Like a Battery Electric Vehicle, a Fuel Cell Electric Vehicle (FCEV) is electrically driven. However, the electricity for the motor is supplied by a fuel cell, which uses hydrogen as a fuel to generate the electricity. The exhaust from the fuel cell is pure water vapor, so there is no environmental impact. The manner in which an FCEV works is shown in Figure 14.1 from the book Technology for a Changing Climate: Net Zero by 2050.
This vehicle works as follows.
Hydrogen gas at high pressure (up to 700 bars) is fed into the fuel tank using a hose similar to what is used for gasoline or diesel is used. However, the connection has to be tight since the gas is at such a high pressure. It takes about 5 minutes to refuel, which is similar to a conventional car, and is much better than the time needed to fully recharge a BEV.
When the vehicle is in operation, the hydrogen gas flows to the fuel cell which generates electricity which drives the wheels of the car in the same manner as a BEV. (The low torque advantages of the BEV apply to the FCEV. In other words, both are capable of jack rabbit starts.)
The fuel cell also charges the battery. It provides power as needed, but it is much smaller than the battery in a BEV.
Water vapor from the fuel cell is discharged to the atmosphere.
Hydrogen has a high heat of combustion on a weight basis. Referring to the energy values in Table 1.2, compressed hydrogen at 700 bars has an energy density of 5.9 MJ/L (megajoules per liter). Gasoline/petrol, on the other hand, has a value of 31.1 MJ/L. However, the fuel cell/electric motor arrangement is more efficient than an internal combustion engine. Therefore, for the same range, the fuel tank for a hydrogen car will need to be around three to five times bigger than for a conventionally powered car.
The following diagram is taken from the U.S. Department of Energy website. It shows fuel tanks that are considerably bigger than would be found in a conventionally-fueled vehicle, but that still fit within a normal layout.
Ranges of commercially available models are:
Hyundai Nexo 756 km
Toyota Mirai 500 km
Honda Clarity 650 km
At the time of writing the Tesla 3 had a range of 430-570 km depending on the model chosen. The range of an FCEV relative to a BEV is greater because the vehicle is lighter due to its smaller batteries. Both types of vehicle have regenerative braking — energy is returned to the system when the brakes are applied.
The range of an FCEV can be increased by adding more hydrogen storage tanks. Doing so will increase the size of the vehicle to accommodate the extra tankage. Otherwise, the performance of the vehicle is not significantly altered. Such is not the case with BEVs. Increasing range means that additional heavier batteries are required. Since this extra battery capacity adds substantially to the weight of the vehicle, there is a drop off in incremental range, as shown in Figure 14.3. Also, the weight of the batteries places heavier loads on other parts of the vehicle, particularly the suspension, transmission and the braking system.
BEV Range vs. Battery Weight
Although hydrogen offers many advantages as a fuel, its use has been limited due to a lack of refueling stations. For example, in the United States early in the year 2021 there were about 44 hydrogen stations in California now, whereas there were more than 1600 Tesla BEV recharging stations nationwide. The shortage of hydrogen fueling stations means that people do not purchase the vehicles, which means that there is less incentive to built more refueling stations, and so on.
Hydrogen is a highly flammable gas that has significant safety concerns. However, because it is already widely used in industry, there is a lot of experience as to how to handle it safely.
The handling of a hazardous material can be summarized in its safety diamond, as described in the discussion to do with Figure 2.6. Hydrogen’s safety diamond is shown in Figure 14.4.
Hydrogen Safety Diamond
Red (flammability) ‘4’ — extreme hazard.
Yellow (instability) ‘0’ — minimal, normally stable.
White (specific hazard) ‘-’ — none.
Blue (health) ‘3’ — serious, full protective suit and breathing apparatus should be worn.
The associated legend reads,
Colorless, odorless, highly flammable gas. Stored as a compressed gas in cylinders. Simple asphyxiant (reduced oxygen available for breathing). Eye and skin contact with the compressed gas may cause frostbite.
The safety diamond shows that, unlike many other fuels, hydrogen does not have specific toxicity problems. However, the following safety issues are a concern.
Hydrogen has a wide flammable range in air, which means that it can ignite more easily than other fuels. Therefore, ventilation and leak detection are particularly important.
A hydrogen flame is nearly invisible, so special flame detectors may be required.
Some metals can become brittle when exposed to hydrogen.
With regard to FCEVs, it will be important to ensure that the hydrogen tank has excellent crash resistance.
Hydrogen cars are relatively expensive both to purchase and operate. It is likely that someone purchasing a FCEV will pay a premium of around $20,000 as compared to a conventional vehicle. Also, it is relatively expensive to refuel the vehicle. However, early adopters of new technology are generally not strongly influenced by initial cost.
It is likely that both capital and operating costs will decline as more FCEVs take to the road.