Fuel-cell hybrid vehicle
A hybrid vehicle model consisting of a PEMFC
stack and a supplemental battery source is developed on AMESim. The
specifications of the selected vehicle are given in table 1. The V-ECU
determines the power balance between PEMFC and the battery depending upon the
requested torque by the vehicle. As the current obtained from PEMFC depends on
the stack voltage, the voltage required by the battery changes with time. In
the event of sudden acceleration, the battery backup should be able to
complement the PEMFC due to its relatively slow response. The PEMFC system
recharges the battery when the SOC drops below lower limit of SOC.
The key components of PEMFC system considered
are: PEMFC stack, air-feeding system and thermal management system (TMS). Due
to the high computational time of multi-dimensional fuel cell model,
one-dimensional PEMFC model is used.
Heat transfer- Heat
generated in fuel cell is obtained by subtracting enthalpy of water formation
from the electric work generated. The heat transfer take place by both the
modes: i) convection- between gas in the channel and GDL; ii) conduction-
between collecting plate and GDL. The coolant helps in maintaining fuel cell
temperature by extracting the heat through convection.
Water transport- the variation of membrane water
content is given by-
The Nerst potential of each cell is given by the
The activation overpotential is given by the eq.
The ohmic overpotential is given by eq. 37
The essential segments of the humidifier are
discretized into three control volumes:
The concentration gradients in all these control
volumes drive the water diffusion. Along the flow volume of the tube, the water
vapor from the shell control volume is transferred to the membrane and finally
it diffuses in the tube control volume.
To capture the distribution of dynamic
characteristics of heat exchanger, it has been split into following components:
The flow configuration was selected to be cross
flow. Diving the heat exchanger into quasi-two-dimensional control volumes
enables us to obtain local states in parallel & perpendicular direction if
Blower output temperature is governed by-
Energy efficiency of a vehicle is function of its
driving cycle as it governs the necessary torque to drive the vehicle. In case
of urban city driving (like FTP-75) there are frequent starts and stops due to
various traffic regulations. Therefore, quick dynamic response of power source
is necessary to meet the high-power requirement of the vehicle. As PEMFC is unable to satisfy this dynamic
requirement due to limitation of supply rates of reactants, a battery back-up
is provided to fulfil additional power.
Current- The input current, being proportional to
the air-flow rate, varies during the cycle which is depicted in fig.
Humidifier- (description of things in humidifier)
As the air
passes through the blower, a sharp decrease in relative humidity is found at
the beginning. This
is caused because increase in magnitude of saturation pressure by temp increase
is much higher than increase in partial pressure by pressure increase.
The sharp decrease in RH indicates that there is a very small amount of water
left in the air at the blower exit. This dehydration can result into degradation of fuel
cell and hence dwindle the performance of fuel cell. Hence, a humidifier is
necessary, post blower to maintain the performance of fuel cell. The air
gains moisture content and becomes almost saturated at fractional dist. Of 0.6
as it travels through the length of the humidifier.
temperature differences in PEMFC stack (becomes higher) and air after blower
(relatively lower) causes the dry air to absorb heat from wet air. Hence, dry
air temp gradually increases along air flow direction. The difference in
vapor and heat transfer rate between dry air and wet air results in variations
of air temperature and relative humidity in the humidifier. In this way, the
dry air entering the FC stack is heated and humidified. Thus, the use of humidifier
improves the fuel economy of FCHV by helping the system to reach its efficient
operation zone in terms of temperature and humidity quickly.
The TMS comes into action when temperature of
stack coolant at outlet exceeds 70 °C. the coolant is then passed through the
heat exchanger which removes the excess heat from the coolant and reduces its
temperature down to 65 °C.
For minimizing ohmic overpotential in the stack,
the membrane is hydrated initially. Later, to avoid the flooding in the channel
due to increased humidity, the temperature is increased to 70 °C.
As stated earlier, we have coupled fuel cell
stack with battery to power the hybrid vehicle. The power obtained from battery
is dependent on the power extracted from PEMFC stack. The power from PEMFC
stack depends on the hydrogen lower heating value (LHV), heat generated in the
stack and the useful electrical energy produced. The electrical energy generation
is a function of vehicle velocity and the road conditions. As we can see, the H2
usage rate is proportional to the FTP-75 velocity cycle. (fig.)