... about the conventional power generation
sources. Most of that stems from the fossil
fuels essentially solar energy concentrated
and stood over millions of years. Now, it
is available to mankind, but as we have already
seen that that would be exhausted in a very
short span of time. For example, 80% of the
petroleum resource will be exhausted in a
matter of 60 years only. So, this way we can
see that the fossil fuels are close to exhaustion
and we need to turn to the non-renewable,
the renewable sources. Renewable means the
ones that are, that do not exhaust like the
solar energy, like the tidal energy, like
the geothermal energy.
I mean if you, if you talk in terms of the
absolute meaning, obviously solar energy is
exhaustible, but in a matter of something
like another 10 billion years. So, it is obviously
too long in human time scale. Similarly, geothermal
energy is also a finite resource, you might
argue, but it is also finite in a sense of
mathematical finiteness, but for comparison
with the human time scale that is obviously
infinite. So, these are the renewable sources
of energy. So, you have the non-renewable
sources of energy, the fossil fuels and the
renewable sources of energy.
So, now we will take the renewable sources
of energy.
So, renewable sources, so
what are the renewable sources? Obviously,
the first thing that comes to one's mind
is the solar energy. Solar energy can be used
both as heat, also as a direct conversion
to electrical energy through photovoltaic
means and we will treat both,
so, solar thermal and solar photovoltaic.
Then we have, what other source of energy?
Tidal energy; then you have geothermal energy.
Can you imagine anything more? The thermal
energy as observed in the vast expanse of
the oceans can also be used. That is ocean
thermal energy conversion. That is of course
solar energy but nevertheless available in
a specific form. What else? Important; wind
energy. What do you have? What more do you
have? Can you think of anything else? Well,
there are also energy in the waves.
Right now, I cannot think of anything more,
so let us take these as our things that we
will have to treat in the next half of the
course. So, solar energy that is what we start
with. Solar thermal, in fact and we will go
to solar photovoltaic, then we will treat
tidal energy, then we will treat wind energy,
then we will go to the others, because these
are solar energy. Both thermal form as well
as photovoltaic, are now commercially used,
viable and economically competitive. Wind
energy is so commercially used, viable, economically
competitive. Right now, India has something
like 3000 megawatts of wind power capacity.
So, it is already big. Tidal energy in India
does have few good spots where tidal energy
can be used. So, these are the things that
we should concentrate on and then geothermal,
wave, ocean thermal, we will treat, but somewhat
cursorily.
So, now we come to solar thermal energy. Well,
solar energy is actually an admixture of various
frequencies, electromagnetic waves of various
frequencies. A portion of that is visible
in the visible range and you also have the
infrared radiation, which is directly heat.
You also have the ultraviolet radiation. So,
all that put together is the solar energy
that is available and there are as I told
you two ways of utilizing that, either converting
the whole energy into heat or using a part
of that directly to produce electrical energy.
So, we will first consider the ways of converting
into heat.
Now, suppose you have got a plate on which
the solar energy falls. Obviously, as per
common experience the plate will be heated
up. But then, that is you know very primitive
and intelligent people should always think
of how to maximize this. So, how to maximize
the conversion of all those frequencies which
are not really heat energy, heat waves, heat
is only the infrared part of it, into heat.
How to convert and how to make that conversion
most efficient? Can you tell?
Well, what does the conversion depend on?
Suppose it is a white plate and it is a black
plate, which one will convert more? Obviously,
the black plate, right. What is the difference?
It is that the white plate reflects most of
the visible range, while the black plate will
absorb all the visible range. So, since much
of the energy is in the visible range, so
you get a larger amount of energy absorptions.
So, first thing is that it should be black.
Second, what is the material, what should
be the material which is using, you use as
the absorbing plate?
Now, it could be wood, blacken it, still it
is black; it could be iron, steel. So, what
should be the criterion of choice? Wood is
a, is wood good for this, this purpose? No,
why not? It is non-conductive, it is a bad
conductor and ultimately this heat that is
absorbed has to be given out to something.
So, it has to be good conductor of heat. Naturally,
intuition tells you that it should be metal.
What metal? Which metal is a good conductor
of heat?
Aluminium, silver.
Silver, my goodness; you make, people will
steal. People will take the whole solar collector
and run away if you make them in silver. So,
what kind of, we make them with, the next,
the next, next good one?
Copper.
