SOLAR TODAY - July/August 2009

inside ases ^| chair’s corner
Keeping Your Cool
Comfort without air conditioning is a folk art.

By JOHN REYNOLDS,
FAIA

Hot weather is upon us, with higher electricity bills for air conditioning. Can we reduce electricity use, avoiding the stiff-neck discomfort of constant forced cold air? Can our bodies and buildings follow natural thermal cycles that respond to the climate?

John Reynolds, FAIA, is chair of the American Solar Energy Society Board. Contact him at chair@ases.org.

Shade is a first line of defense. We’ve all experienced tree-shaded streets that are so much cooler than sunny ones. The same applies to our buildings. When planting trees for summer shade, think about admitting winter sun (avoid evergreens to the south), blocking winter wind (use evergreens to the windward side) and celebrating the seasons. Some deciduous trees provide colorful bark in winter, blossoms in spring, shade and food in summer, color and food in the fall. Trees provide seasonal, large-scale shade for the whole building envelope.

Windows are most vulnerable to solar gain. To shade them, consider deciduous vines. With a much smaller root ball, vines can shade high-rise facades as well as apartment balconies. Fashioned by their support structure, they serve as living awnings. They can celebrate the seasons and provide a local source of food. My pole green bean vines grow very fast and cast dense shade for my south windows. And fresh-picked green beans are a delight.

John REYnoldS

Windows are most vulnerable to solar gain. To shade them, consider deciduous vines.

Architectural shades for windows work for businesses and homes. Not that long ago, awnings over retail and office building windows were quite common. They can be cranked open or closed as necessary. They block sun when temperatures are too hot, admit it when too cool and protect from glare and rain. Facades are enriched with three-dimensional projections and with the fourth dimension of time, as shadows slowly move with the sun. What elegant richness, in contrast to slick, flat, glass facades.

Overhangs above south-facing windows are popular, due to their peculiar advantage at this orientation: In summer months (spring to fall equinox), if the overhang shades to the bottom of the south window at noon, it will also shade to the bottom of the window at all other hours. In winter months, the shadow of the overhang at noon is at its deepest; in the morning and afternoon, the window gets more sun than at noon. So the south overhang shades in the summer and interferes less with sun in the winter. Designers of these fixed overhangs face a dilemma: The spring equinox is much cooler than the fall equinox. If you design for complete shade in hot August, you also block the sun in cool April. I favor fixed overhangs with deciduous vines; the overhang shades from May through July, and vines do the rest. The vine leafs out when the weather is warm and

retains its leaves until late fall’s cool weather.

External shades of all types are subject to wind and weather, and in winter, they can threaten those below with icicles. It is so much easier to shade inside the window. But compared to an externally shaded window of the same size, completely unshaded windows will gain twice as much summer solar heat if facing south and about 3. 7 times as much heat if facing east or west. Unshaded horizontal skylights gain a whopping 7 times as much summer solar heat. Adding either drapes or Venetian blinds inside an unshaded window will cut heat gains, but the sun has already entered. The exterior shade is clearly best at excluding solar heat.

The heat sinks available to cool a building are air (the drier the better), earth (especially when moist) and the cold night sky. The common air conditioner (an air-to-air heat pump) uses outdoor air as its heat sink. The passive cooling equivalent is the open window, but that is of little use when the air outside is hotter than we desire inside. In climates where summer night air is quite cool, buildings can be flushed with night air, and thermally massive indoor surfaces can store the “coolth” for the next day’s comfort. In humid climates, the air is not an effective heat sink for passive systems.

The earth’s heat sink is well used by geoexchange systems, also known as ground-air heat pumps. The low-energy equivalent is the earth tube, an underground passage where outdoor air temperature gradually approaches the earth temperature. The deeper this passage is below ground, the greater the difference between the temperature of the hottest summer air and that of the earth. These systems almost always use a fan to propel the air.

The cold summer night sky may seem the least likely heat sink, because roofs are insulated more thoroughly than any other building surface. There are a few low-energy approaches, such as the roof pond and the night roof spray system. The roof pond, advocated by Harold Hay, slides insulated roof panels open by night to expose bags of water to the cold night sky. These bags are in direct contact with a metal ceiling below. The lower the humidity, the better the system performance, as high humidity can produce condensation on the ceiling. The roof night spray system stores water in an insulated tank; by day, the water circulates through cooling panels in the building, and by night, it is sprayed on the roof, cooled by exposure to that cold sky, then re-collected and stored.