Ten Tidbits from passive '85

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A number of significant new developments and discoveries were presented at this year's meeting of the Passive Division of the American Solar Energy Society.

[1] Natural convective airflow is a very effective, self-balancing means of heat transfer in passive solar buildings. A 2° to 12°F temperature difference between rooms can produce a heat transfer rate of 2,000 to 20,000 Btu/hr by means of airflow through natural architectural features. This transfer rate is essentially proportional to the square root of the temperature difference between rooms. Often, this natural distribution is more than enough to keep all spaces comfortable.

Convection seems to be driven by temperature stratification of air in the room with the heat source—a sunspace, for example—so fans that force air down from the ceiling to reduce stratification can actually reduce heat transfer to adjacent rooms. What's more, fans used to force air from one room to another are often ineffective; in one house tested by Los Alamos researchers, the fanned heat transfer rate was actually lower than that produced by natural convection.

In most cases, doorways are adequate for convective airflow. Paired high and low vents in sunspaces are not necessarily more effective than doors; if the vents significantly reduce stratification, heat transfer may be cut. from "Natural Convection Airflow and Heat Transport in Buildings: Experimental Results, " by J.D. Balcomb and G.F Jones of Los Alamos National Laboratory (LANL).

[2] Thermosiphoning air panels prove more efficient when air passages are made as wide as four inches. Narrower air passages were found to reduce flow rates through (and therefore heat output from) the collector. The location of the air passage—in front of, behind, or on both sides of the absorber—had little effect on performance. However, a matrix of expanded aluminum placed in front of the absorber, so that it was near the absorber at the bottom and near the glazing at the top, proved to be the most efficient configuration. from "Measured Performance of Thermosiphon Air Panels, " by T. Allen and J. Hayes of Marlboro College.

[3] The optimum configuration for earth cooling tubes in moderate climates is 12" to 18" polypropylene pipe, 300 to 500 feet long, buried 12 feet deep. Cool tubes can be effective from about Atlanta, Georgia, northward, but relative humidity in the house may increase beyond a tolerable level. For top performance, the tubes should be cycled on no more than half the time using a fan of about 1,000 cfm capacity.—adapted from preliminary simulations done by R. Vieira, P. Fairey, A. Kerestecioglu, and S. Chandra at the Florida Solar Energy Center (FSEC).