project on solarenergy

8/7/2019 Project on SolarEnergy

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C H A P T E R II

I N T R O D U C T I O N

Solar energy is the worlds most abundant permanent source of energy. Strictly

speaking, all forms of energy on the earth are derived from sun. However, the

more conventional form of energy, the fossil fuels received their solar energy

input ones ago and possess the energy in a greatly concentrated form. These

highly concentrated solar energy sources are being used at such a rapid rate that

they will be depleted in not-too distant future. Our most common forms of energy

Petroleum, natural gas and natural gas-liquids, coal and wood. Excepting

wood, all these common sources have finite supplies. The life is estimated to

range from 15 years for a natural gas and nearly 300 years for coal. Therefore,

as these non-renewable sources are consumed, the man kind must turn its

attention to longer-form permanent type for energy sources. The two most

significant such sources are nuclear are solar energy.

Nuclear energy requires advanced technology and reliable utilization

and costly means for its safe and reliable utilization and may have undesirable

side effects. Solar is a dependent energy source without new requirements of a

highly technical and specialized nature for its wide utilization. In additional, it is

free from pollution.

The amount of solar energy intercepted by the earth is about 17x10 18

Kwh/year which is only tiny fraction of the total energy released by the conversion

of 4 million tons of hydrogen per second to helium in the sun.

Solar energy, which is the ultimate source of most from of energy used now,

is clean, safe and exists unviable quantities in many countries. The draw back in

using solar radiation as energy, as have been pointed out, are that it cannot be

stored and it is a dilute form of energy.

In addition to the thousands of ways in which the suns energy has been

used by both man and nature throughout the time to grow food, to see by, to dry


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clothes. Solar energy is used to heat and cool buildings, to heat water, to power

refrigerators; to operate engines and sewage treatment plant, water stills,

furnaces, distillation equipment, crop dryers and sludge dryers powered by solar

energy electrolyzes convert water to clean hydrogen gas (a fuel ).

Solar energy is not a new concept and it has been realized that it has a

tremendous potential. Although abundant, solar energy impinging on the earth is

relatively dilute, also it is inter milting in its nature. The first factor makes any

solar radiation to heat on a practical scale, relatively large. The second factor

makes it necessary to have a thermal energy storage. The utilization of solar

energy can be approached broadly in three ways.

1. Low grade thermal devices making used direct and diffuse radiation. These

devices are further classified into flat plate type and focusing type. These

are easy to maintain , can be operated with local skills but restricted to low

temperature range (100o c )

The focusing type on the other hand can achieve temperature from 200o c

to 3000oc. But these are sophisticated ones and requires highly reflecting

sun tracking devices and high skills of maintenance solar.

2. Direct conversion of solar energy into electric using Photovoltaic cells.

Solar cells with photo voltaic effect principal and solar batteries using photo

chemical effects can be placed under this heading. That day is not far off,

when mankind receives electricity from solar powered satellites. It is

estimated that one large satellite could produce as much power as 1 to 10

Nuclear power reactors in the 1000 to 2000 MW range.

3. Biological conversion of solar energy into transport fuels. All the stored up

fuels that are now inherited by the earth owe their origin to solar radiation

depend. Processes called photo synthesis. Exploitation by this method

includes permentation and pyrolysis of plant material to produce liquid

fuels and gases development from efficient plants using ways to grow up

usual to ferment, producing methane and hydrogen gas.


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India a fast developing country needing cheap energy with mostly rural

population and favorable weather conditions is best suited for solar energy

utilization. Cost of utilization of solar energy can be computed favorably with

other means of energy as cost of standard fuels in India is high.


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C H A P T E R - lll

S O L A R R A D I A T I O N

The first step is to measure the intensity of radiation received outside

the earths atmosphere called extraterrestrial radiation. This endeavor entails

measuring the intensity of X-rays , ultra violet, infrared, radio emissions etc.., all

of which form part of the solar radiation. A general method is to convert the suns

radiation into heat which can be accurately measured.

Components of solar Radiation :-

On entering the earth atmosphere solar radiation splits into three parts.

One part is absorbed by water vapour and ozone, the second part is scattered

by air molecules, water vapour and dust particles. This part the direction of which

is changed by reflection and scattering Is called Diffuse radiation or Sky

radiation. The third part of which reaches the earth unuttered is called Direct

radiation . The total energy reaching the earths surface is the sum of direct and

diffused radiation.

