On July 17th Professor Theodosios Alexander (publishes as T. Korakianitis) was interviewed on BBC’s Radio 4 Today Programme and on BBC’s World Update Programme on hydrogen as a future automotive transportation fuel.

Transportation is a very special segment of the energy market, demanding high power density from fuels and power plants. Hydrogen in itself is not a very practical fuel. It is not an energy source, it is an energy carrier, or vector, produced by electrolysis from water, and every time we convert one form of energy into another we lose some of the potential to do work. The electricity to produce hydrogen comes from electric power stations, which have their own energy conversion efficiencies and emissions characteristics.

At atmospheric conditions hydrogen is a gas of very low density, 13 times less dense than air and about 8,500 times less dense than diesel or petrol, implying we need several times the vehicle volume as the additional volume of the required container for any practical trip, making hydrogen unsuitable for longer-distance transportation use. (For shorter trips to the supermarket, electric vehicles are perfectly adequate). Hydrogen gas stored at high pressures requires heavy pressure vessels, and hydrogen storage in hydrides or nanotubes result in similarly heavy containers. Energy conversion efficiencies of fuel cells from hydrogen to shaft power are comparable to those of modern piston engines. Piston engines are more efficient when liquid fuels are injected directly into the cylinder, and hydrogen itself is densest (in order to minimise the storage tank, even in a fuel cell application) in its liquid form. Liquid hydrogen must be stored at about minus 253 degrees Celsius and 1 to 10 atmospheres of pressure, meaning any liquid hydrogen tank must be significantly insulated from heat gain from the environment, again making the tank heavy and voluminous. Heat transfer from environmental temperature to this cold temperature results in about 3% of the stored hydrogen to boil off per day, so that a full liquid hydrogen tank left unused goes empty due to boil off in about one month. It also takes a long time to fill up a hydrogen tank (for instance it takes several days to fill the tanks of the Space Shuttle with hydrogen).

Thus hydrogen can only be considered as a desirable fuel only if there is abundant renewable electricity to the extent that we do not care about the losses in producing, pressurising, storing and perhaps liquefying hydrogen. This is not a likely scenario. Today only 4% of UK electricity is provided by renewable and green sources, and the government has been forced to consider nuclear power plants in order to supplement the output of coal, gas and oil-fuelled power plants and meet both our electricity needs and our commitments to CO2 reductions. The half life of nuclear waste is of the order of the age of the Pyramids in Egypt, committing future generations to handling the nuclear waste of today.

However, in a future where we have exhausted hydrocarbon fuel reserves, our only source of clean and dense fuel suitable for long-distance transportation will be what we can produce from renewable electricity, in this case a hydrogen derivative, where the hydrogen will have been produced from electrolysis of water. The best way to liquefy this future hydrogen in order to increase its energy density and make it suitable for the energy densities required for transportation use is to chemically bond two hydrogen molecules with a carbon monoxide (CO) molecule, thus producing CH3-OH, (methanol), or a carbon (C) and carbon monoxide (CO) molecule, thus producing CH3-CH2-OH or C2H5OH (ethanol), which in the process will also remove carbon dioxide (CO2)from the atmosphere, thus reducing CO2 effects on climate change. This brings us back full circle to typical alcohol fuels (which can also be derived from second generation biofuels). In this case hydrogen is an intermediate fuel in the production of more-suitable alcohol fuels. All these future transportation fuels will be very precious because of the energy we will have invested in them in order to produce them, and the losses in every step of the production process.

The audio of the two segments, courtesy of the BBC, is in the following files (the 1st halves of the two audio files reviewing Nissan's car are identical, the 2nd halves of the audio interviews are different).

http://www.sems.qmul.ac.uk/news/media/TA-BBC-TodayRadio4-2008jul17.mp3 and http://www.sems.qmul.ac.uk/news/media/TA-BBC-WorldUpdate-2008jul17.mp3