BEVs vs. FCVs: TWO ESSENTIAL DIFFERENCES

THE TROUBLE WITH COMBUSTION

Internal combustion engine vehicles, ICEVs, are powered by burning hydrocarbons like gasoline, diesel, ethanol, compressed natural gas, propane, and mixtures thereof. They pollute by emitting byproducts of the combustion, mainly CO2 as the principal cause of long-term global warming but also with traces of much more toxic emissions that can impact the immediate neighborhood more short-term [1].

Moreover ICEVs are inefficient. This is because they exploit the thermodynamics of a heat engine that converts a temperature difference into mechanical energy. Nicolas Carnot’s theorem, which he proved in 1824, states that the efficiency of any such heat engine is at most the ratio of that difference to the higher temperature. In order to have anywhere near 100% efficiency the lower temperature would need to be near absolute zero, -273 C. This might be feasible on frigid Pluto far from the Sun’s warmth, but not on Earth, and especially not in an ICE whose lower temperature as a heat engine is that of its exhaust as it leaves the cylinder! Car engines are typically well below even the already low Carnot limit, being about 20-30% efficient.

Thermodynamics is the last place efficiency experts should be looking.

ELECTRICITY TO THE RESCUE

Electric vehicles, EVs, are powered more indirectly, namely by converting the chemical energy of an on-board fuel of one kind or another into electrical energy. In turn an electric motor converts this electrical energy (very efficiently with the present state of the art) into mechanical energy.

An important aspect of this chemical-to-electrical conversion is that it is slow, limited by the impracticalities of driving at over 150 mph on most roads or doing twenty consecutive 0-60 mph timing runs. It therefore does not do the sort of damage to the chemicals that faster conversion can cause. This is relevant to the first essential difference below between BEVs and FCVs.

By avoiding combustion EVs are far less polluting than ICEVs, and by avoiding thermodynamics they are also much more efficient.

All this however is merely background for the immediate question of the form the chemical energy of an EV should best take.

CHOOSING THE BEST CHEMISTRY

There are two main choices: battery chemistry and fuel cell chemistry.

Critics of FCVs like Joe Romm, Tony Seba, and the Wikipedia article on fuel cell vehicles have long lists of faults that would seem to suggest many differences from BEVs. By contrast the Wikipedia article on battery electric vehicles was unable to find a single fault with BEVs.

But some of those differences are not essential. Yes there are many more Tesla supercharger stations than hydrogen stations, but that’s more an accident of timing and deployment than an essential difference. Likewise there are many more BEVs than FCVs, but that too is timing and the ratio has been shrinking rapidly over the past 12 months.

TWO ESSENTIAL DISTINCTIONS

I see only two properties of BEVs and FCVs that distinguish them in anything like an essential way. In decreasing order of obviousness:

1. Refueling a BEV entails transforming electrical energy to chemical.

2. The FCV’s independent fuel cell and tank decouples its available power from its energy capacity.

The first distinction is significant because such conversion in either direction without permanent damage is slow, as noted above in the case of conversion from chemical to electrical. Charging a battery without seriously degrading its life it can take an hour or more.

In contrast the energy in an FCV’s fuel is chemical to begin with. It can therefore be pumped into an FCV’s tank much faster than a BEV’s battery can be recharged, in minutes rather than hours (but not seconds because compressing requires energy and is also thermodynamically inefficient).

The second distinction is significant because the available power from a battery is in proportion to its energy capacity. Hence if you want a BEV with a high capacity battery for longer range, you must employ a battery capable of delivering higher power, whether you want that power or not!

The Tesla S P100D’s Mirai-beating EPA range of 315 miles [2] is achieved with a battery capable of delivering 500 kW (670 HP) [3], good for a 0-60 mph time of 2.5 seconds. Tesla makes this almost record-breaking power a feature of its long-range vehicles, as well it should.

However the price for this mind-boggling power that comes with merely respectable range is a 1200 lb battery and an MSRP of $134,000 [4].   On the one hand, among cars that quick the Tesla may well be the cheapest.  On the other, if all you want is its 315-mile range you can’t get it without paying for luxury-level performance even though you may never use it.

