Mysterious power generation

By Marcus Wilson 30/04/2012

One consequence of being a physicist is that you can’t go anywhere without seeing physics calculations that need doing. I’ve just been to our library hunting down books on the medical technique of transcranial magnetic stimulation (TMS), which was an interesting exercise in itself, since one textbook I found also has a chapter on homeopathy. Hmm. So how much do I trust its section on TMS then?

Anyway, our new, glorious library and student centre gets better every time I go in. Not only has a new cafe opened up inside, but there are now screens telling you just how eco-friendly the building is being right now, by displaying data on the building’s power consumption, solar power generation, water consumption and water capture, etc.

So, in the last month (which I assume means April) the building has used 182 792 kWh of power (that certainly makes my electricity bill look tiddly!) but has generated a cool 1 847 531 kWh from its solar panels. Now, I know April has been unusually sunny this year (shame the sunshine couldn’t have come in summer when it was supposed to) but there is something clearly wrong with this figure.

One metre squared of area, under full sun, gets about 1 kW of power on it. That means in about an hour it captures 1 kWh of energy. I don’t know how much of the building is covered in solar panel or other capture device, but I reckon the footprint of the building is about square with a side of 40 or 50 metres, so let’s say about 2000 m2 in roof area. So, if that were covered in solar panel, under full sun it would capture about 2000 kWh in one hour. In April there are 720 hours, so that gives us 1 440 000 kWh of energy.

But I’ve assumed that the panels are illuminated 24 hours a day! That’s clearly rubbish. So halve that, since half the time it’s night. In April, the sun isn’t anywhere near the zenith, so let’s halve that again. We are down to about 400 000 kWh. Then there are cloudy days (albeit not too many this month) which will take it down again, and still a large factor to apply because the power conversion from light to electricity or hot water isn’t 100% efficient. I reckon we might be down to a more reasonable estimate of 100 000 kWh.

So what does the 1 847 531 kWh represent?  It’s either a mistake, or I’m misinterpreting the display.


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