# Kinetic plates

When this “kinetic plate” was installed in 2009, the Guardian published an article which suggested that it would harvest up to “30 kWh per hour” of “green energy” from the traffic passing over it. Rubbish, of course. Firstly (as was acknowledged in a muted disclaimer at the foot of the article) it wasn’t free energy; it was energy taken from the passing vehicles (and thus paid for by their drivers). But what about the 30 kWh per hour claim? That’s the equivalent of harnessing the output from engine of this Peugeot and running it flat out for 15 minutes in the hour.

Really? We can do some quick sums on this. Say the car, with its driver, weighs about 1,400 kg. Suppose that it depresses the plate 10mm (0.01m). If we take gravitational constant as 9.8 N/kg, the energy imparted by the car as it drives onto the plate is 1,400 x 0.01 x 9.8 = 137.2 joules (watt-seconds). That is only 0.000038 kWh. In other words, you’d need getting on for eight hundred thousand cars an hour to achieve 30 kW output, even if the mechanism were 100% efficient, which it won’t be.

Along similar lines the IMechE published an article about a kinetic pavement in 2015. This related to a system for capturing energy from pedestrians, and rather usefully it included some statistics: that 54,267 footsteps had generated 217,028 watt-seconds. I hope all my readers can confirm for themselves that this equates to a mere 0.06 kWh.