mercado de trabajo y emigración entre 1990-2009

As seen in figures 4.1 and 4.7, the next largest cooling load after exterior glass is the load from interior lighting. For really significant reductions in the building’s annual energy consumption, the design can reduce lighting power consumption when sunlight is available.

Daylighting—using less electrical power and more sunlight to light the interior during daytime hours—can make a significant

reduction in annual energy consumption, especially in hot and hu-mid climates, which are closer to the earth’s equator than are cold climates. So sunlight is available for more of a building’s occupied hours compared to cold or moderate climates, offering the potential for even larger energy savings.

Further, it’s often the case that a buildings’ peak electrical power demand comes during clear, hot afternoons during the summer, when cooling loads are at their peak. So it can be especially cost-effective to use daylighting to reduce that peak load. Peak cooling load determines the size and cost and complexity of the cooling systems, as discussed earlier. And on the economic side, the combined peak lighting and cooling load also determines the cost of electrical power for build-ings located in places where power is limited. In those locations the annual energy cost is often determined by the maximum peak power demand rather than by the total Kw consumed.

Every Watt saved in lighting reduces the building’s power con-sumption by at least 1.2 Watts, because the lighting load appears twice in the energy budget. First, it appears as lighting power. Then part of that power appears again as a heat load for the cooling system. The cooling system uses additional power to remove the heat generated by the lights. That additional power consumption will be between 20 and 30% of the power used by the lights, depending on the efficiency of the cooling system.

So the potential annual energy cost reduction from daylighting can be significant. On the other hand, a building which has effective daylighting often looks different from a building in which daylighting cannot be effective. And a building which uses daylight effectively will probably cost more to construct.

Before the first cost even becomes a factor, one common reason that buildings in hot and humid climates have sometimes neglected the potential of daylighting is because the critical enabling decisions are made very early in project planning. If the owner and architectural designer are not aware of the measures needed to take advantage of the climate’s high daylighting potential until after the look and

feel of the building’s exterior are decided, it may be impractical to consider changes. Both regulatory approval and the marketing of the building may already be based on a structure which cannot use daylighting effectively.

Daylighting does not work properly (and does not save any energy) unless the owner understands and agrees that shape, look, and feel of the building’s exterior and its interior are going to be part of the lighting system, and will therefore need to be designed accordingly. Also, the lighting budget will probably rise, because the building will still have to be lit after dark. So the number of fixtures will probably be no different from other buildings. And to actually save power, the lighting output will have to be modulated by a control system as more or less daylight is available.

More specifically, to achieve the most significant reduction in lighting power and cooling load, the building will need:

1. Wide daylighting windows placed high on the walls, near the ceiling of each floor.

2. Light shelves projecting outwards from the exterior wall underneath those windows.

3. Exterior sun shades above those windows.

4. Glazing in those windows which passes a significant por-tion of visible light - ideally more than 60%, while still keeping out most of the sun’s radiant heat.

5. Light-colored ceiling and wall finishes which reflect incoming daylight evenly, without glare, into occupied spaces.

6. Automatic lighting controls which sense the current indoor lighting level near the occupants’ reading surfaces, and which switch off or dim the lights, modulating energy use as daylighting rises and falls.

Excellent advice for detailed design of exterior enclosures and glazing to optimize daylighting can be found in references 6 and 8, are described at the end of this chapter. They provided the photos seen here as figures 4.5, 4.6 and 4.9, and some of the advice they contain is summarized briefly below.

Figure 4.9 shows what the exterior of a building looks like when it is designed for effective use of daylight, along with callouts indicat-ing the key elements of such a design. Each of those elements plays an important role:

Wide windows, located near ceilings

The deeper the daylight penetrates into the building’s interior, the more electrically-powered light it can displace. A small percentage of daylight penetrates very deep into the building, of course. But as a rule of thumb, enough daylight to be effective only penetrates to 1.5 times the height of the window.⁸ In other words, if the tallest part of the window is 6 ft off the floor, adequate daylight will only penetrate as far as 9 ft. into the buildings interior [if the top of the window is 1.8m off the floor, adequate light may penetrate to 2.7m]. So the higher the top of the window, the deeper the light penetration and the more effective daylighting can be.

Also, the wider the window at that tall height, the more light will be able to penetrate into the building. It’s largely a matter of the total window area at it’s maximum height (its width at the top) which determines how much light can penetrate.

So the ideal fenestration for daylighting is a narrow band of windows circling the entire building near the ceiling of each floor.

Fig. 4.9 Windows for daylighting Conventional windows set at eye level do not transmit enough daylighting to allow a reduction in lighting power. The key exterior architectural elements for success are shown in this photo.⁶ Also, interior surfaces need to reflect rather than to absorb the incoming light.

