Operation and Maintenance of Solar Electric Systems

1. SOLAR RESOURCE AND
RADIATION
MODULE 1

2. SOLAR RESOURCES
o The sun is the source of almost all energy on our planet
o The sun’s seemingly endless energy supply is driven by a process
known as nuclear fusion
o The energy produced in the heart of the sun is emitted as
electromagnetic radiation.
o Electromagnetic radiation is emitted in many useful forms including
o microwaves (as used in microwave cooking),
o radio waves (used in telecommunications) and
o visible light
o Solar cell designers focus on capturing the energy carried in visible
light.

3. DIFFERENCE BETWEEN POWER AND ENERGY

4. SOLAR RADIATION TERMINOLOGY
• Peak sun hours (PSH): Daily irradiation is commonly referred to as
daily PSH (or full sun hours)
• The number of PSH for the day is the number of hours for which
power at the rate of 1kW/m2 would give an equivalent amount of
energy to the total energy for that day

5. IRRADIATION
• The total quantity of radiant solar energy per unit area received over
a given period, e.g. daily, monthly or annually.
Insolation
• Insolation: Another term for irradiation.
• The amount of solar radiation incident on a surface over a period of
time.
• Peak sun hours (kWh/m2/ day) are a measurement of daily insolation
Irradiance
• Irradiance: The solar radiation incident on a surface at any particular
point in time measured in W/m2.

8. QUANTIFYING SOLAR RADIATION
• Radiation emitted from the sun is fairly consistent
• Significant variation in the radiation received at the
Earth’s surface is observed due to
• Earth’s orbit (responsible for the seasons),
• rotation on its own axis (responsible for night and day)
and the
• albedo(measure of the diffuse reflection of solar
radiation out of the total solar radiation received by
earth) of certain areas

9. INSOLATION
• The amount of solar energy received by an area over a
day is referred to as insolation
• can be measured in kWh/m2/day or peak sun hours
(PSH)
• Insolation varies widely throughout the year and
doubles in summer months
• Solar module output power greatly increases in summer.

10. AVERAGE
INSOLATION
IN BOSTON

11. INSOLATION
• This data is for the insolation on a horizontal plane,
which is not ideal in a city so far from the equator.
• Inclining a plane to a tilt equal to the angle of
latitude ensures that the plane faces the sun more
directly and receives more insolation

12. PROBLEM
• If sunlight is received at an irradiance of 1000W/m2 for 2 hours,
600W/m2 for 1.5 hours and 200W/m2 for 1 hour, What is PSH?
=1000W/m2*2 hours+ 600W/m2*1.5 hours+200W/m2 *1 hour
= 3100W/m2/day
=3100W/m2/day/ 1000W/m2/day
=3.1PSH

13. EFFECT OF THE EARTH’S ATMOSPHERE ON SOLAR
RADIATION
• When solar radiation arrives at top of the Earth’s atmosphere has
a peak irradiance value of 1367W/m2(solar constant)
• Solar radiation when it reaches Earth’s surface has a peak
irradiance value of approximately 1000W/m2.
• Difference between solar constant and peak irradiance value at the
Earth’s surface is due to the Earth’s albedo
• Earth’s albedo – the amount of solar energy reflected from a
surface on the Earth at that specific location

14. EFFECT OF THE EARTH’S ATMOSPHERE ON SOLAR
RADIATION
• Light is reflected from Earth in a variety of ways:
• Radiation is reflected off the atmosphere back into space.
• Radiation is reflected off clouds in the stratosphere.
• The Earth’s surface itself reflects sunlight
• Average portion of sunlight reflected from Earth’s albedo is 30 %
• Polar regions have very high albedo as ice and snow reflect most
sunlight
• Oceans have low albedo because dark seawater absorbs a lot of
sunlight

15. KEY TERMINOLOGY
• Direct radiation: Solar radiation that passes directly to the Earth’s
surface
• Diffuse radiation: Solar radiation that is scattered or absorbed by
clouds and gases within the atmosphere and then re-emitted
• Diffuse radiation is less powerful than direct radiation
• Air mass: The distance that radiation must travel through the
atmosphere to reach a point on the surface.
• This value varies throughout the day for a location

