1. UNIT II SOLAR ENERGY COLLECTION
Flat plate and concentrating collectors,
classification of concentrating collectors,
orientation and thermal analysis, advanced
collectors.
2. Solar thermal energy systems
• Solar thermal energy systems are broadly
characterized as follows depending on the way they
capture, convert and distribute solar energy.
• (i) Passive solar thermal energy systems
• (ii) Active solar thermal energy systems.
3. Passive Solar Thermal Energy
Systems
• the heating is carried out without any special device
for energy conversion.
• Heat is directly received and used for heating.
Heating the houses, water, cooking, drying,
• These are the low temperature applications of solar
energy.
• simple and cost effective way
• system does not involve mechanical devices or the
use of conventional energy.
4. Active Solar Thermal Energy Systems
• A solar collector holding a heat-transfer medium such
as air or liquid captures the solar radiation which is
then distributed through the building via electric fans
or pumps.
• The technology is simple and used in many possible
applications of low temperature heat use systems.
• The disadvantage of active solar systems is that the
use of this device external power sources can fail
which needs more controls and maintenance.
5. • The typical basic components of an active solar heating
system include the following:
• (i) Solar collector
• (ii) Storage unit
• (iii) Load and
• (iv) Auxiliary source.
6. • The following active solar thermal energy systems
are commonly used:
• (i) Solar water heaters
• (ii) Photovoltaic (PV) cells or solar cells and
• (iii) Concentrated solar power (CSP).
7. Solar water heaters
• Solar water heaters (active) produce thermal energy to
heat water for households, commercial entities and
swimming pools.
• Solar water heaters consist of two parts such as solar
collector and storage tank.
• In warm climates, collectors heat water directly but in
cold climates, a denser fluid is heated and then
transported to a water tank where it heats the water
indirectly.
• The maximum heating temperature varies with collector
model but water temperature can exceed 90°C, suitable
for commercial purposes.
8.
9. What is Thermosiphonic Effect
• A thermosiphon system relies on warm water rising,
a phenomenon known as natural convection, to
circulate water through the collectors and to the tank.
• In this type of installation, the tank must be above
the collector.
• As water in the collector heats, it becomes lighter
and rises naturally into the tank above.
• Meanwhile, cooler water in the tank flows down
pipes to the bottom of the collector, causing
circulation throughout the system.
• This is called Thermosiphonic Effect.
10.
11. solar cells
• Photovoltaic (PV) cells or solar cells are an active system
in which small panels are applied with semiconducting
material.
• PV panels convert the sun's rays into electricity which can
power a variety of individual items from personal
computers and streetlights to water pumps.
• This material, usually made of silicon but potentially other
polycrystalline thin films, generates a direct current when
sunlight hits the panel.
• PV cells can be installed on windows and roof tiles. PV
systems can be tailored to meet a building's energy needs
by adding concentrating or sun-tracking devices, DC-AC
converters and/or battery storage.
12.
13.
14.
15. Concentrated solar power (CSP)
Concentrated Solar Power (CSP) is an active system
distinguished from other solar energy systems by its ability to
function as a utility-scale power plant.
CSP uses the fields of mirrors to concentrate solar energy into
channels holding heat-responsive fluid.
The high temperatures excite the fluid to a point where it
powers a turbine or engine which in turn runs an electric
generator. Without storage facilities, CSP systems can generate
electricity for about eleven hours on a sunny summer day.
However, CSP systems do have the potential to provide base
load power for utilities.
A CSP system that uses oil or molten salt as a medium in the
heat-transfer process can retain the thermal energy in thermos-
like tanks for the use when sunlight is not available.
16.
17.
18.
19.
20. SOLAR COLLECTORS
• Solar collector is a device used for collecting the
solar radiation and it transfers energy to fluid
passing in contact with it. These collectors are
classified into two types. They are as follows:
• (i) Non-concentrating or flat plate type solar
collector.
• (ii) Concentrating (focusing) type solar collector.
21. • In the non-concentration type, the collector area is
same as the absorber area (i.e., the area absorbing
the radiation).
• In concentrating collectors, the area intercepting the
solar radiation is greater than plate collectors and it
provides higher temperature than a non-
concentrating type.
• It is used to produce medium pressure steam. They
use many different arrangements of mirror and
lenses to concentrate the sun rays on the boiler.
22. Characteristic Features of a Solar Collector
I. Type of solar collector: Whether Focusing or non-
focusing.
2. The temperature of working fluid such as low
temperature, medium temperature and high temperature.
3. Whether tracking system is used or not. If used, what
type of tracking system can be used i.e. Tracking type or
non-tracking type and tracking in one plane or two planes.
4. Cost of the solar collector system such as low cost or
high cost.
5. Design of the solar collector system such as complex or
simple design.
6. Layout and configuration of collectors in the solar field.
23. Performance Indices of Solar
Collectors
• Sun light has low density (0.1 kW/m^2 to 1
kW/m^2). Hence, a very large surface area of the
collectors is required for producing rated power of
1 MW The performance indices or important
aspects of solar collector system are
• 1. Concentration Ratio (CR)
• 2. Temperature range
• 3. Absorption/Reflection ratio ()
• 4. Collector efficiency.
24. • Concentration ratio (CR): Concentration ratio is
defined as the ratio of the area of aperture of the
system to the area of the receiver. The aperture of
the system is the projected area of the collector
facing (normal) the beam.
25. Temperature range:
• Temperature range is the range of temperature to
which the heat transport fluid is heated up by the
collector.
• The resulting temperature of heat transfer fluid by the
solar collector is less than 150°C. Focusing the
collectors attains higher temperature concentration
ratios of the order of 1000.
• It can be obtained with heliostats with sun tracking in
two planes.
• Concentration ratios up to 100 can be achieved by
parabolic trough collectors with sun tracking in one
plane.
26. Absorption/Reflection ratio () :
• It has significant effect on the temperature attained
by the heat transport fluid. Absorption/Reflection
ratio is in the range of 1 to 40.
4. Collector efficiency:
Collector efficiency is defined as the ratio of the
energy actually absorbed and transferred to the heat
transport fluid by the collector(useful energy) to the
energy incident on the collector
27. Factors Affecting Solar Collectors
System's Efficiency
The efficiency of the solar collector system is
adversely affected by the
• shadow,
• cosine loss,
• dust etc.
These aspects have a decisive influence on the design
of the collector layout.
28. Shadow effect: Some of the neighboring collector
panel's shadow fall on the surface of the collector
Particularly when the angle of elevation of the sun is
less than 15°.
The shadow effect is reduced when the angle of
elevation of the sun increases. The shadow factor is
given by Surface of the collector receiving light
29. Cosine loss factor
• An important source of 'loss' in solar concentrators arises from
the fact that solar collectors cannot always be aligned normal to
the incident solar rays.
• When a collector is reflecting off-axis, the apparent area of the
collectors is reduced according to the cosine of the incidence
angle.
• Let us consider be the angle between direction of sun rays and
plane perpendicular to the collector's surface as shown in Fig.
• The area of sun beam intercepted by the collector's surface is
proportional to cos . Therefore, solar power collected is
proportional to cos . Here, the term cos is the cosine loss
factor.
• For the fixed type of collector panels, cosine loss varies due to
the variation of the direction of sun rays. Plane perpendicular to
sun rays
30.
31. Reflective loss factor:
• With the passage of time, the collector glass surface
and reflector surface collect dust, dirt and moisture.
• Hence, it gets rusted, deformed and loosen the
shine.
• Hence, the efficiency of the collector is significantly
reduced.
• It can be prevented by a proper maintenance carried
out time-to-time.
32. Physical Principles of the Conversion of
Solar Radiation Into Heat
• The fundamental process for heat conversion is the
greenhouse effect.
