Solar Panels - How do they work?
Most of us know that solar electricity panels generate electricity, however less
of us know how. This knowledge is vital for our engineers when
determining how to get the best out of a solar PV installation. This
article will explain not only how solar PV panels generate electricity
but also how they link together and the important elements of how they
are manufactured and specified.
As a brief summary, solar
photovoltaic panels generate direct current (DC) which is fed into an
inverter which converts the DC into alternating current (AC). This is
then fed into the properties fuse board which distributed the power
around the home.
The overall principle is simple but if you are
to ensure that your system is properly designed please read
on. The information below will put many potential customers into a
position where they know more than their installer. Not a bad position
to be in when there are so many new start companies still finding their
feet!
Semiconductor - The Complex Part...A basic cell is
constructed by applying two specific substances (dopants) to either
side of a silicon crystal. The dopants are chosen to give an electron
rich half (+) and an electron poor half (-). Given the imbalance of
electrons an electric field is created as electrons wish to travel from
the electron rich half(+) to the electron poor half(-). This
arrangement is called a semiconductor.
Sunlight is made up of packets
of light called photons. As these photos hit the semiconductor some
are absorbed, the energy from this packet of light enables an electron
to move (this is called photo excitation). The electron naturally
wishes to move to the half of the silicon cell that had a shortage of
electrons. This leaves a shortage of electrons where the the photon has
just been absorbed. This effectively means that the positive charge
(atoms short of electrons) moves in the opposite direction to that of
the electrons. The movement of electrons can also be referred to as an
electric current. This process continues as long as the energy from the
sun light is strong enough to keep the electrons moving and therefore
electricity is generated.
Module AssemblyThese
semiconductors (photovoltaic cells) are assembled into a solar module.
The solar module consists of rows and columns of photovoltaic cells, a
sheet of toughened glass and a rear laminate that bonds the cells to the
glass. The whole assembly is then usually encapsulated in a ridgid
aluminium frame. This frame stops any undue movements due to external
forces such as wind and snow loading.
The electrical connections from
the cells are brought into a junction box on the rear of the module.
The junction box may contain a number of diodes that will only allow
electricity to flow in one direction. These diodes are designed to stop
'back flow' of electricity when parts of modules are in shade. This
junction box is also the point at which one module is connected into
another or to the inverter.
QualityThe range of
available components varies massively from manufacturer to
manufacturer. The photovoltaics industry can often be compared to the
automotive industry where the quality, performance and cost varies
massively. The key factors when looking at PV modules are to assess the
quality of the following components:-
- Cells - access the manufacturer and performance warranty - look for sun soaked cells
- Laminates
- Junctions boxes and the number of diodes
- Surrounding frame (if applicable)
Peak Power - Tolerances and Degradation
PV
Modules are assembled to reach a specific peak power in Watts. This is
the rate at which a PV module will generate electricity when the sun is
shining brightly (1000W/m2). Peak power is one indication of a modules
performance, it is important to note that two modules with the same
peak power often perform quite differently in practice down to the type
of cells, manufacturing process and associated components (frame,
junction box, diodes, laminate, inverter, etc).
It is also
important to be aware of the tolerances on PV module peak power. Most
manufacturers quote a peak power with a positive and negative
tolerance. For instance if a module is 100W +-3%. This means that the
module will be fully within its manufacturing specification if it
outputs 97W. This means that if you purchase a 4kW system you could
find that you actually have 120W (3% of 4000W) less than you have paid
for. When comparing modules be aware of the tolerance on the peak power
output.
The peak power is also subject to degradation over the
life time of the module. It is not unusual for a module to be output
only 80% of its specification after 20 years. This affects the overall
revenue generated from the PV system over its life time, frustratingly a
significant proportion of this loss can be in the first few years. Be
sure to check the performance warranty of any system you consider.
More
informed readers may be aware of a process called 'sun soaking'. This
is a process utilised by some manufacturers that degrades the PV cells
in the factory prior to the modules peak power being recorded. This
means that the cell will not degrade as quickly in use, the initial
rapid degradation has already taken place before the modules peak power
is classified. This process will give the owner more output over the
life of the system and the manufacturer will be able to provide a better
performance warranty.
Strings - Module Wiring Design
PV
modules need to be wired in a manner that keeps the total output
voltage and current within boundaries required by the inverter.
Different inverters have different ranges of voltage and current input
guidelines. This means that different inverters can operate at
different efficiencies, this efficiency is heavily influenced by the way
the modules are configured. Wiring multiple modules together in series
creates a 'string' of modules. These strings can then be connected
together in parallel which gives multiple strings. Voltages between
strings must match or electricity can flow in the wrong direction. For
instance if a string of 4 modules is connected in parallel with a string
of 5 modules, some voltage could flow from the 5 module string (higher
voltage) to the string of 4 modules (lower voltage) rather than to the
inverter. This can not only happen with a poorly configured system but
also when there is shading across some strings and not others. Modules
are often fitted with diodes to minimize the impact of this imbalance,
diodes prevent electricity flowing in the wrong direction.
Solar Cell Efficiency
When solar cells are being manufactured, utilising different silicon
wafer shapes, types of silicon and integration with other types of
crystals can lead to high efficiency PV modules. Efficiencies range from around 5% up to about 20%.
Efficiency
is a measure of how effectively a process can convert one form of
energy into another. The input energy is the light from sun and the
output is the electricity flowing into the inverter. Given that the
input energy is free (sunlight) the efficiency is not always as
important as it is for other types of system such as the efficiency of a
gas boiler.
For instance a module with an efficiency of 10% is more
than capable of outputing the same amount of electricity as a module
with an efficiency of 20%. The only difference will be the physical
surface area of the module (the 10% efficiecnt unit will be double the
size). For projects when roof space is limited high efficiency panels
should certainly be considered. However when roof space is not limited
it may be wiser to choose a lower efficiency panel made with high
quality components. For example fitting a high efficiency panel system
on a roof with ample space may not lead to maximum output and
reliability over the life of the system.
Our consultants will spend a
significant amount of time assessing your requirements and property
characteristics before recommending a specific type of PV module/panel.
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