Copper, yes; so, copper is the next good one,
so you should use copper. So, normally these
are made of copper then. So, you have to have
a blackened copper plate as the absorber and
there has to be something to take away the
heat, something to be heated. Now, what would
you like to heat? Water, air? So, then naturally
there are two types of collectors really,
the air heaters and the water heaters. Now,
what do you need the air heaters for? Normally,
in the kind of industrial plants where you
need drying, there air heaters are very useful,
because you can heat up the air and use that
for the drying purpose. There is large number
of industrial process that needs drying and
also heat. The hot water is needed both in
domestic situations as well as industrial
situations, so there are two.
Let us first consider the water heaters. So,
naturally water has to be in contact with
this plate. Someway it has to take away the
heat from the plate. How? What is the nicest
way of doing that? How to put the water? Will
you just pour a bucket of water on it? No,
obviously not. So, there has to be some, some
nice way, engineering wise nice way to take
away the heat.
A nice way is imagine that if the plate is,
let me put the plate as a two dimensional
structure, so that you can feel what I am
talking about and then if you have tubes running
through like so, that are welded to the copper
plate, then the heat is absorbed in the copper
plate and that is carried. For example, if
heat is absorbed here, it will carry it to
the tubes and that will go into the tube.
Naturally, the tubes also have to be a conducting
substance, namely copper. So, the copper tubes
are welded to the metal plate and that makes
a good heat contact and you have a reasonably
good absorber as well as a reasonably good
collector of the heat.
There has to be some means of having the water
flow through these tubes. You can do it in
two ways, either you force water through the
tubes by means of some kind of a pump or allow
the water to flow by natural circulation.
In order to have natural circulation, this
has to be inclined, bent, right. So, the whole
thing will have to be bent, inclined and there
will be one kind, one, here in this side there
will be, imagine that it will ultimately be
inclined. In this side there would be a tube
going that is connected to all these and up
there will be another, so that the heat, the
water flows from the bottom to the top as
a continuous process. I will draw the total
picture later.
So, we have understood that we can make a
good absorber as well as a good collector
and the water can circulate. It can be stored,
the hot water can be stored. It can be used,
whatever, but that is not the end of the story.
The heated plate will also radiate. It will
also be in contact with air. That means it
will lose heat in the form of both radiation
as well convection, conduction, everything.
So, the next engineering decision is how to
arrest the heat, not allow it to go to any
place other than the water to which I want
it to go? So, how do you do that?
First, we need to have something that does
not allow energy to be radiated out. Now,
it so happens that there are some substances
that allow the electromagnetic radiations
other than the infrared to go through, but
does not allow infrared to go through. Can
you name one of the substances? Glass, simple
glass; glass has the property of not allowing
the infrared to go through, but the other
radiations are through, because that is transparent.
It is not transparent to infrared, which means
that if you put a plate of glass above it,
true, it will arrest the component of electromagnetic
radiation coming in that is in the infrared
range it will not allow it go through, but
that is a small component of solar energy.
Most of it is in higher frequencies, either
visible or infrared is or ultraviolet is more
or less arrested by the ozone layer, but nevertheless
it does comes through a bit, but most of the
energy is in the visible range, so that is
allowed through. But, when this fellow is
heated up and it radiates back, because the
temperature is not as high as that of the
sun, so it radiates almost the whole of the
energy in infrared and that is, that is arrested.
So, all you need to do is to put a glass cover
on it that will not allow the heat that is
escaping from it. In the form of radiation,
it will not allow it to go out, it will be
arrested.
So, then you can easily imagine that the design
would be something like this 
and there would be a glass cover. Can you
see? This is the side stuff and on top of
it, this surface is a glass cover, as a result
of which, let me, let me draw it like this.
Here you have the metal plate and here is
the glass cover. The amount of energy that
comes through in form of the visible radiation
goes through. The amount of radiation that
is in the form of the infrared is reflected
out, but this amount is very small. What this
fellow reflects, this fellow emits out, is
almost entirely IR, so that is reflected back.
As a result, the heat is trapped in this part,
heat is trapped in this part.
Now, let us come to the question of conduction
and convection. Air is a very bad conductor.