Unfortunately, by itself this arrangement only allows the occupants to see the sky. That’s why daylit buildings usually have two sets of windows stacked on top of one another. The lower window is set at eye level for a pleasant visual connection to the outdoors. The upper window is set close to the ceiling, to maximize the depth of light penetration and it is also wide, to provide a maximum amount of light.

Exterior light shelves

Light shelves which project outwards from the exterior wall just below the daylighting windows benefit the building in three important ways.

First, they increase the amount of light coming through the daylighting windows, so that the penetration depth increases by about 30%—2.0 times the height of the window instead of only 1.5 times the maximum window height.⁸

Next, they act as sun shades for the view windows set below the daylighting windows. That shading usually reduces the solar heat gain though those larger windows enough to meet the strict energy reduction targets of the International Energy Code, and enough to meet the recommendations of the ASHRAE Advanced Energy Design Guides for hot and humid climates (a solar heat gain coefficient of less than 0.31).

Finally, if those projections are attached all along their inward edge, they can reduce the risk of mold and other microbial growth inside the building. An attached projection will force any rainwater that’s flowing down the walls off and away from the windows under-neath them. The joints around windows are the usual places where water gets into the walls to support microbial growth indoors. As one experienced building scientist has often observed: “If the window doesn’t get wet—it can’t leak.”⁹

Glazing which passes visible light but keeps out heat

It is obviously important for daylighting windows to allow as much visible light as possible into the building. A minimum visible light transmission of 60% (VT= .60) is a useful rule of thumb, and more is better.⁸ Clear glass transmits about 80% of visible light, but it’s

important for daylighting windows to have a low solar heat gain coef-ficient, because after all, they are still windows. Without a low SHGC the daylight windows would waste in cooling any energy savings they might gain from daylighting.

The difficulty is that glass with an extremely low SHGC can reduce the visible light transmission far below 50%. Tinted or highly reflective windows are an example, some having a solar heat gain factor as low as 0.20, but which also pass only 24% of visible light (VT = 0.24)

Fortunately, given the hundreds of glazing combinations cur-rently available, with modern glass it’s possible to strike a reasonable compromise between keeping heat out while allowing visible light in.

Double pane, “spectrally-selective” low-e glazing can be obtained with a VT of 0.71 and a SHGC of only 0.38 . Such windows would be quite effective for daylighting while still excluding a great deal of heat.⁸

Sun shades above daylighting windows

On the other hand, even with solar heat gain coefficients as low as 0.38, the daylighting windows will not meet the target values for solar heat exclusion which are suggested by the ASHRAE Advanced Energy Guides. So if the owner wants to meet the target of 30% less energy than the 2004 edition of ASHRAE Std 90.1 the SHGC of all windows will need to be less than 0.31. That means the daylighting windows will also need sun shades.

Light-colored, diffusively-reflective ceilings and walls

If the interior surfaces are dark, they will simply absorb the incoming daylight and re-radiate that energy in the form of heat, eliminating the benefit of the daylighting windows.

To avoid wasting that daylight by turning it into heat before it can be useful to occupants, the interior finish on the ceiling must be light and highly reflective—and also diffusive. Mirrors on the ceilings would reflect nearly all the incoming light, but the glare would be visually unbearable. A matte-finish, white ceiling tile is ideal.

Also, the walls should also be white or at least extremely pale, so they don’t absorb that daylight, either. The look-and-feel of an

antique Mediterranean whitewashed interior is an excellent model.

Those buildings use daylight very efficiently. Light comes through small windows and then bounces around the interior, reflecting off of whitewashed interior walls and providing considerable illumina-tion for occupants during the daytime. That interior design evolved over centuries, before electric lighting became cheap and readily available.

It would be unfortunate if the interior designer decided on a dark-cave-sophisticated-nightclub look-and-feel for ceilings and walls. Dark colors quickly eliminate—with a single coat of paint—

most of the benefits the owner was expecting from his investments in daylighting windows, light shelves and spectrally-selective glass.

Automatic lighting controls

Automatic lighting shutoff for unoccupied spaces is a baseline re-quirement of ASHRAE Std 90.1-2004, regardless of whether or not the architect and owner decide to take advantage of daylighting. But with daylighting, it makes sense to modulate the interior lighting, or at least to bring it on in stages, rather than simply turning lights on and off with a time clock.

Sunlight varies in intensity as the cloud cover changes, and of course as the day turns into night. So in most commercial occupan-cies, successful daylighting includes automatic controls to brighten or to dim the electrical lights as the daylighting levels rise and fall, keeping those changes imperceptible to occupants, or at least ir-relevant to their activities.

In residential occupancies, there may be little need for automatic controls. The residents will either feel the need to turn lights on, or they won’t. But in commercial buildings, where responsibility and authority for controlling lights is not always given to the occupants, automatic controls will be needed to really achieve the desired energy savings.