Solar panels

Solar panels
Page Type Gear Category
Object Title Solar panels
Page By JanVanGenk
Created/Edited Nov 14, 2007 / Nov 18, 2007
Object ID 4257
Hits 13260
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Category Description

This category is intended for portable solar panels for on-the-move outdoor needs, including expedition use. For any suggestions/recommendations about improving this page, please send me a PM.

What is a solar panel and how does it work?

A solar panel is a device that collects and converts solar radiation energy into electricity (or heat). The building blocks for solar panels are photovoltaic solar cells (in the Greek "photos" stands for light; "volt" is a unit of electric potential or electromotive force). A photovoltaic solar cell is a semiconductor (e.g. silicon, germanium,...). A semiconductor has the special property that, when you illuminate it, electrons get a higher energy which creates a voltage difference between the contacts of the solar cell. With this voltage difference, an electrical device can work or a battery can be charged.

Solar panels can be either stiff (traditional semiconducting solar panels) or flexible. Flexible solar panels use the same concepts as stiff panels. They are capable of being flexible because of the production process used: thin film deposition. It is possible to deposit thin layers of silicon onto flexible substrates. The silicon is able to bend along with this flexible substrate without compromising structure because the silicon layers are only micrometers thick. Thin film processing has an advantage over typical manufacturing techniques due to the ease of manufacturing and reduction in manufacturing costs. The process allows manufacturers to create large rolls that can later be cut into smaller pieces. Not only are flexible solar panels inexpensive but they are also lightweight in comparison to their non-flexible counterparts. This makes the flexible solar panel ideal for portability and where available space can be an issue. Practical applications for these solar cells include, but are not limited to, charging AA, AAA, 6 volt, and 12 volt batteries.

Why are solar cells usually dark blue?

Solar cells are usually dark blue to almost black. The most solar cells are made of poly-crystalline silicon, but this silicon is grey. The blue color is caused by a special anti-reflection layer coated on the silicon. Every light particle that is not absorbed by the solar cell, but is reflected on the surface of the solar cell, is lost. Reflected light particles can not be converted into electricity. By adding a special anti-reflection layer, the reflection is decreased. And the color of this special layer is dark blue to almost black. It is possible to make solar cells in all colours, but then the efficiency is lower, because the reflection is higher than when using dark blue cells.

How are solar panels rated?

The following terms are used to describe the performance of a solar panel: volt (V) - a unit of electric potential, watt (W) - a unit of power, and ampere (Amp) - a unit of electric current. Panels are rated in watts of output. In order to compare the output of solar panels, the manufacturers have come up with some standard test conditions (STC) so that all panel ratings are derived after being subjected to the same conditions as every other panel. The STC refer to the conditions under which a panel is typically tested in a laboratory and which approximate operation in full sunlight. The STC are: (1) 1000 watts/square meter (0.645 watts per square inch) of light input, (2) a cell temperature (not air temperature!) of 25°C, plus or minus 2°C (77°F, plus or minus 3.6°F), and (3) an air mass of 1.5 (slightly above sea level). The maximum electric power of a solar panel under these conditions is called a peak power. The wattage rating is thus derived by multiplying the panel's peak power voltage by its peak power amperage (Watts = Volts x Amps).

The STC are rarely found in the daily reality, as the real world conditions are very variable. For example:

  • 1000 watts/square meter of sunlight would only be reached around solar noon, with the panel squarely facing the sun, just after a rain shower has washed all the dust out of the air. "Real world" input is usually around 800 to 850 watts/square meter on a bright day (when you factor in dust and air pollution and consider that the panels are laid flat and are therefore not square to the sun).
  • When you consider that solar cells are dark blue to almost black, they soak up sunshine and get quite hot so they are operating at temperatures considerably higher than 25°C (77°F). This increased cell temperature translates into a voltage drop and therefore less output.
  • The air mass changes as you move from sea level to altitude. The atmosphere is thicker at sea level so more sunlight is interrupted by dust and pollution and less gets transformed into solar electricity. Conversely, the same panel operating at altitude will see more intense sunlight and will produce more power.

On average you will probably only see about 80% of the rated output in "real world" operating conditions.

What affects output of a solar panel?

The output of a solar panel depends on a number of factors. The most important ones are listed below:

  • Solar radiation (light) intensity / Shading. The brighter the sunlight the more power the panels will produce. At a cloudless sky, the beams of the sun fall directly on the solar panel (direct solar radiation). But when there are a lot of clouds (by day), it is still light (diffuse radiation). Light particles (photons) enter the solar cell, and produce therefore electricity. Thus a solar cell generates electricity even when it is cloudy, only the production is less than on a sunny day. Also the shade caused by any objects or even dust will dramatically affect the output of solar panels. Therefore, it is better to place the panel out of the shade and keep it clean!
  • Angle of sunlight / orientation of a panel. The best performance is achieved if a panel is squarely pointed at the sun (cloudless sky). If not, one will lose some of that sunlight to reflection off of the surface of the panel. The optimal orientation for a solar panel in case of different solar radiation types, is the following:
    1) For the direct solar radiation, the panel must be aimed to the South. For the slope of the solar panel, n, a compromise angle between "b-e" and "b+e" should be chosen, with b the geographical latitude of the place where the panel is installed and e the ecliptica (= 23.5 °).
    2) For the diffuse solar radiation, the optimal orientation is when the solar panel is orientated to the whole sky, meaning that the panel lies horizontally. The slope n is 0°.
    3) For the real situation with direct and diffuse solar radiation the ideal orientation is a compromise between 1) and 2).
  • Cell temperature. The solar cells are dark in color and operate at much higher temperatures than the air temperature is. The hotter these cells are, the more of a voltage drop is experienced.
  • Improper wiring. Poorly made electrical connections create resistance and result in less power going to your batteries. Make the connections tight and keep them clean!

How reliable is a solar panel?

A solar panel has no moving parts to wear out and it does not consume any fuel. As long as there is enough light, it will produce electricity. Very little can go wrong with a solar panel short of physical damage.

More information

Solar power glossary
Wikipedia: solar cell
Wikipedia: photovoltaics