"Zero Energy" Exit Signs

Environmental Building News (where I work) reported on photoluminescent exit signs in 2006. With tens of millions of exit signs deployed in North America that use up to 350 kWh each annually (as much as a nicely efficient refrigerator), it's a big deal. EPA estimates are that exit signs in commercial buildings use 30 - 35 billion kWh per year... the output of about a dozen 1000-MW coal-fired power plants. CFL exit signs use far less power and require far fewer lamp change-outs than incandescent. LED exit signs use only a couple watts. Electroluminescent signs use even less power than LEDs. Electricity-free radioluminescent exist signs — the tritium ones noted in another comment — have been around since the '20s, generally have 5 - 20 year lifespans, and disposal is regulated in the U.S. by the Nuclear Regulatory Commission (though it's probably a given that most of them end up in landfills anyway). And PL exit signs, as noted in the post, also need no power at all... sort of.

The manufacturers' recommendations for some of these signs call for a minimum continuous face exposure of of five footcandles (54 lux) of ambient light. But with low-energy lighting designs for interior spaces, the ambient light at the ceiling — where these signs are installed — the face of the exit sign would get on the order of 1 footcandle. Accommodating the charge requirements for PL exit signs in this kind of situation creates a significant energy penalty. It should be noted that the NYC law requiring PL exit markings (in all Class E commercial high-rise buildings over 75 feet in height) isn't actually about exit signs — though maybe the other commenter is talking about a different law that I'm not aware of. The post-9/11 NYC Building Code Reference Standard RS 6-1 and 6-1A didn't change requirements for exit signs above doors, but did create specific requirements for PL exit path markings on stairs, handrails, exit doors, and other places. That law also requires at least 2 footcandles of light on the floors of corridors to maintain the markings' charge, and nixes motion sensors that interrupt continuous light levels of at least that much. What sort of exit signage and marking is appropriate is part of an overall lighting and energy-use design that meets local code requirements and national laws. PL can be appropriate. It isn't always. It can get especially tricky when the concept is removed from general theory and inserted into design reality.