So, we are not really all that worried about
the conduction part. But convection, obviously
the amount of air that will come in contact
with the bottom plate that will heat up, that
will go up. So, a convection current will
set, but then that does not, if this top plate
were not there, that would be going out. But
here, because of the top plate, because the
whole thing is encased, the circulation remains
confined within this region, the small part
between the plate and the top cover and naturally
the heat is again arrested. Now, this particular
phenomenon where you allow the visible range,
visible radiation to come in and do not allow
the infrared radiation to go out, this is
called the greenhouse effect. So, this is
called the ...
Probably we have talked about the greenhouse
effect while talking about the environment,
right, did I? No, I did not, oh. The same
effect also happens in the atmosphere, the
same effect also happens in the atmosphere.
So, the solar energy from the sun is, again
mostly higher frequency energy, energy in
the range of the visible as well as ultraviolet
that comes into the Earth, that heats up the
surface of Earth and that what goes out is
infrared, because it is heated up and the
frequency at which the emission is done depends
on the temperature of the emitting surface
and in case of the Earth that is moderately
heated, so that will, that will radiate out
the energy in form of the infrared.
Now, in order for the temperature of the,
of the planet to remain constant, the incoming
energy must be equal to the outgoing energy.
Now, if the atmosphere of the Earth contains
many of the gases that does not allow infrared
to go out, then the heat will be trapped.
That is exactly what is happening in the atmosphere
today. That is what the problem of global
warming is. But, at the same time if you look
at the planet Venus, you know what actually
happens? In Venus, the atmosphere is about
98% carbon dioxide. That is a greenhouse gas.
As a result, all the energy that comes in
cannot go out, the energy is trapped. As a
result, the planet, even though the distance
from the Earth is not very large, it is almost
like a hell .....
The temperature of the surface is something
that will melt even lead. That has happened
not because of its proximity to the sun. It
has happened because of the greenhouse effect.
So, the Earth is also sort of moving in that
direction, because you can see the climatic
changes that is taking place. So, that is
not the topic of discussion today. We are
discussing this particular effect, you know
the effect, which we productively use in case
of these collectors.
Now, we can, we have now come to see a particular
kind of design of the collector. The plate
is flat and therefore it is called flat plate
collector. We are not concentrating the solar
energy, so it is called a non-concentrating
collector. There are also concentrating collectors,
I will come to that later, but this is the
most economic design that we have, which is
you must have seen on the top of hotels and
other places. I do not know whether these
are in your hostels, because when I was in
a, I was a student my hostel had one. So,
people, people say that it gives hot water
in summer and cold water in winter, but nevertheless,
it did give reasonably good warm water in
the winter season.
So, how will the whole thing look like? It
will look like, firstly it has to be inclined;
it has to be inclined , so let me draw it
this way.
So, through the, this top plate is the glass
and the bottom plate is the absorber plate,
right. Absorber plate will have the, let me
draw it in another color, the tubes of copper
going through. There would be many more of
course, but I am drawing for the sake of simplicity
only four. Now, these will be attached to
a cylinder in this part and there also there
would be another cylindrical storage device
and then there will be, if you want, a storage
tank. Here is a storage tank which will have
to be on top of a, some kind of a, it has
to be elevated. From here water will have
to be put in like this and from here, again
this is full of water, that goes out, so that
there will be a circulation of water like
this, like this. So, here is the water storage
tank.
Now, you might ask what should be the inclination,
how much is inclined? If it is inclined, then
it can be facing North, South, East, West,
any other angle. So, what should be direction
that it faces? South; why because, we are
in the northern hemisphere, right. If you
have a facility of being able to turn it all
the time, then it is not convenient to make
it face South, because then in the morning
it should be facing East, in the evening it
should be facing West, so on and so forth.
But, such a device, it is rather difficult
to do that; I mean somebody has to sit there
and go on turning it all the time, know that
is not very convenient. It is convenient to
make it static. If it is static, then of course,
it will be convenient for it to face South,
because, why? Because, we are in the northern
hemisphere, most of the time the average sun's
inclination is towards the south, so you have
the inclination towards the south. But, that
inclination of course depends on the, depends
on what?
In the, see, the sun's motion is like this
from east to west only in the summer; in the
winter, it is like so. At an angle it moves
and this angle goes on increasing as we go
into the winter. Again, as we go back to the
summer, it becomes more or less like this,
right. So, if you want to make it look at
the sun, then obviously this angle has to
be changed; depends on the season. So, supposing
you have a facility of changing the inclination
once a month, then obviously you will make
it more or less horizontal during the summer
months and in the winter months it should
be facing rather much to the South. That might
not be very convenient. Why because, during
the summer month, it will be almost horizontal.
