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How Solar PV Panels Work

Photovoltaic (PV) Solar panels convert the energy in sunlight to electrical energy.

"Photo" refers to light and "voltaic" to electricity.

A PV cell is made of a semiconductor material, usually crystalline silicon, which absorbs sunlight. You've seen PV cells at work in simple mechanisms like watches and calculators. You've probably even seen them for signs on the road. More complex PV systems produce solar electricity for houses and the utility grid. The utility grid is the power source available to your local electricity provider.

Solar Cells - A Brief History

Although practical solar cells have only been available since the mid 1950s, scientific investigation of the photovoltaic effect started much earlier.  In 1839 a French scientist, Henri Becquerel, discovered that an electric current could be produced by shining a light onto certain chemical solutions.

The effect was first observed in a solid material (in this case the metal selenium) in 1877. This material was used for many years for light meters, which only required very small amounts of power.

Einstein in 1905 and Schottky in 1930 provided a deeper understanding of the scientific principles that was required before efficient solar cells could be made.

A silicon solar cell which converted 6% of sunlight falling onto it into electricity was developed by Chapin, Pearson and Fuller in 1954, and this kind of cell was used in specialised applications such as orbiting space satellites from 1958.

Today's commercially available silicon solar cells have light-to-electricity conversion efficiencies exceeding 15%, at a fraction of the price.

There are now a variety of methods for the practical production of silicon solar cells, such as single crystal, polycrystalline,amorphous, thick film, ribbon, and sliver. Solar cells can also be manufactured using many different materials, for example, copper indium diselenide, cadmium telluride and gallium arsenide.

How are Solar Cells Made?

PV cells are usually made using either single crystal wafers, polycrystalline wafers or thin films of silicon.

monocrystalline

Monocrystalline PV Cells

Single crystal wafers are sliced (approx. 1/3 to 1/2 of a millimetre thick) from a large single crystal ingot, grown at around 1400°C. The silicon must be of a very high purity and have a near perfect crystal structure.

These cells are highly efficient (15-18%) but expensive to manufacture.

polycrystalline

Polycrystalline PV Cells

Polycrystalline wafers are made by a casting process in which molten silicon is poured into a mould and allowed to set. Then it is sliced into wafers.  Polycrystalline PV Cells are slightly less efficient (12-14%) than monocrystalline PV cells.

However, they are also less expensive to manufacture since they use smaller crystals which are easier and quicker to grow than the single crystals used in monocrystalline PV cells.  The lower efficiency is due to imperfections in the crystal structure resulting from the casting process. Almost half the silicon is lost as sawdust in the two processes mentioned above.

amorphous

Amorphous Silicon

Amorphous silicon is the oldest thin film solar cell technology.  Amorphous silicon is made by depositing silicon onto a glass backing substrate from a reactive gas such as silane (SiH4).  This type of solar cell can be applied as a film to low cost substrates such as glass or plastic. The silicon is called "amorphous" because it has a non-crystalline structure that is similar to glass found in windows and bottles etc.

Compared to crystalline photovoltaic cells, amorphous silicon is less efficient (5-6%), but the material costs and manufacturing costs are also much lower.

Other thin film technologies include thin multicrystalline silicon, copper indium diselenide/cadmium sulphide cells, cadmium telluride/cadmium sulphide cells and gallium arsenide cells. There are many advantages of thin film cells including easier deposition and assembly, the ability to be deposited on inexpensive substrates or building materials, the ease of mass production and the high suitability to large applications.

In solar cell production the silicon has dopant atoms introduced to create a postive-type (p-type) and a negative-type (n-type) region and thereby producing a p-n junction. Dopant atoms (or dopants) are atoms of another element that are added to materials to control its conductivity. This doping can be done by high temperature diffusion, where the wafers are placed in a furnace with the dopant introduced as a vapour. There are many other methods of doping silicon. In the manufacture of some thin film devices the introduction of dopants can occur during the deposition of the films or layers.

PV Cells, Modules, Arrays & Systems

The amount of power available from a PV device is determined by;

  • the type and area of the material;
  • the intensity of the sunlight; and
  • the wavelength of the sunlight.

