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.

62

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