Savings from T-5s Questioned

Savings from T-5s Questioned

Your publication is at the very top of my periodical reading list. I am concerned, however, about the article regarding HID vs. T-5 fluorescent lighting (July/August 2000). While I’m totally in agreement with the concept, it appears to me that the 1st Source lighting analysis may be flawed. It is very risky to uncritically accept a manufacturer’s analysis. I believe that 1st Source is comparing a 95% efficient T-5 fixture with a 60% efficient HID fixture. A fairer comparison would have been two 90%+ efficient fixtures, with the fluorescent one requiring six, not four, T-5 lamps. The savings would have been 25% not 50%, and the relamping costs would be higher.

Am I on the right track with these questions/concerns?

Ron Manganiello

Burlington Electric Department

Burlington, Vermont

Editor’s response:

We asked Jim Rogers for his thoughts on the above concerns and provide his response below.Mr. Manganiello’s questions and concerns are valid and were investigated when I prepared similar fixture comparison tables in the E Source report. I did some research and discovered that the “typical” HID fixture has a fixture efficiency of 60 to 75% and generally has a spun aluminum reflector with a satin finish. This type of reflector helps to mitigate problems of maintaining even distribution from the very high lumen point source. The new T-5 fixtures have higher efficiencies, usually well over 90%, because they are more “open” and they utilize specular reflectors, which serve a dual purpose. First, they get most of the light out of the fixture (i.e., higher fixture efficiency). Second, they serve to distribute the light more accurately and evenly.

Recently, I did a similar comparison of the best (most efficient and pulse- start) MH fixture to the typical high-efficiency T-5 fixture. Although this narrowed the gap a bit, the T-5 still comes out the efficiency winner, because the newest T-5 lamp/ballast systems have higher efficacy than the metal halide lamps. Also, the premium, high-efficiency HID fixtures are more expensive than the T-5 fixtures. And, improving the fixture efficiency does not overcome the other HID drawbacks, such as the poor color rendering, much higher lumen depreciation, poor start and restrike times, and the inability to dim efficiently.

Regardless of the tables and theoretical calculations, the proof is in the pudding. I just completed five actual case studies for a local utility, and they verified that converting from existing HID to new T-5 fixtures produces the same or better maintained lumens (actually footcandles) while using only about half the energy.

The HID-to-T-5 conversions have been so positive that one T-5 fixture manufacturer will install up to 12 “demo” fixtures at no up-front cost and will remove them if the customer isn’t happy. Last time I checked, they had not removed even one.

Jim Rogers, P.E.

Chelmsford, Massachusetts