In contrast the Mirai achieves its EPA range of 312 miles with a 204 lb tank (including 11 lb of hydrogen). Independently it achieves its 114 kW (153 HP) of power with a 124 lb fuel cell, good for a 0-60 mph time of 9.0 seconds that would have been considered very sporty a mere two decades ago [5]. The drive train weight therefore totals 328 lbs (408 with the traction battery) vs. the Tesla’s 1200 lb battery pack. And the 2017 Mirai leases for a quite modest $349/month, likely about a quarter of what the P100D would lease for were Tesla to offer a lease option.

A POSSIBLE THIRD ESSENTIAL DIFFERENCE: EFFICIENCY

There is a third difference, involving efficiency, but it remains to be seen whether this is an essential one. The lithium-ion batteries typically used in BEVs have an efficiency about 1.5 times that of a fuel cell. This is easily deduced from the fact that one gasoline gallon equivalent (GGE) of electrical energy (33.7 kWh) can take a Leaf or Tesla about 1.5 times as far as one GGE of hydrogen energy (about 1 kg).

So what? Both types of EV are far more efficient than ICEVs. What does that extra 1.5 factor do for the BEV owner? Obviously it doesn’t buy more range: Tesla had to go way out on a limb to match the Mirai’s range.

Which leaves cost.

So what’s the cost of hydrogen energy vs. electrical energy?

Solar energy is currently wholesaling at about 3 cents/kWh in several parts of the world. 1 kg of hydrogen has 33.7 kWh. Hence in order to match electrical power, taking into account the 1.5 efficiency factor, hydrogen would need to wholesale at $0.03*33.7/1.5 = $0.67/kg.

Is this possible? Anaerobe Systems’ Mike Cox believes his biofuel approach can achieve $0.50/kg. The jury is out, probably for quite a while.

But this overlooks the capital cost of a 300-mile range BEV. While its electrical fuel may well be cheaper than the FCV’s hydrogen fuel, mile for mile, how many years before that savings makes up for the BEV’s insanely higher price?

5? 10? The car’s lifetime? Your lifetime?

You do the math.

A BENEFIT OF INEFFICIENCY

Meanwhile there is a positive side to inefficiency that ICEVs get in spades during winter: vast amounts of heat. While FCVs don’t get that much heat, they get more than enough to keep both cabin and equipment toasty warm. This is why FCVs have much noisier radiator fans than BEVs.

There are two advantages of this inefficiency warmth. First, there is no need to steal power from the fuel cell’s electricity to heat the cabin. (The Mirai’s electrically heated seats are a different matter, though once the cabin has warmed up they shouldn’t be needed.) Second, just as a warm battery runs more efficiently than a cold one, so does a warm fuel cell run more efficiently.  The fuel cell’s advantage in the cold is that its inefficiency provides it with free heat that can keep it operating at peak efficiency, whereas any heat used to keep a battery warm cannot be had for free but as with heating the cabin must be stolen from the battery’s electrical power.

Winter is as cruel to BEVs as it is harmless to FCVs, which start easily and run efficiently at twenty below. Winter is also tough on ICEVs, which rely on batteries to start: the cold sucks the life out of the 12 volt battery needed to start the car, and ignition is harder in a cold cylinder.

The bottom line for efficiency then is that while the jury is out on the cost benefit of the BEV’s higher efficiency, there is no doubt that the FCV’s inefficiency is a great benefit in cold climates.

CONCLUSION

For short range EVs, batteries may be fine. However for long range EVs FCVs have two clear advantages over BEVs: faster refuelling, and an independence of range and performance that greatly reduces both weight and cost. Whether the greater efficiency of BEVs is of any value remains to be seen.

———————–
REFERENCES

[1] http://www.ehhi.org/exhaust06.pdf

[2] Page 32 of http://www.fueleconomy.gov/feg/pdfs/guides/FEG2016.pdf

[3] https://electrek.co/…/tesla-model-s-p90d-ludicrous-power-o…/

[4] https://evannex.com/…/how-elon-musk-just-built-the-quickest…

[5] (a) Road & Track credits the Leaf with 9.4 seconds. (b) One of the eight Mirais that drove from Rocklin to Truckee for the opening ceremony on August 27 was Brad’s from Santa Rosa (didn’t catch his last name but he’s in real estate). He’d bought his Mirai just weeks earlier after learning that a BEV with the same range would cost twice as much. When I told him the Mirai’s 0-60 mph time was 9 seconds he was adamant that his Mirai was much quicker than that. I didn’t want to contradict him because that’s how the Mirai feels to me too, even if it isn’t true.

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