[Greenbuilding] Fresnel lenses box solar cooker

[Topher]

On 6/3/2013 11:28 AM, RT wrote:
> On Mon, 03 Jun 2013 09:24:34 -0400, Topher <topher at greenfret.com> wrote:
>
>> On 6/2/2013 7:08 PM, RT wrote:
>>>  he did mention "concentrating photovoltaic (CPV)" systems that were
>>> becoming viable which utilise Fresnel lenses to concentrate sunlight
>>> so that solar arrays could be reduced to the size of [his] thumbnail.
>>
>> I don't think I want one on my roof though.  My current plans are for
>> about 2000 watts of cells.  That's 8000 watts or so of heat that needs
>> to be dissipated. I come back to the thing I have been saying for 
>> years.  If you aren't
>> making solar CHEAPER, you aren't helping.
>
> I'm not a gizmologist so I've not looked into CPV systems but I do 
> know that the efficiency of PV cells in general drops as the 
> temperatures to which they are subjected, increases.
Yes.
> As such I would imagine that the people developing CPV systems have 
> addressed temperature issues.
Not sure that follows, but even at the theoretical efficiency limit of 
PV, there is going to a *lot* of waste heat in a system like that.

> If I were doing it, I'd certainly be looking at how to utilise the 
> "waste" heat for some beneficial task, say like heating or pre-heating 
> for a DHWH system.
Presumably.  Not a simple engineering problem though.  8,000 Watts = 
7,500 BTUs / sec = 90 gallons per sec (at 10 degree rise).
>
> Also, I'm pretty sure that I remember my neighbour saying that CPV 
> systems are typically designed as tracking systems, my guess is that 
> they'd be dual-axis tracking to maximise efficiency. If that's the 
> case, it's unlikley that there'd be any contact between the PV array 
> and the roof so I doubt there'd be any heat related issues WRT the roof.
The CPV I have seen were all large commercial operations.  So it is not 
clear that it helps homeowners at all.  The reason to use a roof in the 
first place is that it is already shading.  Moving the PV somewhere else 
doubles the shading.  tracking is /already/ more expensive than just 
adding more panels.

> From what I've seen the major cost of any solar power plant has been 
> the cost of the PV panels.
Not anymore (at least for small systems).  For the most recent estimate 
I saw, balance of system was /twice/ the cost of the panels.  YMMV.

> I don't recall their being particularly pricey so I don't expect that 
> the addition of Fresnel lenses to a PV In array would be much of a 
> factor in increasing cost.
The problem is not just the cost of the lenses.  In order to get the 
same output as a 20 meter^2 PV panel array, you need 20 meter^2 of 
Fresnel lenses; and some means of holding it very steady in the wind, a 
precise distance away from the collector; plus be able to move the whole 
unit (including the cooling system), very precisely throughout the day, 
every day.

Gizmological, as you say.

Thank You Kindly,

Corwyn

-- 
Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
http://www.GreenFret.com/
topher at greenfret.com

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