Used in combination with air-conditioning, fans could lower energy use by 30–70 percent:
Compressor-based cooling or air-conditioning (AC) puts increasing pressure on electric grids worldwide. In the USA, the birthplace of the technology, AC accounts for approximately 20% of year-round electricity consumption by American households, and 15% of total electricity use. The widespread use of AC explains in large part why Americans use so much more electricity than Europeans: AC electricity use by an American household equals 60% of all electricity used by the average European household.
Except for the few temperate regions on the West Coast, air conditioners are now standard in most American homes. While only 12% of American households had AC in 1960, this number increased to 87% in 2009. Furthermore, the average air-conditioned home consumed 37% more energy for cooling in 2005 than it did in 1993 – in spite of a 28% increase in AC energy efficiency. Part of the increase in energy use is due to the switch from window units (which cool one room) to central air-conditioning (which cool the whole building), and in part to the growing cubic footage of houses and apartments.
Even worse is the impact of air-conditioning on peak power demand. Obviously, the use of AC is not spread equally throughout the year, but concentrated in the summer months. On very hot days, many air-conditioning units are set to a maximum position, and as a consequence demand for electricity spikes. Hundreds of American power plants and a great many miles of transmission and distribution lines are needed on average only two or three days per year, while they sit idle for the rest of the time. Peak power demand is growing faster than average power demand, and compressor-based cooling is an important reason for this.
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Air-conditioning is the least energy efficient way of cooling people, because it implies that all the air in an enclosed space needs to be refrigerated (and, if necessary, dehumidified) in order to cool the occupants. The larger the space and the fewer the people within it, the more energy it will take to cool each occupant. Like air-conditioning, circulating fans cool people by encouraging heat loss from the body through convection and evaporation. However, unlike air-conditioning, moving air around requires much less energy than refrigerating it.
Moreover, the cooling effect of circulating fans can be applied locally and has immediate effect. Fans circulate air around the body, while leaving the air in unoccupied parts of the space unaffected. Likewise, it is not necessary to keep the air circulating when nobody is around. Upon entering a room, turning on a fan has an immediate effect. Air-conditioning, on the other hand, needs time to cool down a space. As a consequence, a space will often be air-conditioned even when nobody is around, in order to provide immediate comfort when somebody enters it.
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The cooling effect of circulating fans is substantial. An air speed of roughly 1 m/s is capable of offsetting a 3°C (5.4°F) increase in indoor temperature, while an air speed of 3 m/s has a cooling effect of roughly 7°C (12.6°F). [10]
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A 2013 study found that subjects were comfortable up to 30°C (86°F) and 60% relative humidity with an air speed of 1.2 m/s, and up to 30°C and 80% relative humidity with an air speed of 1.6 m/s. At 60% relative humidity, subjects would be comfortable at temperatures higher than 30°C, but these conditions were not investigated. During the experiment, which took place in a climate chamber, subjects were wearing light clothing (0.5 clo) and performed light activity (for example, computer work at a desk).
If fans are so effective and comfortable, why is their use not more widespread?
Because until very recently, international comfort standards limited air movement indoors to a meagre 0.2 m/s in order to avoid drafts.
Air conditioning did away with traditional building techniques:
Traditional buildings in hot climates kept solar radiation out by using heavy construction materials, big eaves, reflective tin roofs, and growing shade trees around the house. Air conditioning did away with all these building elements and stimulated the use of lighter and cheaper building materials. Office blocks with H, T, and L-shaped footprints, which facilitated cross-ventilation, were replaced by massive, square blocks with very deep floor plans. Completely new building types emerged, such as office towers with fully glazed facades or enclosed shopping centres, which would be simply uninhabitable without air-conditioning because of the greenhouse effect.
I can testify for traditional building techniques. In the stinky hot Gascon summers our house remains cool; fat stone walls will do that, 2–3ft thick, including most of the internal ones.
Don’t forget high ceilings, often 14 ft, and tall double-hung windows that opened both top and bottom for circulation. I once taught in an old Carnegie building at Union College, and the high ceilings and tall windows substantially moderated the heat.