Solar energy can be harnessed in three main ways: through solar thermal technologies to convert sunlight into heat, through photovoltaics to directly convert it into electricity, and through photosynthesis to convert it into chemical energy. Some key applications of solar energy include water heating, space heating, electricity generation, and driving chemical reactions. While solar energy is abundant and clean, it also has disadvantages like intermittency and the need to convert and store it before use.

1. 7. SOLAR ENERGY
Topics:
 Solar Energy: Solar radiation measurements,
 Solar Thermal: Flat plate and focusing
collectors, solar space heating and cooling,
solar pond,
 Solar Photovoltaic: Solar cells and storage

2. SUN
Energy received from the sun in 30 days exceeds total energy available in fossil fuels

3. HIGHLY DYNAMIC
Infrared Black spots

4. THE EARTH
Moon’s Shadow
(at the time of solar eclipse)

5. SOLAR ENERGY
Sun is the prime source of all renewable energy

6. SOLAR RADIATION
 Energy from the sun reaches earth’s surface in the form of
solar radiation.
 The Sun is a sphere of intensely hot gaseous matter,
continuously generating heat by thermo-nuclear fusion
reactions, which convert hydrogen atoms to helium atoms.
 This energy radiated from the sun in all directions and a very
small fraction of its reaches the earth.
 The maximum intensity of solar radiation known as solar
constant which is defined as the total energy received from
the sun, per unit time on a surface of unit area kept
perpendicular to the radiation, in space, just outside the
earth’s atmosphere when the earth is at its mean distance
from the sun. The value of solar constant is 1366 W/m2.

7. DIFFERENT COLORS OF LIGHT HAVE DIFFERENT
WAVELENGTHS AND DIFFERENT ENERGIES

8. SOLAR RADIATION SPECTRUM
Originates with the thermonuclear fusion reactions occurring in the sun.
Represents the entire electromagnetic radiation (visible light, infrared,
ultraviolet, x-rays, and radio waves).

9. Total Energy Received
Per unit surface area
(Solar Insolation)
1366 W/m2
Outside
atmosphere
Earth surface

10. SOLAR CYCLE VARIATIONS

11. SOLAR INSOLATION
 The solar radiation received on a flat, horizontal surface at a
particular location on earth at a particular instant of time is
called the solar insolation and usually expressed in W/m2.
 For a given flat horizontal surface, the parameters of the
solar insolation are:
 Daily variation (Hour angle).
 Seasonal variation and geographical location of the
particular surface.
 Atmospheric clarity.
 Shadows of trees, tall structures, adjacent solar panels,
etc.
 Degree of latitude for the location.
 Area of surface, m2.
 Angle of tilt.

12. ANGLE OF INCIDENCE (θ )
 The angle between the incident beam
(Ibn) and normal (ON) to surface (S).
 If surface S is fixed, angle of incidence
θ has hourly variation due to
changing position of the sun.
 Equivalent Incident Flux (IN) normal to
the surface S = component of Ibn along
ON.
IN = Ibn cos θ
 Angle of Incidence θ depends on several variables such as angle of
declination, tilt angle, hour angle, latitude , azimuth angle
associated with the location and orientation of the surface (S) and
the direction of sun rays.
 The fixed type collector surface ‘S’ should be so oriented that it
collects maximum energy during the year.

13. TILT ANGLE OR SLOPE ANGLE (β)
 The angle between the collector surface plane and
the horizontal plane is called the tilt angle or the
slope angle and is designated by β.
 For vertical surface β = 900
 For horizontal surface β = 00
 β is always positive.
 For sun tracking collectors/reflectors, the angle β is
changed automatically to track the sun.
 For fixed type collectors/reflectors, angle β is
constant.

14. ANGLE OF DECLINATION (δ)
 The angle between the line joining
centers of the sun and earth and
the equatorial plane.
 The angle of declination (δ) varies
with season from maximum value
of +23.45° on June 21 to minimum
value of –23.45° on December 21.
The angle δ is zero at two
equinoxes, i.e., March 21 and
September 21.
 The declination angle can be
calculated from the following
expression:
Declination angle (δ)=23.45sin{(360/365) (284 + n)}
where, n = the day of year counted from first January.

