiller to transfer heat from one fluid to the other* 1. >oth cycles include a device to increase the pressure of the refrigerant and an expansion device to maintain the internal pressure difference, which is critical to the overall heat transfer process* ;. Refrigerant vapor is condensed at high pressure and temperature, reecting heat to the surroundings C. Refrigerant vapor is vapori(ed at low pressure and temperature, absorbing heat from the chilled water flow
2.4 DI**ERENCES BETWEEN VAPOR COMPRESSION AND VAPOR ABSORPTION CYCLES
51
The basic absorption chiller cycle is different to the vapor compression chiller cycle in that . The absorption systems use heat energy in form of steam, direct fuel firing or waste heat to achieve the refrigerant effect* 1. The absorption cycle use a liuid pump, 6OT a compressor to create the pressure rise between evaporator and condenser. 5umping a liuid is much
easier and cheaper than
compressing a gas, so the system ta&es less wor& input. /owever, there is a large heat input in the generator. So, the system basically replaces the wor& input of a vapor+compression cycle with a heat input* ;. The absorption cycle uses different refrigerants that have no associated environment ha(ard, o(one depletion or global warming potential 7for example lithium bromide absorption system use distilled water as the
refrigerant9. The vapor compression
refrigeration cycle generally uses a halocarbon 7such as /3-3+1;, /3-3+11, /-3+;Ca, etc9 as the refrigerant* C. 3ompared to compression chillers, absorption systems contain very few moving parts, offer less noise and vibration, are compact for large capacities
and reuire little
maintenance* <. 3ompared to compression chillers, the performance of absorption systems is not sensitive to load variations and does not depend very much on evaporator superheat* 2. 3ompared with mechanical chillers, absorption systems have a low
coefficient of
performance 73O5 N chiller load0heat input9. /oweve r, absorption chillers can substantially reduce operating costs because they are powered by low+grade waste heat. The 3O5 of absorption chiller is 6OT sensitive to load variations and does not reduce significantly at part loads. -rom the standpoint of thermodynamics, the vapor compression chiller is a heat pump, using mechanical energy and wor&, to move heat from a low to a high temperature. An absorption chiller is the euivalent of a heat engine B absorbing heat at a high temperature, reecting heat at a lower temperature, producing wor& B driving a heat pump.
2.9 APPLICATIONS O* ABSORPTION SYSTEMS 52
The main advantage of absorption chillers is their ability to utili(e waste heat streams that would be otherwise discarded. 'n terms of energy performance, motor+driven compression chillers will beat absorption chillers every time. Still there are
vapor specific
applications where absorption chillers have a substantial advantage over motor+driven vapor compression chillers. Some of those applications include= . -or facilities that use lot of thermal energy for their processes, a large chun& of heat is usually discarded to the surrounding as waste. This waste heat can be converted to useful refrigeration by using a DA$. 1. -or facilities that have a simultaneous need for heat and power 7cogeneration system9, absorption chillers can utili(e the thermal energy to produce chilled water. ;. -or facilities that have high electrical demand charges. Absorption chillers minimi(e or flatten the sharp spi&es in a building4s electric load profile can be used as part of a pea& shaving strategy. C. -or facilities where the electrical supply is not robust, expensive, unreliable,
or
unavailable, it is easier to achieve heat input with a flame than with electricity. Absorption chillers uses very little electricity compared to an electric motor driven compression cycle chiller. <. -or facilities, where the cost of electricity verses fuel oil0gas tips the scale in favor of fuel0gas. Darious studies indicate that the absorption chillers provide economic benefit in most geographical areas, due to the differential in the cost between gas and electric energy. 2. -or facilities wanting to use a Jnatural refrigerant and aspiring for L))F certification 7Leadership in )nergy and )nvironmental Fesign9 absorption chillers are a good choice. Absorption chillers do not use 3-3s or /3-3s + the compounds &nown for causing O(one depletion. ?. -or facilities implementing clean development mechanism 73F$9 and accumulating carbon credits, the absorption use coupled to waste heat recovery and cogeneration system help reduce problems related to greenhouse effect from 3O1 emission.
53
Dapor absorption absorption system allows use of variable heat sources= directly directly using a gas burner, burner, recovering waste heat in the form of hot water or low+pressure low+pressure steam, or boiler+generated hot water or steam.
2.; THE BASIC PRINCIPLE O* ABSORPTION COOLING %ater %a ter boils and evaporates at 11 :- ## :3 at standard atmospheric pressure 7C.?psia #.;& #.;&5a 5a9. 9. %hen the pressure pressure is reduced reduced,, water water boils at a lower lower
temper temperatur ature. e. The
follow following ing table table gives gives the total total pressure pressure in inches inches of mercury mercury and
the corresp correspond onding ing
approximate water boiling temperature at different pressures= Table=+ Table=+ %ater %ater boiling temperature at different pressures= A,o'0"! +#!,,0#!
