Page 114 - Applied Photovoltaics

4. Resistive losses in interconnects.A module V mp of 18 V is required when
charging a 12 V lead-acid battery because:
1. ~2.8 V is lost to temperature rises to 60°C.
2. A drop of ~0.6 V occurs across the blocking diode.

3. A drop of 1.0 V typically occurs across the regulator.
4. There can be some voltage loss with reducing light intensity.
5. The batteries must be charged to 14.0–14.5 V to reach their full state of
charge.
In reality, because of the high resistive losses associated with screen printed cells, it is
not uncommon for the V mp to actually rise with reduced light intensity in steady state.
Although the V oc falls logarithmically with reducing light intensity (constant
temperature), the corresponding fall in temperature and reduced voltage loss, owing
to the smaller currents flowing through the cell metallisation, can often lead to a small
increase in V mp .

The life expectancy of solar cells is determined primarily by the quality of the
encapsulation, particularly with regard to protection against ingression of moisture.
The most common failure mechanisms were discussed in Chapter 5.
As also discussed previously, module design and the materials used make a
significant contribution to cell operating temperature, and hence efficiency. Output is
also influenced by aspects such as siting, shading, tilt angle and the self-cleaning
properties of the modules.

6.4 BATTERIES

Australian Standard 4086 specifies requirements for storage batteries in stand-alone
power supply systems (Standards Australia, 1993, 1997). It applies to all types of
batteries, including lead-acid and nickel-cadmium and both vented and valve-
regulated cells.

6.4.1 Types
There are many types of batteries potentially available for use in stand-alone PV
systems, including lead-acid, nickel-cadmium, nickel-metal-hydride, rechargeable
alkaline manganese (RAM), lithium-ion, lithium-polymer and redox batteries (see
also Crompton, 1990; Sauer, 2003; Spiers, 2003). At present, the most commonly
used is lead-acid. There exist many other battery technologies, such as zinc-bromide,
zinc-chloride, magnesium-lithium, sodium-sulphur and nickel-hydrogen, but they are
of little relevance for remote PV systems at this stage. Hydrogen-oxygen storage
systems are of very low efficiency and are not further considered here (Bossel, 2004).

6.4.2 Applications
Batteries can be used for:

1. Power conditioning (e.g. in water pumping systems, discussed later).
2. Short-term storage, to effectively redistribute the load over a 24 hour period.

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