Shading

In 1960, 12 percent of all American homes had air conditioners, a figure that rose to 64 percent by the late 1980s. By 1989, air conditioners were installed in 77 per cent of new single-family homes. The cost of the equipment, labor and energy involved in air-conditioning is climbing rapidly. The environmental cost of chlorofluo-rocarbon (CFC) refrigerants is now being felt by the refrigeration industry and by consumers. To fight these rising costs, designers and builders can look to the time before air-conditioning for natural cooling ideas. In many climates, the right combination of properly implemented natural methods can provide cooling equivalent to mechanical air-conditioning. At the very least, natural cooling allows you to install smaller cooling equipment that will run fewer hours and consume less energy.

Providing shade from the sun is essential for passively cooled buildings, and for passively heated buildings that might become overheated in hot weather. The best shading occurs before the sun's heat reaches the building. If sunrays are intercepted before passing through the glass, the air-conditioning cooling load can be cut in half, saving anywhere from 10 to 50 percent of energy costs. Place the highest priority on the surfaces that receive the most summer heat. That's usually the east and west.

Shading options, in order of effectiveness, are trees and shrubs, trellises, overhangs, awnings, shade screens, window coatings, and interior shades. Trees throw shade over the walls and roof, and will also shade driveways, sidewalks, and patios that can bounce heat to the building. Since big trees give more shade than little ones, devise a site plan that preserves as many existing trees as possible. Then plant new trees immediately after construction. Trees provide a cooling bonus. To keep themselves cool, trees pump water from the ground into their leaves. As this water evaporates from the surface of the leaves, it cools the tree. This evaporative cooling cools the surrounding area, too.

Deciduous trees are best for south yards, because their canopies are broad and dense. Deciduous shade trees and vines can provide shade for low buildings, while allowing more sunlight through when their leaves fall in the winter. Evergreens can work well for north and northwest yards.

The closer a tree is to the building, the more hours of shade it will give. To be effective, trees should be planted between 1.5 and 6 meters (5-20 ft) from the building. Shrubs offer less shading, but also cost less, reach mature size more quickly, and require less space. Shrubs can shade walls and windows without blocking roof-mounted solar panels.

Trellises are permanent structures that partially shade the outside of a building. Clinging vines growing over the trellis add more shade and evaporative cool-

Vine Shading Diagrams
Figure 23-1 Deciduous vines for seasonal shade.

ing. A special trellis to shade air conditioners, heat pumps, and evaporative coolers improves the equipment's performance. Be sure not to restrict airflow to the equipment. Fast growing vines create shade quickly, while trees can take years to provide useful shade. Deciduous vines (Fig. 23-1), such as grape, clematis, and wisteria, lose their leaves in winter, allowing the sun's heat to strike the building. Trellises and climbing plants are a design solution that's attractive and flexible.

Most homes have a built-in shading device where the roof overhangs the building. Overhangs block the high-angle, summer sun, but allow the lower winter sun to strike the building. They are only effective for the upper story of a multistory building, and don't provide relief for east and west windows. Roof overhangs or horizontal-shading devices (Fig. 23-2) at each floor of taller

Sun's rays enter room

Sun's rays blocked by overhang

Figure 23-2 Horizontal overhang.

Sun's rays enter room

Sun's rays blocked by overhang

buildings can block high summertime sun angles on south-facing walls, while admitting lower winter sun.

Horizontal louvers parallel to the wall permit air circulation near the wall and reduce conductive heat gain. Manual or automatic timer-controlled photoelectric controls allow the louvers to adapt to the sun's angle. Slanted louvers offer even more protection than parallel ones, with angles varying to coordinate with solar angles. Louvers hung from a solid overhang protect against low sun angles, but may interfere with the view. Vertical louvers are most effective on the east or west. Egg-crate louvers (known as brise soleil) combine horizontal and vertical elements to produce a lot of shade. They are very efficient in hot climates.

Outside shade screens on windows exposed to direct sunlight prevent sun from entering a window. These devices are often called sun screens, shade cloths, or solar shields. The screens are made from aluminum or plastic and are lightweight, durable, and easy to install. Unlike insect screens, shade screens are specially made to block a certain amount of the sun's energy, usually between 50 and 90 percent. The shading coefficient is the amount of heat that penetrates the screen, with lower numbers indicating that less energy gets through. While you can see through a shade screen, the view is obscured. Fixed sunshades can block sun in early spring, when it is desirable to have sun penetrate to the interior.

Adjustable sunshades, such as awnings, avoid this problem. Manual controls for adjustable sunshades are inexpensive and relatively trouble-free but require occupants to make the adjustment when necessary. Motorized devices also depend on someone to operate them, but are good for large or heavy devices in remote places, such as clerestory windows. Automatic systems with computerized controls can be set to consider the thermal needs of the entire building.

Awnings can cover individual windows or sections of outside walls, and are most effective on the south side of the building. Awnings come in a variety of shapes, sizes, and colors to match many building designs. Fixed awnings block light at a given angle, while adjustable awnings can be rolled up in the winter to allow low-angle sun to reach into the building. Awnings have the disadvantage of blocking the view from the top half of the window.

Exterior shading rejects about 80 percent of the solar energy striking the window. Interior shading devices absorb and reradiate 80 percent of it into the interior, increasing interior temperatures dramatically. Outside louvers can cool off in a breeze, while draperies become part of a heat trap, giving off radiant heat in hot weather.

Interior shading devices like roller shades, blinds, and curtains, absorb solar radiation and convert it to convected heat in the interior air. They also prevent direct solar radiation from striking occupants and furnishings. Interior shades don't block sunlight as well as exterior shades. Blinds and drapes can reduce solar radiation by as much as half, depending upon their reflectivity. They also reduce visual glare from direct sunlight. Interior shading devices don't have to deal with weathering, are exposed to less dirt, and are usually easy to adjust manually.

To give the most benefit, interior shades should have a light-colored surface on the side facing the window, and be made of an insulating material. They should fit tightly to prevent air movement into the room and should cover the whole window.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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