A Graceful Exit
A Graceful Exit
Although certainly not the dominant user of lighting energy, energy-efficient exit signs offer by far the largest percentage savings potential of all lighting retrofits.
Although each sign represents a relatively small amount of installed wattage, they are on constantly-8 760 hours per year. An incandescent sign with two 20-watt incandescent lamps inside consumes 350 kWh/yr, and must be re-lamped more than once annually.
Since exit signs are always lit, the loads they create are predictably coincident with the local utilities peak demand: each 25 million incandescent exit signs correspond to 1 000 megawatts of electrical generating capacity (equivalent to a large power plant). Wide-ranging estimates put the number in the US between 40 and 150 million signs. About 5 million signs exist in France and 1 million in the Netherlands.
Incandescent lamps designed for use in exit signs trade efficacy for long life. A standard 5 000-hour exit sign lamp operates at only 4 lm/watt-no better than Edison's first lamp a century ago. Incandescent exit signs may be the most inefficient lighting application to be found today. The market share of incandescent systems varies considerably around the world. In striking contrast to the 40 or so watts used by outmoded incandescent-based exit signs, new technologies can provide the same service for less than a single watt. Moreover, they tend to be more compact, more rugged, and longer-lived.
COMPACT FLUORESCENTS (CFLs)
CFLs are the most common substitute for incandescent lamps in exit signs. Typical sizes are from 5- to 9-watt pin-based systems for hardwired applications or as screw-based lamps for direct replacement of incandescent lamps. Their lifetime is longer than one year, and considerably longer than the rating based on regular on-off cycling.
LIGHT-EMITTING DIODES (LEDs)
Thanks to recent improvements in the efficiency of solid-state LEDs and the design of fixtures and power supplies, power requirements have been reduced from 5 to 7 watts per face to less than 2 watts. LED life varies, but 20 to 25 years is a reasonable estimate for adequate light production (batteries and transformers must be replaced more often). One drawback is the narrow angle over which LEDs emit light. By locating LEDs in the ends of vertically oriented acrylic "light-pipe" rods and a lens to diffuse the light, energy use can be minimized while increasing beam spread.
ELECTROLUMINESCENT (EL)
Although plagued by early reliability problems, EL exit signs (which produce light when phosphors sandwiched between two electrodes are excited by an alternating current) have improved and can consume as little as 1/3 watt per face. EL signs achieve uniform letter luminance from any viewing angle. Lumen depreciation can limit the acceptable service life of these systems.
TRITIUM
Tritium signs are self-luminous, require no wiring, maintenance, or energy, and can last for about 15 years. Analogous to fluorescent lamps, electrons emitted by the tritium strike phosphors on the lamp wall which re-radiate visible light. Unfortunately, tritium is radioactive and carries with it the all-too-familiar problems of disposal and health hazards. Given the exceptional efficiency of other options, tritium signs save only negligible amounts of energy.
ECONOMICS
Given their continuous operation, cost-effectiveness of exit sign options can be calculated very accurately. As shown in the table, life-cycle costs are about two times higher for traditional incandescent exit signs than for the more efficient alternatives. An efficient solution can save $200 to $300 over a period of 10 years. The exact cost difference depends on various assumptions. Beginning from a 50-watt (and shorter-lived) baseline, for example, almost doubles the life-cycle cost savings.
IMPLEMENTATION
Energy-efficient exit signs can be promoted in various ways. In the US, many building codes call for efficient exit signs. For example, forthcoming codes will require a maximum of 20 watts in Minnesota and 15 watts in Wisconsin. US electric utilities have also focused on promoting efficient exit signs. According to a survey by Rising Sun Enterprises, incentives range from $6 (for a screw-in CFL retrofit) to $135 per sign (for a new EL sign). Savings-based incentives are set at $0.14/kWh avoided (Seattle City Light) or $400 per avoided kW (Baltimore Gas & Electric). In the case of CFLs, most utilities provide larger incentives for hardwire retrofits than for screw-in ones. For new construction, the trend is to define CFLs as the "baseline" (i.e. no rebate) and to reward energy savings beyond this. No such programs are known in Europe or the developing world.
STANDARDIZATION
Standards in many countries require that the entire face of the exit sign be luminous. A pending European CEN standard is likely to call for a background luminance of at least 2 cd/m^2, and the luminance of the white field (text/pictogram) must be at least ten times higher. Written before LED or EL sources were used in exit signs, existing standards have the tendency to impede market entry of these most energy-efficient solutions for exit signs (these new sources are best suited for creating luminous letters). Exit signs are critical to safety. Important factors include luminance (and lumen depreciation characteristics), uniformity, contrast, color, size, lifetime and reliability, and performance in emergency operating mode (e.g. on battery power or in smoky conditions). Standards must keep pace with the ever-changing technological situation, while maintaining or enhancing safety.
Evan Mills
