Hydrogen fuel cells feed air and hydrogen together; when they react and form water, they also produce energy by exchanging protons. In other words, that is a current, which can be fed into a circuit to power a battery or a motor.
Installing a hydrogen fuel cell (HFC) in a bus does not maketh a mature supply chain for this disruptive technology, contends Tom Mason, CEO of Bramble Energy, a 2016 spin-out that is planning to take on existing HFC suppliers like Canada’s Ballard Fuel Systems (see box).
“We want to validate and make sure they are durable before rushing to implement. You can’t do a suck-it-and-see approach,” he adds, forecasting that the technology won’t be mature until the 2030s when it has been produced in the hundreds of thousands. “One thousand buses is nothing.”
Which is not to say HFCs aren’t right for buses; that application is in fact the latest demonstration project announced by the venture capital-funded developer.
Mason argues that, given the cost premium of hydrogen as a fuel compared to electric, HFCs are best used by assets that are limited by both uptime and energy. First, while a battery-electric van will take time to charge, a hydrogen tank can be refilled in minutes. So they best suit vehicles running over multiple shifts that can ill-afford downtime.
Second, fuel cells have an efficiency edge. Fuel cells such as the Bramble unit can achieve up to 60% electrical efficiency, or 50% in a system, compared to the 40% of a diesel (or hydrogen) internal combustion engine, Mason claims. Those figures mean that they use proportionally less of an expensive fuel than an ICE, for the right application, although fuel cell performance does decline over time.
SYSTEMS
Understanding how HFCs work involves much more than just the fuel cell itself, known colloquially as the ‘stack’, for the piles of plates that it comprises. That is as about as complete a power plant as a cylinder block is to an engine. Enabling and supporting the stack is an array of fans, cooling equipment and filters, which Bramble doesn’t do, expecting its OEM customers to take that over, once production scales are reached.
But at this early stage of Bramble’s development, they are the province of development partner Mahle Powertrain. Its head of research and advanced engineering, Jonathan Hall, contends that a hydrogen fuel cell is halfway between a battery and an internal combustion engine. Although the process is electro-chemical rather than thermal, it still features the equivalent of an air intake and exhaust to manage the air supply to the HFC cathode (which supplies oxygen to the cell), and a fuel system in the form of the anode supply, which provides hydrogen to the cell.
Tuning the process of combining those ingredients is essential for the performance and longevity of the fuel cell, Hall explains. “We need to condition incoming air into the cathode side of the stack so it is at the right temperature, humidity and pressure to not overload the stack mechanically or electrically, but also to create the desired amount of power output. On the anode side, we are trying to create the right flow rate and pressure to balance across the membrane for durability and performance,” he says.
“On the hydrogen side, there is often a recirculation loop including a hydrogen pump so some of the H2 that has been through the stack but has not been used is pushed around to start the process again. We monitor hydrogen slip in exhaust to minimise energy losses. This also acts a bit like an emissions system. Any H2 that might be wasted is a running cost and decreases efficiency and also comes up against legal limits for safety.”
Cooling is a challenge, and so there are two coolant loops: one for the stack at its 60-70°C running temperature, and a second, lower temperature for the intake air and electronics. As the electro-chemical reaction produces heat, all excess heat is dumped into the coolant, which can be only 20-30°C warmer than the ambient air, making it harder to reject at the heat exchanger, so the coolers need to be much larger than with an internal combustion engine, which runs hotter.
Another environmental hazard to the cell is dirty air. “Atmospheric pollutants can cause ageing and degradation,” says Hall. “If you get oxides of sulphur, that can poison the HFC. So you need slightly different air filtration methods to adsorb sulphur oxides that are in the air.”
That is because hydrogen fuel cells’ lifetime is limited by the health of the membrane, which has been described as functioning like a window: best when clean, and degraded when dirty. According to Mason, lifetimes are estimated at 20,000-40,000 operating hours.
HYBRID CONFIGURATIONS
Harsh cycling is another factor that can affect the longevity of fuel cells, which is why HFCs are almost always twinned with some kind of battery, like existing hybrid cars, he continues. Two alternative arrangements have been most common to date. One is a range-extender system, where an HFC tops up a large-capacity battery from which energy is extracted to power the onboard traction motor. Another is the approach used by Toyota for the Mirai: a very small battery, 1-2kWh, which is purely for transient response.
The first Bramble-Mahle demonstrator was a 10kW system powering a small Renault Kangoo BEV van shown at the Cenex-LCV show a few years ago. The system is currently being upgraded with a revised stack and higher power.
BOX: THE BRAMBLE GAMBLE
Bramble Energy, a spin-out of University College London and Imperial College, is developing a new type of hydrogen fuel cell made not from graphite or stamped metal, but printed circuit boards. The advantage is that they can be made from a 24x18in plastic polymer wafer that is produced in huge volumes in existing PCB factories around the world.
Current is proportional to surface area of the cell, and voltage depends on the height of the stack. Membranes come from Gore in a roll-to-roll process that is 300mm wide. Anode and cathodes are made from platinum-group metals, in quantities similar to a catalytic converter. Initial designs were air-cooled, but have now switched to water cooling. Since the element copper, which is used for conductors, poisons the proton exchange membrane, it must be covered up. Instead of using gold, as Hyundai does, Bramble has patented and now manufactures a carbon-rich ink, and has patented a screen printing process to apply it.
This sourcing strategy, it contends, gets around the chicken-and-egg problem of not being able to financially justify construction of a factory without orders, and not being able to fulfil orders without a factory. It also allows the company to scale up quickly.
Bramble Energy forecasts becoming profitable by 2027-8, having spent £60m of venture capital funding. Its first customer is a large North American OEM for a 70kW fuel stack in an unnamed ‘mobility’ (but not automotive) application. The price point: under $400/kW, compared with over $1,000/kW claimed of competitors, and a level at which the company says it can still make a profit.