What is well-to-wheel efficiency in an EV?

Well-to-wheel (WTW, or W2W) efficiency is a way to measure the total efficiency of operating a vehicle, beginning with obtaining the fuel at the primary source, the ‘well,’ through the entire delivery chain and in the drivetrain of the vehicle, all the way to the wheels. It was developed for internal combustion engine (ICE) vehicles, but also applies to electric vehicles (EVs).

WTW accurately estimates greenhouse gas (GHG) emissions resulting from vehicle operation. It’s usually measured as grams of CO2-equivalent, gCO2-eq.

Another, more comprehensive estimate of GHG emissions for vehicles is a life cycle analysis (LCA) that includes GHG emissions from vehicle production and end-of-life considerations. For an EV, the ‘well’ is the primary source of electricity, like a natural gas-fired generator, nuclear power plant, or a renewable source.

In many analyses, WTW is broken down into two segments, well-to-tank (WTT, or W2T), and tank-to-wheel (TTW, or T2W) (Figure 1).

Figure 1. WTW analysis combines WTT and TTW emissions performance. (Image: EU Science Hub)

It’s complicated
There are a lot of assumptions when estimating WTW efficiency. For example, the bars in Figure 2 are the International Energy Agency’s (IEA) estimated ranges (the dots are the worldwide average) for gCO2-e emissions of ICE vehicles, hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs) and fuel-cell electric vehicles (FCEVs) at the country level. The ranges indicate that ICE vehicles are not necessarily the most polluting.

A few of the assumptions included:

The range for PHEVs, BEVs, and FCEVs was determined using the minimum and maximum fuel economy values in the IEA’s Global Fuel Economy Initiative (GFEI).
PHEVs have the added assumption that they are operated 60% of the time on battery power and 40% of the time using the ICE.
The emissions from the electricity generated to charge the batteries in PHEVs and BEVs are based on the carbon intensities delineated country-by-country in the IEA’s Energy Technology Perspectives 2020.
For FCEVs, the minimum is calculated based on hydrogen production using dedicated renewable energy, and the maximum is based on steam methane reforming, which is currently the most common method.

Figure 2. gCO2-eq/km for various types of light duty vehicle power trains. (Image: International Energy Agency)

Line haul truck WTW comparison
A line haul truck is a tractor-trailer truck that travels consistently through several cities and returns to home base at the end of each day. Figure 3 compares the T2W and W2W emissions for line haul trucks in the U.S.

When looking at T2W, BEVs are clearly the best. But when considering W2W, the picture is not that simple. For W2W, biofuel offers the lowest emissions, and the U.S. average BEV has the highest emissions. However, if the greener electricity in California is used for the analysis, a BEV truck is the second best, slightly better than a hybrid solution, but not nearly as good as biofuel.

Figure 3. Comparison of line haul truck GHG emissions for T2W and W2W. (Image: Cummins)

Trucks and the GREET model
When considering other classes of trucks, including Class 8 long haul, Class 8 short haul, Class 8 refuse, Class 6 medium heavy duty (MHD) vocational, Class 4 light heavy-duty (LHD) vocational, and Class 2 pickup trucks (PUTs) and vans, the picture is more complicated. (A vocational truck is custom-built to perform a specific function like making deliveries, firefighting, concrete mixing, etc.)

The analysis in Figure 4 used the Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) model from the US Department of Energy (DoE) to produce gCO2-e/mile emissions estimates. It illustrates the well-to-pump (also called W2T) and the vehicle operation (also called T2W) emissions.

The analysis shows that the gCO2-eq/mile for Class 8 long haul and Class 8 short haul vary little between BEVs and ICEVs, primarily due to the high fossil fuel content of electricity generated on the national grid. All other cases show significantly lower gCO2-e/mile for BEVs compared with ICEVs.

Figure 4. WTW GHG emissions estimates of ICEV and BEV based on the GREET model and using the 2019 U.S. electricity generation grid mix. (Image: Environmental Science and Technology)

Summary
WTW is an important tool for comparing the GHG emissions of various vehicle technologies. It’s not an absolute measurement based on several assumptions about vehicle uses, energy sources, etc.

Additionally, WTW varies for light vehicles, line haul trucks, Class 8 trucks, and various vocational trucks. Some analyses conclude that IEC vehicles are not necessarily the most polluting.

References