Direct Radiation under clear sky conditions : -

Owing to scattering and absorption, the solar radiation reaching the earth

is less than that available outside the earths atmosphere. The reduction in

intensity depends upon atmospheric conditions (amount of dust particles, water

vapor, ozone content atmospheric pressure etc.) and solar latitude. The latter

factor determines the length of atmosphere through which the solar beam has to

pass before reaching the earths surface. If the latitude of the sun is small the

length traversed by beam is long. On the other hand, if the sun is at the zenith,

the solar beam traverses through the vertical, the shortest path through the

atmosphere. The path length of the solar beam through the atmosphere is

accounted for in the terms air mass which is a numerical comparison between

the path length which the solar beam actually traverses and the vertical path


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through atmosphere. Thus the air mass m is unity when the sun is at zenith i.e.,

when AI =90o in general

Air mass = Path length traversed

Vertical depth of atmosphere

m = Cosec ( Altitude )

Direct radiation under clear sky conditions and in the wave length ranger of

0.3 to 3.0 m. Low Wave radiation is defined as radiation originating from

sources at temperature nearly ordinary ambient temperature and thus they are

wave length greater than 3.0 m.

Solar Radiation data : -

Solar radiation data are available in several forms and should include the

following information.

1. Whether they are instantaneous measures or values integrated over some

period of time (usually hour or day ) in our analysis we used daily totals.

2. The time or time period of measurement.

3. Whether the measurement are of beam, diffuse (a/r) total radiation and the

instruments used. For our work the total radiation is measured using

pyranometer.

4. The radiation surface orientation horizontal. Transmission factor (Td) of

the atmosphere for direct solar radiation under clear sky c0ndition at sea

level. TD is defined as the ratio between the intensity of solar radiation

received on the earths surface and the intensity of extraterrestrial solar

radiation. For a given value of air mass m and depth of predicable water

w , the transmission factor TD varies linearly with the number of dust

particles.

Solar short wave radiation is defined as radiation originating from sun, at

a source temperature of about 6000o k.


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C H A P T E R - IV

S O L A R G E O M E T R Y

The orbit of the earth and rotation occurs about axis at an angle of about 23

to one another.

To locate the position of the sun with respect to observer. We fix the earth and

find co-ordinates of sun.

The sun is constrained to move with two degrees of freedom. As a result the

location of the sun can be specified by two angles;

1. The solar altitude angle is measured from the local horizontal plane to the

center of the sun. It is measured between a line collinear with the suns

rays and the horizontal plane.

2. The azimuth angle as is measured in the horizontal plane between a due

south line and the projection of the site to sun line on the horizontal

plane. Signs convention used for azimuth angle is positive east of south

and negative west of south.

Zenith (z) : The point at which the earth sphere intercepts the upward vertical

axis passing through the observer.

Nadir (N) : The point of the celestial sphere diametrically opposite to the zenith.

The position of a point p on the surface of earth with reference to suns rays

is known at any instance, if the altitude l hour angle h , and suns declination d

are know.

l is the angular distance of the point north (or) south of equator.

h is the angle through which the earth must turn to bring the meridian of p

directly in line with suns rays. h is measured with reference to mid-noon. Each


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hour is equal to 15of longitude. It is positive in the mornings and negative in the

afternoons. Thus for example at 10 A.M. h= +30and at 2.30 P.M. h= -37.5.

d is the angular distance of suns rays, north (or) south of equator, atsolar noon. d varies from 23 to +23 over a year. On equinoxes (march

22 vermal equinox, September 22 Automnal equinox). d = 0, on summer

solstice (June 22) d = 23and on winter solstice (December 22, d = - 23)

variation of d can either be determined by a plot of d Vs day (or) approximately

by the equation:

d = 23.45 Sin 360 284 + n 365

Where n is the day of the year, for example on Feb.1

n = 31 + 1 = 32

d = 23.45 Sin 360 30 284 + 32

365

= - 16.25S o l a r T i m e : -

Solar radiation calculations must be made with reference to solar

time. Solar time does not coincide with the local time.

Time reckoned from midnight at the Greenwich meridian (Zero longitude) is

known as Greenwich standard time or Universal tome such time is expressed onan hour scale from 0 to 24 . Hence midnight is 0 and mid-noon is 12.

On any particular meridian the local time is less or more than the GST and

varies by 4 minutes per degree change in longitude plus 4 minutes for eastern

longitude and minus 4 minutes for western longitude.

The local time is not set with reference to the meridian of the place. Each

country spans a certain range of longitudes and the time with reference to mean


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meridian is taken to be the local standard time for example our country spans

from 67 E to 92. The Indian standard time is the corresponding to the 82.5 E

meridian (mean) It can thus be seen that IST is a head of GST by 5 hours 30

minutes.

Local time = Standard time - 4 ( Lst - Lloc ) min.

Where, Lst = standard meridian for local zone and L loc = Longitude of place

required.

