10 Quick Tips Regarding Passive Solar Energy

Catch up on the developments and discoveries presented at the annual meeting of the American Solar Energy Society Passive Division.
By the MOTHER EARTH NEWS editors
January/February 1986
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In terms of energy return, it's best to place solar panels on the west or east sides of homes.
PHOTO: FOTOLIA/ELENATHEWISE


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A number of significant new developments and discoveries regarding passive solar energy were presented at the Passive Division of the American Solar Energy Society meeting in 1985.

1. Natural Convective Airflow

Natural convective airflow is a very effective, self-balancing means of heat transfer in passive solar buildings. A two degree to 12 degree Fahrenheit 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

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. Efficient Earth Cooling Tubes

The optimum configuration for earth cooling tubes in moderate climates is 12 to 18 inch polypropylene pipe, 300 to 500 feet long, buried 12 feet deep. Cool tubes can be effective from about Atlanta 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). 

4. Nighttime Radiative Cooling

Depending on cloud cover, humidity, dust, etc., the effective temperature of the night sky in summer may be 15 to 25 degrees Fahrenheit 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. 

5. Optimal Sensible Heat Output

Plants in sunspaces may reduce the sensible heat output into adjacent areas by as much as 50 percent. 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. Effects of Relative Humidity on Skin Temperature

Most people have a skin surface temperature of 92 to 94 degrees Fahrenheit. Above that temperature, convective airflow across the skin makes people hotter. At these temperatures, more than 3/4 of our bodies' cooling is done by evaporation. Because evaporation is affected by humidity, at 50 percent relative humidity, air movement can keep us comfortable at up to 90 degrees Fahrenheit, while at 90 percent relative humidity, it's effective only to 83 degrees Fahrenheit. — From a cooling tutorial presented by P. Fairey of FSEC. 

7. Utilizing A Trombe Wall

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 degrees west mixed with direct gain features angled 15 degrees 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. Selecting A Good Breadbox Solar Water Heater

Breadbox solar water heaters typically stratify by 40 degrees Fahrenheit. 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. 

9. Cooling With A Radiant Barrier

Radiant barriers are one of the most effective means to passively cool buildings. A layer of aluminum foil (one that's highly reflective to far-infrared radiation) placed next to an air space in the attic of a home can cut overall heating and cooling loads by 10 percent in southern climates and by 5 percent as far north as Chicago. The cooling capability of R-19 insulation and a radiant barrier system is roughly equal to that of R-30 insulation. The material used to coat fiberglass board ducting — made of aluminum, kraft paper, and fiberglass reinforcing — makes a good radiant barrier and can be purchased for less than $0.05 per square foot.

Ventilation is still the most effective means of passive cooling, but the absorption of moisture into (and desorption from) building materials leads to overestimates of its effectiveness and underestimates of indoor humidity. A building opened for night ventilation and actively cooled during the day will have a significantly higher relative humidity all times than one that is continuously air-conditioned. Wind-induced ventilation is the most effective form (followed by cross ventilation enhanced by wing walls) and will easily overwhelm stack effects. — From a cooling tutorial presented by P. Fairey of FSEC, and "Advanced Passive Solar Design, " presented by J.D. Balcomb and R. Jones of BSA. 

10. Solar Panel Placement

 In southern regions, such as Florida, more solar radiation falls on the north wall than on the south wall of a building from May through July. But by far the bulk of the solar gain comes from the east and west. For that reason — and because so much heat gain comes from diffuse radiation-southern shading overhangs are not a panacea for cooling. External window shades, reflectors, and vegetative shading on at least the east and west windows are the most effective techniques. — From a cooling tutorial by P. Fairey of FSEC, and "Vegetative Shading . . . in Warm-Humid Climates," by J. Parker and P. Shlachtman of Florida International University. 


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