Masonry Magazine August 1981 Page. 14
TEMPERATURE IN F
160
140
120
100
80
60
40
20
SINUSOIDAL ACTUAL
APPROXIMATION FLUCTUATION
4A.M. NOON BPM. 4AM. NOON BP.M. 4 A.M. NOON
TIME
Exterior Surface Temperature Fluctuation for a Thermal
Storage Wall
FIG. 1
mal energy to permeate the wall. The time lag provides the designer with an approximation of when the useable heat on the interior surface of the thermal storage wall will be available. The time lag may be determined by using Equation 1.
t = (w/2) x √[(π×8)/n] (1)'
where: t = Time lag. in hours.
w = Thickness of brick thermal storage walls, in feet.
8 = Thermal diffusivity, in sq ft/hr.
n = Constant, 24, for a daily cycle, hours.
π = Constant 3.1416.
The thermal diffusivity may be determined by using Equation 8 or Table 1 of Technical Notes 43D. Values for time lag for typical brick masonry materials are given in Fig. 2.
Consider the thermal storage wall shown in Fig. 3, constructed of face brick and thus having a thermal diffusivity, 8, of about 0.024 sq ft per hour. Using Equation 1 or Fig. 2, the time lag may be approximated.
For an 8-in. wall, the time lag is calculated to be:
t = [(8 in./12 in./ft)/2] x √1/[(3.14× 0.024)/24]
t = 0.33 ft x √ 318.47 hr²/ft²
t = 5.89 hr or 5 hr and 53 min
'From Reference 6
20
BUILDING BRICK
15
PAVING BRICK
TIME LAG. IN HOURS
10
FACE BRICK
GROUTED HOLLOW
BRICK
12
16 20
24 28
THICKNESS IN INCHES
Time Lag vs. Thickness
FIG. 2
Similarly:
12-in. thick wall: t = 8.92 hours or 8 hr and 55 min
16-in. thick wall: t = 11.90 hours or 11 hr and 54 min
24-in. thick wall: t = 17.85 hours or 17 hr and 51 min
The time lag is an important factor when considering the thickness of the thermal storage wall. The thickness should be selected so that the heat is retrieved from the wall prior to the wall being recharged the next day and to provide a source of radiant thermal energy when the energy is most needed. If the maximum and minimum exterior surface temperatures of a thermal storage wall are being achieved at 3:00 p.m. and 7:00 a.m., respectively.