17. • Irradiance is a combination of direct and diffuse radiation
• Will depend on albedo (reflected solar radiation) of that particular
location
• On a sunny day, scattered diffuse radiation will contribute to 10 %
of visible light
• On a cloudy day there will be much more scattering of the solar
radiation reaching the Earth’s surface meaning diffuse radiation
will be greater

18. DIFFUSE AND DIRECT RADIATION

19. AIR MASS
• Air mass will also affect the irradiance at a location
• Greater the air mass, higher chance of light being reflected
or scattered, meaning there will be less solar radiation
reaching Earth’s surface
• Air mass of 1.5 is standard condition at which solar modules
are rated
• Air mass one corresponds to conditions when sun is directly
overhead

22. SUN GEOMETRY
• Because of the Earth’s orbit and
rotation, the position of the sun
relative to a solar array is
constantly changing
• Designers use several geometrical
techniques to design an array that
will capture the most solar energy
possible
• The location of the sun is
specified by two angles which
vary both daily and annually.

23. SUN ANGLES
• Solar altitude: The angle
between the sun and the
horizon; the altitude is always
between 0° and 90°.
• Azimuth: The angle between
north and the point on the
compass where the sun is
positioned. The azimuth angle
varies as the sun moves from
east to west across the sky
through the day.
• In general, the azimuth is
measured clockwise going from 0°
(true north) to 359°

24. • In the northern hemisphere solar arrays are normally installed to
face south as the sun is always in the southern sky
• In the southern hemisphere solar arrays normally face north
• In regions between the Tropic of Cancer and the Tropic of
Capricorn this is not always the case, at certain times of year the
sun will be in the southern sky for those in the southern
hemisphere and in the northern sky for those in the northern
hemisphere
• The sun’s altitude is highest on the summer solstice and lowest on
the winter solstice
• During summer, the sun rises to a higher altitude

26. SUN PATH DIAGRAM
• The sun’s path in the sky for any particular location can be depicted
on a two-dimensional surface in a sunpath diagram
• This diagram can be used to determine the position of the sun in
the sky at any time of the day, for any day of the year
• The sunpath diagram is composed of:
• azimuth angles, represented on the circumference of the diagram;
• altitude angles, represented by concentric circles;
• sunpath lines from east to west for different dates in the year;
• time of day lines crossing the sunpath lines;
• location information that refers to latitude.

28. • Sunpath diagrams may look entirely different for
other areas.
• Directly on the equator, the sunpath would range
equally north and south.
• sun only rises to a low altitude in winter and is
always north or south (depending on the
hemisphere)
• The sun will always be farthest north or south at the
solstices in all locations, north in June, south in
December

29. GEOMETRY FOR INSTALLING SOLAR ARRAYS
• The position of a solar module is referred to as its orientation
• This orientation of the solar array is very important as it affects the
amount of sunlight hitting the array and hence the amount of
power produced
• The orientation generally
• includes the direction the solar module is facing (i.e. due south)
• the tilt angle, which is the angle between the base of the solar module
and the horizontal.
• The amount of sunlight hitting the array also varies with the time
of day because of the sun’s movement across the sky

30. GEOMETRY FOR INSTALLING SOLAR ARRAYS

31. GEOMETRY FOR INSTALLING SOLAR ARRAYS

32. GEOMETRY FOR INSTALLING SOLAR ARRAYS

33. GEOMETRY FOR INSTALLING SOLAR ARRAYS
• Solar modules should be installed so that as much radiation as
possible is collected
• Solar modules should be installed facing either true south
(northern hemisphere location) or true north (southern
hemisphere location).

34. MAGNETIC DECLINATION
• When installing a photovoltaic system it
is important to consider the magnetic
declination of a location (also known as
magnetic variation).
• Difference between true north (the
direction of the north pole) and
magnetic north (the direction in which
a compass will point).
• A solar system should face true north or
true south and so the magnetic
declination angle of the location should
be considered