2. Why Solar
Energy?
• The sun is a sphere of
intensely hot gaseous matter
with a diameter of 1.39 ×109
m
• The sun is about 1.5×108 km
away from earth, so, because
thermal radiation travels
with the speed of light in
vacuum (about 300,000
km/s), after leaving the sun
solar energy reaches our
planet in 8 min and 20 s.
2
3. Solar Energy?
• Large, inexhaustible source of energy
• Power from the Sun on the Earth = 1.8 x 1011 MW.
• Environmentally clean source of energy
• Free and available in the adequate quantity
• Dilute source of energy, as the solar radiation flux available rarely exceeds the 1 kW/m2 and the
total radiation over a day is at best about 7 kW/m2 in the hottest region on the earth.
• Low values for the technological utilization.
• Large collecting areas are required which results in excessive costs.
• Availability varies widely with the time.
• Need for storages also adds significantly to the cost.
• Real challenge is of an economic nature.
3
4. SOLAR GEOMETRY
Irradiance (W/m2): The rate at which radiant energy is incident on a surface, per unit area of surface
per second. The symbol G is used for solar irradiance, with appropriate subscripts for beam, diffuse, or
spectral radiation.
Irradiation or Radiant Exposure (J/m2): The incident energy per unit area on a surface, found by
integration over a specified time, usually an hour or a day.
• Insolation is a term applying specifically to solar energy irradiation.
• The symbol H is used for insolation for a day.
• The symbol I is used for insolation for an hour (or other period if specified).
• The symbols H and I can represent beam, diffuse, or total and can be on surface of any orientation.
5. SOLAR GEOMETRY
• Subscripts on ‘G’, ‘H’, and ‘I’ are as follows:
• o refers to radiation above the earth's atmosphere, referred to as extraterrestrial radiation;
• b and d refer to beam and diffuse radiation;
• T and n refer to radiation on a tilted plane and on a plane normal to the direction of
propagation, respectively.
• If neither ‘T’ nor ‘n’ appear, the radiation is on a horizontal plane.
• Radiosity (W/m2):The radiation at which radiant energy leaves a surface, per unit are by combined
emission, reflection, and transmission.
• Emissive Power (W/m2):The rate at which radiant energy leaves a surface per unit area, by
emission only.
• If θ is the angle between an incident beam, of flux Ibn and the normal to a plane surface, then the
equivalent flux falling normal to the surface is given by Ibncosθ.
6. SOLAR GEOMETRY
• Earth as it rotates in 24 h about its own axis, which
defines the points of the north and south poles N and S.
• The axis of the poles is normal to the earth’s equatorial
plane. C is the centre of the Earth.
• The point P on the Earth’s surface is determined by its
latitude ϕ and longitude 𝜓.
• Latitude (Φ): It is the vertical angle between the line
joining that point of location to the centre of the
earth and its projection on the equatorial plane.
• Latitude is defined positive for points north of the
equator, negative south of the equator.
Figure: Definition sketch for latitude ϕ and
longitude 𝜓
7. SOLAR GEOMETRY
• Longitude: The angular distance between a point on
any meridian and the prime meridian at Greenwich.
• By international agreement. longitude is measured
positive eastwards from Greenwich, England.
• The vertical north–south plane through P is the local
meridional plane.
• E and G are the points on the equator having the same
longitude as P and Greenwich respectively.
Figure: Definition sketch for latitude ϕ and
longitude 𝜓
8. SOLAR GEOMETRY
Declination Angle (𝜹):
The angular position of the Sun at solar noon (i.e., when the
Sun is on the local meridian) with respect to the plane of
the equator, north positive;
-23.45° ≤ δ ≤ 23.45°
𝛿 𝑖𝑛 𝑑𝑒𝑔𝑟𝑒𝑒𝑠 = 23.45 sin
360
365
(284 + 𝑛)
10. SOLAR GEOMETRY
Inclination Angle (𝜶):
• The angle between Sun’s ray and its projection
on a horizontal surface is known as Inclination
Angle.
