routeone has published a monthly column examining alternative power sources aimed at reducing buses’ environmental impact. Now, our industry expert rounds them all up and compares their merits
The principal drivers for improving buses’ environmental impact are air quality and global warming. Both are subject to various legislation, and the implementation of that is beginning to be felt at an operational level by the declaration of low emission zones (LEZs).
Global warming arises from the production of greenhouse gases (GHGs). For transport, those are carbon dioxide (CO2) and oxides of nitrogen (NOx), which is principally nitrous oxide (N2O).
Both are major contributors to the greenhouse effect, which reduces the amount of heat lost to space from the atmosphere.
As the concentration of GHGs in the atmosphere rises, so does the average temperature. Since records began in 1880, the average surface temperature has risen by 0.95oC.
Air quality is a local issue. It is affected by emissions from human activities, and road transport is a major contributor. There are air quality issues associated with diesel tailpipe emissions, particulate matter (PM) and NOx.
Both have serious implications. PM enters the respiratory system and it contributes to asthma, emphysema and lung cancer. It is estimated that up to 40,000 premature deaths per year in the UK are caused by poor air quality.
With the above in mind, let us look at the various technologies on offer that can meet the air quality challenge.
Euro 6 diesel engines
The diesel engine has been in transport use since 1935. Rudolf Diesel set out in the 1890s to build the most thermally efficient internal combustion engine, and he succeeded. Only the engine used by Concorde was more efficient, and then only at supersonic speeds. However, exhaust emissions from diesel have always been a challenge. Unburnt fuel, PM and NOx are all undesirable.
Engine and exhaust technology has come a long way to meeting the challenge, and Euro 6 diesel engines meet the requirements for low emission buses.
Diesel is a fossil fuel, and a renewable diesel in equivalent quantities is neither currently nor foreseeably feasible. Diesel’s CO2 emissions are a significant contributor to GHG production, and the realisation is starting to dawn that, after 120 years, diesel has had its day.
Diesel-electric hybrid propulsion
Hybrid power uses a diesel engine driving an electric generator to charge batteries to power motor-generator units to turn the wheels.
It is a cumbersome system. It involves four conversions of the original fuel energy. It is purported to be more efficient that ‘direct drive’ diesel, and it is claimed to give at least a 30% reduction in CO2 emissions compared to Euro 3 diesel engines.
That proposition is based on the motor generator unit recovering energy and returning it to the batteries.
In practice, I believe that this does not work. An urban bus has an average speed of as low as 5mph, offering scant opportunity for regenerative braking. That is borne out by the comparative fuel consumption of diesel hybrids and of straight diesel propulsion.
Transport for London consumption reports show that, in general, single-deck hybrid buses recorded no improvement in fuel efficiency, and in many cases they were worse. Double-deck hybrids were marginally better, but not by 30%.
That hybrids use diesel engines should preclude them from future planning in any case.
Hydrogen fuel cells
Hydrogen appears to be the ideal fuel, because the only emission it generates is water. While using hydrogen in a fuel cell does result in an emission-free vehicle, there is considerable technical complexity involved, and that means high costs.
One of the largest hydrogen fuel cell bus operations is in Aberdeen. A fleet of 20 will eventually be in use and the total project cost is reported to be £19m. That’s hardly a practical proposition where thousands of alternative fuel vehicles must be found to improve air quality and control global warming.
Hydrogen fuel cells are emission free, but they are not yet practical – fuel cell vehicles have been 10 years away ever since 1970.
Electric power
Electric power for buses appears in several forms. Currently the most popular uses battery storage.
Battery electric buses clearly meet the zero tailpipe emission stipulation, so local air quality issues are addressed, but the same is not the case for global issues by a long way.
Batteries currently in use include lithium ion with lithium iron phosphate cathode material. They have acceptable performance and they are capable of meeting the demands of urban bus work.
But – and it’s a big but – manufacturing batteries has a very significant carbon footprint.
Production of 1kW/hr of lithium ion storage capacity results in the generation of 200kg of CO2 (see Journal of Cleaner Production 18 (2010 p1519-1529).
A typical single-deck electric bus has up to 320kW/h of battery storage, and it has thus generated up to 64 tonnes of CO2 before turning a wheel. If the batteries are charged using grid electricity, a further 0.52kg of CO2 per kW/h has to be accounted for, allowing for 15% charging inefficiency.
Manufacturers claim a 4,000-cycle battery life, which makes the lifetime CO2 footprint of one set of bus batteries as high as 829 tonnes. If an electric bus is to have the same life as a diesel, then it is likely that at least two sets of batteries will be required. That takes the lifetime carbon footprint of a battery electric bus to an astonishing 1,658 tonnes – an exorbitant price to pay for zero tailpipe emissions locally. In comparison, an average diesel bus will produce 1,760 tonnes of CO2 over the same time period.
Infrastructure to support electric buses is another question. A fleet of 200 buses that are charged overnight will need a supply of at least 5MW.
Lithium batteries also have to be disposed of. There is little information on the implications of that in terms of carbon footprint or cost, and lithium is also a tricky element to deal with. It is toxic, flammable, water soluble and it it gives rise to highly-corrosive derivatives.
Biogas as a fuel…
Growing numbers of biomethane-fuelled buses are appearing in UK fleets. Strictly speaking, biogas is not a new technology; MAN built its first gas bus in 1935.
Hundreds of thousands of gas buses are in service around the world and their engines are Otto cycle spark-ignition. Developments of internal combustion engines in general have transferred to gas engines, although no SCR systems are required.
Gas engines are as quiet as electric propulsion. Use of biomethane means that they are carbon neutral, and emissions testing at Millbrook Proving Ground has shown that PM and NOx are present in trace amounts only.
Emission performance is well in excess of Euro 6, meeting LEZ standards set by councils that Euro 6 diesel may not satisfy.
Experience also shows that operating costs are lower; up to 40% savings have been seen, although around 20% is closer to the norm.
Additionally, fuel price is more stable over a 10-year supply contract, and infrastructure costs can also be negated with that option.
Purchase price for a biogas bus is around £20,000 more than a Euro 6 diesel.
Conclusions
The choice of viable technology reduces to battery electric or biogas. But despite the allure of zero-tailpipe emissions, battery electric bears closer examination: High purchase cost, battery changes, depot electric supplies, the carbon footprint of battery production, and issues with disposal.
Biogas, on the other hand, is a well-established technology. It is a carbon neutral and renewable fuel, and tailpipe emissions are trace only, according to Millbrook trials.
It can be argued that in areas of poor air quality, the emissions from a biogas bus are cleaner than the air entering the engine, and the purchase cost is the closest to a new diesel bus.
Globally, locally, and financially, they do the best job, and I believe that is should be questioned why public money is spent on any other type of low-carbon bus.
Our Industry Expert speak with authority, and can back all of his facts, but what do you think? Email editorial@divcom.co.uk if you agree or disagree with him.
