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An Energy-Harvesting Scheme That Is Nearly Useless?

Energy harvesting is a hot topic, and it should be. In many cases, it lets a circuit get free energy that is not only available, but 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 the tag 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 Superpedestrian of Cambridge, Mass., 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 the company calls 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.

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 the 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 seem trendy and virtuous, but the laws of physics ignore those aspects.

What does make sense are the many electric bikes I have seen in which 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?

Editor's note: This article was originally published in EDN .

10 comments on “An Energy-Harvesting Scheme That Is Nearly Useless?

  1. Hailey Lynne McKeefry
    October 30, 2013

    I have to agree with you, Bill…dubious at best. I see similar trends in the research field. I read about studies being developed and funding and i think who would pay for that? Apparantly, we are willing as a greater community. I'd be interested in knowing if anyone has, at first glance, dismissed the importance of a technology and later looked back to realize it had changes the world. anyone?

  2. Daniel
    October 30, 2013

    “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.”

    Bill, now a day's peoples are aware about need of energy harvest and various initiations are happening at state/federal government level. For us now (India) it's the time of celebration “Festival of lights”. I had seen in many common places (Public museum, art galleries, palaces etc), where they deployed energy harvesting bicycle. Visitors can ride the bicycle and energy thus created is storing and using for lighting the illumination.

  3. ahdand
    October 31, 2013

    @Hailey: I think there are many who has rejected or ignored technology at the very 1st instance and then realized the importance of it.         

  4. ahdand
    October 31, 2013

    @Hailey: Investing on training and research is a very good move indeed but doing it in a very effective manner is something which has to be monitored closely.              

  5. ahdand
    October 31, 2013

    @ Jacob: Yes indeed since there are many festivals that represent many things and that sets the whole country on light. A good sign for many indeed.

  6. Himanshugupta
    October 31, 2013

    Hailey, You are right that government fundings in these technologies are due to far greater community advantage than for real known advantages over existing technologies, but over time people find usefulness of such technologies. I think energy harvesting technologies is a vast field. The main problems are very limited energy being harvested, storage and price.

  7. FLYINGSCOT
    October 31, 2013

    When I was younger I read about aircraft survivability system whereby the planes wing was build in detachable sections whereby if one wing was damaged (say from unfriendlies) then the other good wing could be damaged intentionally to balance up the flying surfaces.  however when the system was tested instead of the small wing tip being detached by use of a small explosive the whole blinking wing was blown off th the plane crashed.  Whoops.

  8. Hailey Lynne McKeefry
    October 31, 2013

    @Jacob, and why do they ride the bikes? Is it wanting excercise? Or wanting to help? I'd be interested to know the impetus.

  9. Hailey Lynne McKeefry
    October 31, 2013

    @flyingscott, that's a real cautioinary tail about the need for testing!

  10. Daniel
    November 4, 2013

    “Yes indeed since there are many festivals that represent many things and that sets the whole country on light. A good sign for many indeed.'

    Nimantha, if festival organizers are able to follow it, then its fine. Under normal circumstances, it won't last more than one or two festival season.

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