Hydrogen fuel cells sound exotic and confusing — like a technology out of the science fiction films “Minority Report” or “The Fifth Element.”
But the science is actually pretty straightforward. A simple chemical process strips electrons from hydrogen to create an electrical current, then makes water for cooling by combining the hydrogen with oxygen.
Unlike a battery that needs to be charged from the grid, hydrogen fuel cells can power up on the go. They’re also smaller and lighter than battery-electric systems, with the bonus of being just as powerful and longer-lasting.
That makes fuel cell electric propulsion systems attractive for heavy-truck use. Automotive giant Toyota Motor Corp. plans to spend the next year or more testing a system in a prototype zero-emission Class 8 drayage truck on cargo runs from the Port of Los Angeles to warehouses within a 70-mile radius.
Toyota hopes to amass data proving fuel cell electric trucks to be reliable and economically competitive with battery-electric trucks. The technology could be a game-changer in regions like California, where air quality and renewable energy regulations are driving up demand for clean freight hauling.
How It Works
Individual fuel cells typically produce fairly low voltage. Toyota packs hundreds of them into a package called a fuel cell stack, resulting in a system with enough juice to power a vehicle.
Think of each cell as its own small generating station: Pressurized hydrogen goes into one side — the negative, or anode side. The hydrogen gets passed through a permeable membrane coated with a catalytic material such as platinum, which strips the hydrogen of its electrons. On the other side — the positive, or cathode side — two molecules of hydrogen combine with one molecule of oxygen to form water, or H20.
The flow of electrons freed by the catalytic membrane creates an electrical current — slightly less than one volt per cell in the Toyota system.
Separating the electrons from the hydrogen creates considerable heat. The water formed on the cathode side of the fuel cell helps cool things down and becomes the system’s only emission.
Toyota, along with several other automakers, uses a cell known as a polymer electrolyte membrane cell. It’s ideal for vehicles because it keeps relatively cool — 60 to 80 degrees Celsius, or 140 to 176 degrees Fahrenheit.
These cells are easier to keep from overheating than other types — which can operate at up to 2,000 degrees Fahrenheit — and tend to last longer.
Power in Numbers
Propelling a vehicle demands a lot of electricity, which requires a lot of these cells. The Mirai fuel cell sedan — Toyota’s first commercial use of the technology — bundles 370 thin rectangular cells into a compact, 123-pound fuel cell stack.
The Mirai can generate up to 114 kilowatts of power off its stack. That’s equivalent to 153 horsepower. The stack powers an electric motor that, after accounting for transmission losses, produces up to 151 horsepower and 247 pound feet of torque.
“Project Portal,” Toyota’s prototype fuel cell truck, uses two stacks, or 740 cells — taken out of two Mirai cars — and links them under the floor of the truck’s cab. Including the energy stored in the truck’s lithium batteries, the system can produce at least 500 kilowatts of power in short bursts. When the truck is running in a steady state with no excessive demands for speed, its fuel cell stack produces enough electricity to power 130 average American homes.
The stacks are designed to produce slightly more power than a vehicle needs for standard acceleration and cruising. The excess power — plus electricity generated by the vehicle’s regenerative braking system — is stored in onboard batteries and tapped when the vehicle requires an extra burst of juice.
The Mirai uses nickel-metal hydride batteries, but the truck relies on a pair of 6-kilowatt-hour lithium batteries with higher capacity. The twin electric motors in the truck produce more than 670 horsepower and 1,327 pound feet of torque — roughly equivalent to the output of the 2017 Cummins X15 Efficiency Series diesel engine. As with other electric vehicles, all of that torque is instantly available, giving Toyota’s fuel cell truck impressive acceleration.
Like any other electric-drive vehicle on the road, and unlike diesels, these fuel cell vehicles run in near silence and don’t produce harmful tailpipe emissions.
All the Rest
Although the truck gets its electricity from its fuel cell stacks, it uses a complex power control system to adjust to various operating conditions and load demands.
The control units manage the stack output, system voltage and battery. Each is made up of a power inverter, a boost converter and a direct current converter.
When the vehicle is charging its batteries while in regenerative braking mode, the inverter adjusts the power output from the stacks and the motor. The boost converter can pump up system output from 250 volts normally to as much as 650 volts depending on the driving environment. The DC converter tamps down the 250-volt direct current to 12 volts to safely operate low-voltage equipment such as headlights, cooling fans, air conditioning and dashboard instruments.
The unit is cooled by a pair of liquid cooling systems, including radiators and fans, taken from the Mirai.
Toyota built four high-pressure hydrogen storage tanks for its truck, each capable of holding 10 kilograms of hydrogen gas compressed at 10,000 pounds per square inch (70 MPa or 700 bar).
A kilogram of hydrogen contains the same amount of energy as a gallon of gas. Toyota expects the truck’s 40 kilograms of hydrogen to deliver at least 240 miles of range for a 21,970-pound tractor pulling a trailer loaded at 60 percent of capacity — about 36,000 pounds. That’s equivalent to 6 mpg. A comparable diesel drayage rig delivers around 5 mpg, according to a 2013 study by Calstart, the Pasadena-based clean transportation technologies coalition.
The Toyota truck’s hydrogen tanks are made of high-strength plastic wrapped in material reinforced with carbon-fiber, then covered with Fiberglass-reinforced plastic. Toyota tested the tanks by filling them with hydrogen and then placing them in mechanical crushers and bonfires to check whether damage would cause them to explode — they didn’t.