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Passive Cooling

Jan. 28, 2015
There are a couple of issues that need to be addressed before this becomes commercially viable. If they can be solved, this technology could be a real game changer.

Comfort air conditioning, according to ASHRAE, is “treating air to control its temperature, relative humidity, cleanliness, and distribution to meet the comfort requirements of the occupants of the conditioned space.” With conventional space cooling, we generally are removing heat from a conditioned space and rejecting it to an unconditioned space (usually outside). Passive cooling of a building, on the other hand, requires a design based primarily on heat-gain control and heat dissipation, rather than on the mechanical removal and rejection of heat. The idea of passive cooling is to provide a thermally confortable indoor environment with minimal (or no) energy consumption. It’s not a new concept; in fact, a self-taught architect better known for his political activities than for his architectural achievements designed his own home to be passively cooled. Construction of Monticello, the home of Thomas Jefferson, was begun in 1769, and the house was redesigned and remodeled over the next 39 years. Some of the passive design features that allowed a house in central Virginia to remain comfortable during hot and humid summers included skylighted interior stairways that apparently acted as “thermal chimneys” to induce air circulation throughout the upper floors of the house; thick masonry walls to reduce daytime heat gain; partially earth-sheltered outbuildings connected to the main house with covered breezeways that, along with porticos, brought cooler air into the house; and large operable windows that could bring in cool mountain breezes (his was the first plantation in Virginia built on a hilltop, rather than along a river).

Passive cooling is not new to architects, either. During the late 1970s and early 1980s, The American Institute of Architects, Lawrence Berkeley National Laboratory, and the Solar Energy Research Institute were aggressively promoting passive cooling. And in 2013, one of the oldest design schools in the United States—Boston Architectural College—began offering the online course Passive Heating and Cooling Design.

Many of the concepts of passive cooling are relatively simple. Heat gain can be controlled with, for example, proper location and orientation of a building, insulation, and glazing. Some of the factors contributing to effective heat dissapation include many of the design features Jefferson incorporated into Monticello, such as natural ventilation using operable windows and roof openings. Now researchers in Stanford University’s Electrical Engineering, Mechanical Engineering, and Applied Physics departments have published their work on radiative cooling, a passive cooling strategy using no electrical energy. Although it sounds like science fiction, their technique uses outer space as a heat sink. In conventional passive cooling strategies, it is difficult to maintain an inside temperature lower than the outside ambient temperature. Stanford’s technology emits thermal radiation in a very specific infrared wavelength that is transparent to the earth’s atmosphere. Also, it recognizes that if solar heat gain at the “radiator” is adding more heat than the system can emit, it won’t work. So, equipment also functions as a mirror, reflecting 97 percent of incident sunlight so it can cool itself.

There are a couple of issues that need to be addressed before this becomes a commercially viable solution: scale up of the radiator material, which must be tuned to a very precise wavelength, and a method for conducting heat from the interior of a building to the exterior radiator. If these two problems can be solved, this technology could be a real game changer, especially in developing areas relying on off-the-grid solutions.