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Perpetual Motion Redux

CNN online has a regular feature, under the heading Explorers, in which British inventor Trevor Baylis critiques invention ideas suggested by readers. I’ve read the column a few times and Mr. Baylis seems both fair and rigorous in his assessments. He also brings the true inventor’s sense of economics to bear: often his judgments on the viability of an invention are based more on the likelihood anyone would buy it or on the cost to bring it to market than on its technical viability.

One invention he panned on that basis involved a way to generate “free” energy. The would-be inventor knew enough about electricity to know that a magnetic field moving across a wire would induce current; his suggestion was to embed wires under the road and put magnets on the bottom of cars — the magnetized cars moving across the wires would generate electricity, thus tapping the unused potential of all those cars moving back and forth across the country to solve our energy problems. Mr. Baylis pointed out (correctly) that the expense of digging up all those roads and retrofitting all those cars probably made such a scheme far too costly to be practical.

I thought Mr. Baylis did not go far enough in his rejection of this idea. The problem is that this is yet another variation on perpetual motion — and people who think perpetual motion is possible will never believe it is not cost-effective. How can free energy be too expensive to use?  It is yet another symptom of the general scientific illiteracy that plagues the modern world. I sent this to CNN to augment Mr. Baylis’ critique and to encourage him, in the future, to put as much emphasis on the scientific as on the economic.

13 December 2004

Ref: Mr. Baylis’ critique of the extraction of energy from moving vehicles using magnets and a street wire grid.

Mr. Baylis’ critique, while valid in every respect, was primarily economic and will therefore leave some readers (particularly those whose belief in the laws of supply and demand and pricing are ephemeral, and those who believe that moral righteousness is reason enough to ignore any and all economic considerations) thinking that overcoming those merely economic obstacles should be a high priority if the result is “free” energy.

However, there is a more fundamental matter of physics with which any such scheme must contend: the energy is not free. Any energy generated by such a system must come directly from the engines of the cars. Having a grid sucking energy from moving vehicles would have the same effect as riding the brake in those vehicles or driving them uphill (or perhaps more evocatively, it would be as if they were driving into a fierce headwind) — the generators would put a drag on the cars, and the engines would need to work harder to overcome that drag. Electricity production would go up in proportion to the loss of gas mileage.

Since the internal combustion engines in cars are generally considered one of the least efficient ways of converting fossil fuels into energy (both in terms of energy per unit output and in terms of the resulting pollution), the scheme would actually likely result in a net loss.

The one condition under which this scheme could possibly work is if the energy-sucking grids were strategically positioned only in places where people were already trying to shed energy — in places where they are already using their brakes (for example on steep downhills and while approaching red lights — but not green lights). Under those conditions, the energy grid would act like an auxiliary brake, transferring some of the energy wasted by braking into a useful form. However a much more efficient mechanism for utilizing that energy is the “recapture” braking systems proposed for electric powered vehicles (and hybrids), which use electrical generators to provide the braking force, directing the energy of the vehicle back into the battery where it may be used again the next time the vehicle accelerates.

As the laws of economics are, alas, quite often considered mere conventions or evil machinations by the kinds of true believers who posit such perpetual motion schemes, it is important that, wherever possible, such economic arguments be buttressed by explanations of the underlying physical constraints that prevent such schemes from working as envisioned.

© Copyright 2004, 2005, Augustus P. Lowell

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