Transit Unplugged Transit Science: Understanding Hydrogen Fuel Cells
There is some amazing chemistry and physics going on inside those buses
When people hear hydrogen, the Hindenburg disaster immediately comes to mind. Lesson learned, you have to be super careful around gaseous hydrogen. But it’s exactly that ability to store and release energy (lots of it) that makes it a great fuel for everything from trains to trucks to buses. Hydrogen is the most abundant element in the universe. As a gas (H2) it isn’t very stable and really wants to react to become stuff like water (H2O) or other molecules. Which is why you need to be careful with it, just like compressed natural gas (CNG) or other fossil fuels.
But hydrogen has a lot of potential as an alternative fuel because when it combusts (burns) in the air all you get is pure water and heat. Heat is great because it can be used to make steam to drive turbines—or keep buses warm in the winter. The energy released (if you combust it) could be used to drive a piston (i.e. an engine), but for transportation, with the exception of liquid-fuel rockets, we don’t usually burn it, we react it. And that reaction inside a hydrogen fuel cell is what generates electricity that can power pretty much anything.
Tune into Kirt Conrad, CEO of SARTA in Ohio and Tim Sasseen, Dir. of Market Development and Public Affairs at Ballard Power Systems talking about hydrogen, hydrogen fuel cells, and hydrogen hubs on Transit Unplugged with host Paul Comfort.
Let’s get into how hydrogen fuel cells work, how they are different from battery electric EVs, and some of the challenges facing hydrogen today.
How a fuel cell works
The short answer is gaseous hydrogen is reacted with something (usually platinum) to split it into negatively charged electrons and positively charged hydrogen ions. The electrons are picked up and create electrical flow from one side of the fuel cell to the other. That flow then drives a motor. To complete the process the hydrogen, reacts with oxygen from the air to become water (and release heat). Here’s a diagram of how it works:
The trick here is that part in the middle of the anode (negative pole)-cathode (positive pole) sandwich. The electrons can’t get through the proton exchange membrane in the middle and must travel through the anode to the cathode and that way completes a circuit and makes electricity. Ballard Power Systems pioneered the hydrogen fuel cell in the 1990s and has made public transit applications one of its key missions ever since. In practice, fuel cells are connected in parallel as banks to provide enough power to run an electric motor.
If you want to experiment with fuel cells, there are kits available with small fuel cells that power small cars or fans. All you need is distilled water and some light for a solar panel to create the hydrogen. Here’s a video showing how fuel cells work and a small fuel cell powering a fan:
It’s still an electric vehicle
Hydrogen fuel cell vehicles are still electric vehicles, just instead of storing energy in a battery for later (some fuel cell vehicles do have batteries to store excess power) the electricity is generated and used right away. Fuel cell vehicles are like gas-powered vehicles in that respect. You fill your car with gasoline and as the engine consumes it, your car moves. You fill a hydrogen fuel cell vehicle with gaseous hydrogen and it will run as long as there is hydrogen in the tank.
One of the benefits of fuel cell buses is the “waste” heat generated by the fuel cell can be directed to warm the bus in the winter. This solves for something battery electric buses have a problem with: running in cold weather. Instead of needing diesel heaters or drawing on the battery to warm the bus, fuel cell buses can heat a bus while still sending power to the drivetrain.
Because it uses an electric motor, fuel cell buses take advantage of the high torque and efficiencies electric motors have. These simple motors—without transmissions by the way—are easier to maintain and repair than diesel or gas-powered engines. Fuel cell buses have a lot of the benefits of battery electric, but with fewer of the challenges of range and recharging time.
But it’s different than battery electric too
The biggest challenges to battery-electric vehicles are range and refueling (aka recharging). Battery electric vehicles, buses especially, have about half the range of a fossil fuel vehicle. Okay, sure, we can just top off the battery, right? Sure, but that takes time, sometimes lots of it.
Think about how long it takes you to charge your phone; more than a few minutes, right? Battery-electric vehicles can have fast-charging, but it’s still longer than putting fuel in a tank. Most big buses charge batteries overnight in the garage. They can get a little top off stop to stop, but not more than about 70-80% charge. Those top ups are like plugging your phone in to charge for 10 minutes, you get a little boost, but not much. You might be able to hop from outlet to outlet for a while, but to get your phone back to a full charge, you need to let it charge (and not use it) for a few hours.
