The issue of kinetic road plates came up again today. Doesn’t it sound like a great idea – harvest energy from cars passing over a compressible ramp in the road to drive a generator?  Free energy, as long as people keep driving cars.

But hang on – where is this free energy coming from?

Well, since energy cannot be created nor destroyed, it clearly must come from the car’s engine as the only power supply in the vicinity. In order to compress the ramp the vehicle must first drive up it’s incline. The increase in gravitational potential energy created by raising the mass of the car is then converted to kinetic energy as the ramp gently compresses and drops the car back down to road level, driving a generator to create electricity. But, in order to raise the car up the ramp in the first place consumes more fuel, exactly like driving up a hill.

So, rather than harvesting free energy, the kinetic plate is actually stealing a little bit of energy from each car passing over it. That energy must be replaced by burning more fuel; it’s just that the amount of fuel taken from each car is so small that you’d be unlikely to notice.

Now one of the first adopters of this technology is a national supermarket chain – very clever – the supermarket also sells fuel from its own petrol station, so they are onto a double winner. The store gets a free supply of energy from its customers and then sells more energy to the same customers to replace the energy it has stolen from them.