Mahle Keeps Fuel-cell Drive Components Cool and Clean

Fuel cell powertrains in commercial vehicles present “very high demands” for thermal management. But no additional construction space for the cooling system is required compared to a traditional combustion system. (Image: MAHLE)

Fuel cells are on fire — figuratively speaking — as truck OEMs increasingly extol their potential for applications like drayage and even long haul. Toyota recently extended the driving range of its second-generation Class 8 fuel cell truck to 300 miles (read more in the October issue of TOHE); Hyundai Truck & Bus is developing a fuel cell-powered cargo truck that can drive 250 miles and refuel in about 7 minutes (see page 29 in this issue) to complement its fuel cell electric buses; and in November, the Nikola Tre hydrogen-electric truck was announced for European markets, adding to the start-up’s semi-truck portfolio that already includes the Nikola One and Two for the U.S.

Nikola has tapped Stuttgart-based systems specialist Mahle to be its thermal-management development partner and supplier for the Nikola Two’s air-conditioning system for the driver’s cabin, including the 800V electric compressor, and the cooling systems for all drive components. The supplier has increasingly diversified its commercial-vehicle portfolio to include solutions not only for diesel internal combustion engines, but also electrified powertrains, alternative fuels like CNG and LNG, synthetic fuels, and hydrogen fuel cells.

In the development of its Nikola Two fuel cell truck, Nikola Motor turned to Mahle for its expertise in thermal management. (Image: NIKOLA MOTOR)

The primary task of thermal management is to provide optimized media temperatures to meet the demand for the most efficient energy utilization, according to Arnd Franz, Executive Vice President Sales and Application Engineering on the Management Board of the Mahle Group. To address the various thermal requirements — for the fuel cell stack, the traction motor, and the battery and power electronics — separate coolant circuits at different temperature levels are required. System components include electric coolant pumps and fans.

“Mahle has continued to advance innovative technologies for fuel-cell drives in recent years — the complete thermal management system is a key part of this,” said Franz in a release announcing the partnership. “We are therefore now in a position to confidently and competently support Nikola’s dynamic development process and time-to-market.” Market launch for the Nikola Two is planned for 2021.

High thermal demands

Fuel cell powertrains present “very high demands” for thermal management, necessitating more complex cooling systems (multiple coolant circuits) and adapted cooling units, according to Dr. Jörg Stratmann, CEO of Mahle, speaking at the recent IAA Commercial Vehicles trade show in Hanover, Germany.

Fuel cells have a higher efficiency than diesel commercial vehicles. Nevertheless, the heat rejection in the cooling module is higher as there is only a small heat flux via the exhaust gas, Stratmann explained to TOHE. In addition, the coolant temperature needs to be lower than for conventional diesel, so the Delta T to ambient is smaller. “Both together make the cooling tasks more challenging,” he said.

“Practically we have no heat dissipation via the exhaust gas stream, so the cooling systems have to be able to manage this, actually at decreased temperature level of the cooling agents,” he added.

Mahle can transfer a lot of knowledge and experience from conventional combustion engines to fuel cell powertrains, according to Christopher Rimmele of Mahle corporate communications. “The physical laws and the basic components are the same,” he shared. “The latest-generation internal combustion engines already require a complex and sophisticated approach and we developed solutions to meet the demands.”

Deionized coolant is used to cool the fuel cell, to help prevent undesirable current flow if the cell is damaged. Because the charge air coolers must be resistant to ionized water, Mahle developed a soldering process that ensures durability while preventing ionization of the coolant.

Cooling of the Li-ion batteries 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 the primary circuit.

Thermal management challenges for commercial and passenger vehicles are similar — it’s basically a scaled solution, according to Rimmele. “Considering the higher cooling capacity required, the challenge is to develop thermal management solutions that fit into the current vehicle environment. This requires development of a high-performance cooling module and innovative package solution,” he said.

Despite the unique challenges for fuel cells, Stratmann stressed that no additional construction space for the cooling system is required compared to a traditional combustion system.

“The potential for fuel cells in commercial vehicles is very thrilling, indeed. We are trying to really push it ahead,” Stratmann said.

Air management and filtration

Another hydrogen-electric truck announcement: the European-bound Nikola Tre, offering 6x4 or 6x2 configurations and a range of 500 to 1,200 km (310 to 745 mi) depending on options. (Image: NIKOLA MOTOR)

Mahle’s solutions for fuel cells also include air management and filtration, important to prevent contamination in the fuel cell stacks and harmful gases in the air flow. The company is developing a multilayer filter medium: a substrate material for mechanical stability, a particulate filter layer to remove NaCL, a molecular layer to prevent NH3 from entering the fuel cell, an activated carbon layer to absorb unwanted hydrocarbons, and a specially impregnated activated carbon layer to adsorb SO2, H2S, and NOx.

Because fuel cells are sensitive to oil admixtures as well, Mahle has developed an oil-free electric compressor to compress the supply airflow, Stratmann said. This oil sensitivity also applies to the shaft bearing. The new compressor uses special high-speed roller bearings lubricated with grease, and a Mahle-designed gasket prevents releasing grease toward the fuel cell.

In a polymer electrolyte fuel cell, humidity must also be precisely controlled, Stratmann noted, to prevent the diaphragm from drying out but also to ensure there’s no excess water, which could allow gases to enter the catalytic converter. To ensure that the supplied air is humidified reliably, Mahle and “affiliated partners” developed a flat membrane humidifier, with funding from the German Federal Ministry of Economics and Technology. The company explains that 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.

Water separators allow controlled water disposal, with no fluid escaping from the exhaust air duct.

Vehicle acoustics is another consideration for air management, according to Stratmann. To attenuate noise from the e-compressor and from the flowing air, a plastic exhaust air pathway optimized by Mahle for fuel cell vehicles “dramatically reduces” the audible resonance in the 1,200-5,000 Hz range. It also offers a 70 percent weight savings compared with a steel construction, the company claims.

Diagnosis and monitoring of fuel cell stacks is critical to prevent damage and to control crucial input variables such as gas and air supply. Mahle’s 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 stack can be discharged directly via a semiconductor module.