Again there is a difference between local solar time (LST) and the local time and

this difference is called the equation of the time. Equation of the time is the

result of irregularities of the earths orbit.

LST = Local time + equation of time

Equation of time can be obtained from the charts.

Hence LST = Standard time 4 ( Lst Lloc ) + equation of time

Here the second and third terms are in minutes.

Solar Constant : -

It is defined as the amount of energy that would fall perpendicularly

per cm2per minute on a surface placed out side the earths atmosphere at the

earths mean distance from the sun.The value of constant is nearly 1.94 claries.

cm 2 min 1 . The solar constant is thus the amount of energy available to heat

the entire surface of the atmosphere was perfectly transparent and if the sun was

vertically over head.

Whenever the sun is at a lower angle of elevation, its energies spread

by the slanting rays over a great area of the earths surface and the heating of the

earths surface and the heating of each centimeter is correspondingly weaker.

Hence the intensity of solar radiation is depends upon altitude and time of year.

But the total time of energy received in 24 hours depends also on the length of


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the day between sunrise and sun set. Because of this factor, the daily totals of

the available solar radiation are actually greatest at the poles in mid summer,

although the total over the year is about 2 times greater at the equator then at

the poles. For all practical purposes, solar radiation as it reaches the outside of

the earths atmosphere is similar to the radiation form a block body at a temp of

nearly 6000c. This radiation is effectively contained with in a spectrum

extending from about 0.2 m wave length is ultra violet to about 5.0 m in the

infrared, maximum energy falling near 0.45 m in the visible region.

The distribution of energy in the major divisions

Ultra violet 3 %

Visible 50 %

Infrared 45 %

To be more precise the amount of radiation incident on a plane surface

outside the earths atmosphere is not constant, but varies with solar declination

and the distance of the earth form the sun. The variation is the order of 3%

around its mean value of 1.94 cal. cm-2 min-1 I.e., 1353 W/ m2


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C H A P T E R - V

DESCRIPTION AND PERFORMANCE STUDY OF FLATE PLATE

WATER HEATER

Description : -

The plate water heater under our study is a natural circulation type domestic

water heater. It consists of a metal (GI) plate which is painted black on the side

facing the sun and thermally insulated on the edges (wooden supports) and on

the back side glass wool. Above the absorbing plate, spaced an inch, one more

glasses are placed to reduce up ward hear losses. Provision is made to remove

the collected energy by circulating water in tubes which are in thermal contact

with the absorber plate.

Liquid Heating Flat Plate Collectors : -

The absorber plate, the main component of a collector stops the sun light

converts it into heat, and transfers it to the liquid. Its surface is usually painted

black to reduce the heat loss, cover plates such as glass which transmits sun

light, are placed over the absorber. Heat loss out the back side of the absorber is

reduced through the use of insulation. All of these components are often

containerized (wooden or metal boxes) for shipping case of inclination, or

protection from moisture.

Absorber Plate : -

Traditionally absorber plates for liquid systems have been made of

copper aluminum or steel the characteristics are considered an absorber are

1. Availability


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galvanized or stainless, corrosion also can be reduced by adding so called

inhibitors, some of which are chromate based to the water or to the anti-freeze.

Types of Absorber Plates for Liquid Systems : -

There are three basic designs used to bring liquids in contact with the absorber

plate for the purpose of taking away heat. One is Thomson type, openfaced

corrugated sheet over which liquid flows. The second is the tube in plate method,

used in which passages are incorporated into the absorber sheet itself. The third

and the most popular prior to the advent of the tube in plate is the application of

the tubes on to the sheet, either on the backside, protected from the sun, off on

the front side exposed to the sun.

Absorber Surfaces :-

Absorber surfaces (on coatings) and over plates should be considered

simultaneously. There functions are similar and the choice of cover plates is

related to the absorber surfaces. For example it has been found that the use of

selective surface in combination with one cover plate is more efficient than a flat

black paint in combination with the two cover plates.

The primary function of the absorber surface is to increase

the percentage of sunlight, which is retained by the absorber plate. By definition

of black body is a perfect absorber of radiation, all wave lengths at every incident

angle will be absorbed by a black body. Real substances however, will always

reflect sum of the radiation, which strikes them and will do so to an increasing

extent with increasing angle of incidence. A black body is also a perfect emitter of

thermal radiation. Although there are no perfect emitters in nature, most black

colors absorb as much energy as they simultaneously emit about 90 or 95 %.

The ideal for an absorber surface is that it reflects no short wave light radiation (it

absorbs it all) and reflects all long wave radiation (it emits non of it ) such an ideal

surface is called a selective surfaces.