15. HOUR ANGLE (ω)
 Angle traced by sun in 1 hour with reference to 12 noon
(Local Solar Time) and is equivalent to 15° per hour.
ω= 15×(ST-12),where ST is local solar time
At 9 am ω = 15×(9-12) = - 45°
At 6 pm ω = 15×(18-12) = 90°

16. LATITUDE (Φ)
 The angle made by the radial line joining the given
location and the center of the earth, with equatorial
plane.
Tilt Angle (β) and Angle of Latitude (φ)

17. SOLAR RESOURCES
 The Earth receives at an average of 1366 W/m2
energy (January: 1412 W/m², and July: 1321 W/m²)
in the form of electromagnetic radiation from the
Sun
 This is equivalent to over 43 thousand times the
entire power generation rate on the Earth
 But…
 Large portion of this energy is absorbed in the
atmosphere.
 Not available all the time at one particular place.
 Needs to be collected (absorbed) before its
utilization.

18. SOLAR ENERGY CONVERSION
 To use solar energy, some part of the
electromagnetic spectrum must be converted
into two other farms:
 Heat (Thermal Energy)
 Electricity
 The amount of heat or electricity produced
depends upon the technology used and its
efficiency.

19. TECHNIQUES FOR USING SOLAR ENERGY
Solar energy is used in three different ways:
1. By converting solar energy to thermal energy
through solar heater (thermal conversion)
2. By direct conversion of solar energy to electricity
through photovoltaic (PV) approach
3. By converting solar energy to chemical energy
(photosynthesis)

20. SOLAR ENERGY CONVERSION OPTONS
Heating
CO SC
2
Sugar
O H e-
2 2
HO
2
O
2 H2O
Photosynthesis Semiconductor/Liquid Photovoltaics
Junctions

21. APPLICATIONS OF SOLAR ENERGY
 Water heating
 Air heating for agricultural and industrial applications
 Heating and cooling of buildings
 Cold storage for preservation of food
 Cooking of food
 Green houses
 Distillation of water
 Water pumping
 Solar furnaces
 Power generation
 Solar photovoltaic

22. ADVANTAGES AND DISADVANTAGES
 Advantages
 All chemical and radioactive pollutants of the
thermonuclear reactions remain behind on the Sun,
while only pure radiant energy reaches the Earth.
 Energy reaching the earth is incredible. By one
calculation, 30 days of sunshine striking the Earth have
the energy equivalent of the total of all the planet’s fossil
fuels, both used and unused!
 Disadvantages
 Solar energy is not available round the clock.
 Available Solar energy is diffused. Required to be focused
at one point before using (particularly for thermal
conversion).
 Three step approach is required:
1) collection, 2) conversion, 3) storage.

23. SOLAR THERMAL TECHNOLOGIES
 Solar thermal is the oldest solar energy
technology – has been used for centuries
 Solar thermal technologies can be divided in
three types:
 Passive solar building design
 Thermal collectors for water heating, space
heating and other uses
 Solar thermal power plants

24. PASSIVE SOLAR DESIGN
 Passive solar design is a set of practices that
accommodate the local climate by:
 Letting the sun into the building in the
winter
 Keeping the sun out in the summer
 The most important aspect of passive solar
design are
 Building and window orientation
 Insulation and building materials
 Shading

25. HEATING OF LIVING SPACES
 Best design of a building is to act as a solar collector and
storage unit for the purpose of heating. This is achieved
through three elements: collection, storage, and insulation.
 Efficient heating starts with proper Collection of solar energy
that can be achieved by keeping south-facing windows and
appropriate landscaping (location of tree, tall building, etc.).
 Insulation on external walls, roof, and the floors. The doors,
windows, and vents must be designed to minimize heat loss
(double layer panels).
 Storage: Thermal mass holds heat.
Water= 62 BTU per cubic foot per degree F.
Iron= 54, Wood (oak) = 29, Brick = 25,
concrete = 22, and loose stone = 20

