Environmental Engineering Reference
In-Depth Information
nominal power of the photovoltaic generator, since the batteries can provide very
high power according to their sizing.
Yet, batteries have a limited technical lifetime and have to be replaced several
times, over the entire lifetime of the photovoltaic system (which is determined by
the technical lifetime of the photovoltaic generator of approximately 25 years).
Calculated over the lifetime of the system, the summarised battery costs usually
account for 20 to 40 % of the entire lifetime costs and thus represent the major
cost factor, ranking even before those of the photovoltaic generator. Since the
technical lifetime of the battery is largely dependent on the stress profile and the
operation strategy (i.e. battery management), batteries need special attention both
in terms of planning as well as operation of the battery coupled photovoltaic sys-
tem /6-9/, /6-29/. Especially under economic aspects it has to be mentioned that
the lead- based batteries available today are a mass product without a significant
"economy of scale" effect. Additionally, due to the upcoming shortage on the
world resource markets and the resulting price increase it is expected that lead-
based batteries will become more expensive.
For devices (like watches, calculators) directly supplied by photovoltaics, pri-
marily nickel-cadmium-batteries are used. Additionally, nickel-metal-hydride-
batteries, lead-acid batteries, lithium-based battery systems, and capacitor (so
called bilayer capacitor or SuperCaps) are in operation. Photovoltaic supplied
small scale systems and hybrid systems are usually equipped with conventional
lead batteries.
To date, except for the small consumer appliances mentioned above, only lead-
acid batteries have gained importance /6-9/, /6-33/. However, such lead-acid bat-
teries only have a poor gravimetric energy density of 20 to 30 Wh/kg. Yet, this
disadvantage is of minor importance for the use in photovoltaic power supply
systems, since, unlike for example in electrically powered cars, batteries are oper-
ated stationary and are not moved.
Such lead-acid batteries (accumulators) store electric energy in the form of
chemical energy, which is re-converted into electric energy during discharge.
Chemical energy is stored in two electrodes (positive and negative) between
which there is a potential difference. Fig. 6.21 shows a schematic representation
of a lead-acid battery. When fully charged the positive electrode is composed of
porous lead dioxide (PbO
2
), whereas the negative electrode consists of porous,
spongy lead (Pb). The porosity of both electrodes is well above 50 %, and active
masses must have a fine crystalline structure to provide a large active surface. The
electrodes are submerged into ion-conducting electrolytes of diluted sulphuric
acid (H
2
SO
4
). The electrodes are separated from each other by an ion-permeable
separator to prevent short-circuits. The electrochemical process of charging and
discharging triggers the conversion of electronic current into ionic current.
During discharge both active masses are converted into lead sulphate (PbSO
4
)
by consumption of sulphuric acid (Equation (6.3)); this process reduces electro-
lyte concentration. As a result, the physical and chemical electrolyte properties
(including freezing point, conductivity, aggressiveness in terms of corrosion and
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