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Flat-Plate Photovoltaic Balance of System Basics

August 20, 2013 - 4:29pm


Complete photovoltaic (PV) energy systems are composed of three subsystems.

Illustration of the elements needed to get the power created by a PV system to the load (a house). The top portion of the illustration (a) shows a PV module with DC interface and regulation and a battery bank connected directly to the DC load, which is a house. The bottom portion of the illustration (b) shows a PV module connected to a power conditioner. The power conditioner is connected to both the load (a house) and a meter, which is also connected to a power line.
This illustration shows the elements needed to
get the power created by a PV system to the
load (in this example, a house). The stand-alone
PV system (a) uses battery storage to provide
dependable DC electricity day and night. Even for
a home connected to the utility grid (b), PV can
produce electricity (converted to AC by a power
conditioner) during the day. The extra electricity
can then be sold to the utility during the day, and
the utility can in turn provide electricity at night or
during poor weather.
  • 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.

Mounting Structures

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.

Inverters and Power Conditioners

Illustration of three PV modules attached to a mounting rack. The rack is a shaped like a right triangle, and the arrays are mounted on the longest side toward the sun.

A typical PV array mounting rack.

Inverters convert DC power to AC power and are the basic component needed to the convert the DC power produced by PV arrays to meet AC power loads.  Power conditioners possess additional capabilities for processing the electricity produced by a PV system so it will meet the specific demands of the load. Power conditioners may:

  • Convert DC power to AC power
  • Limit current and voltage to maximize power output
  • 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.

Historically, inverters, with no inherent power conditioning functionality of their own, were only a component of certain power conditioners.  More recently, "inverter" has been used to refer to any power conditioner with inverter capabilities. Advanced inverters, or "smart inverters," can help even out power flows autonomously or controlled by utility operators through remote communication. Specific requirements of inverters 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. 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 Storage

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.

Charge Controllers

Photo of an inverter and charge controller, two small electronic boxes, mounted on a shelf.

An inverter (left) and charge controller (right) are the
power conditioning components of a PV system.

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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