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My city council has pledged to try and be carbon neutral by 2030 as part of the Global Covenant of Mayors and are investigating ways to achieve that.

I'd like to bring forth an idea to replace our aging diesel-fueled buses with more sustainable alternatives, especially for the free city-loops that run 11 hour services (7am-6pm).

How much emissions could be avoided by replacing a single diesel-fueled bus with an electric alternative? (Assume an 11 hour route of stop-start traffic over relatively flat land).

How much does this number change when comparing 100% fossil-fuel recharging vs 100% renewable energy?

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    BTW Assuming you don't mean a trolley bus, you can't do "an 11 hour route of stop-start traffic" on one charge. As of 2018 such buses can have a range of over 280 km with just one charge, however extreme temperatures and hills may reduce range (Wikipedia ref 6).
    – user2451
    Dec 19, 2019 at 15:28
  • @JanDoggen - I assumed the range would be less than diesel but that's valuable info, thanks :)
    – Robotnik ♦
    Dec 19, 2019 at 23:05
  • Yeah, you're really talking about a thing that doesn't exist. A bus that's a Tesla is not a thing, because real transit buses are way too kinetic and you run into the Rocket Equation vis-a-vis carrying enough battery. It can work for a bus that's a Prius. It can also work for little community-service shuttles, because they tend to run infrequently and can plug in during layovers.
    – Harper - Reinstate Monica
    Jan 13, 2020 at 20:24
  • @Harper - Tesla no, but they are far from the only electric vehicle company in operation. An electric bus is very much a thing, and as I've learned after asking this question, a proposal similar to what I list above is currently being trialled not too far from my hometown.
    – Robotnik ♦
    Jan 14, 2020 at 5:17

2 Answers 2

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It depends entirely on where your city is and where it gets its energy.

The Electricity Map has live energy data for the Carbon Intensity of the electricity supply. Right now, the CO2 intensity of the electricity supply in grams of CO2 equivalent per kWh (gCO2eq/kWh) across the world is:

  • 319 gCO2eq/kWh in the UK
  • 28 gCO2eq/kWh in Norway
  • 509 gCO2eq/kWh in the USA
  • 800 gCO2eq/kWh in Australia

I understand the UK is mostly using gas powered electricity generation, Norway uses mostly hydro-electric, and the USA relies on coal.

We're going to compare the CO2 emissions per km for the different vehicle types in different places. The energy efficiency of electric buses is around 1.335 kWh/km, which amounts to the following emissions in the above mentioned countries:

  • 426 gCO2eq/km in the UK
  • 37 gCO2eq/km in Norway
  • 680 gCO2eq/km in the USA
  • 1060 gCO2eq/km in Australia

The CO2 emissions for a regular diesel bus are around 1077 g/km, according to this paper.

Switching to electric buses in most countries could avoid between 400 g/km to 1000 g/km of CO2 emissions compared to diesel. In Australia the difference in emissions between diesel and electric is only about 20 g/km.

At this point I notice that you mention "aging diesel-fueled buses". There have been great developments in diesel fuel efficiency in the last few decades. Your city's buses might have emissions a lot worse than the quoted 1077 gCO2eq/km. So even replacing the aging diesel buses with modern diesel buses might reduce emissions by 50%.

Furthermore, it's seasonal too, in the summer the UK's CO2 intensity was as low as 120 gCO2eq/kWh because of solar power and less demand for heating.

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    From the ElectricityMaps website the CO2 intensity of power generation is around 800 gCO2/kWh, which is really bad, 1068g of CO2 per km. That is so bad that it might actually be cleaner to stick with modern diesel buses. If Sydney wants to be carbon neutral, they're going to have to get rid of the coal power stations before they change the buses.
    – Christopher Gilmour
    Dec 19, 2019 at 10:46
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    As of 2016, the US now uses more natural gas than coal (source).
    – LShaver ♦
    Dec 19, 2019 at 15:02
  • Now we can get hung up on semantics, at this exact moment in time in 2019 the MidcontinentIndependentSystemOperator is using 45% coal and 33% gas, the SouthWestPowerPool is using 33% coal and 19% gas (35% wind). Real time datafeeds are so much fun.
    – Christopher Gilmour
    Dec 19, 2019 at 16:17
  • I must disagree. The problem is, you're looking at the energy mix today and cursing the new electric-bus development with that mix for its lifetime. That's wrong for two reasons: first your grid will clean up generally over time, and the bus does benefit from that. Second, the bus will be new load which will require new generating capacity. That new capacity will be the cleanest, and likely, renewable if the government support is in the right place.
    – Harper - Reinstate Monica
    Jan 13, 2020 at 20:18
  • Can you provide any evidence that the Australian electricity grid is cleaning up over time?
    – Christopher Gilmour
    Jan 14, 2020 at 13:18
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To make a calculation you would need to know the fuel consumption of the current diesel buses and the efficiency of the electric buses that would replace them. That information is probably not available to you, so for my answer I will do a best guess based on data from other studies:

  1. This Finnish study done in 2015 claims that "electric buses have potential to decrease CO2 emissions more than 85% in comparison to diesel buses". One caveat here is that it is not clear what was included in this calculation. The presented graph shows no emissions during operation, so charging has to be with 100% green energy.

  2. This blog of the Union of Concerned Scientists (UCS) written in 2018 says that in California "electric buses had 70 percent lower global warming emissions than a diesel or natural gas bus". Now the carbon intensity of electricity in California is fairly low. This carbonfootprint.com document says it's 0.2502 kg CO2e/kWh (but I have seen other, higher numbers for California as well). The presented map on the UCS site also shows that in part of Wisconsin, which is one of the US states with the highest carbon intensity, an electric bus still is 7.4/4.8 = 1.5 times better in terms of emissions than the average diesel bus.

The problem is how to translate this to your situation. The carbonfootprint.com document says that the energy intensity in Australia is rather high. Since you're close to Syndey I'm guessing you're in New South Wales which has 0.92 kg CO2e/kWh for electricity. This is higher than that of poor performing US states such as Iowa, Illinois, Minnesota and Wisconsin (0.7959 kg CO2e/kWh).

I don't know have the exact numbers to back it up, but based on the numbers above I would guess that for your region electric buses would cause similar or just slightly less CO2 emissions compared to the average diesel buses, when charged with the standard local energy mix.

Of course if the diesel buses are old and consume a lot of fuel then the difference with electric buses will be bigger. Also you mentioned that your city pledged to be carbon neutral by 2030, so that would mean that the carbon intensity of your energy mix has to go down a lot and that emissions caused by electric buses will go down too. Even if electric buses cause similar emissions now, it should become much better over time.

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  • It's unfortunate that the carbon footprint report didn't look at the readily-available state level data, and instead used a regional average in the U.S. Iowa and Minnesota use quite a lot of wind energy, but with their smaller populations the average tips toward Illinois which uses lots of coal.
    – LShaver ♦
    Dec 19, 2019 at 15:21
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    @LShaver To be honest I had some doubts about the carbonfootprint.com report. I found rather different carbon intensity numbers for California and Australia in various sources. The intensity of California in the footprint report was the lowest number I found, which is a bit suspicious. Eventually I decided to use that anyway because it's the only source I found that reports on US states and Australia. I assume those numbers are gathered using a similar methodology.
    – THelper
    Dec 20, 2019 at 8:41
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    And thanks for fixing the link! Seems I accidentally added a character to it.
    – THelper
    Dec 20, 2019 at 8:43

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