• Most of the energy , from the sun comes in the form of
light, a shortwave radiation, not all of which is visible to
the human eye.
• When this radiation strikes a solid or liquid, it is
absorbed and transformed into heat energy.
• Visible sunlight is absorbed on the ground at a
temperature of 20°C at a wavelength of about 10 nm but
CO2, in atmosphere absorbs light of this wavelength
and radiates a part of it tuck to earth.
• It results in the increase in temperature of earth. This
process is called greenhouse effect.
33.
34. Greenhouse Effect.
• CO2 envelope present around the globe in the
atmosphere behaves similar to a glass plane and it
forms a big global greenhouse. It tends to prevent
the escape of heat from earth which leads to global
warming. This phenomenon is known as
"greenhouse effect.
• Apart from CO2 gases methane, nitrous oxide,
hydro flourocarbons, sulphur hexaflouride and
water vapour are known as greenhouse gases.
35. Types of Solar Collectors
• There are two types of solar thermal collectors such
as
• (i) Flat plate collector
• (ii) Solar concentrating collector (focusing type).
36. FLAT PLATE COLLECTORS
• Flat plate collectors are of the non-concentrating
type.
• They are particularly convenient for space and
service water heating applications where
temperatures below 90°C are adequate.
• Flat plates can collect and absorb both direct and
diffuse solar radiation.
• They are consequently partially effective even on
cloudy days when there is no direct radiation.
37. Types of Flat plate solar collectors
• Flat plate solar collectors are mainly divided into
three types based on the type of heat transfer fluid
mused. They are as follows:
• 1. Liquid heating collectors
• 2. Air heating or solar air heaters
• 3. Evacuated tubular collector.
38. Characteristic Features
• It absorbs both beam and diffuse radiation.
• It can function without sun tracking.
• It is simple in construction and it requires little
maintenance.
39. Liquid Heating Collectors
It is used for heating the water and non-freezing
aqueous solutions.
It is the plate and tube type collector. The majority of
the flat-plate collectors have five main components.
They are as follows:
• Transparent cover (one or two sheets) of glass,
Teflon, Marlex or Tedlar.
• Blackened absorber plate usually of copper,
aluminium or steel, typically 1-2 mm thick.
40. • Tubes (typically 1-2 cm diameter), channels or
passages in thermal contact with the absorber plate.
They are soldered, brazed or clamped to the bottom of
the absorber plate. In some designs, the tubes form the
integral part of absorber plate.
• Thermal insulation usually of foam, expanded
polystyrene or glass wool typically 5-10 cm in
thickness.
• Tight container is to enclose above components.
41.
42. Working
• As the solar radiation strikes a specially treated absorber plate, it
is absorbed and raises its temperature.
• This raised heat is transferred to fluid which is circulated in the
tube (or channels) with the absorber plate.
• Thermal insulation prevents the heat loss from the rear surface of
the collector.
• The upper glass cover permits the entry of solar radiation as it is
transparent for incoming short wavelength but it is largely opaque
to longer infrared radiation reflected from the absorber.
• The glass cover also prevents the heat loss due to convection by
keeping the air stagnant.
• The glass cover may reflect around 15% of incoming solar
radiation which can be reduced by applying anti-reflective coating
on the outer surface of the glass. The usual practice is to have 2
glass covers with specific ranging from 1.5 cm to 3 cm.
43. Advantages of second glass which is added above the
first one are:
• Losses due to air convection are further reduced. It
is important in windy areas.
• Radiation losses in the infra-red spectrum are
reduced by a further 25% because half of the 50%
which is emitted outwards from the first glass plate
is back radiated.
44. Orientation of Flat Plate Collector
• Due to low collection efficiency of flat plate
collectors, it is uneconomical to arrange sun-
tracking.
• Hence, a fixed type installation is preferred.
• The axis of the pipes is placed parallel to parallel
lines of longitude passing through north and south
poles.
• Axis parallel to line of longitude
• Axis parallel to line of longitude
45.
46. • The collector is placed on a stand under conditions
in which the operation is nearly steady.
• The global solar radiation can be measured using a
Solarimeter on the plane of the collector.
47.
48. • In these tubular or advanced collectors, high
performance is achieved by the use of the following
advanced features such as
1. Vacuum insulation
2. Selective black absorber coatings
3. Anti-reflective coatings films
4. Heat mirror coating
5. Highly efficient removal of the absorbed heat from
the solar collector by the principle of heat pipe.
49. Some advanced flat plate
collectors are discussed below.
• Modified Flat plate collector
• Solar Air Heater
• Evacuated Tubular collector
52. Types
• Solar air heater with non-porous absorber.
• Solar air heater with porous absorber.
The performance of air heaters is improved
by the following ways.
• Roughing the rear of the plate to promote
turbulence and improve the convective heat
transfer coefficient,
• Increasing the heat transfer surface by
adding fins.
54. FACTORS AFFECTING PERFORMANCE OF FLAT PLATE
COLLECTORS
• Incident solar radiation falling on the solar collector.
• Number of cover plates.
• Slope of the flat plate collector which is tilted at an
angle of latitude of the location.
• Absorbing surface of the collector which should
withstand high temperature and corrosion resistant.
• Spacing between absorber plate and cover plate. Internal
heat loss can be prevented by providing more space.
• Inlet temperature of the working fluid.
• Dust deposited on the cover which should be minimised
to obtain high efficiency.
55. Solar collectors
1) Non-concentrating collector (FP)
a) Liquid flat-plate collector
b) Flat-plate air – heating collector
2) Concentrating type
a) Focus type
i) Line focus (one axis tracking)
a) Cylindrical parabolic concentrator
b) Fixed mirror solar concentrator
c) Linear Fresnel lens collector
ii) Point focus (two axis tracking)
a) Paraboloidal dish collector
b) Hemispherical bowl mirror conc.
c) Circular Fresnel lens conc.
d) Central Tower Receiver
b) Non-focus type
a) Modified flat plate Collector
b) Compound-parabolic concentrating (CPC) type
Classification Dr.N.Shankar Ganesh
55
56. Flat plate collectors
For space and water heating
where temperature is below 90°C
flat plate collectors are
convenient.
They are made in rectangular
panels, from about 1.7 to 2.9 sq.m
in area, and are relatively simple
to construct.
Flat –plates can collect and
absorb both direct and scattered
(diffuse) solar radiation.
57.
58. It consists of an absorber plate
(aluminium or copper) on
which solar radiation falls after
coming through transparent cover
(usually made of glass).
The absorbed radiation is partly
transferred to a liquid flowing
through tubes which are fixed to
the absorber plate or are integral
with it.
This energy transfer is the useful
gain.
The remaining part of the
radiation absorbed in the absorber
plate is lost by convection and re-
radiation to the surroundings from
the top surface, and by conduction
through the back and the edges.
Flat plate collector
Flat plate collector
Dr.N.Shankar Ganesh
58
59. The transparent cover helps in
reducing the losses by convection
and re-radiation, while thermal
insulation on the back and the
edges helps in reducing the
conduction heat loss.
The liquid most commonly used is
water.
A liquid flat-plate collector is
usually held tilted in a fixed
position on a supporting structure,
facing south if located in the
northern hemisphere.
In order to reduce the heat lost by
re-radiation from the top of the
absorber plate of a flat-plate
collector, it is usual to put a
selective coating on the plate.
Flat plate collector (Contd.,)
Flat plate collector
59
60. The selective coating exhibits the
characteristic of a high value
of absorptivity for incoming
solar radiation and a low value
of emissivity for out-going re-
radiation.
As a result, the collection
efficiency of the flat – plate
collector is improved.
