* Posts by Hunneric

2 publicly visible posts • joined 15 Nov 2016

'Pavement power' - The bad idea that never seems to die

Hunneric

Gym Eco Mode

Actually, some of the machines in my old gym used to have some sort of Eco Mode where some of the energy you fed into the equipment went to reducing the amount of energy the treadmill itself drew from the grid (a bit).

Oh yes, those machines are so excruciatingly poorly designed that even when their whole purpose is to extract 100-200w, from the user, the machines themselves draw a great deal more from the grid. I couldn't find the spec on line but I did find a piece about a similar thing in this link (Daily Mail Warning)

http://www.dailymail.co.uk/news/article-2430771/Worlds-self-powering-gym-uses-energy-WORKOUT-lights-dont-break.html

For those who don't want to contribute towards Lord Rothermere's clickbait empire, the key sentence is this one:

"The new treadmills will also use 30 per cent less electricity and generate enough energy to power their own information screens"... but only if the user output is more than 100w.

And so they bloody should given that an ipad can run its screen, do some useful stuff and still charge it's battery from 12w

Hunneric

Not as stupid as energy generating speed bumps though

Which are another greenwash standby you see all the time - only worse.

https://www.theguardian.com/environment/2009/feb/08/alternative-energy-speed-bumps

Because sure, a 1.5 tonne car driving over a speed bump generates a bit more energy than a footstep but putting a speed bump in the road affects the behavior of the car and it's driver too. It probably slows down from 22mph to about 18mph before it hits the bump, slows a little more as it goes over the hump itself to 15mph and then speeds right back up.

So, assuming that the bump generates 20 times as much as the pavement -160w perhaps, for half a second or so - that's a whopping 80 joules in the bank. But that energy generation is dwarfed by the amount of energy the car uses to navigate the hump itself. If the 1.5 tonne car accelerates back from 15mph to 22mph between humps then its velocity changes by 3.1m/s. Just the change in kinetic energy is 7.35kJ. Assume a car is 40% efficient at turning dinosaurs into kinetic energy and the speed bump is the worst energy laundering system in the world.

Assuming my fag packet maths are right (and I'm assuming someone will correct me if they're not), you need 18,000 Joules of dirty energy to make 80 Joules of clean energy.