Page 115 - Applied Photovoltaics

3. Longer-term storage, to ensure system availability throughout periods of low
insolation.
6.4.3 Requirements

Battery maintenance can be a major limitation for stand-alone PV systems. Typical
requirements for a battery system to be used for long term storage are:
x long life
x very low self-discharge
x long duty cycle (long periods of low charge)
x high charge storage efficiency
x low cost
x low maintenance.

6.4.4 Efficiency

There is considerable importance placed on the efficiency of batteries, owing to the
relatively high cost of both the batteries and photovoltaic array. Battery efficiency can
be characterised as follows:
1. Coulombic, or charge efficiency—usually measured at a constant discharge
rate, referring to the amount of charge able to be retrieved from the battery,
relative to the amount put in during charging. Self-discharge will affect
coulombic efficiency.

2. Voltage efficiency—also measured at a constant discharge rate and reflecting
the fact that charge is retrieved from the battery at a lower voltage than was
necessary to put the charge into the battery.

3. Energy efficiency—the product of the coulombic and voltage efficiencies.
Typical average charge storage efficiencies are 80–85% for stand-alone PV systems,
with winter efficiencies increasing to 90–95%, owing to:
x higher coulombic efficiencies when the battery is at a lower state of charge
(85–90%)
x most of the charge going straight to the load, rather than into the batteries
(95% coulombic efficiency has been measured experimentally).

6.4.5 Power rating and capacity
The power rating of a battery is defined as the maximum rate of charge and
discharge, measured in amperes (A).
Battery capacity is the maximum amount of energy that can be extracted from a
battery without the battery voltage falling below a prescribed value. The battery
capacity is measured in kilowatt-hours (kWh) or ampere-hours (Ah), at a constant
discharge rate. The rate of discharge affects capacity. PV systems typically have a
300 hour discharge rate which, for lead-acid batteries, gives them approximately
double the capacity specified at a 10 hour rate. Battery capacity is affected by
temperature, falling by about 1% per degree below about 20°C. At the other extreme,
however, high temperatures accelerate aging, self-discharge and electrolyte use.

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