Map of the United States showing the fuel efficiency that a diesel bus would need to have the same life cycle global warming emissions as a battery electric bus in each region.

Electric vs. Diesel vs. Natural Gas: Which Bus is Best for the Climate?

, vehicles analyst | July 19, 2018, 1:05 pm EDT
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Battery electric buses – the people’s electric vehicle – are becoming more and more common. An increasing number of transit agencies – large and small – are making announcements about purchasing electric buses and putting them into operation.

The obvious benefit of electric buses is that they don’t have any tailpipe emissions. A question we often get at UCS is, “What about emissions used to generate electricity for electric vehicles?”

We answered this for buses charged on California’s grid and found that battery electric buses had 70 percent lower global warming emissions than a diesel or natural gas bus (it’s gotten even better since that analysis). So what about the rest of the country?

You many have seen my colleagues’ work answering this question for cars. We performed a similar life cycle analysis for buses and found that battery electric buses have lower global warming emissions than diesel and natural gas buses everywhere in the country.

What the map shows

The map above shows the miles per gallon that a diesel bus would need to have equivalent life cycle global warming emissions as a battery electric bus on today’s grid (really the 2016 grid, the most recent data available).

This means a battery electric bus operated in North Carolina, for example, has the same life cycle global warming emissions as a diesel bus that gets nearly 15 miles per gallon! That’s impressive considering a comparable diesel bus actually gets 4.8 miles per gallon. So, you can operate three electric buses in North Carolina and have the same emissions as a single diesel bus.

Electric buses are better for the climate than diesel buses everywhere in the country

Battery electric buses range from 1.4 to 7.7 times better than a diesel bus, as shown in miles per gallon emissions-equivalency. Another way of saying this is that a diesel bus has nearly 1½ to 8 times the global warming emissions as an electric bus, depending on the region.

And the grid is getting cleaner every year. Emission rates from electricity have steadily declined the last sixteen years. Transit agencies can also choose cleaner power than what’s provided on their grids by installing solar panels and batteries on site or through renewable electricity contracts.

Charged with the national electricity mix, a battery electric bus has global warming emissions equivalent to a diesel bus getting 12 miles per gallon. This is 2.5 times better than an actual diesel bus (4.8 miles per gallon).

They’re also better than natural gas and diesel-hybrid buses

Everywhere in the country, battery electric buses also have lower life cycle global warming emissions than natural gas and diesel-hybrid buses. Charged with the national electricity mix, an electric bus produces 1,078 grams CO2e per mile, while a natural gas bus produces 2,364 grams CO2e per mile and a diesel-hybrid produces 2,212 grams CO2e per mile.

(Note, values for CO2e per mile reflect the same analysis as the miles per gallon emissions-equivalent values shown in the map, just presented in different units).

Chart showing global warming emissions per mile for diesel, natural gas, diesel-hybrid, and battery electric buses.

Natural gas buses have 12 percent lower global warming emissions than diesel buses.* Electric bus emissions range from 29 to 87 percent lower than diesel buses and 19 to 85 percent lower than natural gas buses.

Here’s a table showing the life cycle global warming emissions per mile from electric buses in all regions of the US.

Table showing the global warming emissions per mile of electric buses charged in different regions of the country.

Click to enlarge.

 

 Cleaner electricity means cleaner electric buses

In upstate New York, the region with the lowest carbon grid in the country (roughly 30 percent hydropower, 30 percent nuclear, 30 percent natural gas), a battery electric bus has nearly 90 percent lower global warming emissions than a diesel bus.

A battery electric bus charged in upstate New York even has lower life cycle emissions than the average passenger vehicle on the road (new and old combined)! An electric bus charged there produces about 350 grams carbon dioxide equivalents (CO2e) per mile, while the average gasoline car/SUV in the US is responsible for 500 grams CO2e per mile.**

Other grid regions (California, Alaska, New England, and the Pacific Northwest) aren’t too far off at about 650 grams CO2e per mile, without even accounting for the fact that a bus can carry a lot more people than a car.

The time is right to get more electric buses on the road

With zero-tailpipe emissions and low life cycle global warming emissions, battery electric transit buses offer significant local air quality and climate benefits. The more of these buses that are deployed the better. Encourage your local transit agency to begin exploring electric buses, if they haven’t already. Also encourage your state and federal representatives to provide incentive funding to help get these clean vehicles on the road.

