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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 Assembly

These 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.

Quality

The 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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