How does the natural circulation take place
then? It will not happen, right.
So, normally what is done is it is made static.
If it is static, then obviously you have to
make it inclined at an angle where it will,
it will absorb the maximum amount of energy
over the whole year, even though the sun's
average angle is going up and down. Then,
what should be the angle? What does your school
day geography lessons tell you? Nahi bhai;
it will be exactly the angle of the latitude,
it will be exactly the angle of the latitude,
latitude angle. So, for us it is somewhere
22 degrees. So, if it is in Kharagpur, it
has to be inclined at 22 degrees; that is
it.
So this angle, so that should be latitude
angle if it is static. If it is moving, if
you have the facility of allowing, allowing
the angle to change, then obviously you can
absorb more energy, if you allow it to change.
But then, you will have to have some kind
of a forced circulation mechanism during the
summer months, otherwise it will not work.
There are situations in which you do not need
this hot water in the summer. For example,
in the hotels or the domestic appliances,
where you will need the hot water for taking
bath or cleaning purposes that is mainly needed
during the winter months. So, in those times
you can make the device inclined at an angle
that absorbs the maximum amount of energy,
clear. Any question on this flat plate collector?
Sir, why 22 degrees?
Why 22 degrees? Ah, I have to go back to the
school day geography.
...
Hanh; it is to be at an, at the latitude angle.
Sometimes it is, sun, suppose it is inclined
at that angle, sometimes the sun goes like
this, during the summer; during the winter
it goes further down. So, there is an average
angle. Now, that angle is the latitude angle.
But why it is the latitude angle, that proof
you must have come through in school days.
That is why I did not want to do it all over
again. So, you have the idea about the solar
flat plate collector. Let me tell you that
the solar flat plate collector is commercially
produced in India. There are many companies
that produce these things.
For the purpose of domestic water heating
this is very good. You avoid using the electrical
current during the winter months when you
really need the hot water. Especially in the,
in the cold region in India, for example in
the Himalayas, the places that are close to
Himalayas, this is very good, because there
you have a lot of sunshine even though it
is cold. The cold is because it is elevated,
but you have the same amount of sunshine and
therefore these are very, very useful.
A quick detour from here will let you understand
how to make a solar air heater. In case of
air heater obviously, one major problem is
there. Air is a very bad conductor of heat.
If air is a very bad conductor of heat, the
rate at which it takes the heat away from
the collector will be, will be bad. But still
you want the heat to be transferred. So, what
needs to be done? You will somehow have to
increase the area of interaction between the
collector and the air. In this case, the area
is very small really, because these are flowing
through the tubes and the inside of the tubes
are the only area in which the air can come
in contact with the, with copper, to the heat
absorbing surface. In case of the air heaters
then what do you do?
The other problem is that if the absorber
is a flat plate, then that flat plate means,
it itself has a very bad area. Now, the area
seen by the sun is only the flat surface and
area seen by the air is also the same flat
surface. For the sun it is no problem, but
for air, it is a problem. You need to increase
that area. How can you increase that area?
What?
Sir, fins
Fins, yes, you can use fins.
So, you would imagine it something like this
that you have got a plate and you have fins
like this. This is what you mean? Yes, you
can do that. So, the fins are welded to the
absorbing plate and the air circulates through.
Air does not have natural circulation much.
So, air has to be forced through. That means
there has to be air blower that allows the
air to circulate, but here also the amount
of area that we have, though we have increased,
it is not really terribly high.
How can you further increase? Well, there
is a very simple way of doing that. There
is no reason why you need a flat plate as
an absorbing plate. Anything that is, that
is made of copper and that is, you know that
is seen by the sun will absorb the energy.
So, instead of this, one simple mechanism
is to put iron filings. You know, in lathe
machines, wherever you have, they use lathe,
there is a lot iron filings that come out;
just blacken it and put it here, sprinkle
that. What happens? That itself becomes the
absorbing surface then and since it has a
large surface area, a very large surface area
is exposed to the air. So, you have a fluffy
thing, metal; the whole place is covered with
that kind of thing and air goes through, through
that material. As a result, the heat transfer
is good. But, then also the air faces resistance,
because it is a, it is no longer a very smooth
surface. So, naturally you have to have a
good blower to blow it in. But that is, that
is what is found to be most suitable form
of doing things.
