Understanding fuel cell electric vehicles integration

IN the previous article, I discussed fuel cell (FC) components, which include the power inverter that changes or converts the direct current (DC) to power three-, six- or nine-phase alternating current (AC) power and drive the electric machine.

I also discussed the DC-DC converter, which bucks or reduces the voltage on the high-voltage bus to approximately 14-volts, which will power all the traditional low-voltage systems, such as headlights, wipers and instrumental cluster, etcetera.

2021 Toyota Mirai FCEV

FC vehicle propulsion systems are also designed to take advantage of regenerative braking.

This will permit the propulsion system to recover or recoup braking system energy and recharge the small battery pack with energy that is normally wasted in heat during a braking event. Recapturing expended energy allows the high-voltage components to lower the consumption of H2 and extend vehicle range.

Maximising the use of electrical potential, both used and available, is key to extending the range of the fuel cell electric vehicle (FCEV).

The thermal system of the FC system requires more components when compared to a traditional internal combustion engine (ICE) system; for example, the Toyota Hilux and Toyota Wish vehicles that are common in Zimbabwe.

2025 BMW FCEV Concept

In an ICE system, engine heat is dissipated in three different systems: exhaust, cooling and combustion chamber. The FC is also an engine, but it does not have any rotating components.

All rotating components are external of the FC stack. However, the FC stack produces a significant amount of heat but does not have a combustion chamber with expanding gases to expend heat energy, nor does the FC stack have an exhaust system to transfer heat energy from its plates, which create electricity for external loads.

In an FC system, the heat generated by stack cannot be rejected into a combustion chamber or exhaust system. Therefore, the cooling system must reject all FC stack heat to maintain temperature that will ensure optimal stack operation.

To ensure the high volume of FC stack system heat is removed, more than one heat exchanger (that is, radiator) is used.

In most FC systems, two or three heat exchangers are used to reject FC stack heat.

2025 Mercedes FCEV Truck

Therefore, when servicing an FC stack thermal system, a technician interfaces with a significant number of heat exchangers, liquid pumps, hoses, coolant and a coolant deioniser.

The deionising system removes contaminants from the cooling system that would permit the coolant to become conductive.

Conductive coolant can cause significant operational problems in an FC system.

Because the coolant is transferred into the FC stack and is nearly in contact with the plates, this can cause the coolant to become conductive and permit the coolant to conduct electrical energy into the cooling system.

This would result in the cooling system components becoming conductive conduit and dangerous for the first or second responders or those servicing the system.

If the coolant becomes too conductive, or the FC stack system resistance (in relation to the chassis) decrease, this could also cause an isolation fault condition and a loss of propulsion.

The isolation fault monitoring system monitors the chassis for how much high-voltage energy is leaking to the vehicle chassis.

High levels of high-voltage energy leaking to the chassis will cause unsafe condition for those directly interfacing with the vehicle or its system.

Fuel Cell Electric vehicle concept

One method of reducing conductivity of the coolant in the FC stack is to utilise a deionising filter. This filter will remove contaminants in the system that would permit the coolant to be conductive. The deionising filter is a maintenance item for the system and technicians should be aware of this filter, locate it (if used) and determine its service intervals, according to the original equipment manufacturer (OEM) service information.

Let us discuss how fuel cell power systems are integrated with electrified motor vehicles (EMVs) powertrain systems.

The output electrical power of an FC stack is low voltage and high current; it is transferred to the high-voltage electric propulsion and power electronic systems.

There are modes in the drive cycle in which the total electrical loads will exceed the maximum FC power output.

The driving mode that could cause this scenario to occur is acceleration.

Therefore, the FC high-voltage system will include a small battery pack, comparable to that of a hybrid electric vehicle (HEV), to load level the system. This battery pack will provide additional high-voltage energy to ensure that the high-voltage DC bus does not sag (that is, drop to a low-voltage state).

Some FCEV applications also allow for plug-in power charging, enabling the vehicle to start with a fully charged high-voltage battery, which allows for a full electric mode until the high-voltage has depleted.

Once the high-voltage battery pack is depleted, the FC will start to generate power by applying H2 to anode, and O2 is provided to the cathode to start the power generation event. This process will continue to provide power to the high-voltage battery pack and the electric machine until the vehicle is stopped or the H2 fuel tanks do not have enough pressure to maintain the FC operation.

Hydrogen Charging Station

Refuelling the H2 tank is similar to a conventional internal combustion engine-powered vehicle, as a normal fill-up occurs in under five minutes.

The combination of electric refuelling and H2 refuelling provides the operator with the option of utilising the fuel of convenience to maintain the zero-emission status of the vehicle.

The use of high-voltage electronics to propel the vehicle keeps the vehicle in an adaptable situation that can capitalise on the difference means of fuelling the vehicle.

Because of the varying technology on FCEV application, there is a need for proper communication between the various controllers to provide an uninterrupted driving experience.

Hyundai FCEV Truck

The need for full battery power usage, when available, allows for conservation of H2 fuel to meet the needs of a recharge event after the high-voltage battery is depleted.

This application is similar to a BEV application, as the FC is utilised to provide power to the electric drivetrain.

The major difference is the ability to charge the high-voltage battery after the initial charge has been depleted.

*Taurayi Raymond Sewera is ASE & Autocate Association-certified World Class Master Technician with 39ASEs, ASE Advanced Level Specialist L1, L2, L3 and L4, AMI Accredited Master Electric Vehicles and Master Automotive Manager, ACDC certified Master Hybrid and Electric Vehicles Technician