Powerful step

A powerful step

The other day I was discussing some different ways to power up electronic devices. Interestingly the question came up in a few different forum with different colleagues, independently of each other.

Basically, I was wondering how much power can be harvested – today – when walking? With comparatively standard components.

For example, assume you are out walking. How much energy can you harvest? And use for any type of electronics that you carry with you? Well, there are many sides of that coin: losses in material, how fast you walk, how much the center of gravity is lifted above the ground (less than you think, since we walk in a pendular motion), etc., etc.

First, however, take a look at these links:

and get inspired how things could be done. I like the first one! A way to harvest energy from the people in e.g. a city. They mention for example that 0.5 % of the total energy consumption in a city could be retrieved this way. Perhaps not a substantial part, but at least a contribution. The last one, a bit more "futuristic" offer really high (?) numbers: 7 W.

Academic

Let’s get a bit academic. Look at these papers (and thanks to Martin Nielsen Lönn for digging these out for me) that use slightly different approaches to harvest energy (PZT films and cantilevers).

  • Shenck and Paradiso: "Energy scavenging with shoe-mounted piezoelectrics"
    in 2001 obtain
    8.4 mW in average.

  • Mateu and Moll: "Appropriate charge control of the storage capacitor in a piezoelectric energy harvesting device for discontinuous load operation" in 2006 obtain 100 µJ per step.

  • Moro and Benasciutti: "Harvested power and sensitivity analysis of vibrating shoe-mounted piezoelectric cantilevers" in 2010 obtain something like 450 µW in average.

With a bit of creative thinking and assuming two shoes and room for a couple of these energy harvesters perhaps we can reach numbers in the order of say 25 mW – comparatively "easy". Read more about Paradiso’s and Shenck’s work at

They have a nice list of additional literature to study. Read also about their self-powered wireless pushbutton controller.

Or why not support these guys?

even though I am not sure the project is still up and running.

How much could you get?

tigger-bouncing

Well, how much energy could we get from a walking human being? Let us assume we are not walking. Instead we are constantly jumping around. (A bit like Tigger in Pooh). Assume we lift our center of mass a meter high every jump. Assume we have a well-built person at a stable 100 kg and that he lives in a deep hole where the gravitational force is 10 m per second square.

In this case, the potential energy will build up to

  • W = mgh =
    100 * 10 * 1 = 1 kJ

Assume we do one jump a second and we end up at an average power of

  • P = 1 kW

which is a quite impressive number to be honest! Yes, will be a bit exhaustive after a while. Not only would it look stupid, it would also be a very inefficient and the (jumping) human race is probably extinct by now.

A nice compilation is given here

where you interestingly find that walking requires more power at higher speed than running. With the example person above, at 6 m/s (s)he would require 1 kW. Now, 6 m/s is quite fast, approximately 22 km/h and once again we would look silly at that pace.

Plugging in more modest numbers, assuming a 1-m long leg and "normal" take-a-walk-to-town pace (not the shopping mall pace, which is a factor 100 less), the power becomes something like 100 W in average power according to the source.

path2985-9

Adding all those losses, friction, efficiency one can probably quite drastically slash that number down by a factor 100 to get realistic levels that a harvester can extract for your electronic gadgets: 1 W.

A number that aligns with the links first reported – to some degree. I guess it opens up for the next post. What can we do with 1 W?

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5 thoughts on “Powerful step

  1. Pingback: How much is 1 W | Mixed-Signal Comments

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