The Electro-Kinetic Bandwagon

My first reaction on reading an interesting Guardian CiF piece by Professor David MacKay discussing a method of scavenging kinetic energy from motor vehicles was that the energy belongs not to the scavenger but to the owner or user of the vehicle who, rather than have it scavenged without permission, may want to scavenge it themselves. They can do this by, for example, employing a KERS (kinetic energy recovery system), or “regenerative braking”, as is being done now by some F1 teams, as well as so-called “hybrid” cars, and as is likely to be standard on motor vehicles within a decade or so.

I was therefore somewhat bemused to read that Sainsbury’s intends to install a system to capture kinetic energy from cars using its supermarkets. It seems Ealing Council is also about to shell out large sums on the deployment of “electro-kinetic” technology in place of traditional speed-bumps. And I happen to have an indirect interest in how Ealing Council spends public money!

My second observation was that speed bumps don’t actually slow vehicles down by making them drive up a little hill. We’ll see later that simple physics demonstrates that this couldn’t possibly be the case. No, you slow down because driving fast over speedbumps is bad news for your car and could even harm the people inside it, let alone any fragile objects in the boot.

Still, I suppose it’s possible that a different kind of speed-bump could slow you down by extracting energy. As the inventor’s website points out , in FAQ number 1, the only sitings of the electro-kinetic device that won’t steal energy from the vehicle are those where the car is braking anyway. Let’s hope potential customers have thought this through. If the car isn’t braking anyway, stealing its energy and converting it to electricity is incredibly inefficient. Most of the energy in the fuel has already been lost through inefficiencies in turning hydrocarbons into carbon dioxide, water, trace pollutants and kinetic energy. Most of that kinetic energy will be lost converting it to electricity.

But could the “electro-kinetic” technology even extract enough energy from cars? Will it provide the electrical power output that is claimed?

Now, the term “electro-kinetic” seems more 1909 than 2009. In fact, the technology is 19th century as well, as YouTube shows. Rocket science this isn’t. Nevertheless, we can apply the same laws of physics to “electro-kinesis” as to rocketry.

Like any good MBA, I started out by looking at the financials. The Guardian reported back in February that:

“The ramps – which cost between £20,000 and £55,000, depending on size – consist of a series of panels set in a pad virtually flush to the road. As the traffic passes over it, the panels go up and down, setting a cog in motion under the road. This then turns a motor, which produces mechanical energy. A steady stream of traffic passing over the bump can generate 10-36kW of power.

The bumps can each produce between £1 and £3.60 of energy an hour for up to 16 hours a day, or between £5,840 and £21,024 a year. Energy not used immediately can be stored or fed into the national grid.”

I wondered how much power you’d have to extract from each vehicle to achieve even 10kW for 16 hours a day. Let’s assume one vehicle passes every 10 seconds on average, that is 360 per hour. We have to extract 10kWh/360 of energy from each vehicle to make the projected return, that is around 28Wh of energy. Is that feasible?

Maybe we ought to turn things round and see how many vehicles we’d need. Back to Professor MacKay’s article where he writes:

“Let’s guess that the kinetic road plates extract one fifth of the kinetic energy of the arriving car. For a car weighing one tonne travelling at 20mph when it hits the road plates, the extracted energy comes to 0.002 kilowatt-hours (kWh).”

It appears we have a slight order of magnitude problem. If we only extract 2Wh of energy from each vehicle, we need 14 times (my 28Wh/David’s 2Wh) the 360 vehicles per hour that I assumed. That is, 1.4 cars per second for 16 hours a day. This seems a big ask. Remember the old “2 second (from the car in front) rule” of road safety campaigns? And we can do some more arithmetic: 20mph (let’s be generous and call it 36kph), is equivalent to 36000m/3600s, that is 10mps (metres per second). That is, we have to have one car every 10 metres all day.

