Flat-Plate Photovoltaic Balance of System
Complete photovoltaic (PV) energy systems are composed of three subsystems.
- On the power-generation side, the first subsystem of PV devices (cells, modules, and arrays) converts sunlight to direct-current (DC) electricity.
- On the power-use side, the second subsystem consists of the load, which is the application of the PV electricity.
Between these two, a third subsystem enables the PV-generated electricity to be properly applied to the load. This subsystem is often called the balance of system, or BOS.
The BOS typically consists of structures for mounting the PV arrays or modules and power-conditioning equipment that adjusts and converts the DC electricity to the proper form and magnitude required by an alternating-current (AC) load. The BOS can also include storage devices, such as batteries, so PV-generated electricity can be used during cloudy days or at night.
PV arrays must be mounted on a stable, durable structure that can support the array and withstand wind, rain, hail, and other adverse conditions. Sometimes, this mounting structure is designed to track the sun. However, stationary structures are usually used with flat-plate systems. These structures tilt the PV array at a fixed angle determined by the latitude of the site, the requirements of the load, and the availability of sunlight.
Among the choices for stationary mounting structures, rack mounting may be the most versatile. It can be constructed fairly easily and installed on the ground or on flat or slanted roofs.
There are two basic kinds of tracking structures: one-axis and two-axis. One-axis trackers are typically designed to track the sun from east to west. They are used with flat-plate systems and sometimes with concentrator systems. The two-axis type is used primarily with PV concentrator systems. These units track the sun's daily course and its seasonal course between the northern and southern hemispheres. Naturally, the more sophisticated systems are the more expensive ones, and they usually require more maintenance.
Power conditioners process the electricity produced by a PV system so it will meet the specific demands of the load. Although most equipment is standard, it is important to select equipment that matches the characteristics of the load. Power conditioners may:
- Limit current and voltage to maximize power output
- Convert DC power to AC power
- Match the converted AC electricity to a utility's electrical network
- Have safeguards that protect utility personnel and the electrical network from harm during repairs.
Specific requirements of power conditioners depend on the type of PV system they are used with and the applications of that system. For DC applications, power conditioning is often done with regulators, which control output at some constant level of voltage and current to maximize output. For AC loads, power conditioning must include an inverter that converts the DC power generated by the PV array into AC power. Many simple devices—for example, ones that run on batteries—use DC electricity. However, AC electricity, which is what is generated by utilities, is needed to run most modern appliances and electronic devices.
Electricity is needed at night and on cloudy days, when PV power generation may not be possible. If tapping into the utility grid is not an option, a battery backup system is necessary for energy storage. However, batteries lower the efficiency of a PV system because only about 80% of the energy that goes into them can be reclaimed. They take up considerable floor space, pose a few possible safety problems, and require periodic maintenance. Still, they provide one way to store PV electricity for later use.
Like PV cells, batteries are DC devices that are directly compatible only with DC loads. However, batteries can also serve as a power conditioner for these loads by regulating power. This allows the PV array to operate closer to its optimum power output.
Inverter convert the DC electricity generated by the PV array into AC electricity, and charge controllers protect batteries from overcharging and excessive discharge. Most batteries must be protected from overcharge and excessive discharge, which can cause electrolyte loss and even damage or ruin the battery plates. Most charge controllers also have a mechanism that prevents current from flowing from the battery back into the array at night.
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