‘Let’s put things in perspective,’ he says over the phone from his temporary home in Stanford. ‘The total contribution of aviation to industrial greenhouse gas emissions is two per cent. That’s not much, but the emissions take place at high altitudes and we don’t know what impact they have. There is a lot that remains unclear about the chemistry of the trails that planes leave behind them in the sky. So aviation has a limited impact on climate change, but a real one. We have to do our best to minimize that impact. And there are geopolitical considerations too. If, for whatever reason, the oil supply is interrupted, all our aircraft will be grounded with serious consequences for the economy.
But in the medium term of, say, the next ten to twenty years, aviation will continue to depend on fossil fuels. Their high energy density (a high
volume of energy per kilogramme) and ease of storage is unrivalled. In addition, aircraft engines already work extremely efficiently with these fuels – it would hardly be possible to make them any more efficient. One improvement that can be made on medium-range flights is by using turboprops instead of jet engines, as these have higher efficiency.
I also expect gains to be made by reusing the waste heat produced by the engines. Because even in the most efficient jet engine, 50 percent of the energy is lost in the air as heat. By using special fluids and turbines in the organic Rankine cycle, we can convert some of that heat into power. In trucks it is possible to recover up to 10 percent of the energy in this way.
If, for some reason, we needed to switch to other energy sources, then I think it would be much easier in other sectors than in aviation. In the energy sector (27 percent of total greenhouse gas emissions ed.), we see that change already happening with an increasing share of solar and wind energy. The transport sector (accounting for 12 per cent – ed.) will use electric power in the future. I became convinced of that during my sabbatical in California. Of course, the air travel industry must do what it can to reduce greenhouse gas emissions too, but even so, there is much less potential for this than in other sectors.’