Page 119 - Applied Photovoltaics

6.6 OTHER ELECTRICAL CHARGE STORAGE METHODS

6.6.1 Nickel-cadmium batteries
Nickel-cadmium (NiCd) batteries are commonly used as rechargeable batteries for
household appliances and can be suitable for stand-alone PV systems, especially in
cold climates. They have a number of advantages over lead-acid batteries. They:
x can be overcharged
x can be fully discharged, eliminating the need for oversizing
x are more rugged
x have excellent low temperature performance and can be frozen without
damage to the cells
x have low internal resistances
x can be charged at a much higher rate
x maintain uniform voltage during discharge
x have a longer life
x have low maintenance requirements
x have low discharge rates when not in use.
However, they also have a number of disadvantages. They:
x are typically two to three times more expensive
x have lower charge storage efficiencies (60–70%)
x can require full discharge to prevent ‘memory’ development, and subsequent
inability to deep discharge
x have a much lower capacity increase due to low discharge rates.
Newer designs aim to overcome these disadvantages, and nickel-cadmium batteries
are now being designed and manufactured specifically for PV applications. In some
cases they are cheaper, on a life-cycle cost basis, than lead-acid batteries (see also
Pedals, 1993) although investment costs are around a factor of three higher than for
lead-acid batteries (Sauer, 2003).

6.6.2 Nickel-metal-hydride batteries
Nickel-metal-hydride (NiMH) batteries rely on absorption and desorption of
hydrogen in a metal alloy during charge and discharge. The electrolyte contains an
aqueous solution of potassium hydroxide, most of which is absorbed in the electrodes
and separator. Hence, they can be used in any orientation. Their nominal voltage is
1.2 V, like nickel-cadmium. Compared to nickel-cadmium, their energy efficiency is
higher, at 80–90%, the maximum power is lower, and the memory effect is less
pronounced. Nickel-metal-hydride cells are less tolerant of voltage reversal than
nickel-cadmium, so care must be taken to avoid this condition, especially in multiple
series connections.

It is not very likely that large nickel-metal-hydride batteries will find wide application
in remote PV energy storage due to their high cost, but they are rapidly replacing
nickel-cadmium for portable appliances (Sauer, 2003).

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