26. HEATING OF LIVING SPACES (contd…)
Passive Solar
Trombe Wall
Passively heated home

27. HEATING OF LIVING SPACES (contd…)
 A passively heated home uses about 60-75% of the
solar energy that hits its walls and windows.
 The Center for Renewable Resources estimates that
in almost any climate, a well-designed passive solar
home can reduce energy bills by 75% with an added
construction cost of only 5-10%.
 About 25% of energy is used for water and space
heating.
 Major factor discouraging solar heating is low price
of electricity!!!

28. SOLAR THERMAL COLLECTORS
 Thermal collectors convert solar radiation
into heat
 Main uses are water heating and space
heating for homes and businesses
 Many different types, but they can be
categorized as:
 Flat plate collectors
 Concentrating collectors

29. FLAT PLATE COLLECTOR
 A flat-plate collector is
used to absorb the sun’s
energy to heat (mostly
water).
 Two methods of heating:
passive (no moving parts)
and active (using pumps).
 In passive collectors water circulates throughout
the closed system due to convection currents.
 Tanks of hot water (insulated) are used for storage.

30. FLAT PLATE COLLECTORS (contd…)
 Flat-plate solar collector absorbs sunlight and transfer the
heat to water or a mixture of anti-freeze and water
 The hot fluid can be used directly or indirectly for hot water
and space heating
 Generally used for low temperature applications like
residential hot water heating

31. FLAT PLATE COLLECTORS (contd…)
 A flat-plate solar collector is one of three main types of solar collectors,
which are key components of active solar heating systems. The other
main types are evacuated tube collectors and batch solar heaters (also
called integrated collector-storage systems).
 Flat-plate collectors are the most common solar collectors for use in solar
water-heating systems in homes and in solar space heating. A flat-plate
collector consists basically of an insulated metal box with a glass or
plastic cover (the glazing) and a dark-colored absorber plate. Solar
radiation is absorbed by the absorber plate and transferred to a fluid that
circulates through the collector in tubes. In an air-based collector the
circulating fluid is air, whereas in a liquid-based collector it is usually
water.
 Flat-plate collectors heat the circulating fluid to a temperature
considerably less than that of the boiling point of water and are best
suited to applications where the demand temperature is 30-70°C and for
applications that require heat during the winter months.

32. EVACUATED TUBE SOLAR THERMAL COLLECTORS
 Evacuated tube collectors use a “thermos bottle” type
of collector that prevent freezing and can achieve higher
temperatures
 Used when large volumes high temperature water are
needed like commercial laundries, hotels and hospitals

33. HEATING WATER: ACTIVE SYSTEM
Active System uses antifreeze so that the liquid does not freeze if
outside temperature drops below freezing point.

34. FLAT PLATE SOLAR COLLECTORS
PERFORMANCE
Useful energy gain Qu
Collector efficiency,η =
Solar radiation incident on collector Ac IT
Ac = Collector area
IT = Incident solar radiation on collector (kW/m2 )

35. FLAT PLATE SOLAR COLLECTORS
PERFORMANCE (contd…)
Qu = in − Eout = R Ac ατ IT − UL (Tf , O − Ta )
 
E F  
FR = Collector efficiency factor
α = Absorptivity of collector
τ = Transmissivity of glass cover
UL = Overall loss coefficient
Tf , O = Temperature of fluid in the tubes
Ta = Ambient temperature

36. COLLECTORS EFFICIENCY VS (Tf,o- Tamb)/IT
FLAT PLATE SOLAR COLLECTORS
PERFORMANCE (contd…)

37. COLLECTORS EFFICIENCY VS (Tf,o- Tamb)/IT
FLAT PLATE SOLAR COLLECTORS
PERFORMANCE (contd…)

38. HEATING WATER—LAST THOUGHTS
 Efficiency of solar heating system is always less than 100%
because:
 % transmitted depends on angle of incidence,
 Number of glass sheets (single glass sheet transmits 90-
95%), and
 Composition of the glass
 By using solar water heating in place of a gas water heater, a
family will save 500 kg of pollutants each year.
 Market for flat plate collectors grew in 1980s because of
subsidy.
 While solar water heating is relatively low in the India, in
other parts of the world such as Cyprus (90%) and Israel
(65%), it proves to be the predominate form of water
heating.