A hydrogen fuel cell bus can travel roughly the same distance as a fossil fuel bus, and when it’s time to fill up the tanks, it takes roughly the same amount of time as diesel to refuel. Hydrogen buses fill up just like CNG buses do—with many of the same procedures and safety checks as well. Unlike batteries, when a fuel cell bus is low on fuel, it isn’t going to have decreased performance like battery electric buses do. A fuel cell, like diesel, CNG, or gasoline, runs the same on a full tank as it does on a nearly empty one.
Refueling isn’t the only difference, weather and climate are huge factors in how well a battery electric bus performs. In cold climates (like Canada), when the temperature dips, the usable range for a battery electric bus drops significantly. Plus, to keep passengers warm, and still get the most drivable time from the bus, many battery electric buses were equipped with diesel heaters. Yes, a green, zero emission vehicle needs fossil fuel to keep people warm in the winter.
Battery electric buses are also solely dependent on the electrical grid to recharge, which presents two challenges for agencies. First is if the agency is drawing more power from the grid, and that power comes from fossil fuels (gas, coal, etc), then more fossil fuels are consumed to make more power—increasing overall emissions. Second is a “what if…” scenario all agencies need to think about: emergency preparedness and disaster response. Transit agencies are often tapped by emergency management officials to help with large scale evacuations, but what if the disaster causes the electrical grid to collapse too? If there is no electricity coming in, no buses can go out. This leaves agencies in a sticky position of worrying that, if push came to shove, they might not be able to help evacuate their cities.
These challenges have pushed many agencies to look more closely at hydrogen for the bulk of their fleets. However, hydrogen isn’t without its own environmental challenges. Green as the buses might be when operating, generating the hydrogen for them is sometimes anything but.
The green hydrogen challenge
The biggest challenge right now for hydrogen fuel vehicles is getting hydrogen. The most common way to generate hydrogen on a large scale is steam reformation which takes methane and turns it into hydrogen (and carbon dioxide). This process takes a lot of energy and uses a fossil fuel (natural gas) as the fuel stock for hydrogen (though biogas methane generated from composted organic matter can be used for this process). As demand for hydrogen increases, there is an increasing environmental cost as well, until renewable natural gas is used more commonly for steam reformation.
You can also make hydrogen with electrolysis, which uses electricity to split water into hydrogen and oxygen (you can do this at home with a battery, some wire, and a couple thumbtacks). Electrolysis takes a lot of energy too, but you can get that energy from solar or wind power. Using water and electricity isn’t as efficient as steam reformation at generating hydrogen, but it’s the easier of the two options go green. Agencies like SunLine in California, Champaign-Urbana in Illinois, and Sarawak Metro in Malaysia use solar energy for their on-site hydrogen production.
While going green is a challenge for hydrogen, the bigger challenge is ramping up production so the cost of hydrogen gets on par with the cost of diesel and there is enough hydrogen to fuel expansion on a large scale.
Tipping point for generation and demand
Regardless of how the hydrogen is made, transit agencies need a steady (and reliable) supply to keep their buses on the road. This is the chicken and egg situation the Hydrogen Hub Project in the U.S. aims to solve. With seven hubs across the country the goal is to get more hydrogen closer to more cities so it can be affordably transported to refueling stations. These hubs serve as commercial, scientific, and entrepreneurial ventures to push generation technology forward while also meeting the growing demand for hydrogen.
Hydrogen hubs are the first step towards mass production of hydrogen so it can meet the needs of the entire transportation sector. It’s a crucial process so hydrogen has a better chance of catching on as a widespread fuel for transportation.
Hydrogen fuel is just one of many options
As much as hydrogen fuel cells are an essential part of the transition to zero emissions, it isn’t the only option. As Kirt Conrad CEO of the Stark Area Regional Transit Authority (SARTA) discusses in the recent Transit Unplugged episode, there is room, and need, for other green fuels. Battery electric vehicles are better suited to smaller, shorter runs. CNG is a good transition fuel since there is less carbon per molecule than diesel and can be generated from organic waste. Even bio-diesel, made from waste vegetable oil, puts a dent into the demand for fossil fuels.
Hydrogen fuel cells are poised to be the fuel for applications like transit, rail, and trucking as just one piece of the zero emissions puzzle.
Listen to conversations about hydrogen fuel and fuel cells
We’ve talked a lot about hydrogen on Transit Unplugged. Here are some episodes that dive into the topic in a little more detail:
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