HOME SOLAR PANELS: FACTS and INFORMATION
The term “solar panel
” in general refers to any complete environmentally protected device that absorbs sunlight and converts it into a more usable form of energy, such as electrical or thermal. This term is used interchangeably with the term "module
", while panel may sometimes refer to a collection of modules mounted together as a field installable unit. The two main types of solar modules that you can use in your home are photovoltaic panels
, which generate electricity, and solar thermal collectors
, which convert solar energy into heat. Here you will find basic facts and practical information on photovoltaic (PV) devices.
HOW PV PANELS ARE MADE
A PV module consists of a number of electrically connected photovoltaic cells mounted in a support structure (see the solar panel diagram below).
In a conventional flat-plate module, the PV cells are protected from the top by a transparent surface, typically tempered, low-iron glass.
A structure of a typical flat-plate solar panel (courtesy of EERE
The structural support is often provided by a frame in the back and around the outer edges, usually aluminum. Some manufacturers, such as First Solar, use frameless laminate designs. The cells as well as the top and rear surfaces are held together by an optically transparent encapsulant, such as Ethyl vinyl acetate (EVA). The transparent front surface normally has an anti-reflection coating that reduces the reflection of sunlight.
The commercial modules intended for home use usually have a junction box in the back. It connects to two waterproof cables equipped with lockable connectors. The cables should have marks of the positive and the negative polarities, although usually red is plus and black in minus.
HOW DO THEY WORK
When light hits the PV cells inside the panel, it creates voltage across each cell of 0.5 to 0.7V. In this process, a portion of
radiated solar energy is converted into electrical energy. Individual cells are connected in series inside the module, which adds up the voltage. Some manufacturers also parallel two or more series strings to achieve higher currents. See series and parallel connection
methods for more details. Different brands and models may have different number of cells. In the commercially available solar panels intended for home use, the open circuit voltage Voc
is typically between 30V and 100V. When no load is connected to the module, the circuit is open; there is no current flow and therefore no power transfer. When an external load such as a battery charger or an inverter is connected to the panel’s terminals, it closes the circuit. The free electrons driven by the electric field (described by voltage) can now flow from the minus terminal to the plus terminal through this external circuit. Such a flow of electric charges is called electric current
. Although it is electrons which produce electric current in conductors, there is an old convention to consider the direction of the current as if it were the positive charges which are moving. So, we take the direction of the current from plus to minus.
As the charges flow through a load, they perform work and their potential energy is converted to some other form. Thus, solar power is being delivered to the load.
The diagram on the right shows as an example volt-amp characteristics of Mitsubishi 185W module PV-SUD185MPH5. The output voltage is the highest at no load. When a panel is loaded, its voltage slightly drops as a function of the current. At some point, the voltage drops to zero and current can no longer increase. This maximum output current is called short circuit current (Isc)
Note that power
in watts equals to voltage times current: P=V×I
. The maximum output power that a module can generate is somewhere near the knee of the I-V curve as you can see from the diagram. Note that Voc depends on the properties of PV cell materials and the temperature, but practically does not depend on the intensity of the light. The short circuit current in contrast is also a function of the sunlight. Finally, the generated power depends on all of the above factors and the load.