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re: ideas for fire protection?
8 may 2004
gary   wrote:

>jerry ellinghuysen wrote:

>> we are in the planning stages of building a house in mountainous colorado.
>> house will be extremely fire resistant (metal roof, concrete walls, working
>> on window protection).
>> the weak point is that we have discovered that we will almost certainly want
>> to go pv. currently considering two tracking units.

how about rooftop panels?

>> my question how do you protect this pv system from fire danger?

>here is a thought on a simple protection system:
>
>1) install a drip tube along the top of the pv panels that allows you to 
>run water over the panel surface at a low rate (e.g. 2 or 3 gpm).

you might cool them for more output power or useful hot water.

>2) cover the panels with something like the "fire foil" mentioned in the 
>link above -- i.e. a material that reflects most of the heat away.

i've practiced with "shake and bake" personal wildfire shelters :-)

>space the foil off the panels by a half inch or so to provide a path for 
>the water, and some insulating space.
>
>if you assume that the fire radiates like a 1500f(?) object, and the 
>emisivity of the fire foil type material is about 0.05, and the panel 
>area is about 100ft^2 (both sides) then the heat transfer to the through 
>the fire foil is about:
>
>	q = 0.173 * 0.05 * 100ft^2( ((1500+460 degr)/100)^4 -
>						((100+460 degr)/100)^4)
>         = 127000 btu/hr

that's about 127 pounds of steam per hour, ie 0.25 gpm at 212 f.
 
>if your supply water is 60f, and you want to keep the average 
>temperature under the shield to 160f, you would need a flow rate of:
>
>	q = 127k btu/hr = (gpm*60 * 8.3 lb/gal) (160f - 60f)(1 btu/lb-f)
>
>	gpm = 2.5gpm
>
>at this rate your 1300 gals would last 8.6 hrs.  if the pump could be 
>triggered by a sensor that would detect the onset of fire, it seems like 
>this might be enough?
>
>the basic scheme is that the reflective barrier keeps 95% of the heat 
>from reaching the panels, and the high specific heat of the water allows 
>it to absorb the heat that does get through.

you might mount the panels horizontally, with a reflective wall to the
north to increase the sunpower and a greenhouse poly film air duct over
the panels containing 1-2" of water for panel cooling and water and
space heating. the wall might be metal roofing with reflective mylar
film greased to the south side and a wood's metal link that allows it
to fall over and cover the panels during a fire, at which time water
could pond above the duct, with a float valve to keep the pond full and
an air gap above the pond to allow evaporation...

nick




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