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re: has anyone researched using solar for cement or pottery kilns?
3 jun 2000
>...an 8x8' surface supplying 192k btu/h through a slowly-moving air film
>conductance of 1.5 btu/h-f-ft^2 would need to be 192k/(8x8x1.5) = 2,000 f
>degrees hotter than the air. radiation helps too. 

so it looks like we need more surface or faster air... 

>putting 200k btu/h into 4' of steel through a 1'x1' hot spot would seem
>to make the spot 4x200k/26 = 30,769 f degrees hotter than the steel...

otoh, we might focus the sun onto the whole 8x8' bottom (rusty iron that
absorbs 80% of the sun and radiates with 20% emissivity?) and raise the
reflective insulating hinged cover underneath during the night. if the
heat only needs to travel an average of 2' into the 4'x8'x8' thermal mass,
that lowers the temperature rise to 2x200k/(26x8x8) = 240 f. still large.
at 750 + 240 = 990 f (530 r), we'd lose 0.174x10^-8(0.2)(1450^4-530^4)
= 1511 btu/h-ft^2 by radiation to a 70 f outdoors. that's 96,696 btu/h for
an 8x8' target which needs to collect 200k+99,696 = 300k btu/h of sun from
a 90% secondary reflector using about 300k/250btu/ft^2/0.81 = 1,500 ft^2
of heliostats (eg 24 8'x8' mirrors) with 90% reflectivity. 
 
>...engine blocks might be cheaper than ball bearings,
 
weld them to the bottom plate? if each contains 2 ft^3 of steel with
16 ft^2 of surface in a 4 ft^3 envelope, we can fit 128 of them into an
8' cube, with the oven on top. this would lower the discharge temp drop
to about 192k/(16x128x1.5) = 63 f.

>grainger sells... a $225 224 watt 280 cfm power venter... for use on
>appliances with flue gas temperatures not exceeding 600 f at the blower
>inlet... moving 200k btu/h with a 100 f temp difference might require
>about 200k/100 = 2,000 cfm...
  
we might use 8 power venters to push oven air into the mass, and
let warmer air flow back up through some holes into the oven. this
could help distribute solar heat into the mass, even with no pizza
in the oven. 

so maybe it's doable, albeit unwieldy. 

nick




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