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re: solar heat storage..
25 feb 2001
mike russon   wrote:

>i am currently constructing a solar heating system using air as the
>liquid. in designing a storage bin, would it be wise to not only
>insulate the exterior of the enclosure but also line the interior using
>a heat relflective polyisocyanurate insulation to increase r value as
>well as prevent out gas problems from the wood sides?

outgassing from wood seems an unlikely problem, but the foil would add
about r3, with an airspace on both sides, from the calc below for 1"
atlas energy shield "r7.2" polyisocyanurate foamboard with and without
its 2-sided foil, using equations derived from pages 46-67 of kreider
and rabl's heating and cooling of buildings (mcgraw-hill, 1994.)


10 data 0.05,0.05,1,1'emissivities with and without foil
20 s=1.714e-09'stefan-boltzman constant (btu/h-r^4-ft^2)
30 tin=130'internal temp (f)
40 tou=70'external temp (f)
50 rb=7.2'bulk r-value of foamboard (us--ft^2-f-h/btu)
60 for p=0 to 1'calculate heatflow with and without foil
70 read ein,eou'emissivities
80 hri=4*s*(tin+460)^3/(1/ein)'internal radiation conductance (btu/h-f-ft^2)
90 hro=4*s*(tou+460)^3/(1/eou)'external radiation conductance (btu/h-f-ft^2)
100 hci=.19*dtin^(1/3)'internal convection conductance (btu/h-f-ft^2)
110 hco=.19*dtou^(1/3)'external convection conductance (btu/h-f-ft^2)
120 ri=1/(hri+hci)'r-value of interior surface (ft^2-f-h/btu)
130 ro=1/(hro+hco)'r-value of exterior surface (ft^2-f-h/btu)
140 i=(tin-tou)/(ri+rb+ro)'heatflow from inside to outside (btu/h)
150 dtin=i*ri'interior surface temp difference (f)
160 dtou=i*ro'exterior surface temp difference (f)
170 if abs(i-il)/i>.01 then il=i: goto 100'iterate to 1%
180 rt=(tin-tou)/i'overall r-value (ft^2-f-h/btu)
190 print tin,tin-dtin,tou+dtou,tou,rt
200 next

inside        inside        outside       outside     overall
air (f)       surface (f)   surface (f)   air (f)     r-value

130           119.2         81.2          70          11.4

130           125.9         75.2          70           8.5

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