More About Solar Panels
Solar Panel Efficiency
First let's consider what we mean
by efficiency. Efficiency is a measurement of the
output divided by a certain factor. When talking
about solar cell or panel efficiency, we are gerally
looking at output per given area.
Therefore a more efficient panel will either give
more power or be smaller than a less efficient one.
This does not mean that a more efficient panel is
more economic, in-fact the reverse may be the case.
More efficient panels may be easier to mount, particularly
if space is limited, or a tracking system is being
used.
There are several separate areas
to consider here concerning the efficiency of pv
solar panels, concerning the inherent efficiency
of the cells in the panel you buy, the panel construction,
and how your installation may affect panel efficiency.
For home owners setting up there
own system, efficiency is likely to be less important
than the cost per nominal watt output.
Solar Cell Efficiency
Researchers are continually striving to improve
the efficiency with which solar cells convert light
energy to electrical energy. The record at the moment
stands at an efficiency of around 40%, using multi
junction cells (multiple layers of silicon), each
layer tuned to trap different frequencies (colours)
of light. This type of cell will however be expensive
to produce and in the past has been mainly used
in space where efficiency may be more important.
Researchers also strive to increase the amount of
light entering the cell. Silicon is a naturally
shiney substance and cells are coated with non-reflective
layers to ensure that as much light as possible
enters the cell.
Cells used in photovoltaic panels for electricity
production are usually single junction type with
an efficiency of somewhere around 15%.
Solar Panel Construction
Solar panels will normally have a layer of glass
(protecting the cells) through which the light must
pass before entering cells. There is scope for reducing
light reflected from the glass and also to trap
light that is not striking the panel at 90o.
Some research centres on trapping light from a large
area from various angles and directing it onto a
relatively small cell.
Panel Mounting and
Positioning
The two important factors here are the angle at
which the panels are mounted and the amount of sunsine
falling on the particular location/avoidance of
shade. Small areas of shade on a panel can have
a significant effect on the power produced.
Effects of Temperature
The output of a solar cell, and therefore a solar
panel, is affected by its temperature. As a result
the power output will be reduced by between 0.25%(amorphous
cells) and 0.5%(most crystalline cells) for each
degree C of temperature rise.
Panel temperatures in the summer in warm climates
can easily reach 50oC resulting in a 12% reduction
in output compared to the rated output at 25oC.
This reduction in efficiency may be important to
you if you have a high electricity demand in the
summer.
Partial Shading
Solar panels obviously produce less power when they
are shaded and should idealy be situated where there
wil never be any shadows on them.
There may be situations where this cannot be avoided,
and the effects of partial shading should be considered.
A shadow falling on a small part of a panel can
have a surprisingly large effect on output. Not
only will the cells that are shaded be producing
less power, but as the cells within a panel are
normally all wired in series, the shaded cells afffect
the current flow of the whole panel.
If the affected panel is wired in series (in a string)
with other panels, then the output of all those
panels will be affected by the partial shading of
one panel. Therefore in a situation where partial
shading cannot be avoided, there may be a case for
not having the panels wired in series to produce
the higher voltages that can be used with some inverters.
How Your System
Affects Panel Efficiency
Apart from positioning and angling
your panels in order to capture as much light energy
as possible, there is more.
The inherent characteristics of solar cells results
in current produced by a particular light level
being virtually constant upto a voltage of 0.4 volts.
A solar panel with a nominal voltage of 24 volts
would normally have 72 cells, resulting a constant
current upto 28.8 volts. Above this voltage, current
drops off rapidly, resulting in maximum power output
being produced at around 28.8 volts.
When the panel is connected to the battery via a
simple charge regulator, the voltage will be pulled
down to near that of the battery. The result is
that the panel will only be able to produce it's
maximum power when the battery is near to being
fully charged.
This is where an MPPT (Maximum Power
Point Tracking) Charge Controller can play a part
by maintaining the panels at their optimum voltage
while producing the voltage required by the battery.
What about if you are not using
an MPPT controller? Well if you are in a situation
(possibly in winter) where your batteries rarely
become fully charged, adding an extra panel will
not only add the extra rated power potential, but
the rest of your panels will be more efficient asthey
will be running at a higher voltage.
This effect will to some degree be negated by the
battery being less efficient at higher levels of
charge but remember that it is beneficial to battery
performance and life to be fully charged on occasions.
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Solar panels are large flat objects
that may catch the wind and they are expensive -
so they need to be securely fixed. Also, considering
their value, you may want top make them difficult
to remove!
Which Direction
Should They Face
In most cases, if you are in the northern hemisphere
they should face due south and if in the southern
hemisphere, they should point due north.
You may need to vary this slightly if you cannot
avoid a hill or high trees etc blocking the sun
at one end of the day. If hill to the west results
in you losing the evening sun early, then use your
judgment to point your panels SLIGHTLY towards the
east.
I emphasise slightly as it must be remembered that
the early morning sun will be of little value as
it is so low in the sky.
The Angle to the
Horizontal
The general advice here is that the angle of your
panels relative to horizontal should be the same
as you latitude in spring and autumn, 15 degrees
less in summer, and 15 degrees more in winter.
If at all possible, it will be worth mounting your
panels so that you can change their angle at least
4 times per year.
If you have to mount them so that they are fixed
then go for the same angle as your latitude. You
may however wish to vary this according to which
part of the year is your need the greatest. For
many people this will be winter time though if you
have swimming pool and cooling fans for the summer,this
may not be the case.
Materials Used
Galvanised angle steel is usually appropriate. There
is a slight possibility of corrosion occurring between
aluminium (the normal material used for the frame
of the panel itself) and the zinc used in the galvanising.
However in most outdoor situations that are reasonably
dry, this shouldn't be a problem.
Trackers
Panels will be much more efficient at producing
electricity if they are always facing the sun. Trackers
are available that serve as the panel mounting and
will point them towards the sun.
There are several types to choose from, Passive
trackers use a system whereby a liquid moves as
it is heated by the sun and is used to move the
panel, automatically returning to the correct position
for the morning. This type is likely to be the cheapest,
doesn't have much to go wrong, and is reported to
give satisfactory results.
Active trackers will usually use electric servo
motors to angle the panels and these may be controlled
by a light sensor sensing the position of the sun
or may be controlled by a time moving the panels
to predetermined positions.
The increased production of electricity
that you will get as the result of using a tracker
will be greater during the summer than the winter
due larger arc taken by the sun across the sky in
summer.
This benefit may be mostly when you least need it
and you may feel that the money would be better
spent on a couple of extra panels.
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