How Can School Buses Become More Environmentally Friendly?

The US school bus fleet is made up of nearly 500,000 vehicles, of which 95% run on diesel fuel. Beyond greenhouse gas (GHG) emissions, diesel fuel has been under much scrutiny over the amount of other emissions—particularly nitrogen oxides (NOx) and particulate matter (PM)—that is released into the air. In 2006, the US Environmental Protection Agency (EPA) implemented new diesel standards to limit these emissions. About 40% of the school bus fleet has compliant diesel engine technology that adheres to these new standards. Although school bus fleets are encouraged by the US Department of Education to retire buses after 15 years, many fleets can hold on to them for much longer, leaving the majority of school buses noncompliant.

As these vehicles are retired, school districts and school bus fleets will need to confront how their next vehicles will meet these regulations and improve air quality. These fleets will have a number of alternative fuel options from which to choose.

A Range of Alternative Fuel School Buses

Beyond diesel and gasoline, the other main alternative fuels used for school bus fleets are propane autogas, compressed natural gas, and electricity.

Propane autogas (PAG): Also known as liquified petroleum gas, PAG is a widely available alternative fuel option for school buses that meet EPA NOx and PM emissions standards. PAG school buses have a slight purchase premium over diesel buses. One case study by the US Department of Energy found districts saved almost 50% on fuel and maintenance on a per-mile basis over diesel buses. Additionally, Argonne National Laboratory’s AFLEET tool estimates that propane vehicles can reduce life cycle GHG emissions by up to 15%. GHG emissions can be reduced even further with the use of renewable propane, which can be collected as a byproduct of renewable diesel production.

Compressed natural gas (CNG): CNG is another alternative fuel option that meets EPA emissions requirements for school districts, though has not had as much traction in the market as PAG. CNG per diesel gallon equivalent is typically cheaper than diesel and maintenance costs can be lower for CNG buses, thus providing an opportunity for lower costs over the lifetime of the vehicle. However, CNG buses do come at a premium and require significant upfront infrastructure investments. CNG buses can have a net zero or even negative carbon intensity for its GHG emissions if renewable natural gas is used, depending on the feedstock.

Battery electric: Battery electric buses (BEBs) are also compliant with EPA regulations since they have zero tailpipe emissions. The Union of Concerned Scientists found that BEBs emit less GHGs than both diesel and natural gas everywhere in the US. BEBs can cost over twice as much as a conventional bus, and also requires the purchase of charging infrastructure and installation. The economics of BEBs are inconclusive, though some studies have found that vehicle-to-grid revenue may be the missing link to being cost-competitive.

What About Diesel Buses?

However, new diesel school buses meet EPA regulations as well and do not come at a premium like most alternative fuel buses. Sticking with diesel buses also means fleets don’t need to reinvent their fueling infrastructure and can easily opt for a biodiesel or renewable diesel blend if GHG reductions are a priority. So why should fleets consider other options? Alternative fuel buses can provide opportunities for reduced total cost of ownership through maintenance, fuel savings, carbon pricing, and vehicle-to-grid integration. Fleets investing in new technologies now will better position them to capture these potential savings as the market emerges.