Driven by fears of anthropogenic climate change, whether real or imagined, attempts are continually being made to reduce the use of fuels that result in emissions of carbon dioxide and replace these with other sources of energy that do not, at least directly, result in such emissions. Although much progress in fuel efficiency for all kinds of transport has been made in recent years, there are physical limits to how efficient conventional technology can be. The aim of eliminating internal combustion engines altogether is now not merely a desire but a specific goal, with governments in France and the UK now committing to eliminating sale of all diesel and petrol cars and vans by 2040. Such an ambition might even be possible, for example through the use of hydrogen fuel cells and battery electric storage, although at a substantial cost. Whether similar goals are achievable for other kinds of transport, in particular air transport, is much less certain.
Two major factors to consider when replacing liquid fuel sources with other sources of energy are storage and transport. Storing gaseous fuels is more problematic than liquid fuels because, to contain useful amounts, gases need to be pressurised, typically to very high pressures. Liquefied petroleum gas (LPG), for example in the form of propane or butane, requires storage in pressurised tanks that can themselves be many times the weight of the gas being stored. Eliminating carbon from the fuel completely and using hydrogen instead, which can either be burnt in a modified internal combustion engine or reacted with oxygen in a fuel cell to generate electricity, results in even more problems with storage. A typical storage tank may need to contain hydrogen at a pressure of hundreds of times atmospheric pressure to be able to contain enough gas to power a vehicle for a reasonable range. Containing and transporting gas at such enormously high pressures poses substantial problems, and requires an entire rethink of distribution infrastructure, with safety being an obvious and very significant factor. Replacing petrol stations with hydrogen stations is a task that requires huge investment, and has so far shown little signs of happening any time soon, even though the technology has developed to a stage where it has at least be shown to work in practice.
Electric storage using batteries, the current favourites being based on lithium ion cells, is currently more often cited as being the future of transport, at least for ground-based vehicles, largely due to the apparent success of Tesla electric powered cars. It is worth looking in more detail at how the amounts of energy compare between conventional petrol powered vehicles and current electric vehicles, before seeing whether similar principles could be applied elsewhere. A typical 35 litre tank on a reasonably efficient petrol powered car (such as the Lexus CT200h hybrid, which I happen to drive) will contain about 1,300 MJ of convertible energy (based on an energy density of around 45 MJ/kg for petrol, with a density of around 0.8 kg/l), for a total weight of around 35 kg, including the tank in which the petrol is stored. Now compare this to an electric car such as the Tesla Model S, the latest version of which contains a battery weighing 540 kg and having a storage capacity of 85 kWh, equivalent to 306 MJ. Given the higher efficiency in converting stored electric energy into motion, both cars can travel similar distances (about 300 miles) before needing to refuel. The major difference, however, is that when the Tesla runs out of fuel the battery weighs the same, while the petrol car weighs about 30 kg less. This ‘dead weight’ can be managed in cars, where the weight of the vehicle is around four times the weight of the battery, and with clever design and weight distribution the effect can be made unnoticeable in practice. For airborne applications, however, this dead weight places the idea of replacing liquid fuels with electric storage into fantasy land rather than reality. To take a current example, the Airbus A320neo is currently the most efficient commercial passenger aircraft, and has a fuel capacity of around 21,000 kg. This is equivalent to a fully fuelled energy capacity of around 900,000 MJ, around 700 times the energy stored in the above mentioned petrol powered car. Replacing this with stored electric energy, even accounting for an increased energy efficiency similar to that between petrol and electric cars, would require around 250,000 MJ in electricity to be stored. Using the same batteries as in a Tesla would require a total weight of around 440 tonnes. Given that the empty weight of an Airbus A320neo is around 44 tonnes, an aeroplane with such a battery would clearly never get off the ground. To even consider powering such an aircraft with electricity alone would require an improvement in battery technology resulting in an increase in energy density of somewhere between ten and one hundred times, and even then the problem of having the same weight of battery on landing as at take-off would remain. At the moment therefore, replacing aviation fuel with electricity is firmly in the realms of science fiction, at least for commercial passenger aircraft. Nevertheless, some progress is being made with small aircraft powered solely by electricity and capable of travelling short distances, for example as currently being demonstrated by Munich-based company Lilium.
For larger passenger aeroplanes, current research is focusing on hybrid-electric distributed propulsion systems, using aviation fuel as an energy source with a combination of conventional gas turbine engines, electric machines and electric storage. As with hybrid cars, this is not going to eliminate use of fossil fuels but may result in further improvements in efficiency. Given that we cannot rely on advances in electric storage technology of the magnitude required, what options remain for future air travel if fossil fuel use eventually needs to be eliminated? The answer, as suggested in the title, may well be nuclear.
Although nuclear power on board an aircraft has actually been tried, as demonstrated in the 1950s by the Convair NB-36H, its use for direct aircraft propulsion has never been a practical proposition. As well as the requirement to provide heavy shielding against radiation, the safety considerations of having a nuclear reactor on board are such that the idea is unlikely to be realistic for the foreseeable future. The idea of nuclear power is not, however, impractical because it can be, and in a small way is already, used instead to generate liquid fuel instead.
The advent of gas-to-liquids technology, developed by Shell over the past few decades, has shown that natural gas, principally methane, can be used as a feedstock for the creation of synthetic liquid fuels. Synthetic petrol is already a part of everyday life, with part of the fuel sold on forecourts in the UK today created from natural gas in plants in Qatar at prices that are competitive to those of fuel derived from petroleum. The gas-to-liquid process starts with generating what is known as synthesis gas, a mixture of hydrogen and carbon monoxide created by partially oxidising natural gas. This mixture is converted into liquid hydrocarbons using a catalyst and the resulting product is converted into the desired liquid fuel through the conventional petrochemical processes of cracking and isomerisation. All this technology is currently available and commercially viable. Although it uses natural gas as a source, this is only one step away from being entirely removed from needing fossil fuels, given that methane (CH4) is only one carbon atom away from hydrogen. To remove fossil fuel as a source needs only an alternative source of energy. Moves are being made at the moment to replace conventional aviation fuel with entirely synthetic fuels, using wind power as a source of electrical energy to split water and use the resulting hydrogen as a starting point. This is not going to be a practical proposition for generating anything more than a small fraction of current aviation fuel usage, given the very low energy densities of wind and solar. It does, however, point towards a more practical option that is already available with current technology.
A single kilogram of uranium, enriched to the level required for nuclear fission to take place in a reactor, can provide somewhere between 5 and 80 million MJ before being reprocessed. This is around a million times greater in terms of energy density compared to liquid fuels, and is available essentially without the need for any emissions of carbon dioxide. Using nuclear power as a source of energy, in the form of electricity and heat, could be used to provide the final bridge for gas-to-liquid technology to create realistic amounts of entirely synthetic liquid fuels, using only water and a source of carbon (for example atmospheric carbon dioxide) as a starting point. Using entirely synthetic fuels avoids all of the fundamental problems involved in replacing such fuels with electric storage, and results in no net emissions of carbon dioxide. Nuclear powered aircraft may therefore be the future, just not in the way you might expect it to be.
Although much progress in fuel efficiency for all kinds of transport has been made in recent years, there are physical limits to how efficient conventional technology can be.