• 𝜶 = 𝟎° at sunrise and sunset.
11. SOLAR GEOMETRY
Zenith Angle (𝜽𝒛):
• The angle between Sun’s ray and the
perpendicular (normal) to horizontal plane is
known as the Zenith Angle.
• 𝜶 + 𝜽𝒛 = 𝟗𝟎°
• At sunrise, 𝜽𝒛 = 𝟗𝟎°
• At sunset,𝜽𝒛 = −𝟗𝟎°
12. SOLAR GEOMETRY
Solar Azimuth Angle (𝜸𝒔):
• The angle on a horizontal plane, between the
line due south and the projection of Sun’s ray
on the horizontal plane is known as Solar
Azimuth Angle.
• It is considered as positive when measured
from south towards west.
13. SOLAR GEOMETRY
Slope (β): The angle between the plane of the surface and the horizontal;
0 ≤ β ≤ 180°
14. SOLAR GEOMETRY
Angle of Incidence (𝜽): The angle between the Sun’s ray incident on plane surface and
normal to that surface.
For horizontal surface,
β = 0°
Zenith angle = Incidence angle
15. SOLAR GEOMETRY
Surface Azimuth Angle (γ):
• It is the angle made in the horizontal plane
between the line due south and the
horizontal projection of the normal to the
surface on the horizontal plane.
• It can vary from -180° to +180°.
• The angle is positive when it is measured
from South towards west.
17. SOLAR GEOMETRY
Hour Angle (ω):
• The hour angle ω is the angle through which the Earth has rotated since solar noon.
• It is an angular measure of time and is equivalent to 15° per hour. It also varies from -180°
to +180°.
𝝎 =
𝟑𝟔𝟎°
𝟐𝟒 𝒉𝒓
𝒕𝒔𝒐𝒍𝒂𝒓 − 𝟏𝟐 = 𝟏𝟓° 𝒕𝒔𝒐𝒍𝒂𝒓 − 𝟏𝟐
• Negative in the morning and Positive in the afternoon.
• Here, tsolar is the Apparent Solar Time.
18. SOLAR ENERGY APPLICATIONS
❑Direct Methods
• Thermal
• Photovoltaic
❑Indirect Methods
• Hydrogen Cells
• Tidal
• Wind
• Biomass
• Wave energy
• Ocean temperature difference
19. SOLAR RADIATION
❑The radiation that is important to solar energy applications is that emitted by the sun within the ultraviolet,
visible, and infrared regions. Therefore, the radiation wavelength that is important to solar energy
applications is between 0.15 and 3.0 μm. The wavelengths in the visible region lie between 0.38 and 0.72
μm.
Thermal radiation
❑Thermal radiation is a form of energy emission and transmission that depends entirely on the temperature
characteristics of the emissive surface. Thermal radiation is in fact an electromagnetic wave that travels at the
speed of light (C = 300,000 km/s in a vacuum). This speed is related to the wavelength (𝝀) and frequency (𝒗)
of the radiation as given by the equation:
20. SOLAR RADIATION
Extraterrestrial solar radiation
❑The amount of solar energy per unit time, at the mean distance of the earth from the sun, received on a unit
area of a surface normal to the sun (perpendicular to the direction of propagation of the radiation) outside the
atmosphere is called the solar constant (1366.1 W/m2).
❑When the sun is closest to the earth, on January 3, the solar heat on the outer edge of the earth’s atmosphere
is about 1400 W/m2; and when the sun is farthest away, on July 4, it is about 1330 W/m2.
21. SOLAR RADIATION
❑Terrestrial solar radiation
❑Beam radiation
❑Diffuse radiation
❑Ground-reflected radiation
Solar radiation measuring equipment
21
Sunshine Recorder
25. FLAT PLATE COLLECTOR
Advantages:
▪ It utilize the both direct and diffuse radiation
▪ Stationary design
▪ No moving component
▪ Little maintenance
Disadvantages:
▪ No optical concentration
▪ Higher heat loss
▪ Low Collection efficiency
26. FLAT PLATE COLLECTOR
▪ Absorber plate is made by a metal sheet ranging thickness from 0.2 to 1 mm.
