2009-11-06

Where does the energy for a solar-powered mole scarer come from?

This question was given as an example of a poor science question on the today programme today, as the presenters declared that the answer is obvious to anyone with basic literacy and has nothing to do with science or mathematics.

"For every problem there is a solution which is simple, obvious, and wrong."
— Albert Einstein


Taking this as an example of a solar-powered mole scarer, it consists of a plastic spike, an aluminium stake, a plastic head, what looks like a crystalline silicon solar panel and an unspecified 'sonic device'. It also probably contains a rechargeable battery, as moles are crepuscular, but I'll ignore that as unknown.

Unfortunately the manufacturer doesn't specify the dimensions or weight, so I'll just do a finger in the air guess.

Assuming that it's around 200 mm long, 150mm of 25mm aluminium tube is around 100 grams aluminium (0.6 kg/m for 25mm tube). The plastic I'd estimate at 100 grams, and the solar panel around 80mm square. The sonic device is assumed to be a wound copper speaker or motor rather than piezo electric, given that it's producing subsonic frequencies, probably around 25 grams.

Taking embodied energy as 300 MJ/kg for aluminium, 100 MJ/kg for plastics, and 100 MJ/kg for the sonic device (copper and plastic).

The total energy requirement to produce a PV panel is 1,060 kWh/m², though that is for full systems, and we've already accounted for the plastic case, so assume 600kWh/m² = 2.16 GJ/m².
 
Item Unit Energy Quantity Energy
Solar cell 2,000 MJ/m² 0.0064 m² 14 MJ
Plastic body 200 MJ/kg 0.1 kg 20 MJ
Aluminium stake 300 MJ/kg 0.1 kg 30 MJ
Sonic device 200 MJ/kg 0.025 kg 5 MJ
Total 69 MJ

At the moment, none of this embodied energy is likely to have been generated by solar power.

A solar cell that size will deliver around 1.5 watts maximum, and assume a seven year lifecycle and, since we're in northern Europe, a 10% duty cycle ( assuming half the hours a year it averages 20% full power ).

1.5W × (60s × 60 × 24 × 365 ) × 7 × 0.10 = 33MJ

For most small devices which aren't in use every day, a solar panel is closer in net effect to a battery which only works when the sun is shining rather than a power source - like a chemical battery, it has energy put into it in manufacture, and you get the same amount of energy back over its lifespan.

I haven't considered end-of-life costs, but already only a third of the 100MJ energy for the solar powered mole scarer is solar.

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2 Comments:

Blogger phyllisjanes said...

Is the demand enough to justify mass manufacture of these devices?

- solar pv panels analyst

Tuesday, 8 November 2011 at 06:59:00 GMT  
Blogger Pete Kirkham said...

Not really, no.

Tuesday, 8 November 2011 at 19:41:00 GMT  

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