Useful for Students of B. Sc (Ag.) and B. Tech. (Agril. Engg.)

1. Md. I. A. Ansari
Department of Agricultural Engineering
(e-mail: irfan26200@yahoo.com)
Renewable Energy and Green Technology

2.  A solar collector is device that converts
solar radiation into heat and transfer
that heat to a fluid.
 It is used for heating water, air, etc.
 It can be used for supplemental space
heating in residential and commercial
buildings.
 It can be used for producing steam.
Solar Collector

3. • In a solar collector, absorber surface is
painted with black colour to make selective
surface.
• Selective surface: A surface which
exhibits the characteristics of a high value
of absorptivity for incoming solar radiation
and a low value of emissivity for outgoing
re-radiation are called selective surface.
• Black chrome is used as coating material.

4. Types of Solar Collectors
1. Non-concentrating or non focusing type solar
collector
 Flat Plate solar Collector
 Evacuated Tube solar Collector
2. Concentrating or focusing type solar collector
 Line Focusing type
 Point focusing type

5. Fixed Solar Collectors
Fixed collectors are much simpler and
remain fixed over a day’s time. Their
position or orientation, may be adjusted on
a seasonal basis.
 Fixed collectors are less efficient than
tracking collectors. They are generally
preferred due to less cost to buy and
maintain. They are normally used for low
temperature application.

6. Tracking Solar collectors
A tracking collectors are controlled to
follow the sun throughout the day.
 A tracking system is rather
complicated and generally only used for
special high-temperature applications.

7. Non-Focusing or non-concentrating
Type Solar Collector
• In the non-concentrating type, the collector
area (i.e. the area that intercepts the solar
radiation) is the same as the absorber
area (i.e., the area absorbing the
radiation).
• In this type, the whole solar panel absorbs
the incident light.

8. Flat-Plate Collector
 When no optical concentration is done, the
device in which the solar energy collection is
achieved is called a flat-plate collector.
 The water temperature less than 100 ◦C can be
achieved from this system.
 Flat-plate collectors are the most widely used
collectors in the world for domestic water-
heating systems and solar space
heating/cooling.
 The first accurate model of flat plate solar
collectors was developed by Hottel and Whillier
in the 1950's.

9. • The flat plate collector mainly consist of a
casing, absorber plate, transparent glass
covers, insulating material and fluid passage
tubes.
• The transparent cover of glass or plastic
allows short-wave solar radiation (Beam,
diffuse radiation) to enter the box and fall
on the black plate, but it prevents the long-
wave (thermal) radiation emitted by the black
plate from being lost.

10. • The solar radiation is absorbed by
absorber plate, which is coated with
black absorber paint.
• The fluid tubes, which are connected to
absorber plate made of copper or
aluminum, absorb the heat and transfer
to the water passing through the tubes
and gets heated.

11. • The collector box supports all the
components and provides weather
protection.
• The insulated box reduces heat loss from
the back or the sides of the collector.
• Flat plate collectors are used for wide range
of low-temperature applications such as
cooking of food, water heating, room heating,
grain drying, etc.

12. Flat Plate Collector

15. Characteristics of Flat Plate Collector
Used for moderate temperature up to 100 ºC
Uses both direct and diffuse radiation
Normally do not need tracking of sun
Mechanically simple
Uses: water heating, building heating and air-
conditioning, industrial process heating, boiler
feed water, pasteurization.

16. Evacuated Tube Solar Collector
• The evacuated tube collector consists of
an array of evacuated glass tubes.
• Each glass evacuated tube consists of two
glass tubes.
• The outer tube is made of extremely strong
transparent borosilicate glass and the inner
tube is also made of borosilicate glass, but
coated with a special selective coating, to
absorb solar radiation with minimal re-
radiation.

17. • The air is evacuated from the space
between the two glass tubes to form a
vacuum, which eliminates conductive
and convective heat loss and water
temperatures up to 100°C can be reached.
• The black coating on the inner tube
absorbs the solar energy and transfers it
to the water.
• The water on upper side of vacuum tube
becomes hot and thus lighter, so it starts
moving upwards in the tank.

18. • At the same time cold water, which is
heavy, comes downward from the tank
and is stored at the bottom.
• The phenomenon is called as natural
thermosyphon circulation, which occurs in
every tube.