39. CONCENTRATING SOLAR THERMAL
COLLECTORS: CONCENTRATION RATIO
S. Concentration
Concentrator-Receiver Combination
No. Ratio
1. Plane reflector – plane receiver 1 to 4
2. Conical reflector – cylindrical receiver 4 to 10
Parabolic cylindrical reflector-cylindrical
3. 10 to 100
receiver
4. Paraboloidal reflector-spherical receiver Up to 10000
kW/m2 insolar radiation on surface
Concentration Ratio =
kW/m2 on surface of focus of collector

40. CONCENTRATING SOLAR THERMAL
COLLECTORS (contd…)

41. CONCENTRATING SOLAR THERMAL
COLLECTORS (contd…)
 Parabolic dish
collectors use optical
mirrors to focus
sunlight on a target
 can achieve very
higher temperatures,
but are more
expensive and
complex Small size Collectors are more
economical in hilly area (Ladakh)

42. INAUGURAL FUNCTION OF WORLD'S LARGEST
SOLAR COOKING SYSTEM (SHRI SAIBABA
SANSTHAN TRUST, SHIRDI 30-07-2009)

43. CONCENTRATING SOLAR THERMAL
COLLECTORS
 Parabolic trough
collectors also use
optical mirrors to
focus sunlight on a
linear target, usually
a tube with a
circulating fluid in it
 Used for power
generation

44. PARABOLIC DISHES AND TROUGHS
Collectors in southern CA
Because they work best under direct sunlight, parabolic dishes
and troughs must be steered throughout the day in the direction
of the sun.

45. SOLAR-THERMAL ELECTRICITY:
PARABOLIC DISHES AND TROUGHS
 Focus sunlight on a smaller receiver for each device; the
heated liquid drives a steam engine to generate electricity.
 The first of these Solar Electric Generating Stations (SEGS)
was installed in CA by an Israeli company, Luz International.
Output was 13.8 MW; cost was $6,000/peak kW and overall
efficiency was 25%.
 Through federal and state tax credits, Luz was able to build
more SEGS, and improved reduced costs to $3,000/peak kW
and the cost of electricity from 25 cents to 8 cents per kWh,
barely more than the cost of nuclear or coal-fired facilities.
 The more recent facilities converted a remarkable 22% of
sunlight into electricity.

46. MIRRORS

47. CONCENTRATING SOLAR THERMAL
COLLECTORS
 Tracking mirrors
focus sunlight on a
stationery “power
tower” to
generate very high
temperatures
(~1000o F)
 Used to generate
electricity

48. SOLAR THERMAL POWER PLANT

49. BINARY CYCLE SOLAR THERMAL POWER PLANT
1. Solar collector 2. Hot water reservoir 3. Head exchanger
4. Cold water reservoir 5. NH3 Gas Turbine 6. Generator
7. NH3 condenser 8. NH3 Pressuriser 9. Cooling Tower

50. SOLAR-THERMAL ELECTRICITY
 General idea is to collect the light from many reflectors
spread over a large area at one central point to achieve high
temperature.
 Example is the 10-MW solar power plant in Barstow, CA.
 1900 heliostats, each 20 ft by 20 ft
 a central 295 ft tower
 An energy storage system allows it to generate 7 MW of
electric power without sunlight.
 Capital cost is greater than coal fired power plant, despite
the no cost for fuel, ash disposal, and stack emissions.
 Capital costs are expected to decline as more and more
power towers are built with greater technological advances.
 One way to reduce cost is to use the waste steam from the
turbine for space heating or other industrial processes.

