LETTERS TO THE NEW SCIENTIST
Costly iron age12 November 2005
The only merit in car engines fuelled by nanoparticles of iron or any other metal seems to be that the metal fuel may be safer than hydrogen (22 October, p 34). But hydrogen or carbon monoxide is needed to recycle the fuel and reduce it to its original metal element after it has been oxidised. If hydrogen were used for the recycling, we would be every bit as dependent on it as if the car were running on hydrogen.
Environmentally, the main problem with hydrogen is that you need another source of energy to produce it from water. And the only practical sources of that energy we have so far are coal plants, other hydrocarbon-burning plants, nuclear power plants and hydroelectric dams.
To produce CO requires the burning of hydrocarbons or coal. And the end-product of using toxic CO to reduce the oxidised fuel to pure metal would be CO2.
Skipping the recycling stage is not feasible either. The total process of mining ores, extracting their metals and making nanoparticles from the metals requires more energy than can be extracted from the end product. And I doubt that the problem could be alleviated by scavenging junkyards and dump sites.
Portland, Oregon, US
The use of powdered metal suggests a possible solution to another problem: how to tap into the enormous quantities of energy falling as sunlight on the world's hot deserts.
Sunlight can be concentrated with mirrors to raise steam and generate electricity (www.eere.energy.gov/solar/csp.html) but centres of population, where the energy is needed, are typically a long way from where there is most sunlight. Conduction losses mean that it is not economical to transmit electricity directly over long distances and, while hydrogen may be generated by the electrolysis of water and used as an energy vector, it is awkward to handle and expensive to transport by tanker or pipeline.
If powdered metal or boron can be produced by treating the relevant oxide with hydrogen (or if these things can be made directly from the oxide using the energy of concentrated sunlight), then they may serve as a very convenient medium for transporting solar energy to the places where it is needed. The oxide produced when the metal or boron powder is burned may be collected and returned to the solar power plants for recycling.
Menai Bridge, Anglesey, UK
Note: Since I wrote this, I have discovered that my remarks about transmission of electricity over long distances were too pessimistic (see Getting the energy to where it is needed).
If we are going to use powder to propel cars, why not use flour? As any silo worker knows, this provides a very explosive mix when suspended in air, eminently capable of powering an internal combustion engine. This would be greenhouse-neutral and much more efficient than fermenting the grain into ethanol for fuel. The technology required would surely be simpler than the metal-fuelled engine.
University of Sydney, Sydney, Australia
From issue 2525 of New Scientist magazine, 12 November 2005, page 26.
In connection with the second letter, see "Concentrating solar power".