Passive solar lighting
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Passive solar lighting is the use of natural illumination instead of artificial lighting systems. (See also Daylighting)
In the U.S., about 22% of an average building's electric bill is for lighting. Inefficient lighting systems can use up to 40% of the electricity consumed by a poorly-constructed modern commercial building that does not utilize sunlight for lighting.
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[edit] Windows
A window is a free daytime passive solar lighting system, used for over two thousand years (since glass was first invented in Rome). Romans recognized the Right to Light as early as the 6th century, and English law supported solar rights in the Prescription Act of 1832.
The problem with windows is that even the best ones have far less resistance to undesirable heat transfer, than a well-insulated wall.
When there is nothing but glass between exterior temperature extremes and interior living space, windows can waste far more energy than leaving efficient lights turned on.
[edit] Skylights
Skylights are often used for daylighting. Skylights admit more light per unit area than vertical windows, and distribute it more evenly over a space. The thermal performance of skylights is affected by stratification, i.e. the tendency of warm air to rise and collect in the skylight wells, which in cool climates increases the rate of heat loss. Poorly-constructed or incorrectly-installed skylights may leak, and single-paned ones may weep with condensation. Skylights may also be more prone to breakage (from hail, falling tree limbs, etc.) than vertical windows.
The skylight topic is controversial. Although roof-angled glass provides good daylighting, it can also create a solar furnace in the summer (when the sun is nearly perpendicular), and then lose more solar gain than it captures (when the low winter sun reflects off it,and warm interior air rises to touch the cold glass on winter nights). Heating and cooling costs for a building are normally greater than the value of skylight daylighting, especially when modern energy-efficient lighting systems are employed.
Skylight Drawbacks:[1]
- Significant source of heat loss or heat gain
- Can constrain design of building shape and orientation
- Difficult\Complicated to specify
- Point of condensation
- Uncontrolled, uneven illumination
- Susceptible to water leakage
- Susceptible to ventilation leakage
- Not appropriate for low ceilings
- Difficult to relocate or reconfigure
- Suitable for downlighting only (i.e. N/A to directional lighting or uplighting)
- Does not maximize the use of available sunlight
- A source of light pollution at night
- Cannot be easily turned off
- Security concerns
Transparent glass and plastic have little structural strength. Vertically, they bear their own weight because only the thickness is subject to gravity. As the angle tilts off vertical, an increased area (the sloped cross-section) must resist gravity. Glass is brittle - It does not flex much before breaking - To counteract this, you must increase thickness, or structural supports - Both increase overall cost, and reduce solar gain potential. Sloped glazing is exposed to the weather, leaks, hail, ice-and-snow load, wind, and material failure. Excess solar gain, harsh lighting, and undesirable heat transfer thru sloped glass are difficult to control. “Therefore, vertical glazing is the overall best option for sunspaces.”[2]
[edit] Light Tubes
There is one modern skylight alternative. A well-insulated light tube (round-or-square light duct) with a highly-reflective interior coating (such as polished aluminum) can be installed between the roof and the ceiling. There is glass (or an inexpensive crystal-clear acrylic light diffuser with a diamond-shaped surface like those in some fluorescent light fixtures) covering the bottom of the tube that is flush with the ceiling surface. This prevents warm interior air from rising up the tube and being lost on cold winter nights.
The top of the tube (on the roof) has weather-sealed transparent glazing that allows daylight to come in and be reflected downward to the ceiling below. This roof-top glazing can be vertical North-facing glass (for diffused lighting), and should avoid east, west, or roof-angled orientation (especially important in the summer). A 45-degree mirror inside the light tube reflects the northern light down the tube to light up the ceiling below. Weather-tight insulated installation is important, to minimize undesirable heat transfer, exterior air infiltration leaks, and dust build up inside the tube. Light tubes are designed to look like light fixtures, but they actually provide free subdued natural daylight with no electricity. At night, energy-efficient electric lights (just above the diffuser) can create the illusion of daylight after sunset.
Light tubes may be particularly useful for isolated non-living areas (with no windows), so that electric lights are not needed in the daytime for closets, laundry room, bathrooms, interior staircases, or other dark areas that need daylight.
[edit] Sawtooth Roof
Another roof-angled glass alternative is a "sawtooth roof" (found on older factories). Vertical roof glass faces away from the equator side of the building, to capture diffused light (not harsh direct equator-side solar gain). The angled portion of the glass-support structure is opaque and well insulated with a cool roof and radiant barrier. The sawtooth roof lighting concept partially reduces the summer "solar furnace" skylight problem, but it still allows warm interior air to rise and touch the exterior roof glass in the cold winter, with significant undesirable heat transfer.