Further improvement in the
collection efficiency is obtained
by evacuating the space above
the absorber plate and leads to
the design of an evacuated tube
collector.
Flat plate collector (Contd.,)
Flat plate collector
60
62. An evacuated tube (Copper)
collector (ETC) consists of a
number of cylindrical modules
mounted side-by-side on a
common frame.
It consists of two concentric glass
tubes, which the annular space
between them being evacuated.
The outer surface of the inner
glass tube is selectively coated.
The incoming solar radiation is
absorbed on this surface and
partly conducted inwards
through the tube wall.
Evacuated Tube Collector
Schematic diagram of an
evacuated tube collector
Dr.N.Shankar Ganesh
62
63. The inner tube is filled with
water, and the heat is transferred
to the water by thermosyphon
circulation.
It is to be noted that the heat loss
by convection to the
surroundings is reduced
significantly due to vacuum in
the annular space.
This results in an improvement
in the collection efficiency.
Evacuated Tube Collector (Contd.,)
Schematic diagram of an
evacuated tube collector
63
64. A schematic cross-section of a
conventional flat-plate collector
for heating air is shown.
The construction of such a
collector is essentially similar to
that of a liquid flat-plate collector
except for the passages through
which the air flows.
These passages have to be made
larger in order to keep the
pressure drop across the
collector within manageable
limits.
Solar Air Heater
Solar Air Heater
Dr.N.Shankar Ganesh
64
65. Air to be heated flows between
the cover and the absorber plate
which is fabricated from a metal
sheet of 1mm thickness.
Cover is either made of glass or
plastic of 4 mm to 5 mm
thickness, glass wool of
thickness 5 cm to 8 cm is used for
bottom and side insulation.
Full assembly is encased in a
sheet metal box and kept
inclined at a suitable angle.
Solar Air Heater (Contd.,)
Solar Air Heater
Dr.N.Shankar Ganesh
65
66. The face area of a solar heater is
about 2 m2, matching the heat
requirement.
The value of heat transfer
coefficient between the absorber
plate and air is low and operating
efficiency of a simple air heater
is also low.
Solar Air Heater (Contd.,)
Solar Air Heater
Dr.N.Shankar Ganesh
66
67. THERMAL ANALYSIS OF Flat Plate
Collector
• As per the heat transfer concept, the sum of
absorbed, transmitted and reflected energy is equal
to unity
76. ADVANTAGES OF FLAT-PLATE
COLLECTORS
• It has the advantages of using both beam and diffuse
solar radiations.
• It does not require orientation towards the sun.
• It requires a little maintenance.
• Flat plate collectors are simpler than concentrating
reflectors.
77. DISADVANTAGES OF FLAT-PLATE
COLLECTORS
• The temperature attained by the working fluid is
low.
• The construction is heavy in weight.
• Conduction heat loss is more as the area is large.
• Initial installation cost of the collector is more.
78. APPLICATIONS OF FLAT PALTE
COLLECTORS
• It is used in solar water heating systems.
• It is used in solar space heating and cooling
systems.
• It is used in low temperature power generation.
• It is used in solar heating dryers.
79. SOLAR CONCENTRATING
COLLECTOR (FOCUSING TYPE)
• introducing a reflecting surface (concentrator) between
solar radiation and absorber.
• It uses the optical system in the form of reflectors or
refractors for concentrating.
• In order to get a maximum concentration, an arrangement
for racking the sun's virtual motion is required.
• An accurate focusing device is also required
80. A solar concentrator consists of a
•Focusing device,
•Receiver system and
•Tracking arrangement.
• As a result of the energy concentration, fluid can be heated to the temperature of
750°C or more.
81. Difference between flat plate and
concentrating collector
• The flat plate collector concentrates only the direct
radiation coming from a specific direction
• The concentrating collector collects all types of
radiation.
• It is due to the fact that the diffuse radiation arrives
from all directions and only a very small proportion is
from the direction for which the focusing occurs.
82. Types of Concentrating Collectors
• Solar concentrators may be classified on the basis of
whether tracking system is installed or not and type
of tracking system installed.
(a) Tracking type: Continuous or intermittent
• One-axis design
• Two-axis design
(b) Non-tracking type
83. Solar concentrators may also be classified on the
basis of optical components.
• Reflecting or refracting type collector
• Imaging or non-imaging type collector
• Line focusing or point focusing type collector
84. Based on' the number of concentrating collector
geometries, the solar concentrators may be classified.
• Parabolic trough collector
• Minor strip reflector
• Fresnel lens collector
• Flat plate collector with adjustable mirrors
• Compound Parabolic Concentrator (CPC).
85. Parabolic trough collector
• The solar radiation coming from the particular
direction is collected over the area of the reflecting
surface and concentrated at the focus of the
parabola.
• The solar radiation is focused along a line.
• It consists of a cylindrical parabolic reflector and a
metal tube receiver at a focal plane.
86.
87.
88. • The dimension of parabolic trough collector can be
varied over a wide range.
• The length of a reflector unit may be roughly 3 in to 5
m.The width about 1.5 m to 2.4 m
• Higher temperature up to 300°C can be achieved.
• Parabolic trough reflectors have been made of highly
polished aluminium of silvered glass of a thin film of
aluminized plastic on it firsts base.
• The reflected light is focused on is central line of the
parabolic trough collector.
89. • The tube located along the centre line absorbs the
heat and the working fluid is circulated through the
pipe.
• The absorber tube may be made of mild steel/copper.
• A cylindrical parabolic trough may be oriented in any
of three directions: East—West, North—South or
polar.
• Trough type collectors arc generally oriented in
East-West or North-South directions.
• The North-South orientation permits more solar
energy to be collected than East-West arrangement.
92. • A paraboloidal dish collector brings solar radiation to a
focus at a point.
• A dish (concentrator) 6.6 m diameter has been made
from about 200 curved mirror segments forming a
paraboloidal surface.
• The absorber (receiver) located at the focus is a cavity
made of zirconium-copper alloy with a black chrome
selective coating.
• The heat-transport fluid flows into and out of the
absorber cavity through pipes bonded to the interior.
• The dish can be turned automatically about two axes so
that the sun is always kept in a line with the focus and
the base of the paraboloidal dish.
• Thus, the sun can be fully tracked as required all times.
93. Mirror Strip Reflector
• In this collector, a number of plane or slightly curved
(concave) mirror strips are mounted on a flat base.
• The angle of the individual mirrors is arranged in
such a way that they reflect solar radiation from a
specific direction on to the same focal line.
• The angle of the mirrors must be adjusted to allow the
change in the sun's elevation while the focal line
remains in a fixed position.
94.
95.
96. Fresnel lens collector
• It has a refracting type focusing collector.
• It utilizes the focusing effect of a Fresnel lens.
• To be fully effective, the Fresnel lens must be
continuously aligned with the sun in two directions both
along and perpendicular to its length.
• In the Fresnel lens collector, the solar radiation is
focused into the absorber from top rather than from
bottom as in a parabolic type.
97. • Cross-section of Fresnel lens through collector For a
trough type collector, the lens is rectangle about 4.7
m in overall length and 0.95 in in width.
• It is made of acrylic plastic and it can be probably
produced in quantity at low cost.
98.
99.
100. Flat Plate Collector with Booster
Mirrors
• It is the simplest type of concentrating collector.
• It consists of a flat plate facing south with mirrors
attached to its north and south edges.
• Reflectors reflect the total radiation in addition to
beam radiation incidence on the receiver.
• Mirrors are also called booster mirrors. It has a
maximum concentration value less than four.
101. • The concentration ratio of such solar concentrators is
relatively low and hence, it is not widely used.
• For a single collector, booster mirrors can be used on all
the four sides.