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The performance is measured by its absorbtivity ( ), by its emissivity

(E) of long wave thermal radiation and by the ratio of absorbitivity and emissivity

( /E).

Cover Plates :-

A cover plate is the generic term applied to a rather large group of

transparent materials. Which are used to the absorber plate and which are

usually located about one inch from it. The short wave sunlight penetrates

through the transparent material (transparent to sunlight), hits the absorber

plate, stops, and turns into long wave infrared. The previously transparent

material is now relatively opaque to these long waves and the heat is trapped.

The basic knowledge of the solar spectrum makes an understanding of the

function and choice of cover plates somewhat easier. Ultra violet light is less than

0.4 microns in length infrared is longer than 0.7 microns, visible light is the

spectrum between these two. Long wave infrared (heat) is usually considered to

be 3 to 20 microns in length or longer glass is almost totally opaque to these long

waves.

Glass, one of the most popular choices for cover plates, varies in its solar

transmittance according to the angle of incidence of the solar radiation.

Measuring Instruments : -

Thermo couples : -

Thermo electric thermometry is based on the rigorous relation that

exists between the thermo-emf generated by a thermocouple and the

temperature to which it is exposed. Thermo couples are widely used in various

fields of science and technology to indicate temperatures down to 2500 c.

Although they can indicate temperatures down to -200 c, thermo couples are

used in the low temperatures on a more limited scale than resistance thermo

meter. In the high temperature region (above 1300 c - 1600 c ) thermo couples

are mainly used for short duration measurements.


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The point is that the corrosive properties of process media

increases with rising temperatures and service life of thermo couples rapidly

decreases.

Basic theory of Thermo Couples: -

This history of thermo couples dates back to the discovery by see

back in 1825 that an electrical current flows in a continuous circuit of two

dissimilar metals if different temperatures are maintained at there junctions.

Thermo emf is related in a specific manner to the temperature at the junctions of

two dissimilar metals.

A and B are two dissimilar metals (say Cu and Platinum) each

homogeneous over its length t and to are the temperatures of the junctions.

When junctions (1) is heated and the junction (2) is left cold a current begins to

flow, direct from B to A at the hot junction and from A to B at the cold junction. If

junction (2) is heated, the direction of current will be reversed. This is thermo

electric current and the emf caused by the difference in temperature between the

two functions is called thermo emf. The device which produce it is called

thermo couple or thermo electric transducer.

Copper constant Thermo Couples : -

Although not used in industrial instruments, copper constant

thermo couples are a convenient aid in laboratory practice and some times, in the

shop for measuring temperatures in the interval form - 200c to + 350 c.

Temperature Probe : -

One junction of copper constant an thermo couple wore is made anf

it is fixed on a piece of copper plate using metal seal. This copper plate is

attached to the bottom of a 1 G. I. Pipe again by using M-Seal. The other end of

the thermo couple wire is drawn through the pipe with this end of the wire, one

cold junction is made and it is maintained at 0c by using ice. Copper wire is fixed

to the copper plate and constant wore is fixed to the cold junction. The two

terminals are connected to the potentiometer. This probe is passed into the hot

water tank and temperature difference is measured in mill volts in


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2. Heat transfer coefficient of the heat exchanger process must be

maximized. As the thermal equilibrium,

qo = qa = ql

Flat plate collector can take the advantage of the diffuse component of

scattered solar radiation as well as the direct component.

The power absorbed/unit collector area for either one of these components

is simply the product of the absorptions coefficient the effective transmittance Te

and the solar radiation falling on the tilled surface RH an evaluated for the

particular component (R is the geometry factor that converts the solar radiation

falling on a horizontal surface to the solar radiance falling on a tilted surface).

This qa = Te R H + Tc R H

l - for direct

ll - for diffuse

Heat loss to the environment is made up of a conduction loss from the back of

the absorber plate through the insulation material (losses through the edge of a

well designed collector are so small that can be ignored) and an upward radiation

conduction convention loss through the cover plates.

The rate of energy loss through the insulation per area qb is dependent upon

the thermal conductivity K and the thickness D the insulation as well as the

difference between arithmetic mean temperature of absorber T and the

temperature of the back of the frame Tb.

qb = k/d (T Tb)

Ambient temperature Ta is used instead of Tb in practical calculations. Since it

is easy and economical to provide good insulation of the rear surfaces. The heat

loss is usually negligible compared to upward heat loss.

The cover plate are for practical purposes opaque to the long wave

length thermal radiation corresponding to the operation temperatures of the

converts and the absorber surface. The upward rate of energy transfer between

any two of these surfaces as an example, the absorber surface and the first cover

is therefore given by

qu = c (T T1)5/4 + 1 (T4 T41)

1/E +

1/E -1

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