Solar
Power
Whether
on a solar-powered calculator or an international
space station, solar panels generate electricity using
the same principles of electronics as chemical batteries
or standard electrical outlets. With solar panels,
it's all about the free flow of electrons through
a circuit.
Silicon
is the raw material used to make solar cells. It's
the second most abundant element on Earth.
There
are three main types:
Monocrystalline
or single crystal cells : The first generation of
solar cells
excellent conversion rate (12 - 16%) (23% under laboratory
conditions)
BUT, making them is a painstaking, therefore expensive
process
another drawback - it takes a lot of energy to obtain
pure crystal
Polycrystalline
cells lower production costs, requiring less energy
to make
11 - 13% conversion efficiency (18% in the lab)
Amorphous
a more recent technology (mid-70's) lower production
costs, but unfortunately also lower efficiency (8
- 10%) (13% in the lab)
This process can use very thin layers of amorphous
silicon (0.3 - 1.0 microns compared to 500 microns
for the other types). Using a vacuum spraying process,
very thin layers can be applied on glass, metal or
even flexible plastic surfaces. Amorphous silicon
is usually the kind used in consumer goods such as
calculators and watches.
Amorphous
panels need about twice the surface area to produce
the same amount of electricity, and their output deteriorates
more quickly over time, but they react better to diffuse
and fluorescent light and work better at higher temperatures.
A single solar cell
always produces a VOLTAGE of approximately 0.5 volts,
regardless of its size. For higher voltages, you have
to connect individual cells in SERIES to add their
voltages. The larger the solar cell, the greater the
CURRENT will be. Current is measured in AMPERES.
You can also connect cells in PARALLEL to increase
current.
The most common solar panels are for 12 V applications.
To reach that voltage, 24 cells would be sufficient,
but for charging batteries and in order to compensate
for voltage drops due to various factors, a PV panel
normally contains between 28 and 40 cells for a higher
voltage. You don't really need to think about the
individual cells. All you need to know is that they
are protected from humidity and the elements inside
the panel, which works as a whole.
The panel has to deliver
more than 12 volts to charge a 12-volt battery. Voltage
can be compared with water pressure in a hose. If
the "pressure" of the electrons isn't high
enough, the electricity can't "penetrate"
the battery.
Voltage can drop for
several reasons:
At high temperatures.
(Unlike thermal solar energy, PV works less well when
it's very hot! In tropical climates, choose higher
voltage panels.)
As a result of long wires. It's important to keep
your wiring between your panels and other parts of
your installation as short as possible.
Diodes can also cause small voltage losses, as we'll
see later.
Just as voltage can be likened to water pressure in
a hose, current can be likened to the flow, or the
amount of water (or electrons) passing through. A
thin hose will take longer to fill a swimming pool
than a thicker hose with the same pressure.
A panel that produces
2 amperes sends twice as many electrons as a one-ampere
panel. When talking of PV panels, you usually refer
to their POWER (measured in WATTS).
VOLTAGE (electrical
"pressure") is measured in VOLTS
CURRENT is measured in AMPERES.
POWER (WATTS) is calculated by multiplying these two.
VOLTS x AMPERES =
WATTS
How Much Will My Panels
Produce?
One square meter of
solar panels can produce up to 150 watts of maintenance-free
power for up to thirty years. They even work on diffuse
light on overcast days, albeit with less output. The
voltage produced by PV panels remains roughly the
same regardless of the weather, but the current (amps)
and the power (watts) will vary.
The most important
variable to bear in mind when planning a photovoltaic
installation is the power output, which will basically
depend on four factors:
the
peak power of your panels (measured in peak-watts
or Wp)
light
intensity
the
number of hours of exposure to the sun and
the
angle of exposure to the sun
The Intensity of Sunlight
A panel's power is
expressed in peak watts, the number of watts it will
produce in optimal conditions, i.e. at noon in direct
sunlight in cold weather. Maximum sun intensity is
1.000 W/m2.
The following factors
will influence the amount of sunlight reaching the
PV panels:
1.Weather conditions
(cloud cover, fog etc.)
