Which Passive Cooling Strategy
Is Right for You?

Before refrigeration technology first appeared, people kept cool using natural methods: breezes flowing through windows, water evaporating from springs and fountains as well as large amounts of stone and earth absorbing daytime heat. These ideas were developed over thousands of years as integral parts of building design. Today they are called "passive cooling." Ironically, passive cooling is considered an "alternative" to mechanical cooling that requires complicated refrigeration systems. By employing passive cooling techniques into modern buildings, you can eliminate mechanical cooling or at least reduce the size and cost of the equipment.

Passive cooling is based on the interaction of the building and its surroundings. Before adopting a passive cooling strategy, you must be sure that it matches your local climate.

In his book Sun, Wind and Light, G.Z. Brown identifies four passive cooling strategies: natural ventilation, evaporative cooling, high thermal mass and high thermal mass with night ventilation. (See descriptions of these strategies.) All these passive cooling strategies rely on daily changes in temperature and relative humidity. You can identify the passive cooling strategies that are appropriate for your building site by using a bioclimatic chart.

This bioclimatic chart defines four passive cooling strategies based on temperature and relative humidity. You can use this chart to determine which passive cooling strategies are appropriate for the climate at the building site.

First, find the following local weather information for each of the months of the year:

  • average maximum temperature
  • average minimum temperature
  • average maximum relative humidity
  • average minimum relative humidity

You can find this information in weather records kept by most local airports. Also the National Climatic Data Center (704-271-4800) publishes this information in Comparative Climatic Data. An extensive list of climate data can be seen on the World Wide Web at www.ugems.psu.edu/~owens/climate.html.

On the bioclimatic chart, plot two points for each month. The first point is the minimum temperature and the maximum relative humidity (RH). The second point is the maximum temperature and the minimum RH. (Note that the highest temperature is paired with the lowest RH and vice versa.) Connect these points with a line. Plot a similar line for each month. Each line represents the change in temperature and RH over an average day.

Passive cooling strategies are shown on this version of the bioclimatic chart as overlapping zones. When your lines cross zones, it indicates that this strategy may work for your climate. Some months may lend themselves to several different strategies. To reduce cost, you would probably choose one or two strategies that are compatible with each other and the building design.

The design strategies suggested by this version of the bioclimatic chart are appropriate only for residences and other buildings with small internal heat gains. Internal gains for a residence are assumed to be 20,000 btu per day per person.

These passive cooling concepts address getting rid of heat that accumulates in buildings. Of course, you'll also want to reduce heat gains in the first place with high insulation levels, heat blocking windows, proper solar orientation and good shading from building elements and vegetation.

Passive solar heating can also be assessed using the bioclimatic chart. Passive solar heating is usually an appropriate strategy when the plotted lines fall anywhere below the comfort zone. More information about applying this chart to passive solar heating region is published in Sun, Wind and Light.

Adapted with permission from John Wiley & Son, Inc., Sun, Wind, and Light, G.Z. Brown with illustrations by V. Cartwright, ©1985 John Wiley & Sons, Inc. G.Z. "Charlie" Brown is Head of the Department of Architecture, School of Architecture and Allied Arts, University of Oregon.

Passive Cooling Strategies

Natural ventilation depends solely on air movement to cool occupants. Window openings on opposite sides of the building enhance cross ventilation driven by breezes. Since natural breezes can't be scheduled, designers often choose to enhance natural ventilation using tall spaces within buildings called stacks. With openings near the top of the stack, warm air can escape, while cooler air enters the building from openings near the ground. Ventilation requires the building to be open during the day to allow air flow.
High thermal mass depends on the ability of materials in the building to absorb heat during the day. Each night the mass releases heat, making it ready to absorb heat again the next day. To be effective, thermal mass must be exposed to the living spaces. Residential buildings are considered to have average mass when the exposed mass area is equal to the floor area. So, for every square foot of floor area there is one square foot of exposed thermal mass. A slab floor would be an easy way to accomplish this in a design. High mass buildings would have up to three square feet of exposed mass for each square foot of floor area. Large masonry fireplaces and interior brick walls are two ways to incorporate high mass.
High thermal mass with night ventilation relies on the daily heat storage of thermal mass combined with night ventilation that cools the mass. The building must be closed during the day and opened at night to flush the heat away.
Evaporative cooling lowers the indoor air temperature by evaporating water. In dry climates, this is commonly done directly in the space. But indirect methods, such as roof ponds, allow evaporative cooling to be used in more temperate climates too. Ventilation and evaporative cooling are often supplemented with mechanical means, such as fans. Even so, they use substantially less energy to maintain comfort compared to refrigeration systems. It is also possible to use these strategies in completely passive systems that require no additional machinery or energy to operate.

©Copyright 1997 Iris Communications, Inc.

Source: Oikos

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