• If the mirrors are set at the proper angle, they reflect the
solar radiation onto the absorber plate.
• The mirrors cut off part of the scattered radiation have
reached the absorber plate and only a part of the scattered
radiation falling on mirrors will be reflected onto the
absorber.
102. • When a number of collectors are combined in two or
more rows, the rows must be set further apart in the
north south directions to allow for the additional sun
shading.
• The efficiency of a boosted flat plate system can be
increased if the angle of the flat mirrors can be
changed several times during the year.
• The advantage of such a system is that it makes use of
the diffuse radiation in addition to the beam radiation.
103.
104.
105. Compound Parabolic Concentrator
(CPC)
• It is a non-focusing type but the solar radiation
from many directions is reflected towards the
bottom of the trough.
• Due to this characteristic, a large proportion of the
solar radiation including diffuse (scattered)
radiation entering the trough opening is collected on
a small area.
• A CPC with two facing parabolic mirrors.
106.
107.
108. • In addition to collecting both direct and diffuse
radiations,
• an advantage of this collector is that it provides
moderately good concentration although less than a
focusing collector in an east-west direction without
adjustment for sun tracking.
109. ORIENTATION OF
CONCENTRATING COLLECTOR
• For obtaining maximum productivity of a solar collector,
a solar collector should be provided with the correct
orientation and tilt angle.
• The plane of a solar collector should always be
perpendicular to sunshine to absorb the maximum
quantity of solar energy.
• However, the sun shines on the Terrestrial surface
depending on time of day and year.
• Therefore for installation of solar collectors, it is
necessary to know the optimum orientation in space of an
absorber of a solar collector.
• For an assessment of optimum orientation of collectors,
earth rotation around the sun and around its pivot-center,
and as distance change from the Sun are considered.
110. • The following factors are considered for aligning
solar collector to the optimum orientation:
• (i) Latitude angle
• (ii) An hour angle
• (iii) Declination
• (iv) A tilt angle to the horizon
• (v) An azimuth.
• The latitude angle shows, as far as the place is to the
north or to the south from the equator, and makes a
corner from 0 ° to 90 °, counted from the equator
plane to one of poles — northern or southern.
111. • The hour angle transfers local solar time to number of
the degrees which sun passes on the sky.
• On definition the hour angle is equal to zero at
midday. Earth turns on 15° for one hour.
• In the morning, declination is negative whereas in the
evening, it is positive.
• The declination of the sun depends on earth rotation
around the sun as the orbit of rotation has an elliptic
form and the axis of rotation too is inclined, the
declination changes within a year from value 23.45°
to — 23.45°.
112. • The declination becomes equal to zero two times a
year in days of a spring and autumn equinox.
• The tilt angle is formed between the horizontal plane
and solar panel.
• For example, at installation on an inclined roof, the
tilt angle of a collector is defined by a steepness of a
slope of a roof.
113. • The azimuth characterizes a deviation of the
absorbing plane of a collector from the southern
direction.
• An azimuth = 0° for a solar collector precisely
oriented on the south. The incidence angle of solar
radiation can be determined by the equation
114.
115. • The solar collectors focused in the southern
direction and mounted at an angle from 15° to 25°
concerning the horizon, allow to reach the
maximum value of absorption of sunlight in India.
• Solar devices in this way are able to capture the full
trajectory of sun and get maximum efficiency.
• But even at certain deviations from these conditions,
solar devices can develop enough energy.
• Installation with a small tilt angle is more effective
in case of solar collectors to focus on south.
116. Parabolic Trough solar Collector
orientation
• Parabolic Trough solar Collector (PTC) is often
oriented with its axis horizontally in north-south or
east-west direction.
• Australia lies in southern hemisphere. The solar
panels will be faced towards north to get maximum
efficiency.
• India is located within both the Eastern
Hemisphere, like the rest of Asia, and the
Northern Hemisphere.
117. • The optimum tilt angle is calculated by adding 15
degrees to your latitude during winter, and
subtracting 15 degrees from your latitude during
summer.
• For instance, if your latitude is 34°, the optimum
tilt angle for your solar panels during winter will
be 34 + 15 = 49°
120. ADVANCED CONCENTRATING
COLLECTOR SYSTEM
• For Concentrating Solar Power (CSP) to be a
significant contributor to utility-scale base-load
power, the industry must achieve the drastic cost
reductions and performance increase.
• So, the heavy glass mirrors can be replaced with
long-lasting reflective films supported by a
lightweight and rigid structure.
121. It can be done in the following ways such as
• Developing a new set of technology elements including
advanced reflective films, optically accurate reflector
panels, low-cost space frames, adaptive optics and
accurate tracking drives.
• Designing and building a large format heliostat design
which is used for high and ultra high concentration.
122. ADVANTAGES OF CONCENTRATING
COLLECTORS
The reflecting surface of the concentrating collector
requires less material and structurally simpler than flat-plate
collectors.
The absorber area of a concentrator system is smaller than a
flat-plate system.
The area from which the heat is lost to surroundings is less
than flat-plate collectors.
It can be used for electric power generation.
The total useful operating time per year is large for a
concentrator system than a flat' plate collector.,
The reflecting surface of the concentrating collector
requires less material and structurally simpler than flat-plate
collectors.
The absorber area of a concentrator system is smaller than a
flat-plate system.
The area from which the heat is lost to surroundings is less
than flat-plate collectors.
It can be used for electric power generation.
The total useful operating time per year is large for a
concentrator system than a flat' plate collector.,
123. • Initial installation cost of the collector is less.
• The amount of heat which can be stored per unit
volume is large.
• Heat storage costs are less for concentration systems
than flat-plate collectors.
• Higher temperature of the working fluid is attained
with a concentrating system.
• No anti-freeze is required to protect the absorber in a
concentrator system.
124. Disadvantages of concentrating
collectors
• In concentrating collectors, only the beam radiation is
collected because the diffuse component cannot reflect.
• Costly orienting systems must be used to track the sun.
• Additional maintenance is required to retain the quality
of reflecting surface against dirt, weather and
oxidation.
• It is non-uniform flux on the absorber whereas the flux
in flat-plate collectors is uniform.
• Optical and interrupt losses are in energy balance.
125.
126.
127.
128.
129.
130.
131.
132.
133. When higher temperatures are
required, it becomes necessary to
concentrate the radiation.
This is achieved using line-
focusing or concentrating
collectors.
The collector consists of a
concentrator and a receiver.
The concentrator shown is a
mirror reflector having a shape
of a cylindrical parabola.
It focusses the sunlight on to its
axis, where it is absorbed on the
surface of the absorber tube and
transferred to the fluid flowing
through it.
Focusing or Concentrating Collectors
133
Solar Radiation
Focus
134. Solar radiation coming out from
the particular direction is collected
over the area of the reflecting
surface and is concentrated at the
focus of the parabola, if the
reflector is in the form of a trough
with parabolic cross section, the
solar radiation is focused alone a
line.
Mostly cylindrical parabolic
concentrators are used, in which
absorber is placed along
focus axis.
Line Focusing Collectors
134
Solar Radiation
Focus
135. A concentric glass cover around
the absorber tube helps in
reducing the convective and
radiative losses to the
surroundings.
In order that the sun’s rays should
always be focused on to the
absorber tube, the concentrator has
to be rotated.
This movement is called tracking.
In the case of cylindrical parabolic
concentrators, rotation about a
single axis is generally required.
Fluid temperatures up to 400 ºC
can be achieved in cylindrical
parabolic focussing collector
systems.
Focusing or Concentrating Collectors (Contd.,)
Cylindrical parabolic
concentrating collector
135
137. A paraboloidal dish collector
brings solar radiation to a focus at
a point actually a small central
volume.