▪ Tubes ranges in diameter from 1 to 1.5 cm.
▪ Tubes are soldered, brazed, welded or pressure bonded to the bottom of the absorber
plate.
▪ Tubes may be bounded to the top or are in-line and integral with the absorber plate
Metal:
▪ Copper;
▪ Aluminum sheets fixed to copper or galvanized steel tubes with pressure bond;
▪ mild steel or galvanized steel sheets with galvanized steel tubes;
▪ stainless steel sheets with built in channels.
27. FLAT PLATE COLLECTOR
Header pipes
▪ Lead the water in and out of the collector and distribute it to the tubes.
▪ Same material as the tubes
▪ Diameter: 2 – 2.5 cm
Transparent Cover
▪ Plain or toughened glass of 4 – 5 mm thickness
▪ Use one or two covers with spacing ranging from 1.5 to 3 cm
28. FLAT PLATE COLLECTOR
Insulation
▪ Bottom and sides are insulated
▪ Mineral wool, rock wool, or glass wool with
covering of aluminium foil
▪ Thickness ranging from 2.5 to 8 cm
Collector Box
▪ Made of Aluminium, steel sheet or fibre glass
Size
▪ Face area of collectors are around 2 m2.
▪ Length along with the slope and larger than the
width
30. EVACUATED TUBE COLLECTOR
▪ The Evacuated or Vacuum tubes collector
consists of a number of rows of parallel
transparent glass tubes connected to a header
pipe and where the heat transfer fluid (usually
50% Propylene Glycol) circulates and absorb
heat generated by tubes.
▪ These glass tubes are cylindrical in shape.
Therefore, the angle of the sunlight is always
perpendicular to the heat absorbing tubes
which enables these collectors to perform well
even when sunlight is low such as when it is
early in the morning or late in the afternoon, or
when shaded by clouds.
▪ Evacuated tube collectors are particularly
useful in areas with cold, cloudy wintry
weathers
31. EVACUATED TUBE COLLECTOR
▪ Evacuated tube collectors are made up of a
single or multiple rows of parallel, transparent
glass tubes supported on a frame.
▪ Each individual tube varies in diameter from
between 1" (25 mm) to 3" (75 mm) and
between 5′ (1500 mm) to 8′ (2400 mm) in
length depending upon the manufacturer.
▪ Each tube consists of a thick glass outer tube
and a thinner glass inner tube, (called a “twin-
glass tube”) or a “thermos-flask tube” which
is covered with a special coating that absorbs
solar energy but inhibits heat loss.
▪ The tubes are made of borosilicate or soda
lime glass, which is strong, resistant to high
temperatures and has a high transmittance for
solar irradiation.
32. EVACUATED TUBE COLLECTOR
▪ Inside the each glass tube, a flat or curved
aluminium or copper fin is attached to a
metal heat pipe running through the inner
tube.
▪ The fin is covered with a selective coating
that transfers heat to the fluid that is
circulating through the pipe.
▪ This sealed copper heat pipe transfers the
solar heat via convection of its internal heat
transfer fluid to a “hot bulb” that indirectly
heats a copper manifold within the header
tank.
33. CONCENTRATING COLLECTOR
▪ Higher temperature, 100 - 400° C or above
▪ Concentration is achieved by using a reflecting arrangement of mirrors or a reflecting
arrangements of lenses.
▪ The optical system direct the solar radiation onto an absorber of smaller area which is
usually surrounded by a transparent cover.
▪ Because of the optical system, several losses also introduced, such as absorption and
reflection losses in the mirrors or lenses, and losses due to geometrical imperfections
in the optical system.
