A concrete-block thermal mass with large air-concrete surface area

David Delaney
Ottawa, November 14, 2003

This page describes the concrete block part of the two-part heat store described in A two-part heat store with large heat-transfer surface area, large thermal mass, and low resistance to air flow [1]

I spent some time with a CAD program searching for a suitable design for a thermal mass constructed from common concrete building blocks. The thermal mass needed to be highly permeable to air flow vertically and horizontally in any north-south vertical plane, and needed to have a large air-concrete surface area per block.  This note presents the best tradeoff I could find between permeability, a large air-concrete surface area, and stability.  The block stack described here has an average air-concrete surface area of 3.46 ft2 per 8"x8"x16" concrete block. I found one stacking method that produced 4.5 ft2 per block, but it looked too unstable.

Warning: I am neither a professional engineer, nor do I have professional training in the design of structures. Use this design only after you have consulted a professional engineer about its safety and conformity with your local building code.

A thermal mass having any amount of surface area for heat transfer can be constructed from any suitable material simply by using enough pieces of the material and arranging them in space so that air can circulate around them. The difficulty is to get the required surface area without using too much mass or space, or paying too much for containers and supporting structures.  Big pieces of material have a smaller surface area per unit of mass than small pieces. A thermal mass constructed with big pieces of material will require more mass of the material to achieve a given surface area than a thermal mass constructed with small pieces of the material.  As pieces of material accumulate, it is usual to get to the needed thermal mass long before getting to the needed heat transfer surface area, unless quite small pieces of material are used.

Although a concrete building block has a much smaller ratio of surface area to mass than a small stone, a concrete block stack has the advantage over a bin of small stones that it provides its own supporting structure. It may be enclosed by insulated walls that need to support no more than themselves and the roof of the enclosure. A bin of stones needs strong walls.

If a stratified heat store is needed, the thermal mass has to admit air easily through at least one of its vertical sides. For my purposes, both the north and south sides have to be highly permeable.  The requirement for permeable sides complicates the design of a bin of stones. By comparison,  a concrete block stack can easily be made permeable to both horizontal and vertical air movement.

The construction of a concrete block stack uses material handling procedures  that are familiar to all builders. Very few builders are familiar with the construction of stone bins.

The thermal mass described here consists mainly of an extremely common concrete building block, the 8"x8"x16" two core stretcher block. (The holes in a concrete building block are called "cores"). Blocks weighing 32 pounds each are assumed here. Less dense, lighter, blocks of the same dimensions are available. They are less desirable for this application. The supplier may deliver light blocks unless the 32 pound weight is specified. Heavier blocks may be available, and may be more desirable if they have the same surface area, and depending on their cost.

In North America, the dimensions of concrete building blocks include an allowance of 3/8" for grout. The actual dimensions of an 8"x8"x16" block are 7-5/8"x7-5/8"x15-5/8".  Since the thermal mass is dry stacked (no grout),  I use actual dimensions here.

I tried designs that had a larger surface area than the one presented here. They all looked too unstable. This design is stabilized by including some extra-long "blocks" in the form of bond beams like the ones masons construct as lintels to hold up the mass of masonary above a window. Bond beams are assembled from concrete bond beam blocks,  steel reinforcing bar (rebar), and grout.  A bond beam block (specifically a knock-out bond beam block) looks like this:
Knock-out bond beam blocks will be considerably easier to use than the more common U channel bond beam blocks, because it will be easier to keep wet grout from falling out of the ends of the beam. The only grout used in the construction of this otherwise dry-stacked thermal mass is the grout that fills the bond beam blocks. The bond beams are easily constructed in place after laying the bond beam blocks and before laying the layer above them. The bond beam blocks are firmly supported by the layer below them before they are tied together by the rebar grouted into them.

Layer patterns and masses.

Analysis of air-concrete surface area.

Analysis of permeability.

A specific design might use one or several stacks like the one described here, or the stacking method described here might be adapted to produce stacks of other sizes.

For the context of this work see references [1], [2],[3], and [4].

[1] A two-part heat store with large heat-transfer surface area, large thermal mass, and low resistance to air flow http://geocities.com/davidmdelaney/thermal-mass/two-part-heat-store.html
[2] Solar thermal energy for housing
[3] Thermal mass of drums of water on top of a stack of concrete blocks
[4] Organizing the air flow between a thermosyphon solar air heater and a thermal mass located above it, http://geocities.com/davidmdelaney/flow-organiser/flow-organiser.html

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