W$"!# o%'%n& +o%n" (J*)
?2# mm+/g 7 atm9
11:
?2 mm+/g 7#. atm9
<:
1<.2 mm+/g 7#.;C atm9
!#:
?.2 mm+/g 7#.# atm9
C<:
2.< mm+/g
C#:
3onsider 3onsider a closed vessel placed placed under a vacuum of say, 2.< mm /g 7refer 7refer to the figure figure below9. Assume the closed vessel contains a high uality absorbent material such as dry silica silica gel, and a heat transfer transfer coiled coiled tube through which which warm water is circulated. circulated. %hen %hen water is sprayed on the outer wall of o f the heat transfer tube= . 't gets boiled at low temperature C#:- 7C:39 7C:39 under vacuum, and in doing so, absorbs heat from from the runnin running g water water in the heat transfe transferr tube. tube. 7The sprayed sprayed water water is also also called called the refrigerant9. 1. The running water in the heat transfer transfer tube is optimally cooled euivalent euivalent to the heat of evaporation.
54
-igure= 1.C Dapor Dapor Absorption System with Silica el Absorbent
The vapors produced, as a result result of evaporation, will immediately immediately be absorbed by the silica gel. >ut when the silica gel reaches the limit of its absorbing capacity, the process continuity cannot cannot be maintai maintained ned.. To ensure ensure a continu continuous ous process, process, some means means of conver convertin ting g the absorbent to its original concentration is necessary. 'n comm commerc ercia iall prac practi tice ce,, sili silica ca gel is repl replace aced d with with an aue aueous ous abso absorb rbent ent solu soluti tion. on. 3ontinuing with the same explanation, as the aueous absorbent solution absorbs absorbs refrigerant vapors* vapors* it becomes becomes dilute diluted d and has less ability ability to absorb absorb any furthe furtherr water water
vapor vapor.. To
complete complete the cycle and sustain sustain operation, operation, the dilute solution solution is pumped to higher pressure pressure where where with applicat application ion of heat, heat, the water water vapor vapor is driven driven off and the re+conc re+concent entrat rated ed absorb absorbent ent is recycl recycled ed bac& to the absorb absorber er vessel. vessel. The released released
refri refrigera gerant nt vapor is
condensed condensed in a separate separate vessel and returned returned for evaporation. evaporation. The simplified simplified diagram diagram here illustrates the overall flow path.
55
-igure=1.< Dapor Dapor Absorption System with Aueous Absorbent
$ost commercial commercial absorptio absorption n chillers use pure water as refrigeran refrigerantt and lithium bromide bromide 7Li>r9 7Li>r9 as absorbent absorbent salt. Another Another common common refrigerantBabso refrigerantBabsorbent rbent pair is ammonia ammonia as the refrigerant and water as the absorbent. There are other refrigerantB absorbent combinations* but in this course will focus on lithium bromide DA DA$.
2.< HOW ABSORPTION MACHINE WORKS Absorption Absorption system system employs employs heat and a concentrated concentrated salt solution solution 7lithium 7lithium bromide9 to produce chilled water. 'n its simplest design the absorption machine consists of C basic components= . enerator 1. 3ondenser ;. )vaporator C. Absorber
56
-igure=+1.2 Dapour Absorption 3ycle ust li&e the vapor+compression refrigeration cycle, the absorption machine operates under two pressures B one corresponding to the condenser+generator 7high pressure refrigerant separation side9 and the other corresponding to evaporator+absorber 7low
pressure
absorption process in vacuum9. -or air+conditioning applications, the evaporator+absorber is at a pressure of 2.< mm/g and temperature of about C#-. The pressure on the high+pressure side of the system 7condenser9 is approximately ten times greater than that on the low+pressure side to allow the refrigerant to reect heat to water at normally available temperatures. Typically the condensation of water in the condenser+ generator ta&es place at a pressure of ?< mm/g and temperature of about ;-.
2.= *UNCTION O* COMPONENTS G!n!#$"o#: The purpose of the generator is to deliver the refrigerant vapor to the rest of the
system. 't accomplishes this by separating the water 7refrigerant9 from the lithium bromide+ and+water solution. 'n the generator, a high+temperature energy source, typically steam or hot water, flows through tubes that are immersed in a dilute solution of refrigerant and 57
absorbent. The solution absorbs heat from the warmer
steam or water, causing the
refrigerant to boil 7vapori(e9 and separate from the absorbent solution. As the refrigerant is boiled away, the absorbent solution absorbent solution returns to the
becomes more concentrated. The concentrated
absorber and the refrigerant vapor migrates to the
condenser.
