How Green – and How Safe – is a Fleet of Hydrogen Fuel-Cell Buses?

As part of its goal to become a zero-emission transportation provider, SEPTA announced last week its plans to purchase 10 buses powered by hydrogen fuel cells, on top of their more than 1,000 traditional hybrid buses. Though the technology has been part of experimental programs in cities since the late 1990s, it has been slow to catch on locally, with more than 90% of the world’s hydrogen-powered buses running in China as of 2020, according to the International Energy Agency .

Joshua Snyder, PhD , associate professor of chemical and biological engineering, has studied electrochemical energy conversion and storage for nearly two decades. His work on improving the life, performance and cost of fuel cells has been supported by the Lawrence Berkeley National Laboratory , the Department of Energy and the National Science Foundation , among others. Snyder recently shared his insights on the benefits and challenges of fuel cells for the Engineering Change blog.

How does a fuel cell differ from a traditional combustion engine?

In a combustion engine, a fuel, which is most commonly derived from non-renewable hydrocarbon deposits, is burned in a confined space to generate motion in a vehicle. Combustion engines convert the chemical energy in fuels to kinetic energy. However, their efficiency is limited as the conversion of that chemical energy to kinetic energy is not perfect, and some of that energy is lost as heat. A hydrogen fuel cell powered car is driven by an electric motor, whose base efficiency is higher than that of an internal combustion engine. The electric power for that motor is provided by the hydrogen fuel cell. This fuel cell tricks the “combustion” of hydrogen into giving off its energy in the form electricity instead of heat. An electrochemical reaction effectively splits the hydrogen molecule into two protons and two electrons. Those electrons are then used to power the electric motor. Unlike combustion processes which are intrinsically limited by the undesirable loss of energy as heat, fuel cells can potentially be operated at 100% efficiency, meaning 100% of the chemical energy in the fuel will be transformed into electrical energy. While the output of a combustion engine is polluting carbon and nitrogen containing gases, the only output of a fuel cell driven electric motor is water.

What are the benefits of hydrogen fuel over a typical battery?

The two main advantages of using a hydrogen fuel cell for powering vehicles over batteries is their fast refueling and increased range. This is particularly relevant for larger vehicles such as buses and tractor trailers, which are characterized by larger vehicle weights. For increased vehicle weight and range, the required battery pack weight increase begins to become infeasible. However, for a fuel cell powered vehicle, because hydrogen and its storage tank weight are essentially negligible in comparison to the vehicle weight, the vehicle range can be increased to a large extent without increasing the overall vehicle weight. “Recharging” of a fuel cell vehicle by refilling the hydrogen tank can be completed in a time similar to the time required to fill a tank with gasoline.

Why does hydrogen fuel make sense for fleet vehicles?

Beyond vehicle price, the main factor limiting widespread fuel cell vehicle adoption is the lack of a hydrogen refueling infrastructure. Hydrogen fuel cells make sense for larger fleet vehicles such as busses because that are likely to be filled at a single maintenance location, likely run by a local governmental entity. This means that we can integrate these fleet vehicles into a location immediately, with minimal required infrastructure investment.

How much “greener” would a bus fueled by a hydrogen fuel cell be than one powered by diesel?

Green hydrogen has the potential to be a zero emissions fuel. Which means it would have none of the environmentally polluting emissions associated with burning diesel fuels. Hydrogen is classified as green if it is sourced through the electrolytic splitting of water into hydrogen and oxygen. There are other grades of hydrogen such as grey which comes from the steam reformation of methane or other hydrocarbons, blue if that reformation process is followed by a direct carbon capture process, or pink if nuclear energy is used to thermally split water into hydrogen.

What the classification of green hydrogen leaves out is the source of the electricity used to split water and make that hydrogen. If that electricity is coming from a gird that is powered by burning coal or methane, there will be a significant carbon output tied to the creation of that hydrogen. If that is the case, your bus run on “green” hydrogen will technically have a carbon footprint. However, if we can drastically increase the input of renewably sourced energy, from solar, wind, and hydro, into our grid, we can greatly decrease the carbon output associated with the use of hydrogen fuel. We have the technology to make this happen, we just need a coordinated effort with buy-in from private and government entities to increase the rate of renewable energy installations and upgrades to local and long-distance electrical grids to make it a reality. We are moving in this direction, and we are truthfully not far away from a transportation sector with greatly reduced carbon emissions.

What are some of the challenges facing adoption of fuel cells, and what are engineers doing to address them?

The greatest challenge associated with adopting hydrogen fuel cells for transportation vehicles is the lack of a national hydrogen refueling infrastructure. The technology required to create this infrastructure exists today, but it is a large undertaking to integrate this infrastructure on a national scale. With the passing of the last Infrastructure bill in Congress, large amounts of money were set aside for further development of a hydrogen economy. The Department of Energy is focusing that money on addressing the existing technical limitations in hydrogen production, storage, and transportation and initiating the development of several Hydrogen Hubs that are meant to be model hydrogen production and distribution centers that will guide further deployment across the country. Even with this, further engineering work is needed to improve technologies on all ends of hydrogen production with the goal of further decreasing the cost of that produced hydrogen.

How has hydrogen fuel technology evolved over the last several years?

The general process and technology for generating hydrogen fuel and extracting energy from hydrogen fuel has not changed for some time. However, in the last decades we have made great progress in improving the efficiency of these processes, bringing the cost of green hydrogen down. We have also made great progress in the safe storage and utilization of hydrogen. The advances in carbon fiber reinforced tanks or solid hydrogen sorbents have come to a point where the risk of a catastrophic hydrogen release is nearly nonexistent. Integrated safety systems make the storage and utilization of hydrogen in a vehicle no more dangerous than an internal combustion engine running on gasoline or diesel or a lithium-ion battery powered car.