Sunday, 24 June 2012

Pedestrian energy isn't a walk over

Ticket barriers at Kings  Cross -  a potential energy generator
It is possible to generate electricity by capturing 'spare' energy from people walking about.  Simon Langton Grammar School now has some floor tiles which do just that (see  Could footsteps ever power Olympics? from the BBC June 2012). However, it is hard to see how the yields from this sort of technology can ever be more than a gimmick. This is what my beloved has to say about it - most of the rest of this post is his.


Consider a one-person size  600 mm x 600 mm (2' x 2') paving slab. Basic physics means that each time the slab is depressed the maximum energy produced is F*s (force times distance travelled). I guess the maximum travel must be about 1 cm, anything more is going to feel like a very wobbly paving slab, and risks trips and falls with subsequent law suits. As for the force, to make life easier, I'll presume 1000 N. This is the weight of a 102 kg (16 stone) person (who'd have to be taller than 2.02 m (6' 7.5") to avoid being overweight). This is larger than most people, but as I'm after an upper bound, this will do.

Each depression therefore supplies 10 J. In reality, some of this will get put into the spring mechanism that restores the slab to an undepressed state, and no generator is going to be 100% efficient, but I'll ignore that and presume it generates 10 J of electricity.

The next problem is how often does it get pressed? Wikipedia tells me that a military quick march is about 120 strides per minute (it varies by regiment). Let's presume we have an array of slabs, and a vast horde of people doing a quick march over this array, spaced 1 slab apart in both directions with their left and right feet marching on adjacent columns of slabs. This way every slab is depressed 120 times a minute, twice a second, generating 20 W, equivalent to ~0.5 kWh/day. Remember that this is an upper bound. [Nicola adds, UK total energy usage is 80 kWh/person/day]

In reality, people do not weigh 102 kg and act like a May Day parade in the old Soviet Union, but this gives us a good upper bound for peak power generation per slab when deployed in a pedestrian area. However, it's nowhere near a realistic average rate of generation. Rather than try to apply guessed fudge factors to the peak rate, I'll look at an example of a place where lots of people go and where they can be funnelled in a way that means they'll walk on a slab.

Wikipedia tells me that last year King's Cross station had 26.255 million entries/exits and 2.15 million interchanges, which corresponds to 30.555 million passengers getting on/off trains there. This is fractionally less than the 31.536 million seconds in a year, but close enough that we can presume one passenger per second arriving or departing on average. More realistically assuming that the average passenger weighs 600 N (61 kg) and can be manoeuvred to step on a slab, that would give us 6 W on average. However, in reality it's not safe or even feasible to bottleneck all passengers through a 600 mm gap. The best we can do is to put the slabs where they have to pass, which means the ticket barriers. There are approximately 50 of these in the station, which means an average power rate per slab of 6 J every 50 seconds, or 0.12 W, 2.88 Wh/day, or fractionally over 1 unit of electricity per year. Considering this is in a place with guaranteed footfall where people can be funnelled onto the generators, unlike most places, this is about the best we can hope for.

1 unit of electricity costs about 14p at consumer rates even for 100% renewable energy (prices from Good Energy). The business rate for non-renewable energy will be about 4-5p/unit depending on contract. If we want this slab to pay for itself financially within  a likely 20 year lifetime year it needs to cost no more than £1.00, including installation. Also there is the embodied energy to worry about.

For comparison, solar panel installations these days cost as little as £6300 for a 2 kWp system which in the Cambridge area will yield 1600 kWh/year. That is about £3.90 per 1 kWh/year generated. To be competitive with solar panels, which most people consider expensive and is subsidised by the Feed in Tariffs, the paving slab which generates 1 kWh/year, with all its special electrics and installation would need to cost no more than £3.90. You'd be hard pushed to get just an ordinary slab for that.


Getting energy from people walking is a fun experiment but it isn't a practical way to generate renewable electricity.

2 comments:

  1. About as realistic as Daniel Rirdan's Blueprint in other words.

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  2. I am not a fan of Daniel Rirdan, but I hope you don't mean to discount all renewable energy? For an excellent overview of what is potentially useful and what isn't I recommend the dothemath blog. Tom Murphy summarises renewables and other alternative energy sources here: http://physics.ucsd.edu/do-the-math/2012/02/the-alternative-energy-matrix/

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