In a photovoltaic system, the term 'balance of system' refers to all of the system components except the PV modules. These components frequently account for half of the system cost and, unfortunately, most of the system maintenance. The components consist of structures, enclosures, wiring, switch gear, fuses, ground fault detectors, charge controllers, batteries, and inverters. Of these, ground fault detectors, charge controllers, batteries, and inverters are the components that could benefit most from developments in both technology and basic understanding. A diagram of a typical hybrid system, illustrating some of the BOS components, is shown below.

SNL Grid-Tied Inverter Test Bed

SNL Engineer Evaluates RFI

Some of the design considerations associated with these BOS components are:

batteries--type, depth of discharge, rate of charge, lifetime (in PV applications)
charge controllers--algorithm configuration, charge current
stand alone inverters--load compatibility, power rating, power quality, battery health
grid-tied inverters--islanding, susceptibility to line disturbances, RFI
hybrid inverters--load compatibility, generator compatibility, battery health
Sandia National Laboratories has a BOS program that partners with industry to address these BOS issues and component reliability and cost. This program consists of research and development contracts with industry and a laboratory program that supports industry and provides product benchmarking.

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