 


Methods

Life cycle emission data and models

Life cycle global warming emissions for battery electric buses include those from generating electricity and extracting, processing, and transporting the fuels used to generate electricity. These emissions were compared to the life cycle emissions of diesel and natural gas buses, which include tailpipe emissions and emissions from extracting, refining, and transporting the oil and natural gas.

Emissions from electricity generation are from the US Environmental Protection Agency’s eGRID 2016 database, reflecting emissions from calendar year 2016. Emission rates include transmission losses associated with delivering electricity, roughly 5 percent, depending on the region.

Emissions from extracting, processing, and transporting the fuels used to generate electricity, diesel, and natural gas were determined using Argonne National Laboratory’s GREET 2017 model. This model was also used to determine the tailpipe emissions from diesel and natural gas vehicles.

Methane emissions and global warming potential

Life cycle emissions from natural gas vehicles depend greatly on the extent of methane leaks throughout the fuel’s life cycle and the global warming potential used for methane. Our analysis uses conservative estimates for both, including global warming potentials over a 100-year period from the IPCC’s 5th Assessment Report. Using higher, yet justifiable, assumptions for methane leaks and its global warming potential, the global warming emissions of natural gas buses can change from 12 percent less than diesel (as used in this study) to 20 percent greater than diesel.

Fuel efficiency

A New Flyer Xcelsior battery electric bus.

The bus manufacturer New Flyer makes the same bus (40-foot Xcelsior) in diesel, diesel-hybrid, natural gas, and battery electric versions. These buses have undergone testing by the Federal Transit Agency, allowing for comparison of fuel efficiencies across vehicle type and over the same test conditions.

Fuel efficiencies used in this analysis were as follows: diesel bus: 4.82 miles per diesel gallon; diesel-hybrid bus: 5.84 miles per diesel gallon; natural gas bus: 4.47 miles per diesel gallon equivalent; and battery electric bus: 2.02 kWh per mile, which accounts for a 90 percent charging efficiency.

The on-road fuel efficiency of any bus will depend on the specific route, including the vehicle’s speed, number of stops, and terrain; passenger load; auxiliary uses of energy, e.g. air conditioners or heaters; and the inherent efficiency of the engine or electric motor, which varies by manufacturer.

Standardized testing of the New Flyer buses shows that electric buses are four times more energy efficient than natural gas buses. In contrast, a study of electric and natural gas buses operated on the same routes by Foothill Transit in Southern California showed electric buses had eight times better fuel efficiency.

Note, converting the fuel efficiency of the electric bus (i.e., 2.02 kWh per mile) into an equivalent miles per diesel gallon (using the amount of energy contained in a gallon of diesel, 129,488 British thermal units per gallon), gives an equivalent fuel efficiency of the battery electric bus of 18.8 miles per diesel gallon equivalent.

Comparing this fuel efficiency to a diesel bus reflects only how much energy the two vehicles use over the course of a mile. The MPG numbers shown in the map above go several steps further and include upstream emissions, which is why we refer to them as “the equivalent life cycle global warming emissions from a diesel bus with X miles per gallon efficiency.” A mouthful, but a critical distinction.

Fuel efficiency is representative of global warming emissions if you’re talking about the same type of vehicle using the same type of fuel, e.g., comparing a diesel bus made by company X to a diesel bus made by company Y. But, if you want to compare two different types of vehicles, e.g. a battery electric bus and a diesel bus, the upstream emissions associated with the fuel or electricity production need to be accounted for, as reflected in the values on the map.

* This result is similar to the finding that natural gas buses have just 9 percent lower global warming emissions than diesel buses in our previous analysis, which was specific to California.

** This result makes use of our life cycle emissions analysis for passenger vehicles and the average fuel efficiency of these vehicles on the road.

 

Photo credit: MJW15 CC BY-SA 4.0

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  • Rudolf

    OK, now let’s get real and factor in the emissions generated when the battery pack is produced. You do know that producing a battery for one of those busses generates many years worth of tailpipe emissions and I am assuming that the bus does use zero-emissions electricity which it virtually never does. Those comparisons are a sham as they leave out the most salient factors. Seems like there is a general unease with hard facts in the EV circles. We dont even talk of biomethane use yet.

