Air Conditioner Shading

Air Conditioner Shading

A study by the Florida Solar Energy Center (FSEC) on the effects of shading outdoor air conditioner condensers indicates that energy savings benefits are modest at best--less than 5%. Although the study involved just three sample cases, and researchers describe the experiments as somewhat inconclusive, the results suggest that if shading is not done properly, the risk of interrupting air flow to the condenser could more than offset any benefits.

FSEC's recently released report, Measured Impacts of Air Conditioner Condenser Shading, states that an extensive literature search uncovered no empirical research on the specific issue of condenser shading, although estimated energy savings based on assumed temperature reductions have varied from 2% to 10%. According to the report, "In spite of the lack of information, the recommendation to shade air conditioner condensers is often repeated and widely perceived as important."

FSEC's study involved experiments on three Central Florida homes, each of which had an unshaded condenser located on its west exposure. Air conditioner energy consumption was measured for a four-week period before and after each condenser was shaded, and the study was performed over the course of two summers. In each location, the condenser was shaded during midsummer, and the landscaping or shading device was designed and installed with the intent to provide as much afternoon shade as possible, without obstructing air flow into and out of the condensers. The parameters measured included ambient air temperature, condenser inlet temperature, insolation on the condenser, and air conditioner electrical use. Thermostats and operation of the systems were held as constant as possible to reduce variation.

At the first site, the condenser was shaded with three 8-ft trees, with care taken to provide maximum shade while minimizing interference with air flows. Before and after measurements for periods of similar weather conditions showed a reduction in maximum afternoon temperature by the condenser inlet of 3deg.F relative to ambient conditions. However, energy savings could not be measured, because the thermostat developed a drift that made a meaningful comparison impossible.

The second location had a site-built wooden trellis above a packaged unit. The condenser takes in air from the sides and expels exhaust air horizontally across the ground. The inlet air temperature was reduced by only 0.2deg.F (not a significant number, given uncertainty in measuring such a small change). A small reduction in energy use of 3% was measured. The researchers attribute this to reduced direct solar gain to the exterior air handler section of the unit that was originally exposed to the sun.

The third site had extensive landscaping added to shade the condenser and surrounding area. Contrary to previous experiences, the temperature around the condenser rose 0.9deg.F higher than ambient temperature between 12 p.m. and 6 p.m. FSEC Senior Research Scientist Danny Parker said of the third site, "We did extensive landscaping but obviously we didn't put the plants far enough away from the unit, because the post-treatment temperature near the condenser inlet was higher relative to the ambient air temperature, and energy consumption was greater." He said this probably occurred because the air flow to the inlet was obstructed, and warm exhaust air from the unit was trapped by the landscaping.

Parker noted that condensers move a high volume of air, about 900 cubic feet per minute (cfm) per ton of cooling, and with most residential air conditioners at about 3 tons, this means that the condenser would process 160,000 ft3 of air per hour. To make this more meaningful, this would represent a cube of air with sides 55 ft across! Thus, a large volume of air surrounding the condenser may need to be shaded in order to effectively reduce energy consumption. Of course, there can be significant savings from reduced cooling load due to shading of the building. (See "Shade Trees as a Demand-Side Resource," HE Mar/Apr '95, p. 11.) Parker also noted that the cooling mechanisms of plants are complex (for example, direct shading, evapotranspiration, reduction of incident wind speed, and the effect of night sky radiation), so their impact on temperature and air flow can be difficult to predict.

FSEC suggests that a better strategy for keeping the condenser cool may be to place it on the north side of the building with no immediately adjacent shading, but with larger shade trees planted to drop the air temperature in the larger meso-climate. The report also states, "This work is not conclusive, but three experiments with relatively unimpressive results seem to indicate that this oft-referred to opportunity may not be all that productive unless care is taken to assure that air flow to the condenser is not affected by any shading scheme."
--Ted Rieger
Ted Rieger is a Sacramento-based freelance
writer who specializes in energy issues.