Solar Panels

Photovoltaic Systems

Solar PV (Photovoltaic) panels use sunlight to create electricity that you can use for lighting and running household appliances.

A PV panel is made from a number solar cells. A cell consists of layers of silicon, a semi conducting material. Sunlight creates an electric field across the silicon layers creating usable amount of direct current (DC) electricity.

Optionally we can feed the DC current into an inverter to turn the DC electricity into a 120V or 240V AC (alternating current) electricity needed for household appliances.

Solar modules are silent and generate no greenhouse gases, saving about 8 tonnes of carbon dioxide per one 1 kWp (Kilowatt peak) installation. . A KWp is the amount of power generated. An average house in the UK would require a 1.5kWp to 2kWp system

Solar Cells

A solar cell is a device that converts sunlight into electricity. Typically solar cells are made from a semi-conducting material like silicon. When a light photon with high enough energy hits the silicon material electrons can be knocked loose from their atoms allowing them to flow through the material. At the same time positive charges are created which flow in the opposite direction creating electricity.

Solar Cells can be produced from many types of materials. However silicon is the most common material used today. The three main types of commercially available cells are:

• Monocrystalline cells
• Polycrystalline cells
• Thin file – amorphous silicon and materials such as copper indium-deselenide, cadmium-telluride and gallium-arsenide.

For our purposes solar cells are assembled to create solar modules (typically 72 cells). Solar modules in turn are linked to create a photovoltaic array big enough to create commercial electricity. Modules can be connected in series for additive voltage or in parallel for additive current. Modules are typically connected in an array in series or parallel/series to create a desired Voltage/Current.

The output of a module is measured in Watts which is derived by multiplying the module’s peak power voltage by its peak power amperage (Watts = Volts x Amps). Peak ratings are based standard test conditions (STC) of 1000 Watts/square metre of light input, a cell temperature of 25 degrees C (77 degrees F), and an air mass of 1.5 (AM1.5G). These standard test conditions are used throughout the industry.

Wafers

a) Monocrystalline - Czochralski (CZ) method

High purity silicon is melted in a quartz crucible. A seed crystal, mounted on a rod, is dipped into the melt and pulled out slowly in a rotational movement creating a single crystal ingot.

The ingots are then sawed into thin wafers of about 200-400 micrometers thickness.

b) Polycrystalline

Silicon is melted in large ceramic crucibles to form an ingot. Each ingot is cut into smaller bricks which are then sawn into wafers.

Polycrystalline silicon material is stronger than the monocrystalline version so can be cut into thinner wafers. It also has a slightly lower wafer cost and is less labour intensive to produce. However, the grain boundaries in polycrystalline silicon hinder electron flow reducing the cell efficiency.

Making of Modules:

The picture illustrates the basic manufacturing processing.

a) One block of silicon crystal is cut into very thin wafers. These wafers are the basis for PV cells.

b) The wafer is treated by a special process called doping. Doping involves the addition of small amounts of chemicals which give the PV cell its special properties. The wafer is then a PV cell ready to convert sunlight into electricity.

c) Cells connected together in a part of a PV module

Currently we offer two types of modules:

1. Monocrystalline
   
   This material is also called single crystalline because the basic material is produced by growing one large crystal. The process to produce monocrystalline silicon is relatively slow and energy intensive compared to processes used for other silicon based PV materials. Therefore modules, which contain PV cells made of monocrystalline silicon, are more expensive but their efficiency, conversion of sunlight into electricity, is the highest of the silicon materials.
   
   
2. Polycrystalline
   
   This material is also called multicrystalline silicon. This material is produced by growing many silicon crystals together. After this the production process is similar to the previous picture. The different crystal growth process for polycrystalline silicon produces PV cells which have slightly lower efficiencies but they are also cheaper than the monocrystalline alternative.

The table below shows typical conversion efficiencies of silicon based PV modules:

A lower efficiency means more PV modules are needed for the same electricity output. However, for the same electricity output the costs of all of these materials are similar. Efficiency is a measure of the electrical energy output from the module or system as a fraction of the light energy (sunlight) input into the module or system.