A dish 6.6 m in, diameter has
been made from about 200 curved
mirror segments forming a
paraboloidal surface.
The absorber located at the focus
is a cavity made of zirconium
copper alloy with a black chrome
selective coating.
Point focusing collector (Paraboloidal type)
Paraboloid concentrating
collector
137
138. The heat transport fluid flow in to
and out of the absorber cavity
through pipes bonded to the
interior.
The dish can automatically turned
about two axes (up – down and left
– right) so that the sun is always
kept in a line with the focus and the
base (vertex) of the paraboloidal
dish.
Thus, the sun can be fully tracked
at essentially all the times.
Point focusing collector (Paraboloidal type)
Paraboloid concentrating
collector
138
139. The concentration ratios
(concentration ratio is the ratio of the
concentrator aperture to the energy
absorbing area of the receiver, it
determines the effectiveness of the
concentrator), are very high in the case
of the parabolic system and therefore
can be used where high temperatures
are required.
Point focusing collector (Paraboloidal type)
Paraboloid concentrating
collector
139
140. 1. Reflecting surfaces require less
material and are structurally
simpler than flat plate collectors.
2. The absorber area of a
concentrator system is smaller
than that of a flat plate collector
for same solar energy and
therefore the insolation intensity
is greater.
3. The working fluid can attain
higher temperatures in a
concentrating system because of
the area from which heat is lost to
the surroundings per unit of the
solar energy collecting area is less
than flat plate system.
Advantages of Concentrating Collectors over Flat – Plate Type
Collectors
Paraboloid concentrating collector
140
Flat plate collector
141. 4. Improved collector efficiency due
to reduced heat losses with the
small area of absorber per unit of
solar energy collecting area.
5. The total useful operating time
per year of a concentrating system
is large than FPC as it can be used
for power generation purpose too.
6. Due to high temperature obtained
from the concentrator system the
amount of heat which can be
stored per unit volume is larger.
7. Higher efficiencies due to higher
temperature of working fluid.
Advantages of Concentrating Collectors over Flat – Plate Type
Collectors
141
Paraboloid concentrating collector
141
Flat plate collector
142. 1. Beam radiations is only collected
as the diffuse radiation cannot be
reflected and is lost.
2. Costly orienting systems have to
be used to track the sun.
3. Additional requirements of
maintenance particular to retain
the quality of reflecting surface
against dirt, weather, oxidation
etc.
4. High initial cost.
Dis-Advantages of Concentrating Collectors over Flat – Plate
Type Collectors
142
Paraboloid concentrating collector
142
Flat plate collector
143. The efficiency is plotted as a
function of the operating
temperature and is to be
expected, it decreases with
increasing temperature.
Comparison of Collectors Efficiency
Efficiency of various types of collectors as a
function of operating temperature
Dr.N.Shankar Ganesh
143
144. 1. Water heating
2. Space heating
3. Space cooling and refrigeration
4. Power generation
5. Distillation
6. Drying and
7. Cooking
Thermal Applications
144
145. Solar water-heating systems can
be classified into two categories
1. Natural Circulation
(Thermosyphon) systems and
2. Forced Circulation Systems
Natural Circulation Systems
A simple small-capacity natural
circulation system, suitable for
domestic purposes is shown.
The two main components of the
system are liquid flat-plate
collector and the storage tank, the
tank being located above the level
of the collector.
Water Heating
Small capacity natural circulation
water heating system
145
146. As water in the collector is heated
by solar energy, it flows
automatically to the top of the
water tank and it is replaced by
cold water from the bottom of the
tank.
The water expands after heating
and becomes less dense, and thus
more buoyant than the cooler
liquid in the bottom.
Convection moves the heated
liquid upwards in the system as it
is simultaneously replaced by
cooler liquid returning by gravity.
Natural Circulation
Small capacity natural circulation
water heating system
146
147. Hot water for use is withdrawn
from the top of the tank.
Whenever this is done, cold
water automatically enters at the
bottom.
An auxiliary-heating system is
sometimes provided for use on
cloudy or rainy days.
Natural Circulation (Contd.,)
Small capacity natural circulation
water heating system
147
148. When a large amount of hot
water is required for supplying
process heat in an industry or in
a commercial establishment, a
natural circulation system is not
suitable.
Large arrays of flat-plate collectors
are then used and forced
circulation is maintained with a
water pump.
The restriction that the storage
tank should be at a higher level is
thus removed.
Water from a storage tank is
pumped through a collector array
where it is heated and then flows
back into the storage tank.
Forced Circulation Systems
Forced circulation water heating
system-closed loop configuration
148
149. Whenever hot water is withdrawn
for use, cold make-up water takes
its place because of the ball-float
control shown.
The pump for maintaining the
forced circulation is operated by
an on-off controller which senses
the difference between the
temperature of the water at the
exit of the collectors and a suitable
location inside the storage tank.
The pump is switched on
whenever this difference exceeds a
certain value and off when it falls
below a certain value.
Provision is also usually made for
an auxiliary heater.
Forced Circulation Systems (Contd.,)
Forced circulation water heating
system-closed loop configuration
149
150. In this case, a concentrating
collector array is used to heat
pressurized water or a thermic
fluid to a temperature above 100
ºC and the heated liquid is kept in
a storage tank.
A heat exchanger is used to extract
energy from the heated liquid and
obtain either hot water or low
pressure steam for industrial or
commercial use.
An auxiliary heater is provided in
the line in order to supply make-
up energy in case the solar energy
is inadequate.
Solar industrial process heating system
Solar industrial process heating system
using a concentrating collector array
150
152. Active methods:
An active method is one which
utilizes a pump or a blower to
circulate the fluids involved in
the space heating system.
In this system, water is heated in
solar flat plate collectors (A) and
stored in the tank (B).
Energy is transferred to the air
circulating in the space to be
heated by means of the water-to-
air heat exchanger (E).
Two pumps (C) provide forced
circulation between the collectors
and the tank, and between the
tank and the heat exchanger.
Space heating
Schematic diagram of a space
heating system using liquid flat-
plate collectors
Dr.N.Shankar Ganesh
152
153. Provision is also made for
adding auxiliary heat (D).
Since the solar energy is first
being used to heat water, the
system can be easily modified
to be a two-in-one system
supplying hot water as well as
hot air for space heating.
Space heating (Contd.,)
Schematic diagram of a space heating
system using liquid flat-plate collectors
153
154. Space heating alternate approach
Schematic diagram of a space heating system using solar air heaters
154
155. An alternative approach to space
heating is to heat air directly in
solar air heaters (A).
The heat is then stored in a
porous bed storage (B) packed
with rock, gravel or pebbles.
Energy is extracted and
transferred to the space to be
heated by blowing cool air
through the porous bed.
Once again an auxiliary heater
(D) is provided for supplying
make-up heat.
Space heating (Contd.,)
Schematic diagram of a space heating
system using solar air heaters
155
158. Passive Methods:
A passive method is one in
which thermal energy flows
through a living space by natural
means without the help of a
mechanical device like a pump
or a blower.
The south face of the house to be
heated is provided with a single
or double glazing.
Behind it is a thick, “black”
concrete wall, which absorbs the
sun’s radiation and serves as a
thermal storage.
Space heating (Contd.,)
Space heating by passive
methods – the trombe wall
158
159. Vents (A and B), which can be
kept open or closed, are
provided near the top and
bottom of the storage wall.
The whole unit consisting of the
storage wall with vents and the
glazing is referred to a Trombe
wall.
During the day, both vents A
and B are kept open.
The air between the inner
glazing and the wall gets heated
and flows into the living space
through the top vent.
Space heating (Contd.,)
Schematic diagram of a UTC-
based space heating system
Dr.N.Shankar Ganesh
159
160. Simultaneously, the cooler air
from the room is pulled out of
the living space through the
bottom vent.