[edit] Solarium
In a well-designed isolated solar gain building with a sun room, solarium, greenhouse, etc., there is usually significant glass on the equator side. A large area of glass can also be added between the sun room and your interior living quarters. Low-cost high-volume-produced patio door safety glass is an inexpensive way to accomplish this goal.
The doors used to enter a room, should be opposite the sun room interior glass, so you see outside immediately when you enter most rooms. Halls should be minimized and open spaces used instead. If a hall is necessary for privacy or room isolation, inexpensive patio door safety glass can be placed on both sides of the hall. Drapes over the interior glass can be used to control lighting. Drapes can optionally be automated with sensor-based electric motor controls that are aware of room occupancy, daylight, interior temperature, and time of day. Passive solar buildings with no central air conditioning system, need control mechanisms for hourly, daily, and seasonal, temperature-and-daylight variations. If the temperature is correct, and a room is unoccupied, the drapes can automatically close to reduce heat transfer in either direction.
To help distribute sun room daylight to the sides of rooms that are farthest from the equator, inexpensive ceiling-to-floor mirrors can be used. They are particularly useful on west walls, where you would prefer to have no windows at all.
Building codes require a second means of egress, in case of fire. Most designers use a door on one side of bedrooms, and an outside window, but west-side windows provide very-poor summer thermal performance. Instead of a west-facing window, you can use an R-13 foam-filled solid energy-efficient exterior door. It may have a glass storm door outside. Opening the inner door allows light in. East/west glass doors and windows should be fully shaded top-to-bottom, or a spectrally-selective coating can reduce solar gain by more than three quarters. Exterior shade is better.
[edit] Translucent Insulation
Aerogel is a new alternative to inefficient windows, if all you need is daylight in a room. It has the lowest density of any solid material. It is translucent and has excellent insulating properties, but limited structural strength. Aerogel can be placed between two pieces of structurally-strong translucent fiber-reinforced plastic to create a translucent wall that allows 20% of available light to come through, but has an R-20 resistance to undesirable heat transfer. Images cannot be clearly seen through it, but diffuse daylight does pass through, without sacrificing good passive solar building thermal performance. Transparent insulation can be used effectively on many vertical walls, or on a sawtooth roof, to provide low-cost natural daylight with reduced heat transfer over window glass. Exterior-or-interior shading, shutters, drapes, or shade screens may be useful in some applications.[citation needed]
[edit] Fiber Optic Concrete Wall
Another way to make a secure structural concrete wall translucent is to embed optical fiber cables in it. (See Concrete casts new light in dull rooms) Daylight (and shadow images) can then pass directly through a thick solid-concrete wall. The only drawback is an inability to put insulation on either side of such a fiber-optic concrete wall. One possibility is to insulate it with aerogel after concrete wall construction, for natural daylight with the highest-possible structural security and NO glass windows.
[edit] Hybrid Solar Lighting
Oak Ridge National Laboratory (ORNL) has developed a new alternative to skylights called Hybrid Solar Lighting, which uses a roof-mounted light collector, large-diameter optical fiber, and modified efficient fluorescent lighting fixtures that have transparent rods connected to the optical fiber cables. Essentially no electricity is needed for daytime natural interior lighting.
2006-2007 field tests of the new HSL technology were promising, but the low-volume equipment production is still expensive. HSL should become more cost effective in the near future. A version that can withstand windstorms could begin to replace conventional commercial fluorescent lighting systems with improved implementations in 2008 and beyond. The U.S. 2007 Energy Bill provides funding for HSL R&D, and multiple large commercial buildings are ready to fund further HSL application development and deployment.
At night, ORNL HSL uses variable-intensity fluorescent lighting electronic control ballasts. As the sunlight gradually decreases at sunset, the fluorescent fixture is gradually turned up to give a near-constant level of interior lighting from daylight until after it becomes dark outside.
HSL may soon become an option for commercial interior lighting. It can transmit about half of the direct sunlight it receives.[3]
[edit] See also
[edit] References
- ^ Hybrid Lighting Applications. Retrieved on 2008-01-15.
- ^ EERE Consumer's Guide: Sunspace Orientation and Glazing Angles
- ^ Muhs, Jeff. Design and Analysis of Hybrid Solar Lighting and Full-Spectrum Solar Energy Systems. Oak Ridge National Laboratory. Retrieved on 2007-12-23.