19. Evacuated Tube Solar Collector

21. Characteristics of Evacuated Tube Solar Collector:
 High quality tubes made from borosilicate glass
 Long lasting
 Better performance in winter and cloudy days
 Vacuum jacket insulation
 Heats water to a very high temperature
 Makes hot water available even on partially cloudy
days
 High quality insulation for maintaining high
temperature of water inside the tank
 Compact
 Easy to install, operate and maintain
 Galvanized Iron tank for hot water storage
 Safe and environment friendly

22. Concentrating or Focusing Type solar Collector

23. • Focusing type collector or concentrating
collectors uses mirrors (reflectors) or
lenses (refractors) to focus the collected
solar energy on smaller absorbing surface
to obtain higher working temperatures.
• This type of solar collector is used for
high-temperature applications such as
steam production for the generation of
electricity.

24. • Concentrating collectors have large mirror
systems to focus beam solar radiation onto a
pipe containing water (or other liquid) or onto a
small receiver.
• The mirrors must be moved to follow the
movement of the sun throughout the day.
• These collectors produce higher temperature
than flat plate collectors, but they are more
expensive and more difficult to use.

25. • Types:
Line focusing
Point focusing
• In practice, line is a collector pipe and the
point is a small volume through which hot
fluid flows.

26. Line focusing:
• Parabolic trough collector
• Linear Fresnel collector

27. Point Focusing:
• Parabolic dish collector
• Central tower receiver

28. Cylindrical Parabolic Concentrating Collector
• In this type of collector, the radiation coming on
concentrator is diverted to absorber tube, which
is covered with glass tube to avoid re-radiation
loss.
• Parabolic trough reflectors are made of highly
polished aluminum or silvered glass.
• The heat from the absorber tube is transmitted
to the fluid and gets heated up.

29. • The direction of concentrator is changed
with the sun movement by providing single
axis tracking.
• The temperature attained by the collector
is 100-400 ºC and produce steam for
generating electricity.
• Length of reflector unit: 3 to 5 m width:1.5
to 2.4 m

30. Parabolic Concentrator

32. Parabolic trough solar plant, Egypt. The plant has the total solar
aperture area of 130,800 m2 and expected electricity generation of
34,000 MWh/year. It has been operating since 2011.

33. • Linear Fresnel’ power plants uses long
parallel lines of flat mirrors that concentrate
sunlight to heat water in an overhead
collector tube, producing steam to drive a
conventional steam turbine-generator.
‘

34. ‘Central Tower' or 'Heliostat' power plant uses an
array of flat, movable mirrors (called heliostats) to
focus the sun's rays upon a collector tower
(sometimes called ‘solar tower’).
A power tower has a field of large mirrors that follow
the sun's path across the sky.
The mirrors concentrate sunlight onto a receiver on
top of a high tower.
Power Tower System with Heliostats

35. • Molten salt (40% potassium nitrate,
60% sodium nitrate) is used as a heat
transfer fluid and thermal energy storage
in solar power plants.
• A computer keeps the mirrors aligned so
the reflected rays of the sun are always
aimed at the receiver, where temperatures
well above 1000°C can be reached.
• High-pressure steam is generated to
produce electricity.

38. Power Tower System with Heliostats

39. Solar power plant, Spain is operational since 2007 and produces
23,400 MWh/year. Aperture size is 74,880 m2. The light is
concentrated on the top of the 115 m high tower.

40. Parabolic Dish System
• A parabolic dish collector is similar in appearance
to a large satellite dish, but has mirror-like
reflectors and an absorber at the focal point.
• It uses a dual axis sun tracker.
• A parabolic dish system uses a computer to track
the sun and concentrate the sun's rays onto a
receiver located at the focal point in front of the
dish.
• Parabolic dish systems can reach 1000 °C at the
receiver, and achieve the highest efficiencies for
converting solar energy to electricity in the small-
power capacity range.

42. Parabolic Dish Collector

46. Concentration ratio: It is the ratio of area of
concentrator aperture to the area of solar
energy absorber.
The concentration ratio can be also
represented by the energy flux ratio at the
aperture and at the receiver.
concentration ratio=average solar flux over
the receiver/ solar flux over the aperture
It determines the effectiveness of a
concentrator.
a
r
A
C
A

C = Concentration ratio
Aa = Aperture area (m2)
Ar = Receiver area (m2)

48. • Problem: In a concentrating collector, the
aperture area is 2.2 m2 and absorber area
is 100 cm2. Calculate the concentration
ratio.
• Solution: Concentration ratio= aperture
area/ absorber area
• =2.2/(100 x 10-4)=220 Ans.

49. Efficiency of Conversion
• The efficiency of conversion η is an
important measure of system performance.
• When applied to solar energy conversion
systems, efficiency of solar energy
conversion (η) is defined as the ratio of the
useful output power (delivered by the
conversion device) to the incident power (of
the solar radiation).
• η=[ useful output power x 100]/ incident
power