Masonry Magazine August 1981 Page. 15
TEMPERATURE
120
110
127.0 F
106.57
97.0
100
90
TEMPERATURE
FLUCTUATION
925F
85.5
80-
81.0
72.0
70-
71.5 F
67.4
58.5 F
60-
64.7
52.0
51.0
50-
50.6
40-
30
24.8
20-23.0
10-
0
40.3
Calculated Temperature Fluctuation and Temperature Gradient
for a 12-in. Thermal Storage Wall in Washington, D.C.
FIG. 3
EXTERIOR AIR FILM
DOUBLE GLASS
AIRSPACE
BRICK THERMAL STORAGE WALL
INTERIOR AIR FILM
TEMPERATURE GRADIENT FOR
MAXIMUM EXTERIOR DESIGN
TEMPERATURE
TEMPERATURE GRADIENT FOR
MINIMUM EXTERIOR DESIGN
TEMPERATURE
tively, the maximum and minimum interior surface temperatures might be expected to occur at approximately:
Wall
Thickness
Max. Temp.,
occurring at:
8 in.
9 p.m.
12 in.
12 midnight
16 in.
3 a.m.
24 in.
9 a.m.
Min. Temp..
occurring at:
I p.m.
4 p.m.
7 p.m.
1 a.m.
The equation for time lag assumes a uniform sinusoidal thermal energy wave and will thus only be accurate when the maximum and minimum temperatures occur 12-hr apart. This information does provide guidance for selecting wall thickness. The 8-in. wall will probably not provide sufficient time lag to have the thermal energy reaching the interior when it is most needed. The 24-in. wall will probably have a time lag which does not allow the thermal energy to permeate the wall prior to the wall being recharged by the solar radiation the next day. However, in areas where day-to-day solar radiation levels are not sufficient to adequately supply the desired interior temperature, thick walls, 20 to 24 in. thick, may be desirable. Typically, walls appear to be the optimal thicknesses when satisfactory time lag is achieved for diurnal cycles. However, the time lag is not the only factor used to determine wall thickness. The interior surface temperature of the thermal storage wall should also be considered.
Exterior Surface Temperatures
The minimum and maximum exterior thermal storage wall surface temperatures must be estimated in order to calculate the interior surface temperature fluctuation. The interior surface temperature fluctuation may be determined by using the simplified heat transfer equations.
Minimum. The minimum exterior surface temperature may be approximated by determining the steady-state temperature gradient through the thermal storage wall system from the air film at the interior surface of the brick thermal storage wall to the air film at the exterior surface of the glazing material. To determine the temperature gradient the total resistance of the thermal storage wall system, the interior design temperature, and the exterior design temperature must be known. The minimum average daily exterior temperature for the month of January, provided in Table 1 of