Do Code-Mandated Lighting Controls Help Us?
by Jim Dirkes, PE, BEMP, BCxP
While I'm not an expert in lighting controls, I interact with them frequently as a part of our commissioning work. Throughout my many years of designing and troubleshooting buildings' functionality, I have repeatedly inquired about the commissioning of lighting controls.
Way back, the “only” lighting system was daylighting; candles didn't work all that well for many activities. The only “controls” were drapes and shades. Daylighting diminished when gas lights were in vogue and then became optional when electric lights became popular and readily available 100+ years ago. Daylighting took a hit with increased attention on energy conservation, since windows normally increase energy use. Windows experienced a small renaissance-of-sorts with LEED Daylighting credits and when people finally became disenchanted with working in "caves.”
Fast forward to today: Current codes mandate maximum lighting power AND occupancy sensors AND daylighting-based controls**. With almost universal adoption of LED lamps, we're applying a lot of lighting control technology to LED lighting - the lighting which uses 50% of the power compared to the previous generation of lamps. More money, more complexity, more maintenance for pretty modest savings. Where's the balance? Let's do a comparison.
Savings Calculations
* Cost of electricity (average) is assumed to be $.12 / kwh
** Energy codes say that if you have more than 20 square feet of window or almost any skylight, you need to add controls which dim the lights based on the amount of sunshine / daylight. On top of that, almost every room in a new building also requires some form of occupancy sensor which turns the lights out when no one is there.
Savings Analysis
Converting to LED? That seems like a slam dunk; savings are great, and the payback is good.
Occupancy sensors make sense from the perspective that people can be careless, and lights get left on when they could be off. Savings are modest but believable.
Also, consider that occupancy sensors are simple controls. In some cases, the sensor is integrated with the wall switch and requires no additional wiring. In other cases, sensors are mounted on the ceiling and require a couple of wires to the wall switch. They are low cost to install and maintain, though throughout an entire facility, the quantity could become significant.
Daylighting controls are not so simple or inexpensive. You need a sensor mounted near the window, you need lamps designed for dimming, a controller and control wiring to each fixture, programming, setup, (often) software, and a wifi network, not to mention a commissioning agent must test a sampling of them. This is more than your grandmother's wall switch by far. Anecdotally, rooms with larger windows sometimes don't even use the daylighting controls because people turn the lights off on a sunny day - they like the "feel" of daylight. Savings are modest, similar to occupancy sensors, but complexity and costs remain high.
Are There Alternative Approaches?
Energy codes (most of them are based on ASHRAE Standard 90.1) allow you to demonstrate compliance via a whole-building energy model and, when you do that, you do not need to comply with all of the prescriptive requirements (eg, lighting). What if we made an energy model that compared “full lighting controls” to another option that deleted some extra, more costly controls? What if that model demonstrated that cost savings were, in fact, minimal, and an alternative approach could substantially reduce complexity and construction costs? We're up for that! This is our plan for upcoming modeling projects where we are engaged early enough to affect design decisions. I’m betting that the model (including the cost to create it) shows impressive cost savings and payback for a less-complex approach.
In addition to integrating lighting with occupancy, there is the option to also integrate the controls with HVAC. If no one is in the room, not only can lights be turned off, but you need not maintain the normal temperature setpoints OR minimum room airflow OR minimum outdoor air for ventilation. This is powerful synergy for irregularly occupied rooms(But we’ll wait for another day, another blog, and an energy model for that one.
Going forward, it is my hope that we can weigh the value of simplicity with savings when considering lighting controls. While certain functionalities may be possible, are they helpful for the end use? Return on investment isn’t only monetary, it also comes by way of user experience.
Further Savings Calculations
*ASHRAE predicts that occupancy sensors save ~10% annually, so we're using that as our basis for savings
**Sunshine varies a lot from city to city! Let's assume that it's decently sunny for 40% of occupied hours (enough that lights can be dimmed) and the average dimming is 30%.