Ten Tidbits from passive '85
(Page 2 of 3)
January/February 1986
By J. Parker and P. Shlachtman
[4] Depending on cloud cover, humidity, dust, etc., the effective temperature of the night sky in summer may be 15° to 25°F lower than the air temperature. This potential sink for nighttime radiative cooling is already used to good effect in roof pond systems and is likely to be put to use in the future for other advanced passive cooling approaches. from "Dessicant Enhanced Nocturnal Radiation—A New Passive Cooling Concept," by P. Fairey, R. Vieira, and A. Kerestecioglu of FSEC.
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[5] Plants in sunspaces may reduce the sensible heat output into adjacent areas by as much as 50%. Though there were several contributing mechanisms found during testing, the main factor was transpiration of moisture by the plants. Most of the latent heat of evaporation bound in the moisture was lost to exfiltration and therefore never became available as sensible heat. If, however, humidity were needed in the house, the net loss would be much less. The authors hasten to add that their test results shouldn't be interpreted as an indictment of plants; however, the information should be considered when sizing sunspaces for space heating. from "The Effect of Plants on Sunspace Passive Solar Heating, " by E. Best and R. McFarland of LANL.
[6] Most people have a skin surface temperature of 92° to 94°F. Above that temperature, convective airflow across the skin makes people hotter. At these temperatures, over three-quarters of our bodies' cooling is done by evaporation. Because evaporation is affected by humidity, at 50% relative humidity, air movement can keep us comfortable at up to 90°F, while at 90% relative humidity, it's effective only to 83°F. from a cooling tutorial presented by P. Fairey of FSEC.
[7] Trombe walls are very effective at delaying solar heating until the evening hours, when heat is most needed. Trombe walls built for this purpose should not be vented, should have selective surfaces on the outside, should be about a foot thick and made of the densest material possible, and may be equipped with a single, diffusing glazing in most climates. Trombe walls angled 15' west mixed with direct gain features angled 15° east balance the timing of heat release. from "Advanced Passive Solar Design, " presented by J.D. Balcomb and R. Jones of Balcomb Solar Associates (BSA).
[8] Breadbox solar water heaters typically stratify by 40°F. Because water temperatures are so much higher at the top than at the bottom, these collectors should always have their inlets at the bottom and outlets at the top. from "Stratification and Performance Characteristics of a Solar Breadbox Water Heater, " by Scott Burns, Pacific Missile Test Center, and Elias Zeeni (student) and Hahl Guven (assistant professor) of San Diego Stag University.
