Michael’s guest for Episode 121 of Cleaning Up was Rob Miller, Professor of Aerothermal Engineering at the University of Cambridge and Director of the Whittle Laboratory. The Whittle Laboratory is conducting world-leading research in pursuit of making net-zero aviation a reality.
With the aviation sector among the hardest to abate in the net-zero transition, Michael wanted to know whether electrification or hydrogen represents the best chance to keep the world flying in sustainable fashion. Also on the agenda were the complexities of producing - and defining - sustainable airline fuel, and the scale of the risk posed by aircraft contrails.
Michael Liebreich So, Professor Miller, could you start by telling us what the Whittle Laboratory does?
Rob Miller The Whittle Laboratory is focused on the decarbonisation of air travel and land-based energy, and accelerating that process. We work in the boundary between industry and academia, with companies like Rolls Royce, Mitsubishi and Boeing. We’re one of the leading laboratories in that field of aerothermal technology, and the laboratory brings in just under 10% of the University of Cambridge's industrial income.
ML Let me just give the background, Rob, of how you and I met. I've been doing a lot of work on hydrogen, and I've been saying that for long-haul aircraft, it just won't work. You reached out on Twitter and you corrected me. What is the answer with long-haul and hydrogen?
RM Well, the trap that people fall into is that when we design a kerosene aircraft, we set the volume to be optimal for a kerosene aircraft. But if you redesign the aircraft from scratch, and move the tanks inside the aircraft, the weight of the tank, and the hydrogen, is still around half that of Jet A, of kerosene. So, because the weight at takeoff of the plane is lower, the size of the wings goes down and the size of the engines go down, and the drag and the weight of those components drop. We think that these sorts of aircrafts will be within plus or minus 20% of the fuel burn per passenger kilometre of current aircraft. Additionally, because the percentage of the aircraft weight that is fuel goes up, and because hydrogen tanks are lighter, hydrogen has an advantage at flying long-range flights rather than short-range fights. So, I think there is a real potential in that space. Another nice thing about hydrogen fuel cells is that, because of the low temperatures, they don't produce nitrous oxides or soot.
ML So it can be done, but in fact, the showstopper is getting liquid hydrogen to airports.
RM Well, I'm not sure it's a showstopper. You'd have to bring in 60 liquid hydrogen tankers per hour into Heathrow. Having said that, that's only half the number of HGVs that drive into Heathrow at the moment. The difficulty here is there are no easy solutions. Buying those tankers, those 60 tankers an hour to transfer liquid hydrogen to Heathrow every day would cost you probably about a billion, two billion pounds. To build wind turbines and solar arrays to produce electro-fuel to come into Heathrow is about 100 billion pounds.
ML What's the situation with electrification? What are the physical limits there?
RM If you consider that we'll still be using lithium ion batteries in 2050, then you reach a limit of probably about 600 kilometres for battery electric aircraft. You're probably talking in 2035 about 400 kilometres operating range. The thing about battery electric aircraft is they are range limited, but not size limited. And so, you might see a move towards bus-sized aircraft, if the business model drove them that way, between major cities.
ML And what about battery lifetime? Because you're presumably going to be pushing them pretty hard, are you not?
RM That's absolutely right. You are fast charging them, and you're using them in repeated cycles every day. And at about 2000 to 3000 cycles, if you're really hammering them, you have to change the batteries, and so the battery becomes like a fuel. And if you don’t control the after-use of those batteries, the climate impact is sort of getting up to the CO2 impact of the flight. Now, if somebody gets a Nobel Prize in batteries, then it might accelerate it suddenly. But I don't see, having reviewed most of the battery technologies that are available, that any of them are close at the moment.
ML Okay, so at short distance, we’re going to go electric. What happens beyond that? If we're talking about the kind of 400 kilometres to 4000, ultimately, you could still just use Jet A. You could just use a sustainable airline fuel made either as an electro-fuel, or a biological based fuel, using the same aircraft as we do right now. Which is going to be cheaper?
RM I don't like the term sustainable aviation fuel, because it contains within it two very different types of fuels: on one end, a biofuel, and on the other an electro-fuel, made with renewable electricity, and the cost and complexity of those two are very different. If you buy today a sustainable aviation fuel, a biofuel, it's made from used cooking oil, effectively. And the actual limit on the feedstock means that, at maximum, that can produce 5% of the world's aviation fuel. Now, if we move to other biofuels, we have about enough feedstock in the world to produce all of today's aviation fuel, but that's about it. And other sectors are going to need that feedstock. On the other end, electro-fuel sounds very attractive, because you keep the planes the same. But the problem is that the cost of those fuels will be about three to eight times the cost of Jet A, pre-pandemic. And that's likely to double the price of your ticket. And more worryingly, the electricity required to do one single transatlantic flight is about 130% of a UK annual electricity bill.
ML So, is this why you're going towards PBTL, that’s Power and Biomass-to-Liquid?
RM Yes. One of the attractions of PBTL is that it doesn't require the uncertainty and cost of direct air capture. So, it's still energy intensive, and it's still expensive, but it's a sort of sweet-spot, in that it produces three times more fuel from your feedstock, and the technologies are there to do it today. But a lot of these solutions, going further out towards 2050, are dependent on how much electricity you have available. Some of our modelling up to 2035 years has shown that virtually anything you do in aviation pulls resources away from other sectors. And if you're limited in those resources, you can actually make the problem worse.
ML Contrails is one of the topics that I wanted to go onto. Because the climate impact of flying, we're told, is only about half the CO2 in the fuel?
RM If you take a hundred-year average, and you integrate up the climate impact, you find that the contrail has about the same impact on the climate as the CO2 itself, because some of the exhaust of the aircraft effectively forms persistent cloud, which traps in and reflects heat. Only about one in 20 flights produces a really bad persistent contrail, but that one in 20 flights could be - and the uncertainties on these are large - doing as much damage as the hundred years of CO2 of all the other flights. But there's an opportunity here, because changing altitude in the flight can stop the contrail forming. It increases fuel burn very fractionally, but it can stop the contrail forming. And this gives us a real opportunity for a short-term technology that could have a massive impact on the climate.
ML In my view, aviation, the ability to fly around the world to different places, is so important, culturally, socially, economically. Do you think that we are going to be able to crack this?
RM I am optimistic, but we are going to have to fly less in the West. Only 11% of the world's population flew in 2018, and we can't deny the rising middle class in Asia, Africa and South America the right to travel. So even if we massively cut air travel in the West, air travel is going to grow in the world. I'm very positive that when we really put our mind to it, we can produce an aviation sector which has zero climate impact. But there's going to have to be a policy that makes people fly in these new ways, because Jet A is always going to be cheaper as a way of flying. Governments need to think about how they incentivize the right behaviour and new ways of developing policy which can absorb this whole-systems thinking. In some ways, that will be harder than the technology problem.