RogerM":39deyek2 said:
I don't think panels are any different to other electronic devices in this respect in that they are more sensitive when kept cool.
Their internal resistance rises with temperature, I believe.
... the sensitive collectors on space telescopes are cooled with liquid helium that keep them at maximum sensitivity.
That's to reduce thermal noise induced in the CCD arrays that are the sensors. It's been done that way for decades, and you can buy bolt-on
Peltier heat pumps for the sensors on amateur telescopes to do the same thing. In fact you usually can't get very good (astronomic) results without them (but there are simple ways of removing *some* noise via software).
It's a confounded nuisance in my Canon 30D camera. For stupid "marketing" reasons, all the major camera manufacturers have settled on black as a camera colour, and sunshine makes mine go very noisy in the darker parts of the picture. I take spherical panoramas as a hobby, and exposure is always a compromise. I found out with great sadness last year in the Italian Lakes, that the thing is pretty useless in direct summer sunshine if I want shadow detail. There's no easy way round the problem - keeping the camera cool causes condensation on the cold surfaces. I may yet cover it in baking foil!
I believe that there is a new generation of panels that are water cooled to improve efficiency, and that the power consumption of circulating the water is more than made up by the greater efficiency caused by the cooling.
It's a very sensible idea, but another way of looking at it is that the conversion efficiency of photovoltaic solar collectors is poor, so you can improve the efficiency by heating water in the same panel simultaneously.
Bluntly, photovoltaic performance is pisspoor. Yahoo Answers* give the theoretical energy density of sunshine on the Earth's surface (avg. in the USA) as roughly 1kW/sqm. Let's be reasonable here: assume 50% efficiency as a conversion target, that's say 6kWh per 1sqm panel per day.
What's the area of solar panels on your roof and where do they point?
In contrast, our solar water heating is pretty good in optimum conditions like today. I usually have the cutoff temperature in the tank set at around 65degC, for human safety (for some reason, people don't expect scalding water from the hot taps!).
Assuming a cold start for the tank (which isn't uncommon as I have a bath and the rest of the family showers first thing), it's usually at storage temperature by lunchtime (midday BST) in the summer:
Code:
Start temp = 15deg
Cutoff = 65 degC
.: delta = 50degC
Tank capacity: 300 litres
.: Energy input by lunchtime = 15,000 kcal
or = 62.76 MJ
or = 17.43 kWh
Obviously it doesn't have internal resistance to worry about!
So in theory it could do about 35kWh for roughly half the year,
if I could store the hot water, and if it was as efficient as that on cloudy days (it isn't). But it would also run a lot hotter too, if it was safe to do so, so I've no idea how efficient it could really be.
We have 8sqm on the roof, so that's:
17.43(kWh) / 6(hours) / 8(sqm) = 0.36kW per panel.
I don't think 36% efficiency is at all bad for a system that cost about £3,000 all-in, and for which the only bit that'll ever wear out is a bog standard central heating pump.
Someone please check my maths, and someone, please do the sums for photovoltaics!
E.
*can't be bothered to search further!
:!: You could have a rainwater harvesting system - channel all your household roof water, and roof water from your 'shop, greenhouse, garage into one channel to feed it, and put a waterwheel on it!
