As the world grapples with the spectre of the so-called “hockey stick” graph of climate change, there have been a variety of solutions proposed to the problem of carbon emissions from sectors such as transport which have become inseparable from the maintenance of 21st century life. Sometimes these are blue-sky ideas that may just be a little bit barmy, while other times they make you stop and think: “That could just work!”.
Such an idea is that of replacing the diesel engines in trucks with electric motors powered not by batteries but from overhead cables. An electric tractor unit would carry a relatively small battery for last-mile transit, but derive its highway power by extending a pantograph from its roof to a high-voltage cable above the road. It’s extremely seductive to the extent that there have even been trials of the system in more than one country, but does it stack up to a bit of analysis?
Time’s Up For Those Big Rigs
One thing that should be obvious to all is that moving our long-distance freight around by means of an individual fossil-fuel-powered diesel engine for every 38 tonne or so freight container may be convenient, but it is hardly either fuel-efficient or environmentally friendly The most efficient diesel engines on the road are said to have a 43% efficiency, and when hauling an single load they take none of the economies of scale afforded to the diesel engines that haul for example a freight train. Similarly they spread any pollution they emit across the entirety of their route, and yet again fail to benefit from the economies of scale present in for example a power station exhaust scrubber. However much I have a weakness for the sight of a big rig at full stretch, even I have to admit that its day has passed.
The battery technology being pursued for passenger cars is a tempting alternative, as we’ve seen with Tesla Semi. But for all its technology that vehicle still walks the knife-edge between the gain in cost-effectiveness versus the cost of hauling around enough batteries to transport that quantity of freight. Against that the overhead wire truck seems to offer the best of both worlds, the lightness and easy refueling of a diesel versus the lack of emissions from an electric. In the idealised world of a brochure it runs on renewable wind, sun, and water power, so all our problems are solved, right? But does it really stack up?
The trouble with evaluating claims about overhead wire electric trucks is that there is little comparable from which to draw parallels. Long-distance electric trains have been around for over a century, but though they make infinitely more sense for very long distance transport they are not analagous enough to a myriad of individual routes for a direct comparison. Likewise electric urban transport in the form of trams and trolley buses are old enough to have been invented, abandoned, and rediscovered, but their use cases of city transport over set routes doesn’t match that of a free-ranging truck. Perhaps it’s better to look at the costs involved both in providing the distribution infrastructure and the extra generating capacity.
Out Comes The Hackaday Back of An Envelope
Just how much energy does a truck user per mile anyway, and what effect would all the trucks going electric have on the grid? Time for a back-of-envelope calculation. This 2017 paper from Oak Ridge National Laboratory (PDF) puts some figures on the table, deriving a 1.89 kWh per mile figure for a battery electric truck versus as 2.02 kWh figure for its diesel equivalent. This disparity is due to predicted recovery of energy through regenerative braking.
Since the Guardian piece linked at the top of the article applies to the UK, a quick look at the British government’s road freight statistics reveals 152 billion tonne kilometres of freight were moved by road over 18.7 billion kilometres traveled in 2018, with an average haul length of 108km or 67.1 miles. This gives us 173,148,148 of those 67.1 mile journeys, and taking in the Oak Ridge energy figures, 23,468,846,276 kWh of diesel or 21,958,474,981 kWh of electric energy consumption. That’s a yearly figure, so dividing by 365 and taking a dubious assumption that those journeys are spread over 12 hours of daytime, we arrive at 5,013,350.45 kW of extra generating capacity. 5.013 GW may be enough to get you back to the future four times, but it’s not inconsequential in generating capacity terms.
To give an idea of the cost, and taking the rosy view that all this capacity will be renewable wind power, a 3.5 MW wind turbine costs £3.13 million to install. To generate 5.013 GW we would need 1433 of them for which we probably have space offshore, so we’d have to find an extra £4,485,290,000 ($5,895,667,014.05). £4.5 billion is a lot of money but it’s not out of sight for a government that’s spending over £100 billion on a high-speed railway at the moment, even if they may soon have some economic uncertainty to contend with.
How about the cost of putting up those electric cables? For that we don’t have any comparisons as there are no large road networks that have been converted to overhead wires. But we do have a parallel in the railway system, as the ongoing electrification of Isambard Kingdom Brunel‘s Great Western Railway from London to South Wales. It’s mired in controversy and has moved significantly from its original cost estimate and scope, but in 2017 the 129 miles from London to Cardiff were estimated to cost £2.8 billion, or somewhere over £21.7 million per mile. The likely size of the UK road network to be converted is quoted as 4,300 miles, so that gives us a final bill of £93.31 billion, or about $122.506 billion. Add to that our by now relatively paltry-sounding £4.5bn for those wind turbines, and we reach a final figure of £97.81 billion (about $128.67 billion).
So our back-of-the-envelope calculation for a countrywide network comes in at a shade under £100 bn, definitely in the same ballpark as the high-speed rail project that only serves London and Birmingham, about 125 miles apart. I’m sure that there will be other costs and that Hackaday readers will pick up on me should I have made any calculation errors, but I must admit to being pleasantly surprised at how relatively affordable that is for a country. Cynical long-time watchers will tell you that everything the UK government touches comes in at twice the price, but even at £200bn it’s not out of sight for the benefit it might deliver.
It May Be Affordable, But Does It Scale?
Doubters will of course point to the size and density of the United Kingdom versus the wide open spaces of for example the American mid-west as evidence for why it could not possibly work over distances greater than those in a small country. To them I would point to the experience in the railway system. For many decades now I have been able to take an electric train (with a few changes) for thousands of miles from the Atlantic coast of the UK through the Channel Tunnel across Europe and into Russia, and since 2002 from Moscow further via the Trans-Siberian Railway to China and as far as Vladivostok. It’s not really an easy journey at over a week, but I can take an electric train journey and then a ferry crossing from near my hackerspace in Milton Keynes to arrive in Japan, and that’s not an inconsequential distance. There is nothing about the technology which makes it impossible or impractical over a large distance, and since it has been around for a long time there’s nothing unproven about doing so.
In investigating the viability of electrified road transport we’ve found to our surprise that it could indeed be viable, and to demonstrate this we’ve leaned heavily upon the analogous experience in the rail industry. But in doing so we’ve inadvertently demonstrated something else, that railways can more successfully be electrified over very long distances. Perhaps the real story here is that what might work best to decarbonise freight transport using electricity would be to electrify the rail lines for freight as the branches of a transcontinental tree, and to treat the regional road networks as its electrified fine roots and leaves rather than try to electrify every road. After all, an electric locomotive can move a hundred loads at once.
Header image: Scania.