Renewable Energy Efficiency

Renewable electricity. Wind turbines and PV solar cells or solar thermal plants produce electricity directly, so using this intermittent electricity to charge BEV batteries is more efficient than electrolyzing water to make hydrogen for a fuel cell EV. Electrolyzers are typically 75% efficient, so making hydrogen from water loses 25% of the energy in the original electricity.

As shown below, we estimate that a wind turbine must produce 123 kWh of electricity to power a battery EV for 250 miles (assuming that a full-function, 5-passenger BEV with advanced Li-Ion batteries could achieve 250 miles range), while a fuel cell EV would require 207 kWh of wind electricity to make enough hydrogen for 250 miles range.  Thus it takes 67% more wind energy to power a FCEV than a BEV....BEVs are 67% more efficient..

Incremental system cost. However, efficiency is not the only criterion for choosing between these two options, since the fuel (wind or solar) is free.  Cost is the major driver. As shown above, the incremental cost of adding one battery EV plus associated charging circuits is estimated at $16,500 per vehicle.  The major cost is the BEV itself; this vehicle cost estimate is based on detailed vehicle costing work by Kromer & Heywood at MIT.

For the fuel cell EV, we need to consider the upstream costs of building slightly larger wind turbines(to account for the lower system efficiency of the FCEV pathway), larger electrolyzers, compression & pipelines systems, and high pressure compression and storage at the fueling site. As shown above, adding the incremental costs of the larger wind turbine and hydrogen systems plus the incremental costs of the FCEV itself adds up to $7,700 per FCEV. Thus fuel infrastructure is more than twice as expensive to add one BEV than to add one FCEV even when accounting for the cost of additional wind power and hydrogen infrastructure.

Incremental system cost vs. range. As the range of the BEV is reduced and the vehicles become lighter, the incremental cost advantage of the FCEV plus wind/hydrogen system decreases as shown below. The dashed lines show the incremental cost of the BEV and the FCEV.  The upper solid lines show the cost of the vehicles plus their respective fueling infrastructure.  The break-even range is approximately 140 miles.  For vehicle ranges greater than 140 miles, the FCEV plus fueling system cost is much less than the BEV cost.  For ranges less than 140 miles, the BEV system costs less.  This is another indication that BEVs will only be economic for ranges less than 100 to 140 miles for full-function 5-passenger vehicles.

Energy storage. These costs ignore one other advantage of hydrogen over electricity: electricity must be used as soon as it is generated.  Except for small pumped hydro facilities, underground compressed air and a few large battery bank demonstration projects, there is negligible capacity to store electricity.

Hydrogen, on the other hand, can be readily stored.  There are several hundred miles of hydrogen pipelines in the US where industrial complexes share hydrogen. These pipelines can store excess hydrogen; when demand is low, the pipeline pressure is increased, cramming more hydrogen gas into the pipes.  Dedicated high pressure tanks can be added to increase hydrogen storage capacity at relatively modest cost compared to adding batteries to store electricity.

Storage is particularly important for intermittent renewables. Ideally we would like to store energy from wind and solar to use when the wind is not blowing and the sun is not shining. The batteries on BEVs and the hydrogen tanks on FCEVs can provide some storage capacity. However, when all the batteries that are plugged in at any instant of time are fully charged, and when all hydrogen tanks are full, no more energy can be saved.

This is where hydrogen has additional long-term benefit. We have estimated the cost of adding stationary batteries compared to the cost of adding stationary hydrogen storage capacity as a function of the storage time required.  For very short storage times (less than one day), batteries are more cost effective, since storing hydrogen requires the purchase of expensive electrolyzers, compressors and bulk storage tanks. But as storage time increases, more batteries must be added at great expense, but the hydrogen system needs only to increase tank capacity, which is relatively inexpensive. In the chart below, we plot the ratio of battery cost to hydrogen storage cost as a function of the storage time in days. Three curves are shown corresponding to vehicles with 100, 200 and 300 miles range.  More stationary batteries are required with longer range vehicles, since more energy is required to accelerate these heavier vehicles. As shown, the cost of stationary batteries exceeds the cost of the hydrogen storage system whenever energy must be stored for more than 1.3 to 1.8 days.

If the wind doesn’t blow or the sun doesn’t shine for more than three or four days, then the cost of stationary battery storage is more than twice the cost of hydrogen storage.

[For the future, we plan to analyze a complete system of FCEVs and BEVs to determine the storage capacity of a hydrogen pipeline network, and the corresponding inherent economic advantage of hydrogen pipeline storage.  This analysis will tie hydrogen pipeline storage economics with typical wind and solar intermittency statistics for different regions of the country. The hydrogen pipelines are already included in our cost model to distribute the hydrogen, so their value as a storage medium is essentially “free,” or already paid for in the hydrogen purchase price.]

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