Environmental Engineering Reference
In-Depth Information
Table 4.7 Data for Various Types of Rechargeable Battery
Lead-acid
NiCd
NiMH
NaS
Positive electrode
PbO 2
NiOOH
NiOOH
S
Negative electrode
PbO
Cd
metals
Na
Electrolyte
H 2 SO 4 +H 2 O
KOH+H 2 O
KOH+H 2 O
β -Al2O3
Energy density (Wh/l)
10-100
80-140
100-160
150-160
Energy density (Wh/kg)
25-35
30-50
50-80
100
Cell voltage (V)
2
1.2
1.2
2.1
Charge/discharge cycles
500-1500
1500-3000
about 1000
about 1500
Operating temperature (°C)
0-55
-20 to 55
-20 to 45
290-350
Self-discharge rate (%/month)
5-15
20-30
20-50
0
Wh efficiency
70-85%
60-70%
60-85%
80-95%
there would be periods without any power for a solely photovoltaic-powered
system, which clearly is not desirable.
Storage systems can be classified into short-term storage for a few hours
or days to cover periods of bad weather and long-term storage over several
months to compensate for seasonal variations in the solar irradiation in
summer and winter. Since long-term storage is extremely expensive, the
photovoltaic generator is usually oversized so that it also can provide sufficient
energy in wintertime. Another solution is hybridization with wind or diesel
generators.
Secondary electrochemical elements are mainly used for storage over short-
and medium-term periods; they are usually called batteries. For economic
reasons, the lead-acid battery dominates the current market. When higher
energy densities are needed due to weight considerations, for example, in
laptop computers, other batteries such as nickel-cadmium (NiCd) or
nickel-metal hydride (NiMH) are used. Other batteries such as
sodium-sulphur (NaS) have been tested for use in electrical (battery-powered)
vehicles but are no longer being developed. Table 4.7 summarizes the data for
various types of rechargeable battery.
Lead-acid battery
Today, the most common battery for electricity storage is the rechargeable
lead-acid battery. The main reason is cost. The car industry, especially, prefers
lead-acid batteries. So-called solar batteries have a slightly modified structure
compared with car batteries and achieve longer lifetimes. However, the
principle structure of the solar battery is similar to the car battery. It has two
electrodes. In the charged state, the positive electrode consists of lead dioxide
(PbO 2 ) and the negative electrode of pure lead (Pb). A membrane embedded in
a plastic box separates the two electrodes. Diluted sulphuric acid (H 2 SO 4 ) fills
the empty space between the two electrodes. A fully charged lead-acid battery
has an acid density of about 1.24 kg/litre at a temperature of 25°C, and this
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