Design Con 2015

An energy-harvesting scheme that is nearly useless?

-October 23, 2013

Energy harvesting is a hot topic, and it should be. In many cases, it lets a circuit get "free" energy that is both available and also would be thermally dissipated or otherwise wasted. Examples include using ambient vibration to power data-gathering sensors via piezoelectric elements, or collecting RF energy in the air for similar uses.

But there are also times the harvesting tag is used, I think, as a come-on for projects that are of very dubious usefulness, validity, or technical reality. It's even more attractive if you can also associate with another hot topic, such as hybrid vehicles.

One such case is the energy-harvesting bicycle, developed a few years ago at MIT and now getting serious funding. It's certainly an interesting engineering-design challenge, but as a real product: sorry, you can count me out.

Yet the Boston Business Journal recently reported that Cambridge, MA-based Superpedestrian has received a little over $2 million to commercialize its so-called "hybrid bicycle" which turns any bike into an electric hybrid (see "Spark backs $2.1M round for Superpedestrian"). It does this by using a special rear wheel, which they call the Copenhagen Wheel, to capture the energy lost during braking.

The Copenhagen Wheel from Superpedestrian is designed to turn any bicycle into an energy harvester via regenerative braking.

We know that regenerative braking is a valid way to capture energy, as millions of HEV and EV cars have shown. But that's a very different situation. On a bicycle, most of the energy that the rider expends is lost to rolling friction, mechanical friction, and especially air resistance, all of which are non-recoverable. Air resistance is the worst loser, since its losses go up as the square of the speed, and thus a significant amount of pedaling energy can never come back.

Further, most bike riders simply don't use their brakes that much, if they are paying attention to the road and overall situation; instead, they adjust their pedaling cadence to slow down. Sure, there are times when you have to brake, and even brake hard, but how much energy is there to recover that way?

Nonetheless, the attraction of getting something for nothing, whether real or imagined, is very strong. If this design did the obvious, and used the special rear wheel as a small generator, I could understand. Modest pedaling requires between 50 and 150 W, so it would be possible to expect the rider to do another five to ten percent and get some modest energy to store - but then, that would be real work on the rider's part, instead of the lure of getting free energy via regenerative braking.

I suspect that the added weight of the special Superpedestrian wheel requires more energy to keep going than the system can capture, if you do a proper energy input/output balance. These so-called hybrid bicycles may seen trendy and virtuous, but the laws of physics ignore those aspects.

What does make sense are the many electric bikes I have seen where a medium-size rechargeable battery is clamped to the down-tube or seat-tube of the frame, and the rear hub is replaced with a small motor. This allows the rider to pedal but also get an assist, or stop pedaling and go 10-15 mph (15-25 km/hr) while taking a pedaling break.

Have you seen trendy schemes which didn't work, or can't pass the "makes sense" reality test? Have you been involved in any? How did that work out?

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