Environmental Engineering Reference
In-Depth Information
Table 4.1 Properties of accumulators
Lead-acid
NiCd
NiMH
NaS
Energy density, Wh/kg
25 ... 35
35 ... 55
50 ... 80
80 ... 150
Power density, W/kg
100
200
300
90 ... 160
Operating temperature, C
10 ... 60
20 ... 55
20 ... 45
290 ... 350
Cell voltage, V
2
1,2
1,2
2,1
Self-discharge, %/month
5 ... 15
20 ... 30
20 ... 50
0
Useful full cycles
600 ... 1500
2000
1000
1000
Energy efficiency, %
70 ... 85
60 ... 75
65 ... 85
80 ... 95
that their development was discontinued, and the once intended vehicle application
has come out of sight.
4.4.3 Electrical Energy Storage
4.4.3.1 Double-Layer Capacitors
Conventional capacitors are a storage element for electrostatic energy in principle;
as devices for storing energy in larger amounts they are not suitable. This limitation
has been overcome by the development of electrochemical double-layer capacitors
(EDLC). They are known under different trade marks, e.g. UltraCaps, SuperCaps
or BoostCaps. EDLCs have two electrodes and a fluid electrolyte. When a voltage
below that of electrolyte decomposition is applied, ions collect on the electrodes
of opposite polarity in layer thickness of only a few molecules. The very low gaps
together with large electrode areas lead to very high values of energy density. The
electrostatic energy is:
E q = 1
2 CU 2
Cell capacitances of 5000 F and above are commercially available, with rated
voltage/limiting voltage of 2,5/2,8 V; allowing energy densities of up to 4,7 Wh/kg.
The useful energy is determined by the difference of the squared voltages charged
and discharged, the latter set e.g. at 1,2 V. The series resistance is very low, so
that power densities of up to 10 kW/kg can be reached. Balancing the series con-
nected cells requires some equalizer effort, and power electronic devices are in-
dispensable for adaption to the supplying system. On the other hand EDLCs have
a long lifetime, allowing 500.000 charging/discharging cycles. Although their en-
ergy density is below that of electrochemical accumulators, with their easy han-
dling, good operating temperature range (
35 ... 65 C) and high reliability they
are becoming a favourite choice for storage applications [Ha196] and especially
for vehicles [Schn02]. Regarding wind energy systems they are also considered as
short-time storage elements; in [Kin04] a compensation device using an EDLC bank
with a buck-boost converter is proposed.
 
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