So, in this case you will have the design
something like this. You have the casing anyway,
you have to have the casing; you have to have
the top glass plate. But here, but here you
have those iron fillings and then, you have
one hole in this. This side is a copper side,
side plate; one hole through which air is
blown in, another hole here through which
the air is taken out that is it. The air naturally
goes through all this and goes out. In case
of air there is no question of storage, but
we do have storage means supposing there is
an industry that works on drying. Most of
the food processing industries need some kind
of drying, so you need hot water, hot air.
So, that hot air can be produced this way,
but of course, there can be cloudy days, there
can be, you know, lack of sunshine or you
may want the work to continue till the evening.
So, in these cases there has to be some storage
of the heat.
Normally what is done is that that air that
is going through that goes through, goes to
a cylindrical place, where a lot of the stone
pebbles are kept. So the, sorry, it should
be like this, not this; these are the stone
pebbles. So, it goes through the stone pebbles,
as a result, when it is hot, it heats up the
stone pebbles and then when it reaches a good
temperature, this whole thing is insulated.
So, the same air goes through and that is
used, but the heat is stored there. As a result,
when you do not have sufficient sun sunshine,
if you still have the air going through, the
air can still absorb the heat from the stone
pebbles which is in that case, a storage of
heat and then that can be still used. So,
you can have a good hot water source, hot
air source, all through the day even when
there is a lack of sunshine, clear.
So, this is how things are commercially used
in industrial appliances, where you need hot
air. But, these all are flat plate collectors;
these are flat, there is no concentration.
But, you can also have a concentrating collector,
where you really need temperature to be higher
than what you can achieve with the help of
the flat plate collectors. By the way, how
much temperature can be reached by flat plate
collectors? Well, if the flow rate is low,
then you can even get steam. You can even
get steam if the flow rate is low. We have
flat plate collectors in our department's
laboratory and we do, we can get steam out
of that. But then, the flow rate has to be
slow enough, very low.
If it is a reasonable flow rate, the temperature
at the outlet will obviously depend on the
amount of solar radiation that is absorbed,
as well as the flow rate. If the flow rate
is large, the temperature will not be large.
Now, on an average you can assume, if the
incoming water is at 25 degrees, which is
the average temperature you can say, the outlet
temperature can be anywhere between 60 to
80 degrees. That is very common and heating
up from 60 to 80 degrees is very easy in a
flat plate collector. By the way, how much
energy is there on the surface of the Earth?
That means how much solar energy is received
by the surface of Earth, any idea? Obviously,
it varies from season to season. In winter
it is less, in summer it is more. In cloudy
days it is less, in clear days it is more.
Nevertheless, on an average, the value is
very easy to remember. It is like approximately
slightly less than 1 kilowatt per meter square.
So, the amount is approximately 1 kilowatt.
So, 1 kilowatt means the domestic electric
heaters mostly are 1 kilowatt heaters. You
have seen those 1 kilowatt heaters and over
a meter square area, the same amount of energy
is really received and depending on the efficiency
of the thing, you can obtain that amount of
heat from a ... So, this is generally called
one sun. So, when we say this is receiving
0.5 sun means, it does not mean that that
you are cutting the sun into half and then
taking part of it, no; that is not, not it.
It is essentially half the average value which
is 1 kilowatt per meter square. That is a
unit that you should remember.
Now, in applications where you need steam
or in applications where you need a, I mean
water, but very close to boiling temperature,
there the flat plate collectors may not be
the best option, because the temperature does
not go up so high. So, you need concentration.
That means you need to concentrate the solar
energy. Now one, okay, how do you concentrate
the solar energy? Obviously, we develop mirrors.
You should not think of lenses, of course.
Why because, if you want to make lenses, it
has to have a big bulk. It has to be a convex
lens. It will have a big bulk. The bigger
this is, it will have very large bulk; naturally,
the cost will be prohibitively high. So, we
never have concentration with the help of
lenses. The lenses held before papers and
burning papers that you did in your school
days that is fine for a school day experiment,
but for the practical purpose of generating
heat that is not good. Of course, there are
ways of overcoming that problem of bulk of
the mirror because, what is actually happening
is, suppose you have only this. Will it concentrate?
It will, because it is doing the same thing
really; it will concentrate.
