MIL-HDBK-1003/19
allowing all possible combinations of those variables (Note: 3 x 3 x 3 x
3 = 81 combinations). The principal design variables and associated
values are:
Am/Ac = 3, 6, 9
THICK = 2, 4, 6 (inches)
R-value = 0, 4, 9 (deg.F-ft2-hr/Btu)
NGL = 1, 2, 3
where Am/Ac is the ratio of the thermal storage mass surface area to the
solar collection area, THICK is the thermal storage mass thickness in
inches, R-value is the thermal resistance of the solar aperture with night
insulation in place, and NGL is the number of glazings in the aperture.
The thermal storage mass in the direct gain systems is high density
concrete with the following properties:
density
[rho] =
150
lb/ft3
specific heat
c
=
0.2
Btu/lb-deg.F
thermal conductivity
k
=
1.0
Btu/deg.F-ft2-hr
The concrete is assigned a solar absorptance of 0.8 and an infrared
emittance of 0.9. Twenty percent of the transmitted and internally
reflected solar radiation is assumed to be absorbed on non-massive surfaces
and rapidly convected to the room air.
Properties of other building materials that can provide thermal storage
are listed in table VI. Any of these other materials can be substituted for
the concrete in the reference designs. The procedure is to simply select
the reference design that has an EHC closest to the design under
consideration and the same NGL and R-value. It is not necessary to match
the parameters THICK or Am/Ac. If the contemplated design does not have
an EHC close to one of the reference values, linear interpolation may be
employed on the values of F and G.
If interpolation on the EHC is used to determine F and G, then Uc is
read from either of the reference designs involved in the interpolation.
(The values of Uc will be identical because both systems involved must
have the desired NGL and R-value.) The best estimate of [alpha] is obtained
from the reference design having the desired NGL and an Am/Ac ratio
closest to the design under consideration.
The effect on performance of decorative coverings placed over mass
surfaces is included in the analysis by multiplying the EHC by the factor:
[alpha]/(1.31 [multiplied by] Rd + 0.8) ,
(Equation 5.9)
where Rd is the thermal resistance or R-value of the decorative covering
and [alpha] is the solar absorptance of its surface; this factor was derived
on the basis of steady state energy balance research reported in the ASHRAE
Journal.
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