2. How high the sun
is in the sky
3. The number of daylight
hours
1) As to the first
factor, oversimplifying somewhat, a 50 watt panel
should produce 50 watts for each hour of sunshine
at 1.000 W/m². It will produce about half that
amount (25 watts each hour) when exposed to 1/2 the
light (500 W/m²). Diffuse light passing through
thin clouds might mean 300 W/m². In very bad
weather conditions with thick, dark clouds, light
intensity could fall to 100 W/m² with only 5
Watts produced per hour.
2) The second factor,
the height of the sun over the horizon varies with
the seasons. When the sun is very high in the sky
(summer), its rays travel through the atmosphere more
quickly over a shorter distance than when it's low
in the sky (winter). Light is scattered more and becomes
more diffuse when passing through fog or pollution.
A spot that gets plenty of sun 9 months of the year
might be shadowed from November to January due to
obstacles (trees, chimneys, rooftops etc.).
3) The third factor
creates the greatest problems for those who don't
happen to live close to the Equator, i.e. the difference
in the number of hours of sunlight between the seasons.
This is a huge subject that we'll have to take a closer
look at later.
Looking for the Sun
It's always best to
best to have your panels facing south at the ideal
tilt angle depending on your latitude and the time
of year. (Magnetic south as indicated by a compass
is actually 16² west of true south.)
THE SUN'S RAYS SHOULD
BE PERPENDICULAR TO THE PANELS.
SUNLIGHT SHOULD HIT THEM AT A 90° ANGLE.
The ideal situation
in Europe is to have a south-facing roof at an angle
between 40 and 60 degrees, or, even better, a flat
roof (or surface) on which your panels can be adjusted
at will. You may decide to deviate from these values
for convenience or for esthetic reasons, in order
to fit them into the existing architectural structure.
The future of PV will depend to a large extent on
the harmonious integration of panels in buildings.-One
example of this: In an apartment building in Denmark,
where they wanted to install glass sides in the balconies
(to limit heat loss), they realized that they could
just as easily install frameless PV panels at a minimal
additional cost. The loss in output due to the vertical
position and the less-than-ideal location (facing
south-west) was estimated at 30%.
Some people use sophisticated
panel mounts called "trackers" that follow
the path of the sun during the day. These automatic
systems can increase output 50% in the summer and
20% in the winter, but this only increases the difference
in output between the seasons. They are also expensive.
The main reason against using them in Europe is the
tremendous amount of diffuse light.
1. You can adjust
your panels' position manually to get the best tilt
angle for each season. Take your latitude and add
15° for the winter, and subtract 15° for the
summer. At the spring and autumn equinoxes, the best
angle is equal to your latitude.
2. If you leave your
panel in a fixed position, you can decide to leave
it at the best angle for the winter to help even out
seasonal performance.
3. At the Equator,
a panel can be placed horizontally for the most intense
rays at noon. In Central Europe, when the sun is 30°
above the horizon, however, this same position would
mean a loss of about half of the sun's intensity (equivalent
to 500 W/m²) compared to a tilt angle of 60°.
4. The sun is 70°
above the horizon on June 21st in Belgium, but 20°
in the dead of winter. (10° in Stockholm)
5. It is advisable
to have at least a 15° tilt to avoid rain accumulating
on your panels. A greater angle will help keep them
free of snow.
6. Snow on the ground
is a welcome sight in winter -it increases diffuse
light considerably!
Grid-Tied Solar Systems
Grid tied systems
do not use batteries! The panels work by producing
DC power during sunlight hours and converting it to
AC power through an inverter. The inverter then feeds
the load required by the home, before pushing the
remaining AC power through a "net meter"
out to the grid.
Net meters are provided
by your utility company when you install the system.
The net meter is a bi-directional electric meter that
spins forward as usual when you are using more power
in your home than you are producing with your panels
(at night, for instance), and spins BACKWARD when
you are producing more electricity than you are using
in your home. This is how you generate credits with
your utility!
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