Passive and Hybrid Cooling Research
The term “passive cooling” generally denotes the dissipation of heat from buildings by the natural processes of radiation, convection, and evaporation which do not require the expenditure of any nonrenewable energy. In many cases, evaporation and convection can be significantly enhanced by the use of motor-driven fans or pumps, which consume small amounts of electrical energy and the word hybrid has been adopted to characterize such processes.
Much of the early work in passive cooling, notably that of Harold Hay in Arizona and California, was empirical in nature, carried out at the investigator’s expense to prove the effectiveness of systems which were the result of intuition rather than analysis. With the entrance of the Department of Energy into the arena of solar space conditioning, funds became available to finance research into the basic nature of the passive cooling processes. Among the most important of those studies has been the work at Trinity University, under the direction of Dr. Eugene Clark, in the field of radiative cooling under both clear and cloudy skies. The Trinity team has shown, by vahdated computer simulations, that nocturnal heat rejection from roof tops can successfully dissipate solar heat gains for residences in virtually the entire United States.
The warming effect of the high nocturnal atmospheric temperatures which prevail in the southern part of the United States has been studied by Trinity and by Martin and Berdahl from the Lawrence Berkeley Laboratory of the University of California. The term “convective intrusion” has been added to the vocabulary of passive cooling and means of combatting this phenomenon has been reported. Givoni has shown, by work done in Israel, that the use of thin polyester films which are transparent to infrared radiation can be helpful until the film temperature falls to the local dewpoint. At this condition, the film becomes covered with moisture which effectively blocks the escape of radiant heat.
Much of the work done in recent years has been analytical and quantitative in nature, relying upon extensive and expensive instrumentation. Combined with the earlier empirical work carried out before the DOE came into existence, a firm foundation has been laid for the use of passive cooling processes in the residences which will be built in the post-DOE era.
KeywordsRadiative Cool Passive Cool Humidity Ratio Solar Heat Gain Nocturnal Cool
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