Now, what is actually concentrating? Is the
bulk being used in the concentration? No;
it is essentially the angle, right, it is
essentially the angle. So, if instead you
take this and make it this way, here it was
this angle, so you make this angle then you
come down; here it was this angel, so you
make this angle and you come down. Here it
was this angle, make this angle and you come
down. So, if you do it this way, then obviously
you have the same thing. Only in these small
parts it will not be the spherical structure
that you had here, instead you will have a
state structure. Naturally, in this case it
was concentrating on to a point, in this case
it will concentrate on to some area all right,
but nevertheless it will do the same thing,
right. Now, this lens is called the Fresnel
lens. Notice that the s is silent. It is called
the Fresnel lens.
You must have seen these lenses in the overhead
projectors. Have you seen that? In the overhead
projector, transparency projectors, the top
glass is essentially a Fresnel lens that does
the concentration. So, if you look at it carefully,
next time you go to a seminar room you take
a look at that lens carefully. You will find
that that is a Fresnel lens. But then, Fresnel
lenses are also rather difficult to manufacture
in large areas. So, in relatively smaller
sizes, Fresnel lenses can be used, but normally
concentration, concentrating collectors are
done by reflecting means, no refraction. In
this case, it was doing it by refraction.
Refraction is not economically viable. Even
through Fresnel lenses are better economically
than the bigger lenses, but they are finally
not economically viable.
So, you have to have some kind of a reflecting
surface and supposing the solar energy is
coming this way and I want to reflect it on
to here. What kind of reflecting surface do
I need? Con, convex mirror? Concave mirror,
okay; concave is you know, it can be spherically
concave, it can be hyperbolic and parabola,
what?
Parabola.
Parabola, yes; it has to be parabola and this
point has to be the focus. So, it has to be
a parabolic mirror, whose focus is here and
then this will be reflected on to the focus.
So, imagine that since this is a three dimensional
structure, though I am drawing on a sheet
of paper, it is actually coming over a 3-D
region, so this will have to be a kind of
trough, right, bowl which is shaped in a parabolic
shape and the inside has to be reflecting
surface. Now, reflecting surface is not very
difficult to make, because steel if polished
well is a good reflecting surface. You do
not really imagine that this will be made
of glass and with a, you know silvered side;
no, that does not, that is good for flat mirrors,
but you cannot really make things shaped like
this, made of glass. It will be breakable
very soon.
So, these are not made of glass really; these
are made of steel, but made of this size,
this shape.
But obviously, making something this particular
shape is you know, in terms of manufacturing
process, rather difficult. In terms of manufacturing
process, how do you make, how do you manufacture
a parabolically shaped mirror? It is not easy.
That is why the people who manufacture these
things or the companies that manufactures
these things, make these things pretty expensive.
These are really used for optical purposes
in case of, yeah, satellites.
...
..… are pretty bad, you know they are not
parabolic, come on. Just they reflect forward
and there is no advantage for the company
to make it perfectly parabolic. So, it is
not properly parabolic, it is more or less
parabolic. But it is small area, but here
we are talking about big areas. Obviously,
you cannot use it that way. The other problem
is that in this case you have to have something
that was not necessary for the flat plate
collector that is tracking. You need to track
the sun, okay, you need to track the sun.
So, what does it mean? You have to, if the
sun moves, you have to always move the whole
mirror, otherwise the concentrator will not
be at this point.
So, at this point, I need to talk about two
types of concentrators one is the imaging
concentrator, the other is the non-imaging
concentrator. The parabolic concentrator is
actually imaging concentrator. So, it actually
forms the image of the sun on the focus. Not
exactly on the focus, it produces the image
on the, on the focus if the solar radiation
comes axially. If it does not come axially,
it does not form the image on the focus. The
image then, image position then moves. So,
there are two ways of tracking them. Either
you have to move the whole mirror or you have
to move the absorbing material. The absorbing
material can also be moved, but nevertheless
you have to have tracking. How the tracking
is done, I will come to that little later.
Just understand that at this stage that you
need tracking for an imaging concentrator
and the parabolic concentrators are imaging
concentrators and in this case you will need
two axis tracking, right. In this case you
need two axis tracking, because it has to,
it has to move all the ways. So, both North
South as well as East West it has to move
and two axis tracking is somewhat expensive.
That is why people often do not prefer these
kinds of devices, unless they are going for
a very big establishment, where the two axis
tracking also becomes economically meaningful.
