The solar panel

An individual solar panel is made of many solar cells. The cells are electrically

connected to provide a particular value of current and voltage. The individual

cells are properly encapsulated to provide isolation and protection

from humidity and corrosion. There are different types of modules available on the market, depending on the power demands of your application. The most common modules are

composed of 32 or 36 solar cells of crystalline silicon.

These cells are all of equal size, wired in series, and encapsulated between glass and plastic material, using a polymer resin (EVA) as a thermal insulator. The surface area of

the module is typically between 0.1 and 0.5 m2. Solar panels usually have

two electrical contacts, one positive and one negative.

Some panels also include extra contacts to allow the installation of bypass

diodes across individual cells. Bypass diodes protect the panel against a

phenomenon known as "hot-spots". A hot-spot occurs when some of the cells

are in shadow while the rest of the panel is in full sun. Rather than producing

energy, shaded cells behave as a load that dissipates energy.

In this situation, shaded cells can see a significant increase in temperature (about 85 to

100ºC.) Bypass diodes will prevent hot-spots on shaded cells, but reduce the maximum voltage of the panel. They should only be used when shading is unavoidable. It is a much better solution to expose the entire panel to full sun whenever possible. The electrical performance of a solar module its represented by the IV characteristic curve, which represents the current that is provided based on the voltage generated for a certain solar radiation. The curve represents all the possible values of voltage-current.

The curves depend on two main factors: the temperature and the solar radiation received by the cells. For a given solar cell area, the current generated is directly proportional to solar irradiance (G), while the voltage reduces slightly with an increase of temperature. A good regulator will try to maximize the amount of energy that a panel provides by tracking the point that provides maximum power (V x I). The maximum power corresponds to the knee of the IV curve.

How to choose a good panel?

One obvious metric to use when shopping for solar panels is to compare the

ratio of the nominal peak power (Wp) to the price. This will give you a rough

idea of the cost per Watt for different panels. But there are a number of other

considerations to keep in mind as well.

If you are going to install solar panels in geographical areas where soiling

(from dust, sand, or grit) will likely be a problem, consider purchasing panels

with a low affinity for soil retention. These panels are made of materials

that increase the likelihood that the panel will be automatically cleaned by

wind and rain.

Always check the mechanical construction of each panel. Verify that the

glass is hardened and the aluminum frame is robust and well built. The solar

cells inside the panel can last for more than 20 years, but they are very fragile

and the panel must protect them from mechanical hazards. Look for the

manufacturer's quality guarantee in terms of expected power output and mechanical

construction.

Finally, be sure that the manufacturer provides not only the nominal peak

power of the panel (Wp) but also the variation of the power with irradiation

and temperature. This is particularly important when panels are used in arrays,

as variations in the operating parameters can have a big impact on the

quality of power generated and the useful lifetime of the panels.