What does a low-carbon bus mass transit system look like?

Posted in: Energy and environmental policy, IPR internship, Public services, Science and research policy

Jac McCluskey is an IPR Intern and a third year Economics student at the University of Bath.

In March 2019, Bath and North East Somerset Council (BANES) declared a climate emergency. As part of the resolution the Council resolved to:

  • declare a climate emergency;
  • provide leadership to enable carbon neutral BANES by 2030;
  • enable citizen engagement;
  • oppose expansion of Bristol Airport.

A major part of this from a practical policy point of view is to encourage the use of mass transit. At present, the mass transit system of Bath uses mostly normal diesel buses. Transport contributes 29% of emissions in BANES, and Somerset Live recently highlighted the importance of changing the propulsion systems of mass transit vehicles in reducing diesel-based propulsion. So, what does a low carbon bus transport system look like?

According to the Low Carbon Vehicle Partnership “Low carbon buses are defined by the Government as those producing 30% less emissions of greenhouse gases (GHGs) than a normal diesel bus”. The carbon emissions of diesel internal combustion (ICE) buses can be reduced by improving the efficiency of their engines to comply with Euro 6 emissions standards. However, innovations are costly, and become more difficult as the opportunities for improvements diminish. For buses to emit substantially less carbon emissions, an alternate propulsion system is required. There are 2 main options: Battery electric bus (BEB) or hydrogen fuel cell electric bus (FCEB).

Data from the International Transport Forum suggests that the estimated lifecycle GHG emissions per vehicle kilometre of BEB and FCEBs is currently around 30% and 40% lower than the standard ICE bus respectively. Furthermore, the emissions savings of BEB and FCEBs could increase to 80% and 85% respectively, should electricity and hydrogen come from 100% renewable processes.

Emission reductions can come at a cost, and bus operators must consider all relevant factors when choosing which buses to include in their fleet. For example, compared to standard diesel ICE buses: BEBs tend to have a higher purchase price, significantly less range, and aren’t as capable at climbing hills; and FCEBs tend to have a higher purchase price, higher maintenance costs, and lower availability.

Given the relative capabilities of BEB and FCEBs, BEBs could be used in flat urban environments, and diesel ICE buses in all other circumstances, until FCEBs become more reliable and lest costly. Urban environments suit BEBs because opportunity charging infrastructure could be implemented to overcome the BEBs inferior range; BEBs are more efficient than ICEs in stop-start traffic; and BEBs have zero tail pipe emissions. BEBs are also on their way to becoming more profitable than diesel ICEs because of lower maintenance costs and higher availability.

FCEBs can be used as a direct replacement for diesel ICEs because they can have a similar range and top speed to diesel ICEs and are no worse than ICEs on inclined routes. This assessment is consistent with the European Commission’s statement that “in transport, hydrogen is a promising option where electrification is more difficult”.

Realistically, decisions are made after considering the effect of technological development. For example, investment in BEBs could be wasteful if significant improvements materialise within the next five years. Many of the costs associated with BEBs are attributed to their batteries. Improvements to the lifetime, range, and manufacture of batteries would have a significant effect on the cost effectiveness of operating an electrified fleet.

Electric propulsion systems also bring the possibility of manufacture designs which lower maintenance costs. For example, in 2016 Umea municipality operated buses which had modularised key components, which could be replaced quickly with limited skill. This is particularly beneficial to operators who regularly use their buses along narrow routes, as they would usually face large fines if a breakdown obstructed traffic flow. There have also been experiments which evaluated the possibility of BEBs utilising exchangeable batteries, which were mechanically replaced (within 1 minute) every 20km (Choi et al, 2015). Such ideas accentuate the possibilities associated with BEBs.

BEBs have been found to be significantly less efficient if they have been overcharged. However, real-time monitoring systems have been developed to prevent this occurrence, as well as reduce electricity bills by organising charges in a way that avoids incurring peak demand energy prices where possible. BEB operators could also achieve ancillary revenues if Vehicle to Grid (V2G) chargers become more advanced. V2G technology enables operators to discharge energy back into the national grid. This can come at a profit if the vehicle is charged overnight when energy prices are low.

The UK is one of the European leaders for low-carbon buses, primarily because of their involvement in the CUTE (Binder et al, 2006), HyFLEET, CHIC (Muller, 2017), and ZeEUS projects, and Transport for London’s (TfL’s) Ultra-Low Emission Zone (ULEZ) and Low Emission Zones (LEZs). According to TFL, their fleet now includes over 200 electric buses, and “from autumn 2020 all new single deck buses entering the fleet will be zero emission (at tailpipe)” with “a mix of hydrogen buses and electric buses”. Other areas of the UK have also made significant progress. For example, Nottingham City Transport (NCT) currently operates the world’s largest fleet of Bio-Gas double decker buses (120), amounting to 38.5% of their entire fleet.

Most authorities recognise that, since the market does not internalise the external costs of GHG emissions, the government must intervene to incentivise more sustainable business practices. The standard strategies would be to either: place a Pigouvian style tax on carbon emissions; subsidise low-carbon operations; or issue tradeable emission permits amounting to the total quantity of GHG emissions the government will tolerate. Each option requires a valuation of the cost of GHG emissions.

However, these calculations are extremely difficult given the inherent uncertainty of outcomes. As a result, estimates for the social cost of carbon (SCC) can range from as low as $12 to as high as $805 (Ricke et al, 2018). The EU has instead used IPCC forecasts to set a target of no more than 1.5°C increase in world temperatures, and subsequently calculated the carbon tax which would be consistent with this objective. This intervention promotes efficiency because the tax and tradeable permits allow the markets to decide the most cost-effective way to reduce emissions.

Interventions to be avoided include direct regulation, such as banning the production of diesel ICE buses. It would be very difficult for any government to forecast which industries are most able to reduce their emissions through innovation. As previously discussed, emissions savings can come at a cost. Direct regulation prevents the market from making an informed decision over whether the emissions savings in one sector are less costly than in another.

Government policies which are designed to encourage sectors which they predict to succeed are also ill advised. In practice, mass investment in either BEBs or FCEBs rather than providing a ‘fair playing field’ could distort the market by incentivising firms to invest in a technology which they believe to be less promising.

In conclusion, I expect the future mass transit systems to be mostly electrified, with the possibility of FCEBs in instances where BEBs are not feasible. Euro 6 diesel ICE buses may retain their use until technology progresses (specifically with regards to battery cost, lifetime, and range). However, this process may be rushed if governments are pressured to meet climate targets. The question for BANES is if they will rely on market-driven innovations or regulatory interventions and/or subsidies to bring about a rapid change in mass transit bus propulsion systems.


All articles posted on this blog give the views of the author(s), and not the position of the IPR, nor of the University of Bath.

Posted in: Energy and environmental policy, IPR internship, Public services, Science and research policy


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