Let’s carry on, though. David has guessed “that the kinetic road plates extract one fifth of the kinetic energy of the arriving car”. Now, that seems a lot me. I’m not a physicist, undergraduate level maths for biologists is as far as I ever got, but I do recollect that kinetic energy is a square function (1/2 mv^2 if I remember rightly). So to lose 1/5th of my energy is equivalent to losing around 10% of my velocity. As a driver I imagine I’d have to hit a rather large kangaroo to be slowed by 10% from 20mph. And a little thought about the other term in the kinetic energy equation – mass – suggests that in fact I’ve underestimated. Applying conservation of energy, a car weighing 1 tonne would have to collide with a very large kangaroo, in fact one of 1/5th the combined mass of the car and the kangaroo – 250kg – to lose the energy required, assuming the car and the kangaroo end up travelling with the same velocity after the collision. Hey, who says physics can’t be fun?

How much is 2Wh anyway?  Remember, we’re generating this every second when a car passes over the “electro-kinetic” device, so our power output is 7.2kW (note we’re already a bit less than the 10kW claimed).   In other words, the energy scavenged from a car should be able light 72 100W lightbulbs – the kind that are so extravagent they’ve now been banned – or 7.2 single-bar electric heaters.  Suspicious?  I am – it may not be Hiroshima, but it’s a spectacular energy conversion nonetheless.

The description of the “electro-kinetic” device tells us that the principle is in fact to extract potential energy from the car, rather than kinetic energy directly. The car’s kinetic energy is used to drive up a ramp, which falls, driving the mechanism in a pit below. This is most clearly seen in the YouTube video. The picture illustrating David MacKay’s CiF piece doesn’t really tell you much. In fact, I’m beginning to wonder if that picture actually shows the final product – it may just be one of those metal plates that are often used to stop people and cars falling into holes in the ground!

How far, then, would a 1 tonne car have to fall to release 2Wh of energy? The relevant equation is that for potential energy:

PE = mgh

where energy is in Joules, m is in kg and h in metres.  g is the gravitational constant, approx 10ms^-2.

As every Cambridge physics professor knows, 1Wh = 3600J.

Therefore we have:

7200J = 1000kg * 10 ms^-2 * h

h = 7200/10000m = 0.72m or 72 centimetres.

Yeap, if my arithmetic is correct, with perfect efficiency your car would need to drop 72 centimetres to provide 2Wh of energy to an electro-kinetic device. Or to slow down by 10% from 20mph – this calculation explains my hunch about how speed-bumps actually work!

Now, if I was going to assemble the best engineers to design such an “electro-kinetic” device, I wouldn’t promise more than around 30% efficiency.  But even if the electro-kinetic engineers can exceed that dramatically, your car would have to drop at least 1 metre to provide 2Wh of energy.  You should experience a short period of freefall on the way to Sainsburys.  But don’t worry, it’s all in a good cause!

Have another look at the YouTube video. It looks to me like the car drops more like 10 centimetres. And the prototype is not driving 72 100W lightbulbs, but 10 of what look like 12W halogen lamps, which only light up when a car is actually passing over the ramp. At least the video only claims “500-800W” output. It’s not clear whether this is the peak or average – which rather matters. Even if we’re talking about continual output, how could the prototype be scaled up to “5 to 10kW”? The only way I can imagine is – rather than dropping cars through 1m – to drop them through 10cm 10 times, i.e. to bump along a series of electro-kinetic ramps. Pack your eggs carefully!

I have to say that my conclusion is to suspect a similar mix of well-meaning, but naive public procurement crashing into commercial interests to that which has led to the biofuel disaster. Garnished with the hopeless optimism that accompanies new technologies, of course. During the dotcom boom I saw at least one sales estimate that was overoptimistic by 2 orders of magnitude (100 times), so I wouldn’t be entirely surprised to find a similar phenomenon affecting “green” technology. I hope Sainsburys and Ealing Council have validated the claims for “electro-kinesis” – and only deploy it where cars have to brake, providing an escape route, of course, for vehicles with KERS!