Masonry Magazine August 1981 Page. 13
BIA Technical Notes
on Brick Construction
Brick Institute of America 1750 Old Meadow Road. McLean, Virginia 22102
PASSIVE SOLAR HEATING WITH BRICK MASONRY
UNVENTED THERMAL STORAGE WALLS-PART V
Abstract: The thickness of thermal storage components for passive solar energy systems has typically been determined by empirical methods. The empirical approach is usually sufficient to determine the thickness of the thermal storage component for direct gain systems, unvented and vented thermal storage wall systems, and attached sunspaces. A rational approach to aid the designer in selecting unvented thermal storage wall thicknesses and to describe the mechanics of how the thermal energy wave permeates a thermal storage wall is provided.
Key Words: Absorptivity, bricks, emissivity, energy, masonry, passive solar heating systems, thermal conductivity, thermal diffusivity, unvented thermal storage wall systems.
INTRODUCTION
The thickness for most thermal storage components for passive solar energy systems is currently determined by empirical procedures. Guidelines for empirically sizing and selecting the appropriate thicknesses of the thermal storage components of passive solar energy systems are provided in Technical Notes 43A. The provisions in Technical Notes 43A are for the thermal storage components exposed to sunlight. The designer should keep in mind that the total amount of brick masonry within a building assisted by passive solar heating should be at least one and one-half cubic feet for every one square foot of surface area of glazing being used as the collector. This total amount of brick masonry not only includes the brick masonry exposed to sunlight, i.e.. thermal storage wall, but also any additional brick masonry that may be unexposed to sunlight, i.c., interior walls, floors, fireplace/chimney assemblies, which are used to decrease interior temperature fluctuations by providing a thermal flywheel. The empirical minimum one and one-half cubic feet of brick masonry for every one square foot of collector area should result in minimal interior temperature fluctuations and thus reduce the auxiliary heating system loads.
Empirical procedures for determining the appropriate surface area and thickness of components are usually sufficient for most passive solar energy system applications. The required surface area of unvented thermal storage walls may, however, be more accurately determined using the design procedures provided in Technical Notes 43B. This approach usually requires several iterations, by selecting various thermal storage wall surface areas and calculating the expected performance, until the calculated optimal performance is achieved.
Simplified heat transfer equations may be used to aid the designer in selecting the thickness of unvented thermal storage walls and to give the designer a better understanding of the way thermal energy flows through the thermal storage wall. These simplified heat transfer equations assume sinusoidal heat flow in one direction and steady-state heat loss. These equations also provide a basis for determining thermal storage wall thickness by a rational approach. This approach is based on the thermal diffusivity of brick, which is discussed in Technical Notes 43D. Although the analysis given in this Technical Notes does not predict actual performance, it does provide guidance for selecting thermal storage wall thickness. The accuracy of the prediction decreases as exterior temperatures deviate further from the 24-hr sinusoidal heat flow pattern used in the simplified equations. A typical variation is shown in Fig. 1.
UNVENTED THERMAL STORAGE WALLS
General
Determining how the thermal energy wave permeates an unvented thermal storage wall is relatively easy to approximate by using the simplified heat transfer equations. This procedure may be used to assist the designer in selecting the appropriate thermal storage wall thickness. An accurate analysis would require an hour-by-hour energy balance considering the conduction, convection and radiant heat losses. An hour-by-hour hand calculation procedure is not feasible and the actual exterior temperature and amount of solar radiation for each hour is not generally available, even for computer analysis. Thus, in addition to the assumptions for the simplified heat transfer equations, the average maximum and minimum exterior surface temperatures of the thermal storage wall must also be assumed.
Time Lag
One of the principal factors for determining the thickness of the brick thermal storage wall is the time lag, or the number of hours necessary for the wave of ther-