Con!n,!#: The purpose of condenser is to condense the refrigerant vapors. 'nside the
condenser, cooling water flows through tubes and the hot refrigerant vapor fills the surrounding space. As heat transfers from the refrigerant vapor to the water, refrigerant condenses on the tube surfaces. The condensed liuid refrigerant collects in the bottom of the condenser before traveling to the expansion device. The cooling
water system is
typically connected to a cooling tower. enerally, the generator and condenser are contained inside of the same shell.
E+$n,%on D!%!: -rom the condenser, the liuid refrigerant flows through an expansion
device into the evaporator. The expansion device is used to maintain the pressure difference between the high+pressure 7condenser9 and low+pressure 7evaporator9 sides of the refrigeration system by creating a liuid seal that separates the high+pressure and
low
pressure sides of the cycle. As the high+pressure liuid refrigerant flows through
the
expansion device, it causes a pressure drop that reduces the refrigerant pressure to that of the evaporator. This pressure reduction causes a small portion of the liuid refrigerant to boil off, cooling the remaining refrigerant to the desired evaporator temperature. The cooled mixture of liuid and vapor refrigerant then flows into the evaporator.
E$+o#$"o#:
The purpose of evaporator is to cool the circulating water. The evaporator contains a bundle of tubes that carry the system water to be cooled0chilled. /igh pressure liuid condensate 7refrigerant9 is throttled down to the evaporator pressure 7typically around 2.< mm /g absolute9. 58
At this low pressure, the refrigerant absorbs heat from the circulating water and evaporates. The refrigerant vapors thus formed tend to increase the pressure in the vessel. This will in turn increase the boiling temperature and the desired cooling effect will not be obtained. So, it is necessary to remove the refrigerant vapors from the vessel into the lower pressure absorber. 5hysically, the evaporator and absorber allowing refrigerant vapors generated in the
are contained inside the same shell,
evaporator to migrate continuously to the
absorber. A,o#!#:
'nside the absorber, the refrigerant vapor is absorbed by the lithium bromide solution. As the refrigerant vapor is absorbed, it condenses from a vapor to a liuid, releasing the heat it acuired in the evaporator. The absorption process creates a lower pressure within the absorber. This lower pressure, along with the absorbent4s affinity for water, induces a continuous flow of refrigerant vapor from the evaporator. 'n addition, the absorption process condenses the refrigerant vapors and releases the heat removed from the evaporator by the refrigerant. The heat released from the condensation of refrigerant vapors and their absorption in the solution is removed to the cooling water that is circulated through
the absorber tube bundle.
As the
concentrated solution absorbs more and more refrigerant* its absorption ability decreases. The wea& absorbent solution is then pumped to the generator where heat is used to drive off the refrigerant. The hot refrigerant vapors created in the generator migrate to the condenser. The cooling tower water circulating through the condenser turns the refrigerant vapors to a liuid state and pic&s up the heat of condensation, which it reects to the cooling tower. The liuid refrigerant returns to the evaporator and completes the cycle.
59
2.? LIST O* COMPONENTS USED . $irrors for reflecting the rays of sun. 1. $ild steel stands for supporting the parabolic dish and other components. ;. 5ressure pipe for oining the components. C. 3'T' -'_ adhesive is used for fixing the mirrors with the fiber sheet. <. -iber sheet is used for covering the area in the parabolic dish. 2. -our liter steel tan& as a receiver on which five sides are covered with glass wool, which is capable for wor&ing for 1##o3. ?. 5D3 pipe. !. $ild steel tube. @. Stainless steel tube. #. 5ump for spraying water.
60
2.1@ METHODOLOGY AND OBSERVATION STANDARD OPERATION PROCEDURE
. 1. ;. C. <.
Set up the model in proper place of available sunlight source. Set the focus of the parabolic dish. 3are should be ta&en to ensure if there is any lea&age. 3hange the direction from east to west of parabolic dish in every hour. Remove the dust from the reflector.
PER*ORMANCE ANALYSIS OBSERVATION
A detailed investigation using different instruments had been carried out to access the performance of the collector. Temperature sensor was used to measure the temperature of refrigerant in the inlet and outlet of different components.
61
2.11 OBSERVATION TABLE:
62
CHAPTER73 3.1 ECONOMIC ANALYSIS 5arabolic dish )vaporator enerator 3apillary 3ondenser Absorber
C### <## <## ;## ;## ;## =?@@
To"$'
63
3.2 MERITS AND DEMERITS MERITS . 1. ;. C. <. 2. ?.
maintenance cost is low. Long life. 6oise less operation. 5ollution free. Relatively good efficiency as compared to flat plate collectors. /igh temperature can be obtained by using parabolic dish. Low initial cost.
