Environmental Engineering Reference
In-Depth Information
1000
6
IC engine
Fuel cells
4
100 h
EV goal
2
Li-ion
PHEV-40 goal
100
PHEV-10 goal
6
Ni-MH
4
Lead-acid
HEV goal
2
10 h
10
Capacitors
6
4
36 s
1 h
0.1 h
2
3.6 s
1
10
0
10
1
10
2
10
3
10
4
Specific Power (W/kg)
Acceleration
FIGURE 6.6
Plot of energy versus power for various energy systems. (From http://berc.lbl.gov.venkat.
Ragone-consruction.pps)
for diverse applications. This means that a VRB system can produce more
kilowatts of power simply by resizing the stacks and/or supply more energy
by increasing the size of the electrolyte storage tanks. The system theoreti-
cally has no limit to the amount of energy it can provide.
One may therefore ask, “Given all these advantages, why don't we see
more VRBs in operation?” The answer mainly involves energy density, size,
cost, possible irreversible precipitation of V
2
O
5
, and the infancy of the tech-
nology that makes most companies hesitant to invest in technology that has
not been fully tested. The energy density of a VRB is limited by V
2
0
5
and
has been measured to be ~167 Wh/kg. Figure 6.6 provides a comparison of
energy densities of other systems. For example, a 600-MWh vanadium redox
flow battery system would require 30 million liters of electrolyte. If stored in
6-m high tanks, its footprint would be the size of a football field. In general,
the following conditions must be met for the operation of vanadium redox
flow cells:
• Electrodes require high electric conductivity and good wettability.
• Charging voltage must be limited to a maximum of 1.7 V to avoid
damage to the carbon current collectors.
• Good electrical contact to the bipolar plates and current collectors is
essential and best achieved when the activation layers are thermally
bonded to the current collector.
• Access of oxygen to the negative electrolyte compartment must be
avoided.
