There are several experimental hydrogen-powered aircraft, but they are not in commercial use. The object is to reduce pollution. Conventionally fuelled aircraft engines emit pollutants including carbon dioxide, nitrous oxide, sulphur dioxide, carbon monoxide, unburnt fuel and particles including soot.

If hydrogen is used in place of kerosene in a conventional jet engine, the only pollutant emitted is nitrous oxide. The exhaust is mainly water vapour.

A better way is to supply the hydrogen to a fuel cell, where the hydrogen is combined with oxygen from the atmosphere to produce electricity.

The exhaust is water vapour, which is so clean it can be drunk.

The direct current produced by the fuel cell is converted to alternating current electronically and used to power one or more brushless electric motors to drive a propeller or ducted fan.

The output of the fuel cell may be supplemented by a rechargeable lithium-ion battery to provide additional power when needed, for example, durlng take-off.

However, there are a number of problems. Hydrogen is a gas at everyday temperatures, so to carry enough on an aircraft to give a reasonable range you have to compress it and store it in heavy pressure vessels or cool it to below minus 253℃ to convert it into a liquid.

You can store much more by liquefying it than by compressing it. When liquefied, hydrogen is only a third of the weight of conventional fuel, but it occupies four times the volume.

The fuel tanks for liquefied hydrogen have to be slightly pressurised, which makes them spherical or cylindrical. These can't be carried in the wings as in conventionally fuelled aircraft and must be stored in the fuselage or external streamlined tanks.

A liquid hydrogen fuel tank has to be well insulated to slow the rate at which the hydrogen boils off and to prevent ice forming on the tank.

Hydrogen as a gas is not available in significant quantities so has to be manufactured. Most of it is made by a chemical process from natural gas or charcoal.

The first produces carbon dioxide as a by-product and the second results in carbon monoxide, so neither can be considered green. Hydrogen can also be produced by running an electric current through water, but this is much more expensive than other methods.

Researchers are working on non-polluting methods of producing low-cost hydrogen, together with a means of storage that does not involve low temperatures or high pressures.

Biofuels produced from organic matter may prove to be a better alternative.

Found in the Daily Mail's "Answer to Correspondents" column, Wednesday, 31st October, 2018, contributed by Denis Sharp of Hailsham, East Sussex.