Solar Photovoltaic
Balance-of-system (BOS)
Apart from the photovoltaic array, the other components required to make up a photovoltaic system are referred to as the balance-of-system (BOS). The BOS includes combiner boxes (junction boxes), charge controllers and storage batteries for standalone systems, inverters (for grid-connected systems or systems supplying AC loads), mounting structures, wiring, switchgear and fuses, surge arrestors, earth-fault protection devices and so forth. The BOS may account for up to half of the capital cost of a photovoltaic system (for small systems) and most of the maintenance cost. Major components of BOS are described below.
(a) Charge controllers (for standalone systems)
A charge controller is a device used in standalone photovoltaic systems for regulating the current flowing from the photovoltaic array to the storage batteries. At high irradiance, the photovoltaic array keeps charging the batteries and when the battery is "full", the charge controller disconnects the batteries from the photovoltaic array. The configuration of a shunt-type charge controller is shown below (the other type is series-type).
A MPP (maximum power point) charge controller incorporates a DC-to-DC converter such that the photovoltaic array can operate at a voltage which can deliver maximum output power at the prevailing solar irradiance.
(b) Inverters
An inverter is a device to convert electricity from DC to AC. Since solar panel output and battery output are DC, an inverter is needed in a photovoltaic system intended to supply AC loads. Standalone inverters are used for standalone photovoltaic systems, and grid-tie inverters are used in grid-connected systems.
Power loss occurs during the DC to AC conversion process. The conversion efficiencies of inverters of different designs are generally in the range of 85% to 95%. An inverter of good conversion efficiency should be used to reduce energy loss. Besides, the quality of the AC generated (such as the harmonic contents, voltage and frequency stability) and the protection features (such as short-circuit protection, overload protection, deep discharge monitoring for standalone inverters) should also be considered when choosing an inverter.
Grid-tie inverters are discussed in the This link will open in a new windowSolar photovoltaic - Grid Connection
section.
Above: A wall-mounted grid-tie inverter
(c) Storage batteries
Storage batteries are essential for standalone photovoltaic systems as electricity generation is not constant and is not controllable (i.e. depends on solar irradiance). With storage batteries, electricity generated by solar panels can be stored up and used to power the electrical loads when there is no sunlight.
Lead-acid batteries, in particular the deep-cycle battery is the most popular battery type for photovoltaic systems as they are designed to be charged and discharged frequently and can handle heavy discharges time after time. The life-span of deep-cycle lead-acid batteries varies from 3 to 8 years. In photovoltaic systems, the storage capacities of battery systems are generally in the range of 0.1kWh to 100kWh.
Battery capacity is usually expressed in ampere-hour (Ah). In theory, a battery rated at 50 Ah will delivery electricity at 1 ampere for 50 hours or 2 amperes for 25 hours. (The storage capacity of a battery in kWh is equal to the Ah rating times voltage rating divided by 1000. The storage capacity of a battery system in kWh is the sum of the kWh capacities of the individual batteries.) When sizing the capacity of the battery system, an important consideration is the "days of autonomy", which is the number of days that the battery system is capable of supporting the electrical loads without being recharged in cloudy and rainy days.
The major problem of lead acid batteries is ageing, meaning an increase in internal resistance with age. Ageing of lead acid shortens its service life. Another problem caused by the use of lead acid batteries is the potential environmental hazard caused by lead upon disposal.
