Blog: Big Batteries or Smart Infrastructure?


We recently performed a study of heavy vehicle operations within Cambridge.  (See the publication here.)  Cambridge is a small city – easy to study since we live here!  Nevertheless, the heavy vehicle movements are not too different to other small cities in the UK.

We analysed the ‘Park and Ride’ (P&R) double-decker bus routes; all of the refuse collection routes around the city; and some home delivery operations run by supermarkets from their stores located near the edges of the city.  For each of the vehicle operations we recorded the existing routes using GPS receivers and we measured the real drive cycles (speed vs time) of the vehicles. We used these measured drive cycles to predict the energy requirements using simulation models of conventional diesel vehicles.  We also simulated electric vehicles that could be used to replace the conventional vehicles.

Our aim was to understand the relationship between the size and cost of the batteries needed in the electric vehicles and the charging infrastructure provided along the routes.  Our thinking was influenced heavily by the successful trial of electric buses in Milton Keynes https://www.milton-keynes.gov.uk/highways-and-transport-hub/bus-and-taxi/bus-projects/electric-bus  These buses are charged wirelessly using inductive pads buried in the ground at the two ends of the route, for 10-15mins at the end of each journey.  This so-called ‘opportunity charging’ means that the buses have much smaller batteries than they would need if they had to carry all their energy for the whole day.  It also means that charging the buses overnight in the depot only requires modest charging facilities instead of very high powered chargers and large, dedicated electricity substations.

So what did we discover?

Well it turns out that the strategy used by the Milton Keynes buses is definitely the way to go!  Our study indicated that to store enough energy for a whole day of operation (without heaters), the double-decker buses would require batteries of 500kWh – 800kWh, depending on the route; with a corresponding battery mass of 4 tonnes – 6.5 tonnes.  (This can be compared with the largest battery available for the Tesla Model S which has a capacity of 100kWh.)  However, using the opportunity charging approach, the same double-decker buses would only need batteries of 100-150kWh; with a mass of approx. 1.2 tonnes: about the same as the mass of the fuel tanks and drive-train of the equivalent diesel vehicle.

Consequently, it would not be possible to run double-decker electric buses on these routes, unless they had opportunity charging – because about half of the allowable payload mass (ie most of the passengers on one of the decks) would be used up by the 4t-6.5t batteries!

But the kicker is that the opportunity charging system is also much lower cost.  Li-ion batteries are expensive.  Large Li-ion batteries are very expensive!  Having one opportunity charger at each end of the route turns out to be the lowest cost solution by far.  (Having more chargers located at normal stops along the route is not worthwhile because buses don’t stop long enough to charge significantly.)

We used the real costs of the Milton Keynes trial to estimate the capital costs of electric double-deckers and their batteries and the charging infrastructure (including the cost of equipment and installation as well as the purchase and connection of any chargers needed at the depots).  The Cambridge P&R routes each have 5 or 6 buses and the total capital cost of buses and charging system for each route is expected to be about £2.5m for the big battery solution, but only about £1.5m for the opportunity charging solution… a saving of approx. 40% of the capital cost.

The CO2 savings by either solution would be substantial.  By 2040 the electric buses will emit 75%–83% less CO2 than today’s diesel buses (assuming the UK Governments projections for decarbonisation of the electricity grid.)

We found that the opportunity charging strategy also works well for refuse collection vehicles.  If they charge their batteries each time they go back to the waste centre to dump their load, every few hours, the batteries can be kept small: 150kWh-200kWh, leaving room for a full payload.

On the other hand, the small 3.5t home delivery vans can carry sufficient battery storage to take them around their entire route without any difficulty… so they can simply charge each time they go back to the supermarket to refill.  No special charging arrangements required.

And the conclusions?

Getting the right electric vehicle charging infrastructure into our city streets is a key step towards a low carbon future for heavy vehicles.  If local authorities take the initiative and put appropriate charging infrastructure into cities, the buses, rubbish trucks and other urban freight vehicles can have relatively small batteries, with low embodied energies and the charging infrastructure can be modest.  If they leave the decisions to market forces alone, we are likely to end up with expensive, heavy, big battery solutions – with large embodied energies and very high over-night power charging requirements in depots.

Now is the time to be thinking about building suitable electrification infrastructure in our cities…. See my previous blog!

My thanks to Doros Nicolaides for performing the simulation studies described in this blog.  Good luck with your PhD examination!

David Cebon
12/3/2018

Further Information

Kontoua, A. and Miles, J.  ‘Electric Buses: Lessons to be Learnt from the Milton Keynes Demonstration Project’, Procedia Engineering, Volume 118, pp 1137-1144, 2015. https://doi.org/10.1016/j.proeng.2015.08.455