Thus, a natural circulation path
is set up.
Some energy transfer to the
living space also takes place by
convection and radiation from
the inner surface of the storage
wall.
During the night, both vents are
closed and energy transfer takes
place only by convection and
radiation from the inner surface.
Space heating (Contd.,)
Schematic diagram of a UTC-
based space heating system
160
161. The trombe wall design can also
provide summer ventilation by
using vents C and D near the top
of the glazing and on the north-
facing wall.
On a hot summer day, vents B, C
and D would be kept open, while
vent A would be kept closed.
The heated air between the glazing
and the wall would then flow out
through vent C, drawing air from
the living space to replace it.
This in turn would cause air to be
pulled in from outside through
vent D.
Space heating (Contd.,)
Schematic diagram of a UTC-
based space heating system
161
163. One of the interesting thermal
applications of solar energy is for
the purpose of cooling.
Space cooling may be done with
the objective of providing
comfortable living conditions (air-
conditioning) or of keeping a food
product cold (refrigeration).
Water heated in a flat-plate
collector array is passed through a
heat exchanger called the
generator, where it transfers
heat to a solution mixture of the
absorbent and refrigerant, which
is rich in the refrigerant. 163
Space cooling and Refrigeration (contd.,)
164. Refrigerant vapour is boiled off at
a high pressure and goes to the
condenser where it is condensed
into a high pressure liquid.
The high pressure liquid is
throttled to a low pressure and
temperature in an expansion
valve, and passes through the
evaporator coil.
Here the refrigerant vapour
absorbs heat and cooling is
therefore obtained in the space
surrounding this coil.
Space cooling and Refrigeration (contd.,)
164
165. The refrigerant vapour is now
absorbed into a solution mixture
withdrawn from the generator,
which is weak in refrigerant
concentration.
This yields a rich solution which
is pumped back to the generator,
thereby completing the cycle.
The rich solution flowing from the
absorber to the generator is
usually heated in a heat exchanger
by the weak solution withdrawn
from the generator.
Space cooling and Refrigeration (contd.,)
165
166. This helps to improve the
performance of the cycle.
Some of the common refrigerant-
absorbent combinations used are
ammonia-water and water-lithium
bromide, the latter being
essentially for air-conditioning
purposes.
Space cooling and Refrigeration (contd.,)
166
168. SOLAR THERMAL POWER
GENERATION
Solar thermal power generation employs power
cycles which are broadly classified as follows:
• Low temperature cycles.
• Medium temperature cycles.
• High temperature cycles.
169. • Low temperature - flat-plate collectors -maximum
temperatures 100°C.
• Medium temperature - maximum temperature 150 to
300°C
• high temperature - temperatures above 300°C.
• For low and medium temperature ranges, the
thermodynamic cycle preferred is Rankine cycle.
• For high temperature range apart from Rankine cycle,
Brayton and Stirling cycles are also being considered.
173. Low temperature cycles.
• Using Flat Plate Collector.
• temperature range in the order of 60 to 100°C with collection
efficiency of 30 to 50% maximum.
• not suitable to employ Rankine cycle because steam
generation is not possible.
• Organic fluid (commonly Freon group)
• In the heat exchanger, the heat of water is transferred to
butane for boiling it.
174.
175. Medium Temperature Thermal Power
Generation Using Solar Distributed
Collector
• parabolic trough concentrator collectors with line
focus are most commonly used.
• These systems can also use paraboloid dish type
concentrating collectors.
• Cylindrical parabolic concentrating collectors
generate temperature in range of 250 to 700°C with
efficiency of 50-70%.
• High temperature - many flat mirrors produce a
temperature in range of 600-2000°C with an
efficiency of 60-75%.
176. Types of heat engines are commonly being used with
parabolic dish/trough systems.
• Rankine cycle engine.
• Organic-Rankine cycle engine.
• Stirling cycle engine.
• Air-Brayton cycle engine.
177. • hot gas or vapour is expanded in turbine and cooled.
• vapour is further cooled to reject heat and it is finally
returned to its initial state .
• In a distributed collector system, the solar thermal
energy is collected from a large number of sun-
tracking solar parabolic trough type or paraboloidal
dish type cylindrical collectors.
• Each collector transfers heat to a heat-transport fluid.
• This heat transporting fluid available at high
temperature from the collectors is pooled at some
central power station.
• heat transfer fluid- water/steam.
178. • A simple parabolic concentrator solar power generation system using
water as working fluid.
• It consists of a parabolic cylinder reflector to concentrate sun light on
to a collecting pipe within Pyrex or glass envelop.
• A proper sun-tracking arrangement is made so that the maximum
sunlight is focused on the reflector for producing optimum efficiency.
• They usually operate in the lower temperature ranges of about 90 to
315°C.
• In this system, oil having boiling point higher than the boiling point of
water is used to circulate through the absorber tube of the concentrator
collector.
• The heated oil is then passed through the heat exchanger where the
heat is transferred to the water to produce steam.
• The hot steam can then be directly used to power a turbine for
mechanical work which is coupled to an electric generator to generate
electricity.
• The exhaust low temperature steam from the turbine is condensed in a
condenser with the help of fresh cold water circulated by a water
pump. This condensate is fed to the heat exchanger again using a
condensate pump.
179.
180. High Temperature Thermal Power
Generation Using Central Receiver
System
• A larger solar thermal power plant in the range of 50 MW to
200MW comes under Central Receiver Schemes (CRS).
• Such systems are economical in MW range for network
connected plants.
• The high capacity is possible due to high temperature steam in
the central receiver resulting high efficiency of plants. In the
central receiver scheme, several heliostats are located on the
ground level.
• A heliostat is a nearly flat mirror with the provision to track the
sun in two planes.
• The reflected rays are pointed towards a central receiver
mounted on a tall tower.
• A large central receiver plant is usually built up based on
modular concepts. Each plant may have 2 modules to 10
modules and rated at 10 MW to 100 MW.
181. Components of Central Receiver
System
This system can be subdivided into the following
subsystems. They are as follows:
1. The tower with the central receiver on top kit.
2. The heat conversion sub system.
3. The heat storage device.
4. The field of oriented mirrors.
182.
183. CENTRAL RECEIVER
The central receiver at the top of the tower as a heat
absorbing surface by which the heat transport fluid is
heated.
There are two basic receiver configurations as
follows.
• Cavity receiver type.
• External receiver type.
184.
185. Heat conversion sub system
• Liquid water under pressure enters the receiver.
• Then, the heat energy is absorbed by the water and it
leaves as superheated steam.
• Typical steam conditions might be a temperature of
500°C and pressure of 100 atm.
• The steam is piped to a ground level where it drives
conventional turbine generator system.
186. Heat storage device
• fire bricks
• ceramic oxides
• fused salts and sulphur.
The choice of a conventional storage material is
determined by its
• energy density
• thermal conductivity
• corrosion characteristic
• cost and convenience of use.
• operating temperature of working fluid.
187. Mirrors
• The flat mirror surface can be manufactured by
metallization of float glass or flexible plastic sheets.
The mirror must be steerable.
• The glass mirrors would not be capable of
withstanding the wind load which often occurs in
arid lands without any supporting structure.
188.
189. • Gas turbine power plant working on Brayton cycle.
• The mirrors (heliostats) - reflect the direct beam
radiation receiver to produce high temperature.
• Beam radiation incident in the boiler is absorbed by
black pipes in which the working fluid is circulated
and heated.
• The working fluid is allowed to drive a turbine
thereby producing mechanical energy.
• The turbine which is coupled to an alternator
produces electrical energy.
• A suitable heat storage is also provided to supply the
heat energy during the period of cloudiness.
