Environmental Engineering Reference
In-Depth Information
Table 4.19 Hydrogen storage technologies (+, excellent; 0, average;
, poor)
Type
System
mass
(kg)
System
volume
Extraction
ease
System
cost
($)
Fuel
cost
($)
Stand
time
(h)
Maturity Total
Compressed
gas
(5 kpsi)
+
0
+
+
+
+
+
6
Compressed
gas
(10 kpsi)
+
0
+
+
+
+
0
5
Cryogenic
(liquid H
2
)
+
+
0
+
0
+
4
Metal hydride
+
0
+
+
0
3
Carbon
adsorption
(e.g.
nanotubes)
0
0
0
+
0
1
Chemical
hydride
+
+
+
3
power densities with advanced batteries. Much is being written about ultra-
capacitors for hybrid propulsion application as a means to remove the heavy
cycling load from the electrochemical battery, particularly when the battery is
lead-acid.
Ultra-capacitors will be competitive in hybrid propulsion systems when their
cost drops below $5/Wh. To put this into perspective, a typical automotive battery
manufacturing cost is $0.05/Wh a factor of 100 lower, but ultra-capacitors have
many redeeming features. Test data on advanced batteries are pointing to the fact that
advanced batteries such as NiMH and lithium ion have cycle life durability that can
be closely approximated by taking total energy throughput, for shallow cycles only,
and gaining a very good indicator of life in a hybrid environment. However, lead-
acid battery systems do not share this predictability based on cumulative shallow
cycles as a life indicator. The conclusion in Reference 36 is that ultra-capacitors
make eminent sense when combined with lead-acid batteries for any duty cycle
application, but this does not appear to hold for advanced batteries in the case of
shallow cycling. A hybrid propulsion system exposes the ESS to cycling at depths
>
1% with 4% as very typical. A shallow cycle can be defined as charge or discharge
events for which less than 1% of the stored energy is exchanged.
A list of available ultra-capacitor suppliers is given in Table 4.20 along with
product specifications and calculated specific energy and power values. This list is
not exhaustive but representative of the ultra-capacitor market today.
Ultra-capacitor specific energy is determined from constant current testing
conditions. The ultra-capacitor is pre-conditioned to its final voltage and held for
sufficient time so that it is fully charged. Constant current discharge results in a
linear slew rate of cell voltage to zero as depicted in Figure 4.41.
