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Balance of System (BOS): what is it?

What is Balance of Systems?

In a photovoltaic energy system, the solar panel is the queen of the system. All other parts which contribute to the functioning comprise the “balance of system” usually denoted by the acronym (BOS). These will include the:

  • Wiring,
  • Switches
  • Mounting system-mostly needed for supporting the panels but may house other allied equipment.
  • One or many solar inverters- for converting the DC produced by the panels to AC suitable for use or grid connection.
  • Battery bank- Required with sufficient capacity if grid connection is not available (stand-alone systems). Excess Energy must be stored for use when the sun is not available.
  • Battery charger- Charges batteries from solar energy. Maximum Power Point Tracking (MPPT) chargers adjust the current so as to pass maximum power to the battery under given solar output conditions.
  • Inverters- Invert DC to AC.
  • Junction boxes with fuses and relays.
  • Power conditioners- Ensure that the inverter output is well regulated and free of sags, surges and spikes.
  • Metering system- In case of grid-connected systems to record flow of energy from and to the grid.

Optional equipment, depending on nature and scope of system, may include:

  • GPS Solar tracker- To keep the panel always positioned for receiving maximum energy from the sun.
  • Power management software- Will be required in larger systems using multiple strings of panels and inverters, and possibly feeding multiple loads. The software may control alarms for out-of-tolerance conditions, take autonomous switching actions.
  • Solar concentrator
  • Solar irradiance sensors- These are normally justified in large commercial systems for mainteanance and monitoring purposes.
  • Wind speed sensors
  • lenses or optical mirrors (CPV system only)-For concentration of the beam.
  • Or accessories specific to the installation and owner requirements.

Balance of Plant

A similar term to Balance of System is “Balance of plant (BOP)” which is generally used in the context of power engineering and applies to all the supporting components and systems of the power plant which are needed to produce the energy. These may include suitable transformers, inverters, cabling, switching and control equipment, protection equipment, power conditioners, support structures etc., depending on the type of plant.

Cost of BOS

Cost of Balance of System will include the cost of the hardware (and software, if applicable), labor, permitting Interconnection and Inspection (PII) fees, and any other fees that may apply. For large commercial solar systems, the cost of BOS may include the cost of land and building, etc. The cost of BOS can be about two thirds of the total cost.

Downward Trend

While the cost of solar panels is coming down appreciably, the cost of BOS is not showing the same rate of decline. It is understandable because extra effort has gone into the solar cell technology. The solar cell technology is still evolving and improving, and costs are being reduced fast. The balance of systems consists mostly of items which are not specific to solar technology. As an example, the mounting structures are quite usual and the technology may already be mature, benefitting little from further innovation and research.

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