▪ The combined effect of all such losses is indicated through the introduction of a term
called the optical efficiency.
▪ Follow or track the sun
▪ Most of the diffuse radiation is lost because it does not focused.
34. CONCENTRATING COLLECTOR…DEFINITIONS
▪ Concentrator: the optical subsystem which directs the
solar radiation on to the absorber
▪ Receiver: Subsystem consisting of the absorber, its
cover and other accessories.
▪ Aperture (W): it is the plane opening of the
concentrator through which the solar radiation passes.
▪ For a cylindrical or linear concentrator, it is
characterized by the width, while for a surface of
revolution, it is characterized by the diameter of the
opening.
▪ Concentration ratio (C): Ratio of the effective area of
the aperture to the surface area of the absorber.
▪ Values of C vary from unity to a few thousands for a
parabolic dish.
35. CONCENTRATING COLLECTOR…DEFINITIONS
▪ Acceptance Angle (2θa): the angle over which
beam radiation may deviate from the normal to the
aperture plane and yet reach the absorber.
▪ Collector with large acceptance angles require
only occasional adjustments, while collectors with
small acceptance angles have to be adjusted
continuously.
37. CYLINDRICAL PARABOLIC COLLECTOR
▪ Also referred as parabolic through or a linear
parabolic collector.
▪ Basic elements: absorber tube located at the
focal axis through which the liquid to be heated
flows, the concentric transparent cover and the
parabolic concentrator
▪ Aperture area: 1 – 6 m2.
▪ Concentration ratio range from 10 to 80, and
rim angles from 70 to 120°. (The incident
radiation on the reflector at the rim of the
collector makes an angle with the center line of
the collector, which is called the rim angle.)
38. CYLINDRICAL PARABOLIC COLLECTOR
▪ Absorber tube is usually made of mild steel
or copper, and has a dia. of 2.5 to 5 cm.
▪ It is coated with a heat resistant black paint and
surrounded by a concentric glass cover with
an annular gap of 1 or 2 cm.
▪ In the case of high-performance collectors, the
absorber tube is coated with a selective
surface like black chrome and the space
between the tube and the glass cover is
evacuated.
▪ In some collectors, the concentric cover is
replaced by a glass or plastic sheet covering
the whole aperture area of the collector. Such
an arrangement helps in protecting the
reflecting surface from the weather.
39. CYLINDRICAL PARABOLIC COLLECTOR
▪ The liquid heated in the collector depends upon the temperature required.
▪ Usually organic heat transfer liquids (referred to as thermic fluids) are used.
Because of their low thermal conductivities, these liquids yield low heat transfer
coefficients.
▪ The reflecting surface is generally curved back silvered glass. It is fixed on
light-weight structure usually made of aluminium sections.
▪ The proper design of this supporting structure and of the system for its
movement is important, since it influences the shape and orientation of the
reflecting surface.
40. CYLINDRICAL PARABOLIC COLLECTOR
▪ Compared to flat-plate collectors, there are very few manufactures of
concentrating collectors all over the world.
▪ The volume of production is also low.
▪ In India, many experimental collectors have been built and tested. However,
commercial manufacture has not yet begun.
41. COMPOUND PARABOLIC COLLECTOR
▪ Compound Parabolic Concentrators (CPCs) are designed to efficiently collect
and concentrate distant light sources.
▪ With acceptance angle options of 25° and 45°, CPCs are able to accommodate a
variety of light sources and configurations.
▪ Compound Parabolic Concentrators are critical components in solar energy
collection, wireless communication, biomedical and defense research, or for any
applications requiring condensing of a divergent light source.
42. THERMAL APPLICATIONS
Some of the major application of solar energy are as follows:
• Solar Water heating systems
• Solar Space heating system
• Solar based Power Generation Systems
• Space cooling and refrigeration
• Distillation
• Drying
• Cooking system
• Solar pumping system
• Solar air-heater for drying of agricultural and animal products
• Solar furnaces
• Solar green houses.
