Monday, February 08, 2010

Sustainable Energy – without the hot air Professor David Mackay

OK – two things are definite – we all want our time on this planet to be worthwhile. And we kind of want to have a good conscience about not using up too many of the world’s resources. So a book I am reading is Professor David MacKay’s top selling energy book Sustainable Energy – without the hot air.

But – he is Professor of Physics at Cambridge and so, although I did an O Level in Physics a long time ago, I have to concentrate hard to get his arguments. They involve equations.

I recall my Physics teacher (who played the banjo – remorselessly, and we had to listen to him) telling us that the bicycle was the most inefficient machine known to man.

So how does David MacKay explain the efficiencies of bicycles, which he rates higher than cars?

What is the energy consumption of a bicycle in kWh per 100 km? That’s a long way, to me - I am no Lance Armstrong, but theoretically I am still very interested.

In these equations, Professor MacKay explains, for a car, “4” is used to stand for engine efficiency, p the density of air, d is distance, A is the area of the front of the car, the area A=cdAcar is the effective front area of a car and v is its speed.

Anyway, using lots of equations, but primarily dividing the energy-per-distance of a car by the energy-per-distance of a bike, a cyclist going at 21 km/h consumes 3% of the energy per kilometre of the lone driver on a motorway, about 2.4 kWh per 100 km. So the fuel efficiency (your pumping legs) is 30 times better than a cars. David MacKay thinks the area A of a bike is 4 times less than that of a car, as you can fit 4 bikes in the space of one car on a road. You can, but hardly any cyclists ride side by side as it is too dangerous to slow down the cars behind. But I do think 4 cyclists can easily squeeze up past your car waiting at traffic lights, so I will allow him the dividing by 4. He calls these equations a fun means of scaling the efficiencies of bikes and cars. I think they are primarily about making people feel good about using their bikes rather than cars.

If you want to follow all of the equations, I suggest you get a copy of the book – which is packed full of other equations. What I really like about science is that it can prove beyond doubt what your common sense tells you is right and thus provide fuel to combat human nature. Yes, his book aids will power!

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