    • Jimmy O’Dea

      Hi Rudolf,

      We’ve previously investigated the emissions associated with manufacturing combustion and battery electric cars. We found the manufacturing emissions are much smaller compared to emissions associated with operating the vehicles, roughly 10% vs. 90% of emissions, respectively, in battery electric cars. This corresponds to about a 1-year emissions payback period, depending on the electricity grid. See Chapter 2 of this report: https://www.ucsusa.org/clean-vehicles/electric-vehicles/life-cycle-ev-emissions

      • Rudolf

        Hi Jimmy, Its hard to get any reliable data on that. I have seen studies that say 8 years of operation are needed in order to wear off the carbon backpack. Thats a hell of a difference. I am working in heavy transport and we need at least 500 km range on a 40 ton rig and I dont see that coming.

  • AmanaPlan

    Are the emissions involved in manufacturing the different types of vehicles a significant factor in making this analysis? It seems you should at least have had some mention of that factor, unless I missed it in the article.

    • Jimmy O’Dea

      Hi AmanaPlan,

      Thanks for the comment. Life cycle emissions in this study include extraction, processing, and
      transport of fuels plus operation of the vehicles.

      We’ve previously investigated the emissions associated with manufacturing combustion and battery electric cars. We found the manufacturing emissions are much smaller compared to emissions associated with operating the vehicles, roughly 10% vs. 90% of emissions, respectively, in battery electric cars. This corresponds to about a 1-year emissions payback period, depending on the electricity grid. See Chapter 2 of this report: https://www.ucsusa.org/clean-vehicles/electric-vehicles/life-cycle-ev-emissions

  • Brad Couch

    By the way, if the transit buses are burning carbon negative RNG, and are consuming 4 times as much fuel as asserted above, then the net carbon removed from the atmosphere is monumental and a HUGE carbon sink for the environment. PLEASE correct this article, unless of course this article is about propaganda and not transportation and scientific facts!

  • Brad Couch

    Study is bogus because it ignores the cleanest alt fuel available, renewable natural gas aka RNG which is a type of CNG that is chemically identical in the pipeline. As long as you have groups like this pushing agenda’s they will conveniently omit the scientific facts that don’t further their cause. The life cycle analysis of RNG as proven by the California Air Resource Board or CARB, clearly shows the exponentially cleaner nature of RNG. So why would the UOCS ignore it? I can only conclude because it doesn’t fit their narrative. Never let the facts get in the way of a good cause … bogus study I say!

    • Jimmy O’Dea

      Hi Brad,

      You may be interested in our analysis of biomethane life cycle emissions and availability here: http://www.ucsusa.org/biomethane-transportation. We found that electric buses on today’s grid in California have lower life cycle emissions than natural gas buses using biomethane from landfills, the predominant source of biomethane.

      Biofuels, including biomethane, offer a lower carbon option than diesel when there’s not an electric option. But there’s definitely an electric option for buses today. From an emissions standpoint, you may be interested to learn that it is better to use biomethane to generate electricity than use it in natural gas vehicles due to the greater efficiency of power plants and electric motors compared to combustion engines.

      Uses of biomethane must also be weighed against the limited availability of the fuel, which is also discussed in the link above. Given the chemical similarities of biomethane and natural gas, it is also important to note that once biomethane is generated and injected into a natural gas pipeline, its public health and environmental impacts parallel those of natural gas. Small leaks throughout the natural gas distribution system, and catastrophic leaks such as the one at Aliso Canyon, can erode any climate benefits associated with using methane as a fuel.

      • Brad Couch

        Your data is in direct conflict with CARB, UC Davis and US DOE data. BioCNG in California can have between -125% to -400% (1, 2, 3) reduction in GHG. The best an EV bus can do is get close to zero. But it never gets negative as does BioCNG aka RNG on a life-cycle basis. And being GHG negative, the more its used, the more the GHG number generates a credit, eventually off-setting the vehicle and manufacturing GHG cost. No EV on the market today can make that claim. I’d submit to you that this approach is intellectually dishonest!

        So respectfully, I will choose to use CARB, DOE and UC Davis data over UOCS (who in my view shows an obvious bias). And furthermore, landfill based RNG is becoming increasingly less of the over all RNG generated and consumed nationally. Plus you ignore the dirty electric grid issue around the rest of the country. It seems this study cherry picks data and ignores some MAJOR authorities on the subject of RNG. As a nation, we need an all solution approach, not a silver bullet mentality. And to say that when RNG is injected into the pipeline it’s environmental benefits are negated, well that is just wishful thinking as any thinking person can plainly see, if the fuel is clean going it, it doesn’t magically turn dirty by spending some miles in a pipeline. And you bring up fugitive emissions of the natural gas industry, so are you going to acknowledge the data that shows the HUGE reductions in fugitive emissions over the last two decades despite massive production increases? And lastly you say RNG in the pipeline is bad, yet how do you think it would get to electrical generation power plants? You can’t have it both ways. At least you can’t if one is being intellectually honest in one’s approach.

