Energy Efficiency

Vehicle efficiency. Some battery EV enthusiasts state that a BEV has three to four times the energy efficiency of a fuel cell EV. A BEV by itself is more efficient than a FCEV, since round trip battery efficiencies (charging and discharging) may be as high 85% to 90%, while fuel cell efficiency averaged over a typical driving cycle is between 52 and 55%.  Therefore a BEV with the same weight (mass) as a FCEV would have approximately 55% to 73% higher efficiency than the equal mass FCEV. (Note that there is simply no justification for the claim that BEVs are 3 to 4 times more efficient, even when considering only the vehicle energy losses with no penalty for extra battery mass.)

But it gets worse! For a vehicle range above 80 to 90 miles, the weight of a BEV must be much greater than that of a FCEV.  More weight (mass) requires more stored battery energy than hydrogen energy (we assume that drivers will demand roughly equivalent performance in terms of acceleration and hill climbing capability, so extra mass requires more energy to accelerate the heavier vehicle.)

Detailed calculations show that a full-function 5-passenger vehicle with 250 miles range would require approximately 111 kWh of grid electricity for a BEV, and roughly 137 kWh of hydrogen energy for a FCEV of the same size. Thus the BEV would have about 23% higher efficiency than a FCEV, considering only the energy losses on the vehicles (which ignores the energy losses in producing electricity and hydrogen).

Well-to-wheels system efficiency. But we need to take into account the entire fuel cycle, not just the efficiency of the vehicle itself. The total system efficiencies will depend on the source of fuel for hydrogen and electricity. We have analyzed three generic fuel sources: fossil fuels such as natural gas, or coal; biomass, and intermittent renewables such as wind or solar energy. Converting either natural gas or biomass to hydrogen is more efficient than converting natural gas or biomass to electricity.

The well-to-wheels total system efficiency is higher for the fuel cell EV than the battery EV as follows, when each vehicle is designed for 250 miles range:

On the other hand, the well-to-wheels efficiency is higher for the battery EV than the fuel cell EV for intermittent renewables such as wind or solar that produce electricity directly; in this case the FCEV must absorb the losses inherent in converting renewable electricity to hydrogen:

  • With wind energy as the source, the BEV system is 67% more efficient than a FCEV system. (Still not a factor of three or four!)

Incremental vehicle & fuel infrastructure cost. However, efficiency may not be the most important figure-of-merit for renewable energy systems, since the fuel (sunlight or wind) is free. Cost may be the more important attribute. The incremental cost of adding one BEV and its required charging circuits is approximately $16,500, while the incremental cost of adding one FCEV and its share of the necessary hydrogen infrastructure, including the added cost of a slightly larger wind turbine to compensate for the lower FCEV system efficiency, is only $7,700.  The cost of adding one BEV and its fueling system is more than twice the cost of adding one FCEV and its incremental fueling system when the source is intermittent wind electricity.

This incremental cost advantage of the FCEV decreases with shorter range, with both vehicles and their respective incremental infrastructure costs equal for a vehicle range of 140 miles.

Bulk storage cost advantage for hydrogen. The previous cost calculations do not take into account the storage capability of hydrogen. Electricity must be used as soon as it is generated. Hydrogen that is produced at a wind farm or PV array can be pumped into a pipeline and stored for many hours or days. If demand is low, the pipeline pressure can be increased to store hydrogen for later use.  In addition, bulk storage tanks can increase the amount of hydrogen energy saved for later consumption. Given the intermittent nature of renewable electricity, this storage capability should have significant economic advantages for the FCEV.  [This will be the subject of a future analysis.]

[Natural gas system efficiency] [Coal system efficiency] [Biomass system efficiency] [Renewable energy system efficiency]

 

Here is Toyota’s estimate of the well-to-wheel efficiency of a FCEV (40%) an HEV (34%) and a BEV (33%) and an ICV (19%):

(Source: Tatsuaki Yokoyama, “Progress & Challenges for Toyota’s Fuel Cell Development,” 2009 ZEV Symposium, September 21, 2009)

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