On the way to fuel-cell-vehicle (FCV) commercialization Volkswagen has developed several generations of prototype-FCV to proof technological and financial feasibility. The latest prototype called HyMotion3 has a freeze-start capability to at least -15 ?C (prediction), is equipped with 700 bar gaseous-hydrogen-tank technology and has 100 kW of driving power. Further the fuel-cell-system (FCS) comprises an active hydrogen-recirculation and a gas-to-gas-humidification on the cathode-side. The drive train consist besides the FCS of a high-power Li-ion battery to recapture braking energy and to boost energy while accelerating.
Using today’s PEM-FCS for automotive transportation, the cooling-system is one of the most limiting factors in order to achieve appropriate driving conditions since FC-temperature is limited to about 90 ?C. Especially considering steep uphill driving with high loads at raised ambient temperatures such as 40 ?C lets the cooling-temperature easily exceed the upper limit, where the FC can not permanently be operated without increasing degradation. Therefore Volkswagen’s long-
term-goal is to build FC with higher operating-temperature (such as 120 ?C and higher) for better heat-rejection, diminution and abolition of components such as humidifiers which all ends up in the reduction of costs.
Until Volkswagen gets to the point where this high-temperature-PEM-FC can be applied in a FCV, lots of efforts are targeted on optimizing today’s FCS to gain experiences by field-tests.
This talk offers a description of how the cooling-system for the latest prototype-FCV HyMotion3 was optimized and what achievements in comparison to the earlier HyMotion2 were accomplished. The cooling-system is basically built as a circuit where the FC, electrical components and heat-exchangers for gas-tempering are heat-sources. The radiator in combination with electric fans has the purpose to reject heat and keep the inlet-temperature of the FC at 80 ?C and lower.
The large amount of heat that has to be rejected by the radiator in combination with a low temperature difference to the ambient atmosphere requires large radiator-surfaces and high air-side flow rates thru the radiator. The surface area mostly competes against packaging issues and design aspects of the vehicle. Therefore the depth of the radiator-block constitutes the only further degree of freedom, which has to be optimized. There is to find an optimum of block-depth between heat-rejection-surface and pressure-loss.
As instruments for this optimisation a stationary 1D-fluid-simulation of airflow which requires measurements in a climate wind-tunnel as well as a CFD-calculation of front vehicle package are used.
In order to describe the airflow rate of the radiator, the pressure-loss curves of every component such as the radiator itself, the AC-condenser as well as the air louvers have to be determined by measurements in a wind-tunnel test-bench. Further the pressure gain of the fans is measured for different rotational speeds. In addition the heat-transfer-characteristic of the radiator has to be determined for different air- und water- flow rates and temperature-differences. These curves are entered in a 1D-fluid-calculation-tool called KULI, which besides these curves takes the position and geometrical dimensions of the components into account. The most important and yet unknown variable is the build-in resistance of the front-end airflow (figure 1), which comprises the pressure loss of the FCS in the front vehicle. To determine the optimum of radiator-depth, this build-in resistance is approximated as an empirical value taking front end package into account. A parameter-variation offers the optimum radiator-block-depth where most heat is rejected.
upper air-intake build-in resistance air-outlet
lower air-intake fans A/C condenser
Figure 1: Configuration of front vehicle air-flow components in KULI
After building up the FCV, measurements to validate the estimate of build-in resistance are made in a climate wind-tunnel as shown in figure 2, where constant boundary conditions such as airflow and ambient temperature are available. Thereby the vehicle is operated at different velocities, each until stationary conditions are reached. The measurement of coolant flow-rate as well as in- and outlet-temperature of the radiator adds up to the heat rejection at every vehicle speed. As an input to KULI these values lead to the airflow rate thru the radiator and to the build-in resistance of the FCS. It is further on possible to extrapolate the vehicle driving conditions and determine the maximum vehicle velocity at different ambient temperatures and inclinations.
Figure 2: Tiguan HyMotion3 in climate-wind-tunnel
Another way to determine the airflow and the possible heat rejection of the radiator is to use a 3-dimensional fluid-dynamics-tool, which is fed by the same component-characteristics as above in addition to the precise CAD-data of the vehicle front end. Calculations for different vehicle-velocities bring out the airflow thru each component as well as additional outcomes such as leakage flow rate and homogeneity of radiator airflow.
In figure 3 a comparison of air-flow-rate for both vehicle generations is shown. The optimization results in an increase of air-flow by 25 % at a vehicle speed of 150 km/h (fans on), which is even higher at lower speeds. When fans are turned off, air-flow thru HyMotion3-radiator reaches the flow-rate of HyMotion2 using fans at a driving speed of 130 km/h. Thereby the parasitic loss of FCV is decreased since fans must be used less frequently.
HyMotion3 (fan on)2HyMotion2 (fan on)1,8HyMotion3 (fan off)1,6HyMotion2 (fan off)
0,8air-flow in kg/s
0020406080100120140160vehicle speed in km/h
Figure 3: Comparison of air-flow-rate for both vehicle generations
Based on these curves the radiator heat-rejection can be evaluated for given air- and coolant- temperatures. Furthermore the heat that has to be rejected by radiator only depends on vehicle traction power. By taking traction-system- and FCS-efficiency as well as road inclination into account, the amount of heat as a function of vehicle speed can be calculated.
One of Volkswagen’s test procedures for cooling-system-components in series-production assigns
driving up Towne-Pass in Death-Valley (CA) with a vehicle-speed of 60 mph. In doing so the ambient-temperature is 50 ?C at the foot of the mountain and decreases steadily with altitude down to 38 ?C at the mountain peak. This procedure constitutes very rough conditions for FCV-cooling-system since road inclination is up to 10 % (figure 4) resulting in a maximum traction-power of 75 kW.
6006altitude in m
road inclination in %4004
time in s
Figure 4: Altitude- and inclination-profile for Towne Pass (Death Valley)
The results show, that possible heat-rejection at a FC-temperature of 85 ?C, which PEM-FC is capable of, is less than the heat that has to be rejected. As consequence coolant-temperature would rise above the allowed limit.
heat-flow, that has to be rejected80heat flow, that can be rejected at t_FC = 85 ?C70heat flow, that can be rejected at t_FC= 100 ?C
30heat flow in kW
time in s
Figure 5: Variance analysis of heat-rejection for Towne-Pass driving procedure
By increasing FC-temperature to at least 100 ?C, the cooling-system would be capable to reject as much heat as needed for described uphill driving-procedure. Having a FC with a possible coolant-temperature of at least 120 ?C, size of the radiator would be comparable to those of ICE-vehicles.