September 13, 2019

Tesla Model 3 vs Toyota Mirai fuel cell vehicle

In years of writing about the EV scene, we’ve seen loads of “head-to-head” comparisons (do cars really have heads?) between Teslas and other vehicles. We’ve seen Teslas face Porsches, Dodge Chargers, Bentleys and Alfas on the drag strip; we’ve perused performance/price specs for Model 3 and its rivals from Audi, BMW and Mercedes; and we even recently read an analysis that concluded that Model 3 has a lower total cost of ownership than the humble Toyota Camry.

Top: Tesla Model 3 (Wikipedia: Carlquinn); Bottom: Toyota Mirai (Wikipedia: M 93)

As regular readers of this column will already be aware, the Tesla almost always comes out ahead in these match-ups. Tesla is of course the standard-bearer for the EV industry, and the headlines highlight the larger point that electrons are poised to supersede fossil fuels as a method of propulsion. In addition to the comparisons between vehicles, we’ve read scads of studies about the emissions reductions delivered by EVs, and scrutinized teardowns that demonstrated the greater simplicity of electric drivetrains.

We’ve also dissected discussions about the relative merits of hydrogen fuel cells compared to batteries. However, the only side-by-side smackdown that pits a Tesla Model 3 against a Toyota Mirai fuel cell vehicle we’ve seen was written by Paul Martin, a consultant and project manager who works in the chemical process industries.

Like Elon Musk and the other Tesla founders, Mr. Martin has concluded that batteries represent the superior energy storage medium. “I’m quite confident that hydrogen is a dead end for cars,” Martin writes. “My reasons aren’t personal- they’re thermodynamic, and rather difficult to argue with.” (Note that he says “for cars,” leaving open the possibility that hydrogen could prove to be a viable technology for some vehicles - perhaps for long-haul trucking, an application in which the weight and volume of the fuel storage medium is of greater importance.)

Mr. Martin has contributed several articles on the topic to LinkedIn: in one, he presents his thermodynamic arguments in detail, with references; in another, he explains that “renewable hydrogen” is, at least at present, a myth (most hydrogen today is made from fossil fuels).

The main argument against fuel cells is that they are simply less efficient at storing energy than batteries. Mr. Martin points out that “a hydrogen fuel cell vehicle will use, at minimum, 2.4 times as much energy as a battery-electric vehicle of similar size and features. That’s best case, and the reasons for that are thermodynamic and hence ‘difficult’ - in the Japanese sense of that word - to overcome in a meaningful way. We’re not getting that down to 2 times, ever, irrespective of what we invent or how clever we are.”

The main arguments in favor of hydrogen vehicles have been that they offer longer ranges and faster refueling times. Over the past few years, improving battery technology has pretty much made the former argument moot, and BEV boosters expect the second to eventually be neutralized by faster charging tech and/or the widespread adoption of autonomous vehicles.

At the moment however, the FCV’s faster refueling is a fact. “The Mirai refuels faster - in 3-5 minutes once connected to the pump,” writes Martin. “Of course that assumes you have a hydrogen filling station on the route between home and work. In contrast, the Model 3 can be recharged at home, leaving the use of Tesla’s supercharger network for longer trips. But on longer trips, you can expect to spend some time refilling the Model 3’s 75 kWh battery pack. Tesla’s superchargers are about 120 kW peak, and hence should refill the Model 3 to about 80% SOC in about 30 minutes.”

Leaving aside for the moment the larger question of which technology is better for society, our grandchildren and the butterflies, how do the two actual cars compare? This is not a rhetorical question - the Toyota Mirai is available to buy now in California and a few other markets (Toyota sold 1,700 of them in the US in 2018, according to Car Sales Base).

Above: Comparing Toyota's FCV Mirai with Tesla's BEV (RWD) Long Range Model 3 (Source: Paul Martin)

“The Mirai and the Model 3 are similar cars in terms of size, passenger and cargo capacity,” writes Martin. “Surprisingly the Mirai tips in at 200 pounds heavier than the Model 3 even with its comparatively huge 75 kWh battery pack. But in terms of performance, the Model 3 has the edge, by a long shot. Its motor delivers 211 kW and a 0-60 of 5.1 seconds, versus the Mirai’s 113 kW peak power and 9 second 0-60 mph. Yes, the Mirai is an electric car- but its smaller hybrid battery pack isn’t up to the power draw which comes effortlessly (i.e. at a very low C rate) from the Model 3’s giant pack.”

“The Mirai is more expensive to buy, despite reportedly still being subsidized by Toyota, whereas the Model 3 is reportedly already making Tesla a profit - not enough to make it worth selling shorter-range, cheaper Model 3s yet though,” writes Martin. “The ranges of the two cars per the EPA mixed driving cycle are similar - and both are similar in range to that of a comparable gasoline car.”

Martin notes that both cars will lose range in cold climates.

Martin’s primary interest is energy efficiency and greenhouse gas emissions, and here the comparison becomes even more lopsided. Martin’s previous paper indicated that the well-to-wheels energy efficiency of the the Mirai is comparable to that of a Toyota Prius. If you fuel your Mirai with fossil-fuel-derived hydrogen, your GHG emissions per mile will also be similar to those of the Prius, or “modestly better in the best case.” So driving on fossil hydrogen is better for the butterflies than driving a Hummer, but worse than driving a pure EV, which, as dozens of studies have shown, has substantially lower emissions than a legacy gas vehicle.

However, the whole point of hydrogen is that it can be made from renewable or low-emission energy sources (although, as Martin explains in another article, this is a work in progress). How would the environmental benefits of a car powered by renewable hydrogen compare to those of a battery EV?

Like a battery, hydrogen is an energy storage medium, so the answer to this question can be found by calculating the relative efficiencies of the two methods of storing energy. As Martin Eberhard and Marc Tarpenning did so many years ago, Paul Martin has calculated the storage efficiency of hydrogen, and found it wanting.

“As my previous paper makes clear, hydrogen’s 3-step process - hydrogen generation, storage at high pressure and then generation of electricity using a PEM fuel cell - means that its energy storage cycle efficiency is terrible,” Martin writes. “Best case, it means hydrogen will require 2.4 times as much energy as if you used a battery instead. Does that hold when comparing the Mirai and the Model 3, or did I get it wrong in my calculations?”

The mathematically inclined are encouraged to read Martin’s detailed analysis - he notes that he’s tried his best to make the complicated comparisons fair, and is willing to revise his conclusions if he receives credible, referenced alternative information. He tries to take every relevant factor into account, including the efficiency of an EV’s charger and battery pack, the efficiency of making (by electrolysis) and compressing hydrogen, and average grid losses. His conclusion: “the Mirai uses 3.2 times as much energy from source per mile driven as the Model 3.”

“When you look at costs, it’s even worse,” Martin continues. “Retail hydrogen in California sells currently for $15/kg, and that’s hydrogen which is only 33% renewable - the remainder is much cheaper fossil-sourced hydrogen, burdened with a modest carbon tax. Refilling the Mirai costs $75 US - to drive 312 miles. In contrast, you can completely refill a Model 3 on Tesla’s supercharging network for $18, at $0.24/kWh (measured presumably onboard the car’s battery, not from the wall). Using the average retail price of electricity to recharge at home in California ($0.15/kWh), it’s even a little cheaper - $14. Off-peak, at night, presumably it’s even cheaper still.”