190.
191. Analysis of a central receiver
system
• The mirror field in a central receiver system is to be
laid in such a way to suit for both in winter and
summer seasons.
• There is no shade of one mirror on the other mirror.
• Therefore, the heliostats are put apart and a fraction
of the ground (φ) is only covered.
192.
193.
194.
195.
196. Advantages of central Receiver
system
• Very high temperature is obtained to produce steam.
• It provides good efficiency.
• A larger area can be covered by using relatively
inexpensive mirrors rather than using expensive
solar cells.
• Concentrated light can be redirected to a suitable
location via, optical fiber cable.
• For example, illuminating buildings similar to
hybrid solar lighting.
197. Disadvantages
• Concentrated collector systems require dual axis sun
tracking to maintain the sunlight focus at the
collector.
• Inability to provide power in diffused light
conditions.
• Solar cells are able to provide some output even if
the sky becomes cloudy but power output from
concentrating systems drop drastically in cloudy
conditions as the diffused light cannot be
concentrated passively.
198.
199.
200.
201. SOLAR POND
• a pool of very salty water in which convection is
inhibited, allowing accumulation of energy from
solar radiation in the lower layers.
Notas del editor
Examples of basic passive solar structures are greenhouses, sunrooms .
As the sun rays pass through the glass windows, the interior will absorb and reta..1 tie net In passive solar energy systems, the energy collected through solar Stem if. distributed according to a law of thermodynamics. It states that the heat moves frorrvarm. to -011 '1%4s and surfaces. The simple way of transferring heat from passive scl: co; ,i.tori convection mode. Ancient people used passive solar energy systems They build their house: o,:t of :tow or clay which absorbed the sun's heat during the day and stayed warm aftet Jerk, heat throughout the night. Hence, the properties of building materials and vu 1.1.1g both play a role in the energy balance of the system. However, it requires a caretitl design and is often difficult to implement in an older and present buildings those did not he design feature when manufactured. Builders today use similar methods for building new buildings for passively capturing solar energy. Passive solar energy systems include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air. Strategic planning of building location, orientation and materials provide a great control over the inside temperature. Installing large south facing windows, planting shade trees and using Trombe walls which are made of absorptive materials that store heat during the day and slowly release it at night are measures that can be adopted for new and old buildings alike. For example, Sunspaces or glass rooms built on the south side of a building can provide up to 60 % of a home's winter heating. Also, builders construct houses with large double- or triple-paned windows that get direct sunlight to capture and magnify the sun's warmth. The inside air becomes much warmer than the air outside because the windows let in the sun's energy and trap it, gradually raising the temperature.
For example, a solar collector positioned on the roofs of buildings heats the fluid and then pumps it through a system of pipes to heat the whole building.
The most common application of these systems is the production of domestic water heater known as solar water heaters.
These heaters are one of the most commonly implemented renewable energy technologies because of their cost effectiveness and relatively simple installation.
Solar water heaters can reduce conventional energy consumption for heating water by 50% in commercial applications and up to 75% in homes
PV cells are effective in all regions of the country. In a solar PV array, it does not matter how hot the sun is by the time its light and warmth reach the home, as long as some of the sun's rays are hitting the solar panels.
PV systems may or may not be connected to the electric transmission grid. When the number of PV panels are arranged as an array and it is connected to the grid, the excess power can be sold to the electric company.
Commercially available PV panels are up to 22.5% efficient at converting sunlight into electricity in optimal conditions but even in partly cloudy weather they can operate at 80% of their maximum output.
The collectors receive heat from solar rays and give it to the heat-transport fluid. Solar collector surface is designed for high absorption and low emission. So, the utilization of solar energy requires solar collectors.
The solar energy collector with its associated absorber is the essential component of any systems for the conversion of solar radiation into more usable form (e.g. heat or electricity).
This type shows better efficiency than the flat plate type. For the best efficiency, collectors should be mounted to face the sun as it moves through the sky.
A greenhouse is an enclosure having transparent glass planes or sheets. It behaves differently for incoming visible (short wave) radiation and outgoing infrared (long wave) radiation. Once, the sun's energy passes through the glass planes, it is absorbed by some material inside the house and the heat will not be reradiated back outside. Glass, therefore, acts as a heat trap and it has been recognized as a source for the construction of greenhouses.
". Normal Concentration of CO2 in the atmosphere is 0.03%.
It is due to greenhouse effect that the earth maintains an average surface temperature of 15°C.
In the absence of this layer, the earth would be a frozen planet at about —25°C (the temperature of the outer atmosphere), Any further increase in the concentration of CO2 will upset the temperature balance of the universe.
It would cause warming of the globe and have disastrous consequences.
They are made in rectangular panels from about 1.7 to 2.9 m2 in area and they are relatively simple to construct and erect.
The constructional details of simple flat plate collector are shown in Figure 2.3(a) and positioning of this type of collector is shown in Figure 2.4.
It means that the radiation and other conditions are essentially constant for a long time enough for the outlet temperature and useful gain to become steady.
It has additional reflective side faces which provide higher concentration ratio up to 10 and higher temperature of working fluids (up to 200°C) is achieved.
Such a design is aligned in East-West direction and it requires a periodic tilt adjustment. The different optimum depth to base width ratios and cone angles are possible depending on the frequency of seasonal tilt adjustment.
Advantages of solar air heaters:
It is compact, simple in construction and it requires little maintenance.
2. The need to transfer thermal energy from the working fluid to another fluid is eliminated as air is used directly as the working fluid.
3. Corrosion is completely eliminated.
4. Leakage of air from the duct is less severe.
5. Possibility of freezing of working fluid is also eliminated.
6. The pressure inside the collector does not become very high.
The vacuum is created between absorber and transparent glass cover.
the tube cover above the selective surface is evacuated and evacuated tubes ate al-Hinged above the absorber surface so that there should not be any space left between consecutive tubes.
The evacuated tubes provide a vacuum layer above the absorber to reduce the top loss coefficient.
The vacuum layer suppresses the convection heat loss from the absorber to plass cover.
Similar to the flat plate collector, an incident solar radiation is absorbed by absorber surface after transmission through the glass cover and transparent evacuated tubes. After absorption, most of the available thermal energy at the absorber will be first conducted and then it is convected to the working fluid below the absorber.
Rest of the energy is lost by radiative heat loss.
Further, there will be convective and radiative heat losses from the upper portion of the evacuated tubes to the glass cover.
Since the temperature of the upper portion of the evacuated tubes will be small, there will be less heat losses. The working fluid may be either a liquid or air.
Thermal analysis offal-plate collector:
When a body is subjected to solar radiation of intensity I, it is partially absorbed by the body and the remaining is partially transmitted and rest reflected.
The absorption part of energy is used for increasing the temperature of the body, gut, a portion of the energy is lost by conduction, convection and radiation.
For thermal equilibrium, energy losses are equal to the absorbed energy.
Let us assume for thermal analysis purpose, conduction and convection losses are negligible
From above equation, it is clear that the higher ratio of absorption coefficient to emission coefficient will lead to higher equilibrium temperature.
However, the amount of energy collected does not depend on this ratio.
It depends mainly on higher value of a only.
Hence, flat plate collectors are made by materials having higher value of a and painted with black colour.
The performance of a flat plate collector is described by an energy balance equation.
It indicates the distribution of incident solar energy into useful energy gain and various losses.
The useful thermal energy output (Qu) per unit time of a flat plate collector of area A, is the difference between the absorbed solar radiation, q,, and thermal loss.
