An analysis by a team at Argonne National Laboratory (ANL) has found that by 2045, some configurations of battery electric vehicles (BEV) could become almost as energy dense as a conventional vehicle. The team presented their paper at the recent 2016 SAE World Congress.
Hydrocarbon fuels (either fossil- or bio-derived) have high energy densities that are at least 100 times greater than that of a present day lithium-ion battery. Despite projected improvements in battery technology, this form of energy storage is still expected to be significantly less energy dense than gasoline even by 2045. However, the Argonne team argues, the energy density of storage medium (fuel or battery) should not be used as the sole criterion to compare conventional vehicles and BEVs. Rather, powertrain-level energy and power density will be better criteria to compare the propulsion technology used for BEVs and conventional vehicles, they suggest.
This requires assessing the efficiency of the conversion of the stored energy to useful mechanical energy to propel the vehicle.
|Comparison of powertrain energy density at the wheel of BEVs as a ratio to gasoline-powered conventional vehicle. Closer to “1”—the value for the gasoline vehicle—indicates higher powertrain energy density. Vijayagopal et al. Click to enlarge.|
For the study, the team compared several mid-size passenger cars:
A gasoline-powered car that can run 300 miles on one tank of gasoline.
A BEV with a driving range of 100 miles (BEV 100)—common commercially today.
A BEV with a driving range of 300 miles (BEV 300) to match the range of the conventional vehicle.
The researchers considered two variants of each BEV: a BEV with a two-speed transmission (DM), and a BEV with a single-speed reduction between the motor and wheels (Fixed). Vehicle mass, power, and energy requirements changed slightly between these two options.
|Powertrain components that convert stored energy to useful mechanical power output. Vijayagopal et al. Click to enlarge.|
They then modeled out, in five-year increments, changes in vehicle weight reduction, battery technologies, powertrain components, power and energy requirements, and powertrain energy and power densities. For each time period, they produced three assumed values corresponding to high, low, and medium probabilities.
They then calculated the “Ratio of the Powertrain Energy Density”: the ratio of the powertrain specific energy (energy consumed divided by vehicle mass) of the conventional gasoline vehicle to the BEV.
They found a significant decrease in energy consumed by the gasoline vehicle over the period, largely due to the addition of start-stop systems that reduce engine idling.
They also found that current gasoline-powered vehicles require about 10 times more energy input per kg of vehicle powertrain mass compared to the energy requirement of a BEV. Even with the projected improvements, the conventional vehicles in 2045 will still need almost twice as much energy input per kg compared to the BEV 300.
In terms of energy-out density, even though the BEV is more efficient than the conventional vehicle, its greater mass results in spending more energy at the wheel per kg of the vehicle powertrain mass.
This ratio, however, will decrease as batteries get lighter and other components become more efficient.
By 2045, BEV 300s will be comparable to conventional vehicles in terms of the energy spent at the wheel per kg of the powertrain mass.—Vijayagopal et al.
Vijayagopal, R., Gallagher, K., Lee, D., and Rousseau, A., “Comparing the Powertrain Energy Densities of Electric and Gasoline Vehicles),” SAE Technical Paper 2016-01-0903, doi: 10.4271/2016-01-0903