A typical single crystal silicon PV cell of 100 cm2 will produce about 1.5 watts of power at 0.5 volts DC and 3 amps under full summer sunlight, of which the approximation of 1000Wm-2 is used (Wm-2 are the symbols that represent the Watts of energy for every square metre). The power output of the cell is almost directly proportional to the intensity of the sunlight.

As single PV cells have a working voltage of about 0.5 V, they are usually connected together in series (positive to negative) to provide larger voltages. Panels are made in a wide range of sizes for different purposes. They generally fall into one of three basic categories:

  • Low voltage/low power panels are made by connecting between 3 and 12 small segments of amorphous silicon PV with a total area of a few square centimetres for voltages between 1.5 and 6 V and outputs of a few milliwatts. Although each of these panels is very small, the total production is large. They are used mainly in watches, clocks, calculators, cameras and devices for sensing light and dark, such as night lights.
  • Small panels of 1 - 10 watts (and 3 - 12 V with areas from 100cm2 to 1000cm2) are made by either cutting 100cm2 single crystal or polycrystalline cells into pieces and joining them in series, or by using amorphous silicon panels. The main uses are for radios, toys, small pumps, electric fences and trickle charging of batteries.
  • Large panels, ranging from 10 to 60 watts (and generally either 6 or 12 volts with areas of 1000cm2 to 5000cm2) are usually made by connecting from 10 to 36 full-sized cells in series. They are used either separately for small pumps and caravan power (lights and refrigeration) or in arrays to provide power for houses, communications, pumping and stand-alone power supplies (SPS).

If an application requires more power than can be provided by a single panel, then larger systems can be made by linking a number of panels together. However, complexities arise because often very large quantities of power are required at specific voltages, and at a time and level of uniformity than can be easily provided directly from the panels. In these cases, PV systems are used to customise the output of arrays to better cater for the energy needs of the user.

PVSystem

PV systems generally contain the following basic elements:

  1. a PV panel array, ranging from two to many hundreds of panels;
  2. a control panel, to regulate the power from the panels;
  3. a power storage system, generally comprising of a number of specially designed batteries;
  4. an inverter, for converting the direct current to alternating current (eg. 240 Volts AC – like most Australian homes).
  5. backup power supplies such as diesel generators (this is optional).

A framework or structure as well as housing for the system is generally required to dependably support and orientate the array towards the sun and keep the other components dry and clean. Trackers and sensors to optimise the performance of the system are often viewed as optional in a PV system.

PVTrackSys

However to ensure reliable performance, the design and installation of all PV systems in Australia should always be completed according to the Australian Standards:

  • AS 4509 Stand-alone Power Systems Parts 1, 2 and 3.
  • AS 4086.2 Secondary batteries for use with SPS - installation and maintenance.
  • AS 5033 Installation of photovoltaic (PV) arrays.
  • AS 2676 Guide to installation, maintenance ... of secondary batteries in buildings.
  • AS 3011 Electrical installations - Secondary batteries installed in buildings (LV batteries).
  • AS 3010 Electrical installations – Supply by generating set.

StationaryVsTrackedArrays of panels are being increasingly used in building construction (building integrated), where they serve the dual purpose of providing a wall or roof as well as providing electric power for the building. Eventually as the prices of solar cells fall, building integrated solar cells may become a major source of electric power.

Trackers are used to keep PV panels directly facing the sun, thereby increasing the output from the panels. Trackers can nearly double the output of an array. Careful analysis is required to determine whether the increased cost and mechanical complexity of using a tracker is cost effective in particular circumstances.

Photovoltaics: A Diverse Technology

US Department of Energy | Energy Efficiency & Renewable Energy

This video illustrates the diversity of photovoltaic (PV) technology, which is due to innovations in PV materials, reductions in manufacturing costs, and expanding uses of the technology. A brief explanation is provided of how PV cells work, as well as an overview of the different semiconductor materials used for silicon and thin-film cells.

Click on the link below to watch the video - opens in a new window

https://www.eeremultimedia.energy.gov/solar/videos/photovoltaics_diverse_technology

Solar Power Basics

US Department of Energy | Energy Efficiency & Renewable Energy

This video summarizes the process of generating solar electricity from photovoltaic and concentrating solar power technologies. Research, manufacturing, and usage across the United States is also discussed.

Click on the link below to watch the video - opens in a new window

https://www.eeremultimedia.energy.gov/solar/videos/solar_power_basics

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