The flat plate collector efficiency factor (F) is defined as the ratio of actual rate of useful heat collection to the rate of useful heat collection rate when the collector absorbing plate (Tp) is placed at the local fluid temperature (T1),
Heat removal factor (FR) of a flat plate collector is defined as the ratio of the actual useful energy gain to the useful energy gain if the entire flat plate collector was at the fluid inlet temperature (T5) (forced circulation flow).
There is a variation of solar radiation with inclination of the surface for given latitude, orientation, time of the day and day of the year.
Hence, there should be an optima inclination to receive maximum radiation. On the basis of a literature survey, an optima
special form of flat-plate collector modified by introducing a reflecting surface (concentrator) between solar radiation and absorber.
In these collectors, the radiation falling on a relatively ,large area is focused onto a receiver of considerably smaller area.
Latitude is an angle (defined below) which ranges from 0° at the Equator to 90° (North or South) at the poles.
The tilt angle of the photovoltaic (PV) array is the key to an optimum energy yield. Solar panels or PV arrays are most efficient, when they are perpendicular to the sun's rays. The default value is a tilt angle equal to the station's latitude plus 15 degrees in winter, or minus 15 degrees in summer (1).
Low temperature cycles generally use flat-plate collectors so that maximum temperatures are limited to about 100°C.
Medium temperature cycles work at maximum temperature ranging from 150 to 300°C while high temperature cycles work at temperatures above 300°C.
For low and medium temperature ranges, the thermodynamic cycle preferred is Rankine cycle.
For high temperature range apart from Rankine cycle, Brayton and Stirling cycles are also being considered.
The Rankine cycle is a model used to predict the performance of steam turbine systems.
It was also used to study the performance of reciprocating steam engines.
The Rankine cycle is an idealized thermodynamic cycle of a heat engine that converts heat into mechanical work while undergoing phase change.
The Brayton cycle is a thermodynamic cycle named after George Brayton that describes the workings of a constant-pressure heat engine. The original Brayton engines used a piston compressor and piston expander, but more modern gas turbine engines and airbreathing jet engines also follow the Brayton cycle
The Stirling cycle is the thermodynamic cycle on which the Stirling engines work.
n Stirling cycle gas like air, helium, hydrogen etc is used as the working fluid.
All the processes of Stirling cycle are assumed to be reversible process, hence when the gas is heated the engine produces work or power and when work is supplied to the cycle it works as the refrigerator or the heat pump.
The reversed Stirling cycle is used extensively in the field of cryogenics as the cryogenerator to produce extremely low temperatures or liquefy gases like helium and hydrogen.
The exhaust butane vapour from butane turbine is condensed in a condenser with the help of fresh cold water circulated by a water pump.
This condensate is fed to the heat exchanger or butane boiler.
Low Temperature Thermal Power Generation Using Flat Plate Collector.
The flat plate collector system and solar pond are classified as low temperature collectors because of its temperature range in the order of 60 to 100°C with collection efficiency of 30 to 50% maximum.
In this system, it is not suitable to employ Rankine cycle solar thermal power production system because the generation of steam using flat plate collector or solar pond is not possible as the boiling temperature of the water is more than 100°C and so, it cannot be used directly to run the prime mover.
Therefore, some other Organic fluid (commonly Freon group) is used which evaporates at low temperature and high pressure by absorbing the heat from the heated water.
The vapour formed can be used to run a turbine or engine which may generate power whiCh will be sufficient to light the group of houses for rural areas and for irrigation purposes.
A low temperature solar engine- using heated water from flat-plate solar collector and butane as the working fluid is shown in Figure 2.15.
The system has array of flat-plate collectors to heat water up to nearly 70y C. In the heat exchanger, the heat of water is transferred to butane for boiling it.
The high pressure butane vapour runs a butane turbine
Medium temperature thermal power generation systems employ solar distributed collectors in which parabolic trough concentrator collectors with line focus are most commonly used for converting the solar energy into heat energy.
These systems can also use paraboloid dish type concentrating collectors.
Cylindrical parabolic concentrating collectors generate temperature in range of 250 to 700°C with efficiency of 50-70%.
High temperature collectors such as paraboloid type concentrators consist of many flat mirrors produce a temperature in range of 600-2000°C with an efficiency of 60-75%.
This is a modular system consisting of a dish shaped parabolic collector for focusing the solar radiation on d receiver to heat a working fluid coupled with a power generation (engine/alternator) for electricity generation.
It may also be used for producing process heat
In each of these cycles, hot gas or vapour is expanded through an engine or turbine to produce work and it is thereby cooled.
The gas or vapour is further cooled to reject heat and it is finally returned to its initial state for getting energised by solar radiation and thus, completing the cycle.
As noted earlier, distributed solar energy collectors may be preferred for thermal power plants of moderately small capacity.
In a distributed collector system, the solar thermal energy is collected from a large number of sun-tracking solar parabolic trough type or paraboloidal dish type cylindrical collectors.
Each collector transfers heat to a heat-transport fluid. This heat transporting fluid available at high temperature from the collectors is pooled at some central power station.
The heat transfer fluid could be water/steam to be used directly in a steam turbine.
A simple parabolic concentrator solar power generation system using water as working fluid.
It consists of a parabolic cylinder reflector to concentrate sun light on to a collecting pipe within Pyrex or glass envelop.
A proper sun-tracking arrangement is made so that the maximum sunlight is focused on the reflector for producing optimum efficiency.
They usually operate in the lower temperature ranges of about 90 to 315°C.
In this system, oil having boiling point higher than the boiling point of water is used to circulate through the absorber tube of the concentrator collector.
The heated oil is then passed through the heat exchanger where the heat is transferred to the water to produce steam.
The hot steam can then be directly used to power a turbine for mechanical work which is coupled to an electric generator to generate electricity.
The exhaust low temperature steam from the turbine is condensed in a condenser with the help of fresh cold water circulated by a water pump. This condensate is fed to the heat exchanger again using a condensate pump.
A larger solar thermal power plant in the range of 50 MW to 200MW comes under Central Receiver Schemes (CRS).
Such systems are economical in MW range for network connected plants.
The high capacity is possible due to high temperature steam in the central receiver resulting high efficiency of plants. In the central receiver scheme, several heliostats are located on the ground level.
A heliostat is a nearly flat mirror with the provision to track the sun in two planes.
The reflected rays are pointed towards a central receiver mounted on a tall tower.
A large central receiver plant is usually built up based on modular concepts. Each plant may have 2 modules to 10 modules and rated at 10 MW to 100 MW.
In the cavity receiver type, the solar radiation reflected by heliostats enters through aperture at the bottom of the cavity whereas the absorber surfaces are on the exterior of a roughly cylindrical structure in the external receiver type.
Gas turbine power plant working on Brayton cycle.
The mirrors (heliostats) installed on the ground are oriented so as to reflect the direct beam radiation into an absorber or receiver which is mounted at the top of a tower located near the center in the field of mirrors to produce high temperature.
Beam radiation incident in the boiler is absorbed by black pipes in which the working fluid is circulated and heated.
The working fluid is allowed to drive a turbine thereby producing mechanical energy.
The turbine which is coupled to an alternator produces electrical energy.
A suitable heat storage is also provided to supply the heat energy during the period of cloudiness.
Very high temperature is obtained to produce steam.
It provides good efficiency. By concentrating the sunlight, this system can get better efficient than simple solar cells.
A larger area can be covered by using relatively inexpensive mirrors rather than using expensive solar cells.
Concentrated light can be redirected to a suitable location via, optical fiber cable. For example, illuminating buildings similar to hybrid solar lighting.
Concentrated collector systems require dual axis sun tracking to maintain the sunlight focus at the collector.
Inability to provide power in diffused light conditions.
Solar cells are able to provide some output even if the sky becomes cloudy but power output from concentrating systems drop drastically in cloudy conditions as the diffused light cannot be concentrated passively.
