Blog post

Thinking twice about the future of transport

Energy Blog, 14 October 2019

In George Orwell’s scarily prescient novel “1984”, he introduces the concept of doublethink: the ability to hold two apparently contradictory thoughts at the same time, e.g. my wife fancies the svelte, densely coiffured actor Hugh Jackman but has actually married me, a fat bald accountant.

On a less personal scale, the UK Government’s approach to transport seems to be a classic example. They proudly proclaim no new cars or vans powered solely by a petrol or diesel engine will be sold in the UK from 2040 – but at the same time they spend the best part of GBP 500 M per annum on incentivising technology to produce diesel and gas from bio sources such as crops and waste. This is through the somewhat clunkily titled Road Transport Fuel Obligation scheme (RTFO).

That RTFO scheme: in summary a generous premium for each litre of alternative fuels

As you may be aware, the RTFO scheme works as follows; suppliers of fuel for road transport have to source a minimum of 8.5% of their fuel from renewable sources and this will rise to 12.5% by 2032. Producers of fuel are incentivised through subsidies whereby every litre of fuel they produce attracts a payment in the form of a certificate. Different types of feedstock attract different numbers of certificates. For example, a litre of fuel produced from crops attracts only one RTFO whereas the same litre produced from waste attracts two. These certificates trade for about 20p each (coming in addition to the wholesale value of the fuel) so you can see that they are an incentive to produce fuels from crops, straw, waste or cooking oil. This year over 800 million litres of fuel qualified for RTFO support and similar schemes are emerging throughout Europe.

The apparent doublethink is: why is the UK Government subsidising the development of alternative fuels for conventionally powered transport, when another bit of the self-same government is cheerfully announcing bans on conventionally powered cars? I think it’s because both bits secretly know that we are a long way off from a truly electric vehicle fleet – and the reason for this is the underlying immutable and indifferent physics of energy densities per fuel type.

Specific energy explained (or why batteries are really heavy)

Although the next four paragraphs are a bit maths-oriented, there are only couple of actual equations and understanding these is key to the diesel/petrol versus electric vehicle debate. The key concept is specific energy. In layman’s terms, this means how much energy (Mega Joules or MJ) is contained per kilogram (kg) of weight. Batteries compare very badly to petrol or diesel in this regard. Petrol contains 49 MJ per kg, and as anyone who has ever used petrol to try to light a barbecue will ruefully testify (with newly singed eyebrows), that this is a truly astonishing amount of energy by weight. In contrast, the best currently available battery technology contains 0.9 MJ/kg – a factor of 50 less.

To put this in context, my car’s fuel tank holds 45kg weight of fuel – or about 2,200 MJ of potential energy. An electric car would need to carry a battery weighing 1.8 ton to carry the equivalent. Similar laws apply to food and drink, which is why good things like chocolate and beer make you fat (lots of specific energy per gram) but dreary things like lettuce and mineral water don’t.

The relative inefficiency of petrol engines (as acoustically demonstrated by teenage boys in Renault Clios)

Advocates of electric cars will no doubt splutter with indignation at this point and point out the relative efficiency of the corresponding engines. Although petrol has a very high MJ/kg ratio, petrol engines are very inefficient and only convert about 35% of that energy into usable power. The rest is lost mainly as heat or, as is apparent from teenage boys driving past my house in souped up Renault Clios, through the noise from the exhaust.

In comparison, electric motors have a very high efficiency of circa 90% and don’t make as much heat or noise. So, to make a true comparison, you have to multiply the specific energy by the efficiency. However, even factoring this in, batteries still lose. 35% of 49 MJ/kg is 17 MJ/kg, which is still a much bigger number than 0.81 MJ/kg (90% of 0.9 MJ/kg). On an energy per kg basis, then, petrol- or diesel-powered vehicles are much better as a means of storing energy than electric ones, and will most likely remain so for a long time.

Why little electric cars sort of make sense but big electric lorries don’t

This isn’t a big issue for electric cars that do lots of short journeys but it is a huge obstacle for electrifying heavy vehicles such as articulated lorries that travel long distances. They would either have to tow an enormous battery around everywhere, or deal with a short range between recharges. Still, these vehicles currently contribute an awful lot of CO2 per journey compared to a car, making them an obvious policy target. Half a million articulated lorries trundle about on UK roads every year, producing about 20 million tonnes of CO2, compared to 32 million cars producing 70 million tonnes. In the same vein, UK aviation produces another 36 million tons of CO2 – but I think it will be a long time before we see a battery-powered passenger plane.

Is fuel from waste an opportunity for investors ?

The government is no doubt painfully aware of this, meaning that switching these lorries (and planes) to renewable fuels is a quicker route to a low carbon future than relying on the specific energy of battery technology to catch up. Hence the apparent doublethink. Fleet operators are also beginning to realise that maybe, just maybe, bio-powered rather than electric trucks are realistic – and real – progress towards greener transport for the medium term at least. Tellingly, the department store chain John Lewis recently announced they are converting their 500 articulated delivery lorries to biomethane rather than battery-powered alternatives.

All of this creates a big opportunity for operators of both anaerobic digestion (AD) and waste plants. For those of you not too familiar with the technology, AD plants convert organic matter (such as food waste or crops) into hydrogen and methane. These gasses can be compressed and used as fuel. One ton of straw produces circa 250 litres of fuel in the form of liquid gas. Operators of existing AD plants are increasingly looking to the RTFO as an income source. In the same vein, we are now starting to see the major oil companies such as Shell invest significantly in alternative fuel production facilities particularly from waste or cooking oil.

So, although at first glance UK road transport policy seems like a classic doublethink, it probably isn’t. Electrifying things like cars and vans is all very sensible but the immutable maths of specific energy means we are a long way off from being able to do same with the heavy lorry fleet. A relative factor of 50 in different specific energies (or 20, factoring in relative efficiencies of different engine types) between liquid fuels and batteries mean that it will take a long time, if ever, for the latter to narrow the gap. In the meantime, fuels derived from renewable sources will increasingly play a big part in tackling the CO2 emissions produced by the half a million-strong UK lorry fleet – and this can be replicated in a global scale. The UK is leading the world in promoting these developments. So although we are about to cut trading ties with our European friends, we will at least have lots of (now empty) low-carbon lorries on the roads.

Banner picture by Clare Black via Flickr