So, what is the other option? The other option
is if you have one axis tracking, how can
you have a one axis tracking? By making a
linear concentrator. What is the linear concentrator?
Supposing you have got the parabolic surface
like this, but you do not make it round shape,
rather you make it ... This is also parabolic
concentrator. It will concentrate on what?
A line, rather depending on the, it will be
a line if the parabola is a perfect parabola,
else it will concentrate on a region, a region
that has some width like a strip. It will
concentrate on a region like a strip.
In that case, in this case, the other problem
is that what do you make the absorber of with?
The absorber of course has to be something
like here. So, you can have a bowl like thing,
a small container whose outside is blackened,
which is made of copper and inside you will
have the absorbing material. So, the whole
thing has to be packed into a very small region,
small area.
In this case on the other hand, the whole
sun light is concentrated on a strip and therefore,
you can have a tube going through, you can
have a tube going through, which will, whose
dimension, whose diameter should be exactly
the width of that strip. Why does it have
the width? Does it need to have a width? Because,
the surface cannot be exactly made to be a
parabola, there will be some deformities due
to which there will be some deviations that
obviously obtains from the manufacturing process,
so depending on that, there will have to be
a diameter. But, you can always make a simple
tube, copper tube of that same diameter, make
the external side blackened, so that will
be the absorber and the tube itself can carry
the water, the tube itself can carry the water.
So, here much of the problems are naturally
solved.
You have a concentrator, you have the absorber,
you have the same tube. In case of the flat
plate collector, you have to had the absorbing
plate and the tube different; they are, they
are welded together. In case of this, the
linear parabolic concentrator, they are the
same thing. But, you have there the glass
plate. So, how do we put the glass plate?
Simply, simply as a glass cover like this.
So, there are two concentric cylindrical things.
The inside one is a copper cylinder, copper
tube and outside it is a glass cylinder; that
is it, rather simple construction.
Now, let us see some pictures on the computer
screen. Here is a picture of the flat plate
collector, a section of the flat plate collector
and here is the direct irradiance that is
coming, insulation that is coming. A part
of it is reflected and a part of it, as the
top glass cover is heated up, part of it is
lost due to convection. This amount is rather
small, because the heat is mainly trapped
inside and you have these absorber things,
tubes and finally the amount of heat is absorbed
in this plate, in these tubes. So, this is
what is finally available as the available
heat. Some amount of energy is lost through
the convection, some amount of energy is lost
through the radiation, but mainly the radiation
is trapped inside because of the, because
of the effect due to the glass cover.
Now, let us see another picture, where the
actual photograph of this kind of a solar
thermal collector is shown without the glass
cover. Here you see, these are the copper
tubes that act as the absorber and here is
the top big tube which collects the water
that goes through. So, the water flows through
this in the vertical direction due to the
natural circulation and that is how it works.
Let us see another.
Here, this is a picture of an actual photovoltaic,
actual solar thermal flat plate collector.
You would see that here there is a tube running
and here there is another tube running. So,
this tube is connected to all these upgoing
tubes of the solar thermal collector and water
flow in this direction. Let us look at another
picture.
Here you have another picture of the solar
thermal flat plate collector and here is another
that is stationed on top of a building.
These are naturally used for heating up water
mainly for domestic purposes or industrial
purposes and these are very practically used.
Flat plate solar thermal collector - as you
can see that here you have the total absorber
area. Water is coming in through this pipe
and it is going into the pipe that is at the
bottom. Through those relatively thinner pipes,
both made of copper, blackened copper, the
water is flowing up into the relatively thicker
pipe at the top and from there it is going,
from there it is going out. Now, it is actually
stored in this tank, so that the tank water
slowly gets heated up and that can be used
at any point of time. The tank is insulated
at the outside. Now, here you can see that
this surface, the top surface is actually
glass that is used for the greenhouse effect
that I explained in the class. In the bottom
surface, you have the copper plate that acts
as absorber and the copper plate is glued
or it is welded to the vertical pipes that
carry water. So, the solar energy is absorbed
in the absorber plate that gets transferred,
the heat gets transferred into those pipes
and that is how the whole thing works. As
you can see, this is an experimental arrangement
in which the temperature at the inlet is being
measured, the water is flowing in and the
temperature at the outlet is also being measured
and you can, if you go there, you can see
about 20 degrees difference depending on the
flow rate, of course.
