HOW DOES PV WORK?
A typical PV system uses panels of solar cells, made from semi-conducting
materials that react with sunlight to produce electricity. This
electricity is then transferred into the property to power appliances
and provide lighting, or if surplus to requirements, is sold
to the local electricity network.
• The Solar Cell
• Types of PV Cell
• The PV Panel
• The PV System
• On Grid Systems
• Off Grid Systems
The Solar Cell
Solar cells are the power units of every PV system.
A PV cell consists of two thin layers of semi-conducting materials,
usually silicon , that have been treated with chemical substances.
These chemicals react to sunlight when it shines on the cell,
creating an electric field across the layers and producing electricity.
The greater the intensity of sunlight, the greater the flow
of electricity. This process is called the 'photovoltaic effect'
and is explained in greater detail below.
1. The cell is covered with a thin layer of anti-reflective
coating (ARC) to minimize light reflection.
2. The top semi-conducting layer, or 'n' type layer, is
doped with tiny amounts of phosphorus so that
almost every thousandth silicon atom is replaced
by a phosphorus atom. This creates free moving negative charges
3. The base semi-conducting layer, or 'p' type layer,
is doped with miniscule amounts of boron so
that almost every millionth silicon atom is replaced
by a boron atom. This creates free moving positive charges called
4. When the 'n' and 'p' type layers are placed close together,
as they are in a solar cell, the positively
charged 'holes' and the negatively charged 'electrons'
are attracted to each other. As they move into their respective
neighboring layers they cross a boundary layer called the 'p-n
junction'. This movement of negatively and positively charged
particles generates a strong electrical field across the p-n
junction. When sunlight strikes this field it causes the electron
particles and the hole particles to separate, which in turn
creates a voltage of around 0.5V.
5. The voltage pushes the flow of electrons or 'DC current'
to contacts at the front and back of the cell
where it is conducted away along the wiring circuitry
that connects the cells together.
Types of PV Cells
Solar cells can be made from a number of semi-conducting materials.
A semi-conducting material is one that has a limited capacity
for conducting an electrical current and those used in solar
cells are all uniquely suited to producing electricity from
sunlight - the photovoltaic effect.
By far the most commonly used material is silicon, which is
the main component of quartz sand and, after oxygen, is the
second most common element in the Earth's crust.
The performance of a solar cell is measured in terms of its
efficiency at turning solar radiation or 'sunlight' into electricity.
A typical solar cell has an efficiency no greater than 13 -
15% as only a portion of the sunlight energy spectrum can be
converted into electricity and much of the sunlight is reflected
or absorbed by the materials that make up the cell. If this
seems off putting bear in mind that a gas power station has
an energy conversion efficiency of only 35% and that 70% of
the electricity generated is lost during the long distance transmission
to the consumers- you and I.
Here is an overview as to the properties of the types of commercial
solar cell available today.
The PV panel
|Type of Cell
||13% - 15%
||> 30 years
||*Highest efficiency = least surface area required
so most suitable for properties with limited roof area.
*Most expensive due to complicated manufacturing process.
||10% - 13%
||> 25 years
||Most commonly used type of cell as offers good efficiency
at reasonable cost.
|Thin Film / Amorphous
||5% - 7%
||> 20 years
||*Low efficiency requires large surface area. *Made
from flexible material and so can be used on curved
building surfaces *Works better in diffuse light than
mono & polycrystalline
As an individual solar cell only generates a low voltage, approx
0.5V, a number of cells are wired together to form a solar panel
or 'module' that can generate anything between 80-165kWp. Modules
are then connected together to form a PV array that will be
typically fitted onto a southerly facing roof at an angle of
between 30º and 50º in order to receive maximum sunlight. South-easterly
and south-westerly facing systems can be installed with only
a 5% reduction in panel efficiency but panels placed on a northerly
orientation do not receive adequate sunlight to generate sufficient
|PV solar cell
||PV solar panel
||PV solar array
It is extremely important that all the solar panels in a system
are free from shading during the daylight hours. Even the partial
shading of one cell in a panel will lower its power output.
As the cells of a panel are connected in series the weakest
cell will bring the others down to its lowered power level,
which can reduce the efficiency of the whole panel by as much
as fifty percent.
The PV System
PV systems are quoted in kWp (kilo Watts peak), which is the
rating of DC power produced by the system at any one time during
optimum lighting and temp erature conditions. The DC power needs
to be converted into AC before it can be used with household
appliances or is transferred onto the national energy grid.
This is done via an inverter, and, through the exchange process
some power is lost. A well installed PV system should produce
between 750kWh and 850kWh of AC electricity annually for every
1kWp of PV installed.
The panels and the inverter account for a major portion of the
cost of a PV system but there are additional components required
to ensure that the system operates safely, reliably and to current
regulatory electrical. These are shown in the diagram below:
Grid connected PV system
On Grid systems
Grid-connected photovoltaic systems are the most common type
as they make use of the existing mains electricity grid. They
are simpler in design and easier for the installers to fit than
off grid systems. The electricity produced during the daytime
is either used by the property owner, or directed back into
the electricity grid and purchased by a utility company, an
arrangement called 'net metering'. At night, or on dark days
when the panels do not produce sufficient power, electricity
will be supplied via normal utility company grid system.
Off Grid systems
Far less common is an "off grid" or 'stand alone' system, which
produces and stores power independently from the utility grid.
These systems are particularly suitable in remote locations
especially those where the property is more than one-quarter
mile from the nearest power lines. Often the installation of
an off grid PV system proves more cost-effective than extending
the power lines. The electricity generated by the panels is
stored in a bank of rechargeable batteries as DC but in order
to power household appliances an inverter will be required to
convert the stored DC to AC. These rechargeable batteries contain
specialised parts and chemicals not found in disposable batteries
and are therefore larger and more expensive to purchase and