Recently on CleanTechnica there have been a couple of excellent pieces on the challenges with hydrogen fuel cells for vehicles in general, but how would they be for urban transit buses specifically? It’s instructive to look at an apples-to-apples comparisons of diesel, electric trolley and hydrogen buses traveling 100 kilometres in the USA.
Diesel is, on the average grid, the least CO2 emitting choice per this assessment. Its particulate and nitrous oxide emissions in major populated areas still make it questionable as a choice, especially in places with better than average CO2 per KWH on the grid. It’s definitely better in coal-dominant grids than electric trolley buses for CO2 emissions, but hydrogen buses might be appropriate there due to urban air quality and related health concerns.
As the grid decarbonizes, dominantly due to increased renewables, the CO2 balance will change. In eleven states today, electric trolley buses are already equivalent or better than diesel buses in terms of CO2 outputs, and of course emit no particulate or chemical pollutions. These states are California, Connecticut, Idaho, Maine, New Hampshire, New Jersey, New York, Oregon, South Dakota, Vermont and Washington. If grids reduced carbon by 20% across the board, 18 states would be better with electric trolley buses.
The price of operation is an important consideration. While electric trolley buses require overhead wire systems, it’s by far the cheapest system to operate, so for dense urban areas it is most likely to be the cheapest overall. The high cost of operating hydrogen buses makes it unlikely that they will become common regardless of other factors.
Electric trolley buses are the lowest CO2 choice where the grid has already substantially decarbonized, are the cheapest to operate and it has no human health effects. And grids are going to decarbonize. For dense urban areas, electrification of bus transit appears to be the obvious choice.
Electric Trolley Buses
Electric trolley buses running under overhead wires use about 2.73 kWh per kilometre. That means that on the average US grid they will generate about 150 kilograms of CO2 for 100 kilometres. If the grid is using coal only, electric buses will generate about 276 kg of CO2 over 100 km.
Coal produces significant particulate and chemical emissions, gas produces quite a bit less and nuclear, wind and solar generation produce no emissions during operation. The grid is shifting to lower carbon sources of generation for the most part, so we can expect the CO2 number as well as grid particulate and chemical emissions to reduce substantially over time. Idaho’s 78% generation from renewables leads the way and results in an extremely low 0.055 kg CO2e / KWH, one tenth of the average. This is the future of electricity.
During operation, electric trolley buses emit no particulate or chemical pollution in downtown areas, so have no direct health impacts related to the fuel choice. Indirect health impacts of high coal grids must be considered, but they will typically be much more diffuse and lower per capita than diesel emissions directly in dense urban areas.
Of course, this assumes the sunk cost of the relatively straightforward trolley wiring grid. It’s a rounding error on the energy usage and both diesel and hydrogen require infrastructure as well, so this is an acceptable gloss for dense urban areas.
Electricity price is $0.103 USD per KWh per the average 2013 price to the transportation sector. As 273 KWH are required to travel 100 km, an electric trolley bus would have fuel costs of $28.12 USD for this distance.
Diesel buses average about 39 litres per 100 km (6 mpg). Diesel emits about 2.67 kg of CO2 when burned which is the tank-to-wheel output, but that’s 86% of well-to-wheel emissions per this study. As such, it would emit about 121 kg of CO2 over 100 km.
Note: an earlier version of this article used the tank-to-wheel number, not the well-to-wheel number.
Diesel-electric hybrids are only 10% more fuel efficient than diesel according to this source, so they would be only slightly better than direct diesel buses and will not be considered in detail.
Health impacts of diesel buses are important to assess as they run in densely populated areas at the same times as there are a larger percentage of the populace on the street. In the USA, diesel emissions have been dropping substantially due to regulation except for nitrous oxide (NOx), which is key to the formation of ozone and smog in the summertime, according to the EPA’s Health Assessment Document for Diesel Engine Exhaust.
So if a diesel bus drives 100 km, they will emit roughly 60 grams of particulate matter which directly inflames lungs, makes asthma worse and irritates eyes and noses. They will emit about 4,000 grams of NOx, which lingers and combines with volatile organic compounds to create summer smogs which directly impact breathing for children, geriatrics and those with asthma and emphysema. They will emit about 60 grams of effectively unburnt fuel (HC = hydrocarbons) with varying health risks.
By comparison, coal generation has many of these same risks, but coal plants are not idling in city traffic among hundreds of thousands of commuters. In general, their emissions flow downwind of the plants and are more diffuse than those of buses.
As an example, in Chicago buses travel about 300,000 kilometres a day, so if they were all relatively modern diesel buses they would emit about 12,000 kg of smog creating nitrous oxide and a couple of hundred kilograms each of particulate matter and unburnt fuel every day. That would mean in the range of 3.6 million kg of nitrous oxide and 50 thousand kg of particulate matter and unburnt fuel annually. This is a significant amount of health-impacting air pollution.
