The Fundamentals of Photovoltaic Systems
The basic element of a photovoltaic system is the solar cell. Modern solar cells are approximately 4 inches square and are most often made from silicon, a semiconductor.
The photovoltaic effect occurs when sunlight shines on the silicon, freeing electrons and generating an electric current. The electricity is collected and transported by metal contacts on the top and bottom of the cell. The current flows through a wire to provide electricity.
Groups of cells are mounted on a rigid, rectangular plate and wired together to form a module often called a” panel” or “flat-plate collector.” The module is sealed with plastic or glass for protection. Two or more modules connected together form an “array.”
Single cells produce little power and are not often used individually. They can be found, however, in some items such as small yard lights. Individual panels, which usually produce 40 to 60 watts of electricity, power larger devices such as downtown Phoenix bus stop lights. Arrays, depending on the number of panels used, can provide all the electricity for a home or even create a huge generating station.
Modules or arrays are sometimes mounted on tracking systems, which follow the sun across the sky. These devices help maximize electricity production because sunlight shines directly on the PV modules throughout the day. Single-axis trackers move as the sun changes position from the east to west. Two-axis trackers not only follow the suns east to west movement, but also allow for its apparent change in attitude with different seasons. Trackers can increase the energy production of a photovoltaic system by nearly 40 percent.
Electricity storage is a critical component of many PV systems. If power is needed at night or on cloudy days, solar-generated electricity can be stored in batteries. In large PV-powered homes, for example, it is not uncommon to find banks of 50 or more batteries. These battery banks usually store sufficient electricity to power the home through one or two cloudy days and nights.
All photovoltaic, cells produce direct current (dc) electricity. That electricity can be used immediately if the PV cell is connected to a device designed for dc power – many refrigerators in recreational vehicles, for example. However most homes and appliances are designed to operate on alternating current (ac) electricity provided by utility companies. For applications, an inverter, which changes dc electricity to ac must be added to the PV system.
Charge controllers are also important components of many PV systems. These devices protect batteries from excessive charge when the modules produce more electricity than the batteries can store. They also keep batteries from releasing electricity if their charge is too low. Without charge controllers, batteries suffer extreme wear-and-tear and become less effective, last a shorter amount of time and possibly even short circuit.