The whole roofing situation is an expensive problem to solve. You can't
just penetrate a roof of any sort without extensive sealing, etc. All that
costs money. And even if the collectors are anchored parallel to the roof
- perhaps even attached closely to it, the risk is still there that there
will be a pressure differential between upper and lower surfaces that will
make the collectors fly off - perhaps taking some of the roof with them.
As for build-ups of snow and ice; ice being somewhat transparent, will
allow some amount or solar heating to take place within the collector, much
the way it does in your car during the day. Snow too - depending on how
much, may allow this, and at 45degrees tilt, there may be enough melting to
allow it to slide off. All this assumes some degree of thermal insulation,
which defeats the purpose in summer when you need flow-through of air (but
not dust).
As with all solar applications, there are trade-offs. Last thing you (I)
want to do is climb up on the roof to fix things so they work. Could be
that a separate, fairly low (accessible) "billboard" set-up is best - not
too many billboards fly away, because they're designed not to - but again
there's a cost factor.
P.
At 01:56 PM 1/1/2007 +0100, you wrote:
----- Original Message -----
From: "Stephen A. Lawrence" <[EMAIL PROTECTED]>
To: <vortex-l@eskimo.com>
Sent: Sunday, December 31, 2006 5:51 AM
Subject: Re: [Vo]: Musings on grid-independence and personal alternative
energy
>
>
> thomas malloy wrote:
>> Stephen A. Lawrence wrote:
>>
>>>
>>>
>>> John Steck wrote:
>>>
>>>> I might be mistaken, but I think there is a significant performance
>>>> drop at
>>>> low temperatures with solar cells... someone please correct if wrong.
>
> Groping around on the web I ran into an off-hand claim that they
> actually perform better at low temperatures, here:
>
> http://www.wagonmaker.com/power.html
>
> Haven't dug into it any farther at this point.
They do perform better when they are cooled.
>
>
>>>
>>> What's worse, there's a totally catastrophic performance drop when
>>> they're covered with 2" of snow, or an inch or two of ice.
>>
>> Sounds like a job for electrical heater cables to me.
>
> That, plus a 45 degree tilt, might be all that's needed.
Ideally they should be pointed towards the yearly average of the sun's
position in your sky, so in the northern hemisphere they should point
south and be tilted wrt the horizontal with an angle equal to the latitude
(0° at the equator, 90° at the north pole)
>
> Then we'd just need to be sure they were well enough anchored to the
> roof so that the tilted solar panels wouldn't turn into a "solar sail"
> and fly away in the first storm that comes along...
In this respect, if your roof already has a pretty steep pitch, you might
be better off laying them flat on the roof as Robin suggested.
Michel
P.S. Wrt your earlier remark about clouds, I believe PV works rather well
in diffuse light too.
>
>>
>>
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>>
>