Recent news out of Britain does not look positive for diesel vehicles in general.
However, diesel vehicles produce high levels of nitrogen dioxide, which can lead to respiratory disease and has been linked to 7,000 deaths a year.
The suggested number of premature deaths must be put in context that this includes passenger cars and cargo trucks, not just buses, but it’s obvious that diesel buses would lead to some increased premature deaths due to emissions, as well as degraded health and quality of life for many more people.
By comparison, electric buses and hydrogen buses add no particulate or chemical pollution to city streets in close proximity to the populace.
Diesel costs $1.03 USD per litre based on the average US retail over the past three years. Traveling 100 kilometres would require 39 litres and cost $40.17 USD.
Hydrogen is more of a storage medium for energy than a source of energy. It must be created from raw inputs such as water or natural gas.
Hydrogen is normally processed from natural gas. The steam reformation process requires significant energy and hydrogen is a notoriously slippery molecule with very low density in gaseous form. According to a 2001 NREL full lifecycle assessment the total CO2 emissions for a kilogram of hydrogen produced from natural gas is 11.9 kg, with 25% of total emissions coming from process, storage and transport. Other sources peg the primary CO2 generation as up to 10.1 kg CO2e / kg H2, so this could vary up to 13.3 kg of CO2e / kg H2 assuming that the ratio holds. The NREL number will be used regardless, but it’s important to note that it could be even worse.
The gasoline gallon equivalent for diesel is 0.88, and one kilogram of hydrogen has the energy equivalent of one gallon of gasoline. As this source points out, hydrogen fuel cell engine efficiencies are roughly the same and in many cases poorer than diesel engine efficiencies, so they will be considered on par.
Starting with the 39 litres of diesel, then, 10.3 kg of hydrogen would be required to travel the 100 km, so this would have a 139 kg of CO2 emissions burden.
The process for producing hydrogen is polluting, but less so than burning coal or diesel regardless.
Can natural gas hydrogen generation scale? Well, the current U.S. refinery hydrogen production capacity is about 7 million kg daily, which is sufficient for roughly 70 million bus miles daily. Buses travel about 9 million miles daily in the USA. Scaling up hydrogen production for bus use appears to be a viable expansion of capacity of 13% or so. It would appear that it is at least feasible to consider the natural gas option for creating hydrogen for bus fleets.
There’s an obvious statement to be made about electrolysis: turning electricity into hydrogen then turning the hydrogen back into electricity is less efficient than using the electricity directly. The following quote is solely based on electrolysis.
In a recent study, fuel cell expert Ulf Bossel explains that a hydrogen economy is a wasteful economy. The large amount of energy required to isolate hydrogen from natural compounds (water, natural gas, biomass), package the light gas by compression or liquefaction, transfer the energy carrier to the user, plus the energy lost when it is converted to useful electricity with fuel cells, leaves around 25% for practical use — an unacceptable value to run an economy in a sustainable future. Only niche applications like submarines and spacecraft might use hydrogen.
Assuming that this ratio is correct, the bus will effectively use four times as much electricity to travel 100 km and as a result produce four times the CO2 from the grid as the direct electric bus. That means that it will produce roughly 600 kg of CO2 for 100 km. And on a coal-only grid, that turns into 1,105 kg of CO2 as well as significantly higher particulate and chemical pollutants.
Would hydrogen from electrolysis make sense in a low-carbon grid? Considering Idaho with its 0.055 kg CO2e / KWH, all of a sudden hydrogen from electrolysis would put a load of only 60 KG of CO2e into the atmosphere due to a bus traveling 100 km making it much better than diesel. However, the same bus running directly off of electricity would only put 15 kg of CO2 into the air. There is an argument for using off-peak inexpensive wind or nuclear energy to generate hydrogen using electrolysis, but it’s an economic one more than a direct environmental argument.
Hydrogen price is estimated to be $7 as the expected future average for steam reformation and a distribution model similar to gasoline, which as has been shown appears to be the most likely and lowest CO2 model. To travel 100 km, 10.3 kg of hydrogen would cost $72.10 USD. Electrolysis is more costly at an estimated average of $9 USD per kg, or $92.70 for 100 km travel, making it both by far the greatest emitter of CO2 and by far the most costly option.
Hydrogen is the most expensive fuel for buses, and it’s not the lowest greenhouse gas choice either. There’s no good argument for hydrogen fuel cell buses except in deeply polluted cities with coal-only grids, and that niche is shrinking not growing.
Diesel buses are the lowest greenhouse gas choice on average today, but the health impacts of diesel emissions in densely populated urban centres makes them a difficult choice, especially when urban areas are attempting to clean up their air.
Electric trolley buses are the cheapest to operate, have much lower health impacts than diesel buses and are already the best choice for greenhouse gas emissions in 20% of the US states. They will only improve as the grid decarbonizes. The smart money is on electrification of urban buses.