Do hydrogen-electric plug-in hybrids like the Audi A7 h-tron make sense?
For all the friction between hydrogen fuel cell advocates and battery electric vehicle evangelists, the two factions both want the same thing: a low-carbon car driven on electricity rather than oil. Though some may see it as an unacceptable compromise, could the ideal solution be a combination of the two?
Audi shows off the ultimate hybrid
At the Los Angeles Auto Show last week, Audi presented a first-of-its-kind concept that is undeniably intriguing. The Audi A7 Sportback h-tron Quattro, aside from being a mouthful, is a high-performance hydrogen-electric plug-in hybrid that can travel over 300 miles without tailpipe emissions.
It takes a page out of the Tesla dual-motor electric AWD book, with a 114-hp unit in both the front and back. Though the motors are rated at 114 hp, each is capable of a temporary power boost to 152 hp. We know what such a setup did for the Tesla Model S – it is only a matter of time until the system is adopted by other automakers.
What is really interesting, though, is what drives the two motors. An 8.8-kWh lithium-ion battery pack borrowed from the A3 e-tron can provide up to 31 miles (presumably on the lenient NEDC cycle) of driving on battery power. When the battery is depleted, the hydrogen fuel cell range extender kicks in for as much as 310 miles of extra range drawn from four hydrogen storage tanks.
The fuel cell is a stack of 300 individual cells, and can reach efficiencies of 60% with cold-start performance down to -28 degrees C. However, the acceptable temperature range is only 80 degrees C, which Audi says places greater demands on the vehicle cooling system than a combustion engine.
A tri-port DC-DC converter regulates the voltage supplied to the electric motors, as the fuel cell and battery operate at different voltages.
Best of both worlds?
While the A7 h-tron concept is not yet a reality, it is one that is easy to get excited about. Leaving aside efficiency and greenhouse gas debates (as both fuel cell and battery electric cars offer varying but significant improvements over gasoline), a hydrogen-electric plug-in hybrid addresses some of the main drawbacks of battery electric and hydrogen fuel cell vehicles: while the main knock on electric cars is low range and long refueling times, the biggest obstacle for fuel cell cars to overcome is insufficient hydrogen refueling infrastructure.
Since most drivers in the U.S. travel 40 miles or less per day on average, the lithium-ion battery in the A7 h-tron (or a variant with a larger battery) would be sufficient to power most daily driving without ever needing to expend more expensive and less ubiquitous hydrogen fuel. Electricity is the ultimate fuel for urban and suburban driving, as home recharging at night is extremely cheap and convenient.
On the other hand, when the need for a long trip arises, battery power becomes less practical. This is where hydrogen fuel cells hold the advantage: strategically placed hydrogen refueling stations can maximize utility at minimal infrastructure cost, similar to Tesla’s over-the-road model for Supercharger stations.
Hydrogen fuel is quite expensive compared to electricity, but owners are less likely to mind if they fuel up only for the occasional road trip while plugging in for most of their miles around town.
As a concept, it seems like the ideal solution. Battery power for short distances, when plugging in at home is often sufficient, and hydrogen power for long distances, when range and quick refueling times are paramount. Both energy sources send power to electric motors, which compared to the internal combustion engine are undeniably a vastly superior way to move a car down the road. What’s not to like?
The bad news
Unfortunately, we have yet to mention the magic word: cost. Though other obstacles like complexity, packaging and safety certainly come into play (four hydrogen storage tanks, a fuel cell stack, an 8.8-kWh battery, and two electric motors essentially necessitates a clean-sheet design), the biggest barrier to this concept becoming a reality is cost.
It is not a secret that neither batteries nor fuel cell and hydrogen storage systems come cheap. Combining both is bound to result in one expensive vehicle - any such hydrogen-electric plug-in hybrids would be limited to high-end status for quite a while. Considering that a gas-powered A7 can be had for no less than $65,900, the h-tron version would no doubt be out of reach for most.
On the bright side, Toyota has been boasting rather a lot lately about how it has driven down the cost of its fuel cell system, and batteries are going to get substantially cheaper by the end of the decade. There is just too much potential here to be ignored, particularly for large vehicles. Though battery power may not be the ideal solution for pickup trucks or large SUVs, battery power combined with hydrogen may be the answer.
So to answer the title question, hydrogen-electric plug-in hybrids do make sense...but that does not mean they will be arriving anytime soon, if at all. It would require significant cost reductions and a genuine effort on the part of the auto industry to make it happen, but it is well within the realm of possibility.