If you work at Toronto Western Hospital or if you have visited the site, you may know that we have an underground 2-level parking garage below Fell Pavilion. To provide enough lighting for drivers, 21 powerful 250W high pressure sodium (HPS) lighting fixtures were installed on parking garage entrance ramp sidewall, some are outside and some are inside. Those lighting fixtures share a few circuits together so the only way to run the outside ones is to keep all the lights on 24/7. As an energy project manager I felt guilty every time I passed by the ramp area.
HPS was invented by GE in 1960s. The bulb contains a small amount of solid sodium inside a glass tube. When heated up the sodium would vaporize and the lamp would glow yellow. Typical construction is shown as below:
HPS has higher efficiency when compared with many other lighting sources such as incandescent, metal halide, fluorescent and even LED in lm/W. However there is a major drawback: one of the worst colour rendering on the market. Because of this it was mainly used for occasions where colour rendering is not important such as street lighting, parking lot, etc. If you want to know more details about lighting, my coworkers Chad and Michael have talked about different technologies in the past.
As we know HPS lights are very efficient. Its efficacy is about 80-140 lm/W, which is similar to relatively new LED technology. How can we reduce energy consumption with this setup? The secret is lighting quality. If you have ever worked under HPS lights, you would understand: colour of illuminated objects are almost indistinguishable. So a lumen from HPS is not equivalent to a lumen from a lighting source with higher quality. To tell the difference, scientists at the Lawrence Berkley Laboratory (LBL) developed the concept of scotopic lumens. They developed a factor called S/P ratio. This ratio helps convert traditional lumens into actual lumens perceived by the eye under mesopic light conditions and gives a more accurate estimate of the amount of light. According to LBL, typical S/P ratio is only 0.62 for HPS and for 5000K LED it’s around 1.8. This combined with luminaire efficacy makes the LED fixture much more efficient.
After testing some fixtures, Facilities Management and Energy & Environment team determined that a 40W 5000K LED fixture replacement is appropriate for the project. For outdoor lights, since it’s not cost-effective to separate them from circuits for indoor lights, photocells are added to each of those lights so that they will only be on when needed. In total the project achieved 75% reduction on annual energy (kWh) consumption, or over 36,000 kWh. The new LED fixtures also last much longer. Their rated life is 100,000 hours while typical HPS bulbs would last about 25,000 hours, which will greatly reduce maintenance work.
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