DEMERITS . %e need Dery small amount of electricity for running pump. 1. 't wor&s only in sunny days. ;. 3onstant efficiency cannot be obtained 7varies with time or atmospheric temperature9.
3.3 SCOPE *OR *UTURE WORK
64
The following wor& may be carried out to increase the performance of the refrigeration system= . -or increasing the concentration ratio, provide the suitable trac&ing system euipped with sensors. 1. The mirrors used in the parabolic dish should be casted in 2 pieces for the entire cross+section area of parabolic dish.
CHAPTER74 65
CONCLUSION The future of solar refrigeration and air conditioning seems to be a very good proposition and no doubt will find its place in future industrial applications. The maor limiting factor at present is the shape of energy so as to ma&e it available whenever it is reuired, for example at nights and extended cloudy days when we cannot attain a high enough temperature. 'n the case of air conditioning and refrigeration, storage can either be done in the form of heat or as the final product 7cold water or ice9. The latter is a much easier form of storage but it is rather bul&y, for this reason there has been ongoing research in the area of storage in various forms, trying to ma&e use of phase change materials, eutectics, oils, etc., which has the potential of storing large uantities of energy within a small space and over a longer period of time than water. %ith the achievements already made in this field, the technology will no doubt be available for large scale application in the near future. 3oupled with a more elaborate design of the refrigeration system that we hate designed we could go far way in supplementing solar energy for the conventional energy used for these pricesses today. The optical efficiency can be improved by &eeping the reflector clean and polished. The oints of all the components with each other should be of good because lea&age of ammonia is not good for human health.
RE*RENCES 66
. 3ritoph, R.)., Refrigeration in developing countries + the renewable options, proc.st %orld Renewable )nergy 3ongress, Reading, G`, @@#. 1. /arvey, A.>., Study of an intermittent regenerative cycle for solar cooling, 5h.F. thesis, Gniversity of %arwic&, G`, @@#. ;. 3ritoph, R.)., An ammonia carbon solar refrigerator for vaccine cooling, proc.;rd %orld Renewable )nergy 3ongress, Reading, G`, @@C. C. %/O report )5'0 33'S0!<.C, Solar powered refrigerator for vaccine storage and icepac& free(ing, Status summary une @!24, @!2. <. uilleminot, .. et al, Solid adsorption solar refrigeration system= A condenser as part of a solar reactor4, 'S)S Solar %orld 3ongress, /amburg, @!2., pub. 5ergammon 2. >ansal, 6.`., >lumenberg, ., `avasch, /.., Roettinger, T., 5erformance testing and evaluation of solid absorption solar cooling unit4, Solar Energy, 21, 1?+C#, @@?. ?. )xell, R./.>., `ornsa&oo, S., Oeapipatana&ul, S., A village si(e solar refrigerator, Asian 'nstitute of Technology Report 6o. ?;, >ang&o&, Thailand, @!?. !. Saunier, .Y., Reddy, T.a., $ulti+fuel ice+ma&ing machine4, Asian 'nstitute of Technology Report 6o. @#, >ang&o&, Thailand, @!2. @. 'mam Osman Ahmed, Stoetes, ).%., `er&di&, 3., Stol&, A.L., )xperience with a ;&% 0 <# m1 solar driven absorption refrigerator in the Sudan4, 'S)S Solar %orld 3ongress, /amburg, @!2. pub. 5ergammon. #. Gpal, A./., Fevelopment of a solar+energy operated absorption refrigerator for vaccine storage in 5apua 6ew uinea4, 'S)S Solar %orld 3ongress, /amburg, @!2., pub. 5ergammon. .Turner,
L.,
'mprovement
pumping0refrigeration
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of
activated
'nvestigation
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porosity
and
adsorption heat0mass
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characteristics, 5h.F. thesis, Gniversity of %arwic& 7@@19. 1. Favies, .6.L., 'nternal report, Gniversity of %arwic&, @@2. ;. /enning, /.$., /berle, A.., >uilding 3limati(ation with solar+assisted open cycle solid sorption cooling systems + design rules and operation strategies4, Solar research meeting, $unich, @@<. C. /agberg, /., Refrigerant and compressor free air conditioning system for commercial buildings4, 3'>S) national conference, G`, @@<. <. Solar assisted desiccant air conditioning4, 3AFF)T Renewable )nergy 6ewsletter, C0@<, 6ovember @@<. 67