51. POWER TOWER IN BARSTOW, CALIFORNIA

52. DIRECT CONVERSION INTO ELECTRICITY
 Photovoltaic cells are capable of directly
converting sunlight into electricity.
 A simple wafer of silicon with wires attached
to the layers. Current is produced based on
types of semiconductor (n- and p-types) used
for the layers. Each cell produces 0.5 V.
 Battery may be needed to store electrical
energy
 No moving parts, no pollution, do not wear
out, but because they are exposed to the
weather, their lifespan is about 20 years.

53. PHOTOVOLTAIC TECHNOLOGY - BACKGROUND
 Photovoltaic (PV) converts sunlight to DC electricity
using a semiconductor cell.
 The PV effect was discovered in 19th century by
Alexander Becquerel
 Bell labs pioneered early application, especially for
satellites, in the 1960s
 Very small, remote applications emerged in the
1970s and early 1980s
 As cost declined, PV became more common for
larger applications in late 1980s and early 1990s

54. SOLAR PANELS IN USE
 Because of their current
costs, only rural and other
customers far away from
power lines use solar
panels.
 Subsidy is given for these
panels by Central and State
nodal agencies.
 The costs may go down in
coming years in view of
ongoing R&D work
worldwide

55. PHOTOVOLTAIC EFFECT
 Electromagnetic radiation can be viewed as photons
 Each photon has energy E = hν = h c/λ
 Photons travel at speed c = ν λ
 Photons having an sufficient energy can dislodge an
electron from silicon (1.12 eV = 1.794 x 10-22 kJ)
 The electron is accelerated by the electric field
 If a circuit is provided a current will flow

56. PHOTOVOLTAIC CELLS
 When sunlight strikes the solar cell, it “knocks loose”
electrons, which generates a flow of DC current

57. PHOTOVOLTAIC CELL EFFICIENCY
The most commonly used material crystalline silicon, absorbs energy in a small part of
the spectrum. Efficiency depends on how much of the available spectrum can be
converted to electricity.

58. MANUFACTURER’S DATA

59. PV CELLS IN SERIES AND PARALLEL
Series arrangement: voltages add
Parallel arrangement: currents add

60. PV CELL MATERIALS
 The most common PV cells are made from
crystalline silicon wafers
 Other types of materials include thin films like
Cadmium Telluride (CdTe), Copper-Indium-Gallium-
Diselenide (CIGS), amorphous silicon (a-Si)
 The main goals for manufacturers are to minimize
the amount of materials and maximize efficiency
 Today, the best crystalline silicon cells are about 15%
efficient, the best thin films are about 8% efficient.

61. PV CELLS, MODULES AND ARRAYS
 PV cells are connected like batteries to increase voltage and
current output and are assembled in to modules
 Modules become part of larger arrays

62. HOW ARE PV SYSTEM RATED?
 PV modules are rated based on the
maximum power produced in Watts
when the amount of sunlight is 1,000
W/m2
 PV systems are rated based on the
maximum combined power output of
the PV modules
 Since the amount of sunlight changes,
the power output of the system will vary

63. SOALR PHOTOVOLTAIC PANELS

64. EFFICIENCIES OF PHOTOVOLTAIC DEVICES
25
20
15
Efficiency (%)
10 crystalline Si
amorphous Si
nano TiO2
5 CIS/CIGS
CdTe
1950 1960 1970 1980 1990 2000
Year

65. BATTERIES
 Primary Batteries
 can store and deliver electrical energy, but can not be
recharged. Typical carbon-zinc and lithium batteries
commonly used in consumer electronic devices are
primary batteries. Primary batteries are not used in PV
systems because they can not be recharged.
 Secondary Batteries
 can store and deliver electrical energy, and can also be
recharged by passing a current through it in an opposite
direction to the discharge current. Common lead-acid
batteries used in automobiles and PV systems are
secondary battery.

