Daylighting Resources - Energy Issues
If a workplace is well lit by daylight, then electric lighting can be dimmed or switched off, creating a reduction in lighting electricity bills. In some buildings, windows may increase heating bills in the winter because they are poor thermal insulators. Windows may also increase air conditioning bills in the summer if direct sunlight is allowed to enter the buildings.
Energy cost savings in practice are highly dependent on the behavior and cultural values of occupants—if occupants are provided with opaque aluminum blinds and leave them closed all the time to ensure privacy, then no lighting energy savings are possible. If occupants value daylight and the view out, or are motivated to minimize energy costs, substantial savings are possible.
Typical cost of lighting energy
Electricity for lighting typically costs 60 cents per square foot per year, or approximately $60 per year per employee (assuming a typical lighting energy density of 1.5 watts per square foot (W/ft2), office occupancy from 08:00 to 18:00, 100 square feet per employee, 12 cents per kWh electricity cost, and that 0.35 watts of air conditioning are required to remove one Watt of lighting heat).
Sixty dollars per employee is a very low sum compared to the cost of hiring, training, paying, managing and retaining that employee, and so the primary goal in the use of daylight should be to improve employees’ working environment and to cause them no undue annoyance or discomfort. Unsuccessful energy-saving schemes focus only on the cost savings to the company, whereas successful schemes focus on creating pride in the workplace, improved morale and an awareness of shared responsibilities and benefits.
Energy-saving potential of various lighting control systems
The energy saved by any lighting control system varies very widely depending on the size and layout of the office, the size of the windows, and any partitioning systems used. All energy savings are quoted relative to a baseline of the lights being switched on all the time.
- Single-occupant or small shared offices
A simple light switch is an effective energy-saving device because it allows occupants to easily switch their lights on when required, or to leave them switched off if daylight is providing sufficient light (Hunt, 1979). Light switches are appropriate when occupants are controlling only their own lights, or those of a small group of co-workers. Switches should be positioned close to the light fixtures in order to maximize usage (California Energy Commission, 2003). Lighting energy savings of around 30% are typical for small offices, but vary widely depending on the size and type of windows, and the motivation and preferences of the occupant(s) (Newsham, 1994).
Automatic systems that switch off or dim the lights in response to daylight are highly effective in single daylit offices. Reinhart (2002) found that energy savings of 50% were possible in conjunction with a manually-operated blind system, and that savings could reach 70% if the blind were optimally controlled either by occupants or by an automatic system.
- Large open-plan offices
In open plan offices it is difficult to provide light switches close to the fixtures, unless local task lighting is used, or unless an expensive, addressable control system is installed. Occupants of shared space are often reluctant to use light switches for fear of creating conflict with their co-workers (Moore et al., 2003).
A system that automatically controls the lights in the response to the amount of daylight is the most effective way of saving energy in open-plan spaces. Systems that progressively dim the lights are preferred to those that switch them off, because the operation of the automatic system is less noticeable to occupants. Occupants should always be provided with a simple means of overriding the automatic system, otherwise complaints may lead to the system being decommissioned.
Lighting energy savings for automatic systems vary widely depending on the size and type of windows; Reinhart (2002) showed that a typical 30’ deep office with dimming controls could save approximately 40% of lighting energy in conjunction with manually-controlled window blinds. Switching systems are not effective at distances of more than 10-15’ from windows.
- Large partitioned offices
Partitions dramatically reduce the transmission of daylight into an office, and the resulting energy savings, compared with open-plan offices. Reinhart (2002) showed that 72” partitions halved daylight levels in an office 15’ from the windows, compared with 48” partitions. Therefore automatic dimming or switching systems will only achieve significant energy savings in the first row of offices with high partitions.
Skylights are a particularly effective source of daylight because they admit light from the brightest part of the sky (the zenith), but seldom cause glare because they are above the typical visual field of occupants. Skylights are complementary to windows, since windows illuminate the perimeter of a space, whereas skylights can illuminate its center.
The use of skylights can reduce the electric lighting requirement of an entire space during daylight hours to almost zero, but they must be designed to shade the space from direct sunlight (Heschong-Mahone Group, 1998).
Strategies for maximizing energy savings
- Light-colored walls and furnishings increase the amount of daylight in a space and improve visual comfort by reducing contrast. The areas around the windows are especially important.
- Removing clutter from the window sill and the floor around the window, and using a light colored carpet or rug will significantly improve the transmission of daylight into the space.
- Light switches should be clearly labeled to show which light fixtures they control. This will reduce people’s uncertainty and make them more likely to turn lights off, or switch on only the lights they require.
- A simple “light shelf” made of a light-colored material, approximately 18” deep and placed horizontally about 2-3’ below the top of the window will brighten the upper walls and ceiling around the window, reducing contrast and improving visual comfort. This will make occupants less likely to switch their lights on.
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