        1 The Feasibility of Renewable Natural Gas as a Large-Scale, Low Carbon Substitute, Prepared for the California Air Resources Board and the California Environmental Protection Agency by Amy Jaffe, Principal Investigator. STEPS Program, Institute of Transportation Studies, UC Davis *

        2 U.S. Department of Energy. 2016. 2016 Billion-Ton Report: Advancing Domestic Resources for a Thriving Bioeconomy, Volume 1: Economic Availability of Feedstocks. M. H. Langholtz, B. J. Stokes, and L. M. Eaton (Leads), ORNL/TM-2016/160. Oak Ridge National Laboratory, Oak Ridge, TN. 448p. doi: 10.2172/1271651 *.; 2030 Values achievable at $60/Ton

        3 California Air Resources Board, Low Carbon Fuel Standard Pathway Certified Carbon Intensities *

      • Jimmy O’Dea

        Hi Brad,

        Our analysis of life cycle emissions reflect the volume-weighted carbon intensity of biomethane, just as our analysis reflects the grid average carbon intensity for electricity. One could easily choose an electricity pathway entirely from wind or solar or a biomethane pathway entirely from dairy manure, but both of these represent only one aspect of each sector. An even more holistic comparison of electricity and biomethane is grid electricity to grid natural gas, where the latter factors in the amount of biomethane being injected into the pipeline (<<1% compared to fossil natural gas) just like the electricity grid mix factors in the amount of wind, solar, etc. But even looking at biomethane in isolation, we found on today's grid and with today's sources of biomethane (predominantly landfills, see our fact sheet on biomethane, the California Air Resources Board, and the UC Davis study cited above), electric vehicles have significantly lower life cycle global warming emissions. These results reflect carbon intensities from CARB. Thanks for your interest in our work.

  • Pop Fizz

    So I’m not seeing renewable natural gas (RNG) anywhere in this study. RNG buses can have significantly lower emissions than electric buses (negative carbon intensity depending on the source), and RNG is being used in the vast majority of buses in California. Transit authorities need to use multiple fuels for resiliency purposes – electric and RNG for the win?

    • Jimmy O’Dea

      Hi Pop Fizz,

      You may be interested in our analysis of biomethane life cycle emissions and availability here: http://www.ucsusa.org/biomethane-transportation. We found that electric buses on today’s grid in California have lower life cycle emissions than natural gas buses using biomethane from landfills, the predominant source of biomethane.

      Biofuels, including biomethane, offer a lower carbon option than diesel when there’s not an electric option. But there’s definitely an electric option for buses today. From an emissions standpoint, you may be interested to learn that it is better to use biomethane to generate electricity than use it in natural gas vehicles due to the greater efficiency of power plants and electric motors compared to combustion engines.

      Uses of biomethane must also be weighed against the limited availability of the fuel, which is also discussed in the link above. Thanks for the comment.

      • Pop Fizz

        Hi Jimmy,

        Thanks for the response! I read the linked article, but I think a few things are missing. The analysis really just focuses on landfill RNG, which is one of the most carbon intensive of all RNG sources. If you look at other sources, the carbon numbers fall below the battery electric carbon emissions with current fuels/grid, and I suspect would be negative with a few different sources.

        As far as supply goes, I think the numbers in the report are a bit off, since there is almost as much potential for biomethane from livestock manure as landfills (257 vs 284 billion cubic feet respectively, source: https://www.usda.gov/oce/reports/energy/Biogas_Opportunities_Roadmap_8-1-14.pdf) in the US, which has a negative carbon intensity (https://www.arb.ca.gov/fuels/lcfs/fuelpathways/pathwaytable.htm). And the total estimated amount is 2,499 million GGE worth, which is more than enough to cover an estimated 1,300 million GGE (afleet/APTA factbook estimate) of fuel needed to cover all transit buses in the US.

        In a nutshell, we’re going to need every solution in the book to get to 100% renewable-fueled buses, and so far I think RNG has been largely dismissed. The technology is proven, is available today, and arguably can have a better carbon footprint than many other sources, including electric buses. I would encourage you to include it in future analyses, as proponents for it from groups such as your own could help expedite lower emissions and cleaner air for all.