Photovoltaics - Balance of System

Background

We may think of a complete photovoltaic (PV) system as comprising three subsystems. On one side, we have the PV devices (cells, modules, arrays, etc.) that convert sunlight into direct-current (dc) electricity. On the other side, we have the load, or the application for which the PV electricity is intended. Between these, we need a third subsystem to enable the PV electricity to be properly applied to the load. This third subsystem is generally referred to as the "balance of system" or BOS.

The BOS typically consists of structures for mounting the PV arrays or modules and the power-conditioning equipment that adjusts and converts the dc electricity to the proper form and magnitude required by an alternating-current (ac) load. If required, the BOS also includes storage devices, such as batteries, for storing PV-generated electricity to be used during cloudy days or at night.

Mounting Structures

Photovoltaic arrays have to be mounted on some sort of stable, durable structure that can support the array and withstand wind, rain, hail, and other adverse conditions. Sometimes, the mounting structure is designed to track the sun.

Stationary structures are usually used with flat-plate systems and generally tilt the PV array at a fixed angle that is determined by the latitude of the site, the requirements of the load, and the availability of the sunshine.

Tracking Structures

There are two general kinds of tracking structures: one-axis and two-axis. Single-axis trackers are typically designed to track the sun from east to west on its daily route. They are used with flat-plate systems and some concentrator systems. The two-axis type is primarily used for PV concentrator systems. These systems track not only the sun's daily course but its seasonal course between the northern and southern hemispheres. Naturally, the more sophisticated the system the more expensive and the more maintenance it may require.

Power Conditioners

Power conditioners process the electricity produced by a PV system to make it suitable for meeting the specific demands of the load. Although most of this equipment is standard stock, it is extremely important to match the capabilities of these devices with the characteristics of the load. Power conditioners may have to perform these functions:

        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

        Safeguard the utility network system and its personnel from possible harm during repairs

The requirements of power conditioners generally depend on the type of system they are integrated with and the applications of that system. For dc applications, power conditioning is often accomplished 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 direct current generated by the PV array into alternating current.

Storage

In many PV systems, energy will not be used as it is produced but may be required at night or on cloudy days. If tapping into the utility grid is not an option, a battery backup system will be necessary. The drawbacks to batteries are that they decrease the efficiency of the PV system, because only about 80% of the energy channelled into them can be reclaimed. They also add to the expense of the overall system and must be replaced every five to ten years. They take up considerable floor space, pose some safety concerns and require periodic maintenance.

Like PV cells, batteries are direct-current devices and 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. Most batteries must also be protected from overcharge and excessive discharge, which can cause electrolyte loss and can even damage or ruin the battery plates. Protection is usually achieved using a charge controller, which also maintains system voltage. Most charge controllers also have a mechanism that prevents current from flowing from the battery back into the array at night.

Source: U.S. Department of Energy Photovoltaics Program.

For more information on this source please visit National Renewable Energy Laboratory

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