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PASSIVE SOLAR Strategies
Evolve Integration Always consider passive solar strategies in building design. Five basic passive solar strategies include 1) proper building orientation with the long axis of the building oriented east-west to maximize southern solar exposure; 2) number of windows and placement appropriate for regional climate 3) use of energy-efficient glazing and careful window sizing to reduce heating costs in the winter, avoid overheating in summer and promote thermal comfort year round; 4) proper sizing and locating of building overhangs and shading elements to avoid overheating in the summer; and 5) incorporation of thermal mass into the building to store heat.

Energy efficiency is also a key component of passive solar design a building should be well insulated, tightly constructed, and have high quality windows and doors. Proper ventilation is needed: consider operable windows on the east and west sides of the building. The more efficient the building is, the larger the fraction of the heating that will be satisfied by the passive solar gains.

Buildings under 10,000 ft2 in the Rocky Mountain Region, commercial or residential, are ideal targets for passive solar heating in winter and shading in summer. One computer simulation program available for these calculations is ENERGY-10. The ENERGY-10 software tool was specifically designed to analyze passive solar and energy-efficient strategies in buildings under 10,000 ft2. Buildings between 10,000 ft2 and 50,000 ft2 are good candidates for passive solar heating and daylighting. Daylighting reduces electricity use and cooling energy, lessening direct solar gain through glazing selection, orientation, and external shading devices.


Glazing

Division 8 includes a more comprehensive discussion on windows. The energy performance of glazing and windows is characterized by U-Factor, solar heat gain coefficient (SHGC), visible transmittance (Tvis), and air infiltration (cfm/lf). Look for total window and door U-Factors that account for glazing and framing effects. There are two very good sources of window U-Factors and SHGCs available: ASHRAE 1997 Handbook of Fundamentals (Chapter 29, table 5 and Table 11); and the National Fenestration Rating Council Certified Products Directory.

In Colorado, we have over 300 days of sunshine per year. Through proper placement, orientation, and sizing of windows, the solar heat gain through windows and glazed doors warms a space and lowers heating requirements. To prevent overheating, provide shading through landscaping, overhangs, and shading devices. Optimization can potentially lead to downsizing of heating and cooling equipment, resulting in cost savings. The most cost-effective, energy-efficient windows have U-Factors less than 0.4 Btu/hr-ft2-F (R-2.5). While the lower the U-Factor the better, this is not necessarily true with solar heat gain. When solar heat gain is desired, such as in passive solar homes, a window or glazed door with a higher SHGC (greater than 0.5) is preferred. For buildings that are dominated by cooling loads, such as most commercial buildings, products with lower SHGCs (less than 0.4) are recommended.

The most exciting change in windows is the addition of low-E (low emittance) glazing. Low-E glazing reduces the U-Factor of a window that results in lower heat loss. In the winter, the interior surface of the glass will also be 500F to 1000F warmer than that of a standard double-glazed window. The biggest difference between different low-E products is in how much solar heat gain they allow. Many residential products with low-E glazing have an SHGC less than 0.5. For passive solar heating applications, be careful to specify windows with low-E glazing and an SHGC greater than 0.5.

Daylighting

Designing a building to take advantage of plentiful daylight greatly improves the comfort and aesthetic appeal of interior spaces. If done correctly, the use of daylight can dramatically reduce the energy consumption of the building by reducing the time the lights are on. Reducing the electric light use also reduces cooling requirements. Studies have also shown that the occupant's psychological and physiological comfort can be improved by natural light.

A successful daylighting plan can introduce an ample amount of daylight and prevent overheating. This is accomplished by using daylighting strategies that minimize direct solar gain through shading, use of north light and reflected light, and glazing selection that minimizes heat gain but maximizes visible light transmission. Daylighting can be as simple as using proper glazing with Venetian blinds and lighting controls that respond to ambient light levels. Effective daylighting strategies often include clerestories, light shelves on the east, west, and south walls, atria, and cupola structures. Light interior colors work best for reflecting daylight deep into the buildings. Light pipes can also be used to bring light deep into the inner areas of the building with very little impact on the design.
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Automatic lighting controls are necessary for maximum electric light energy savings and peak load reduction from daylighting. These controls can either dim or turn off lights when the natural light level from windows, skylights, or clerestories is adequate for the occupants. Unless the space has been specifically designed to bring daylight deep into the building, daylighting controls are only effective on fixtures within about 12 feet of the perimeter windows. The different types of daylighting controls include on/off, step dimming, and continuous-dimming. Controls are also available that combine daylighting and occupancy sensors. They are available as light switches and ceiling-mounted devices.

Step-dimming and continuous-dimming controls require dimming ballasts, so the initial cost for the lighting system is higher than with on/off daylighting controls. Step-dimming ballasts vary in the number of steps they offer and their energy savings potential. For example, some products only offer two light levels (e.g. 50% and 100%), while others offer up to five light levels, ranging from 10% to 100%. Two-step ballasts are also available for HID lights, which are often used in industrial or warehouse applications. Continuously dimming ballasts allow the lights to respond to the changing natural light levels without making discreet steps. Continuously dimming ballasts are available for fluorescent lamps, HID lamps, compact fluorescent lamps, and incandescent lamps.

Detailed calculations can optimize the daylighting design for minimum energy consumption for heating, cooling and lighting by comparing different glazing types, sizes, and configurations. Several computer simulation programs are available for these calculations.
 













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