Environmental Engineering Reference
In-Depth Information
mass of 300 kg. The fuel cell stack radiator is mounted behind the vehicle grill as in
a conventional car. The traction motor and stack are liquid cooled. In Reference 35
the author recognizes that cold start of the FCHV in cold weather remains a major
obstacle to commercialization. Fuel cell hybrids today are rated for operation in a
rather confined temperature range of 0-40
C [36]. If the environment is outside
that temperature range, the Toyota FCHV will display the message 'sorry, unable to
start'. The Honda Motor Co. FCX has a very similar constraint.
Toyota's FCHV (5th generation) uses compressed H
2
at 34.5 MPa (5,000 psi)
in storage tanks located beneath the floor pan and in the trunk space (Figure 10.49).
The vehicle has a range of 300 km on the Japan 10-15 cycle and 290 km on the US
combined cycle. FC propulsion system costs are still prohibitively high. According to
the Arthur D Little Company, FC manufacturing costs are approximately $100 KW in
high volume at 2001 dollar basis. The US Freedom Car initiative has set an FC system
cost target of $30/kW by 2015. Vehicle costs in that same time frame are expected to
be at approximately $100,000 each.
80
60
40
20
0
100
0
20
40
60
80
100% H
2
40% H
2
Output (% rated)
Figure 10.49 Fuel cell system efficiency (pure H
2
versus reformulated)
10.6.3 Ultra-capacitor model
Basics of ultra-capacitors were described previously. Ultra-capacitors are electrostatic
field energy storage devices, which rely on polarization of the electrolyte in a highly
porous medium at both electrodes. The device is non-Faradic, but ionic transfer does
occur between the electrodes. At the electrode boundary the ions accumulate, forming
a double layer capacitor. Aqueous electrolytes exhibit lower series resistance than
organic electrolytes but easily suffer dissociation and degradation if the applied
potential exceeds 1.2 V. Organic electrolytes, conversely, have higher series resistance
but are capable of operating at 3 V before the electrolyte begins to dissociate.
The model of an ultra-capacitor is very similar to that of an advanced battery.
The highly porous electrodes are modelled as a distributed ladder network of
resistance and capacitance. When characterized with EIS, it has been reported [37]
that ions have finite mobility and diffuse into the smallest of pores in the electrodes
only after long time constants. If the porosity is characterized as macro, meso and
micro, then the resulting frequency dependence of impedance is clearer. At very
low frequencies (dc to mHz), the ions have time to migrate into micro pores and