66. SECONDARY BATTERY TYPES AND
CHARACTERISTICS
Deep Cycle
Battery Type Cost Maintenance
Performance
Flooded Lead-Acid
Lead-Antimony Low Good High
Lead-Calcium Open Vent Low Poor Medium
Lead-Calcium Sealed Vent Low Poor Low
Lead Antimony/Calcium Hybrid Medium Good Medium
Captive Electrolyte Lead-Acid
Gelled Medium Fair Low
Absorbed Glass Mat Medium Fair Low
Nickel-Cadmium
Sintered-Plate High Good None
Pocket-Plate High Good Medium

67. BATTERIES FOR PV
 Battery Capacity
 It is a measure of battery’s ability to store or deliver
electrical energy, commonly expressed in units of
ampere-hours.
 Ampere-Hour Definition
 It is the common unit of measure for a battery’s electrical
storage capacity, obtained by integrating the discharge or
charge current in amperes over a specific time period. An
ampere-hour is equal to the transfer of one ampere over
one hour, equal to 3600 coulombs of charge. For
example, a battery which delivers 5 amps for 20 hours is
said to have delivered 100 ampere-hours.

68. BATTERIES FOR PV (contd…)
 Discharge rate affects capacity
 Typical discharge times
 Industrial, motive applications 10 hours
 Photovoltaic applications 100-300 hours
 Maximum Recommended Depth of Discharge for Lead Acid
Batteries
 Shallow cycling types 50%
 Deep cycling types 80%
 Time to fully discharge
Days of reserve × 24 hours(1 day)
 Time to discharge (rating) =
Maximum Depth of Discharge (%)

69. BATTERY SELECTION CRITERION
 Nominal system voltage
 Charging requirement
 Required capacity
 Ampere-hour capacity at discharge rate
 Daily and maximum depth of discharge
 Self-discharge rate
 Gassing characteristics
 Efficiency
 Temperature effects
 Size, weight and structural needs
 Susceptibility to freezing
 Electrolyte type and concentration
 Maintenance requirements
 Terminal configurations
 Battery life (cycles/year)
 Availability and servicing
 Cost and warranty

70. PV SYSTEMS
 A complete PV system may also include a device to convert DC to
AC power (inverter), batteries to store energy, and a back up
generator
 PV systems can be connected to the electric utility and can be used
to reduce the amount of electricity purchased from the local utility
without using batteries or generators

71. LOW EFFICIENCY AND OTHER
DISADVANTAGES
 Efficiency is far lass than the 77% of usable solar spectrum
(theoretical efficiency).
 Only 43% of photon energy is used to warm the crystal.
 Efficiency drops as temperature increases (from 24% at 0°C
to 14% at 100 °C.)
 Light is reflected off the front face
 Internal electrical resistance are other factors.
 Overall, the efficiency is about 10-14%.
 Underlying problem is weighing efficiency against cost.
 Crystalline silicon-more efficient but more expensive to
manufacture
 Amorphous silicon-half as efficient, less expensive to
produce.

72. FUTURE OF SOLAR ENERGY
 Solar thermal energy is already very cost-
effective for providing low temperature heat
almost anywhere
 PV is very cost effective for providing
electricity in remote areas and in niche
applications
 As the costs of fossil fuels and electricity
increase, PV is becoming more cost effective
compared to electricity from conventional
sources
 The costs of all solar technologies are
declining

73. SOLAR ENERGY ISSUES AND BARRIERS
 ‘Fuel’ is free but the systems are not. Can be
costly to install compared to grid supplied
electricity and fossil fuels
 Certain technologies, like PV, can require
large areas
 Some PV technologies use toxic materials,
although in very small amounts
 Energy storage must be used in some cases

74. FINAL THOUGHT
 Argument that sun provides power only during the day
is countered by the fact that 70% of energy demand is
during daytime hours. At night, traditional methods can
be used to generate the electricity.
 Goal is to decrease our dependence on fossil fuels.
 Currently, 75% of our electrical power is generated by
coal-burning and nuclear power plants.
 Mitigates the effects of acid rain, carbon dioxide, and
other impacts of burning coal and counters risks
associated with nuclear energy.
 Pollution free, indefinitely sustainable.