Wednesday, 4 February 2015

Being realistic about energy harvesting

It is possible to harvest small amounts of energy in surprising ways - but just because it is possible doesn't mean it is worthwhile. I see that Pavegen are shortlisted for a trial on the Canary Wharf estate [1]. Pavegen make floor slabs that harvest energy from our steps. Does that sound like a good idea to you?

My beloved wrote about this before. He estimated that each step would generate at most 10 Joules of energy. I gather actually it is about 7 Joules[2].

Suppose you get one person walking over the slab every 12 seconds, for 12 hours a day. Not many places will have a higher footfall than that. For comparison, this is about twice the rate at which people go through ticket barriers at Kings Cross railway station [3]. At that rate each Pavegen slab generate 2.6 kWh/year [4]. One kWh of electricity currently costs about 14p so you get electricity worth 36p/year.

Each slab is about 0.25 m2[3] and the whole floor costs £1250/m2 [5] so that slab that earned 36p/year has cost £310. The payback time is 860 years.

By the way you can get paving slabs for a patio for about £25/m2.

The same area (0.25m2) of solar panels would generate about 31 kWh/year, or twelve times as much at a quarter the cost [6]. People think PV is expensive! Pavegen is a gimmick, and an expensive one at that.

Of the other companies that were shortlisted, there were several that generate energy from solar power including Polysolar. Their product produces power from transparent windows, by harvesting only some of the light that goes through. I like that a lot more.

[1] Green building innovations compete for Canary Wharf trial 27/Jan/2015

[2] Watt Now reports that the slabs are rated for 6W at 50 steps per minute. That means each step generates 6*60/50 = 7.2 Joules

[3] Kings Cross has 30.555 million passengers/year (see Pedestrian energy isn't a walkover) through 50 ticket barriers so 1670 passengers/barrier/day. One every 12 seconds for 12 hours would be 60/12*60*12 = 3600 passengers/barrier/day.

[4] 12 hours * 60 minutes/hour * 60/12 times per minute * 7 = 25,200 J/day
= 25,200*365/ 3,600,000 kWh/year = 2.6 kWh/year

[4] Harvesting kinetic energy from every step (Pavegen)

[5] The floor tiles that use foot power to light up cities (Guardian) 11/Jan/2015

[6] PV system sizing and yield calculator Assume £2000/kWp.

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