Throughout the world, the number of vehicles equipped with fuel cell drives is increasing slowly but steadily. Large quantities are expected within the next five years, particularly from Asian manufacturers. However, new opportunities are also opening up worldwide for heavy-duty commercial vehicles. Promising avenues include weight savings and extension of the cruising range in comparison with battery-powered solutions. Fuel cell concepts may represent an attractive option for long-distance hauling, which requires heavy loads and large cruising ranges.
When designing commercial vehicles with a fuel cell drive, the high hydrogen storage pressures of around 700 bar are not the only challenge. The extreme requirements in terms of thermal and media management and the sensitivity of fuel cell stacks to contamination and harmful gases in the air flow call for a perfectly harmonized arrangement of the fuel cell peripherals. MAHLE supports the development of commercial vehicles with fuel cell drives that are suitable for large-scale production—thanks to its competence in complete systems, thermal and air management, as well as filtration—and focuses on reducing manufacturing costs and improving operational safety. MAHLE is developing a modular fuel cell systems portfolio based on its current range of components. This is helping to reduce technological barriers and improve the suitability of vehicles with fuel cell drives for the mass market.
Air management for cold combustion
The air management of fuel cells places extremely high demands on the components used. To prevent damage to the cell, harmful gases such as SO2, O3, NOX, and NH3 as well as particles need to be separated reliably. For this purpose, MAHLE is developing a highly effective, multilayer filter medium. A substrate material ensures mechanical stability, a particulate filter layer removes NaCL, a molecular layer prevents NH3 from entering the fuel cell, an activated carbon layer absorbs unwanted hydrocarbons, and an additional, specially impregnated activated carbon layer adsorbs SO2, H2S, and NOx.
Fuel cells are also sensitive to oil admixtures. The electric compressor used to compress the supply air flow must therefore be oil-free. This also applies to the shaft bearing, as even small quantities of oil can cause irreversible damage. The compressor developed by MAHLE therefore uses special high-speed roller bearings lubricated with grease, which are prevented from releasing grease toward the fuel cell by means of a gasket also developed by MAHLE.
The water balance of a polymer electrolyte fuel cell significantly affects efficiency and service life. If the diaphragm dries out, this will lead to a gas breakthrough, while surplus water has the undesired effect of allowing the gases to freely enter the catalytic converter. Therefore, it’s not sufficient to filter the external air supplied to the fuel cell—its humidity must also be precisely controlled. For this purpose, MAHLE—together with affiliated partners and with funding from the German Federal Ministry of Economics and Technology—developed a flat membrane humidifier to ensure that the supplied air is humidified reliably. In the flat membrane humidifier, the exhaust and supply air are in cross flow and separated from the membranes. A moisture exchange takes place above the membrane surface.
Deionized coolant is used to cool the fuel cell, as it is only slightly electrically conductive and will not cause any undesirable current flow if the cell is damaged. As a result, the charge air coolers used must be resistant to ionized water. To this end, MAHLE has developed a special soldering process that not only makes the charge air cooler very durable but also prevents ionization of the coolant.
The specifications often require separation of water, gases, and water vapor downstream of the fuel cell—for esthetic reasons, no fluid water should escape from the exhaust air duct. MAHLE has developed water separators that allow a controlled water disposal.
Vehicle acoustics also affect air management. In particular, noise from the e-compressor and from the air flowing along the air pathway must be attenuated, taking into account interactions between the systems components.
The result of this holistic systems approach is the plastic exhaust air pathway optimized by MAHLE for fuel cell vehicles. It offers a weight saving of around 70 percent in comparison with steel constructions and dramatically reduces the audible resonance in the 1,200–5,000 Hz range, while retaining as much design freedom as possible.
Thermal management
The use of fuel cells is giving rise to more complex cooling systems and larger coolant coolers. This is due to the need for three separate circuits for the fuel cell stack, the battery/electronics, and the electric motor, as well as the overall increase in waste heat and the reduced temperature in comparison with the combustion engine. A higher volume of coolant is required in order to compensate for the lower temperature differential between the internal and external temperatures.
In addition, as there is no belt drive, electric fans are required throughout. All components must also be resistant to the deionized coolant.
Vehicles with a fuel cell drive have a backup battery. The charging and discharging of batteries results in a loss of efficiency, with some energy being converted to heat. As lithium-ion batteries need to operate within a certain temperature range, battery cooling is also required here. This is achieved by a secondary circuit in which coolant flows through a cooling plate under the battery. After the heat has been absorbed, the cooling medium is cooled to the initial temperature in a chiller. The temperature reduction in the chiller is caused by the evaporation of another refrigerant circulating in a primary circuit.
Diagnosis and monitoring
Fuel cell stacks require continuous monitoring during running operation. This prevents damage and also means that crucial input variables, such as gas or air supply, can be controlled. The MAHLE Fuel Cell Monitor module has two microprocessors that process the signals from the fuel cell stack and provide feedback to the central control unit. When required, the voltage in the fuel cell stack can be discharged directly via a semiconductor module. The power distributor and discharge resistor should be housed on a cooling plate to ensure problem-free operation.
Conclusion: fuel cells are a strong alternative for long-distance hauling
The fuel cell compensates for the weaknesses of a battery-powered powertrain—in terms of cruising range and power output—and recharges and refuels much more quickly. The higher initial costs make up only a small portion of the total cost of ownership (TCO) to the end customer. This figure is largely made up of fuel costs (H2) and toll and emissions charges. If these components shift the TCO in favor of fuel cells, it may be possible to replace vehicle fleets very quickly. MAHLE’s advanced engineering work aims to ensure that this concept reaches the required level of technological maturity for the mass market.
