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LD+A The Magazine of the Illuminating Engineering Society of North America

LED Testing & Application  

Seven Ideas for Better LED Education

A recent article in LD+A by Paul Mitchell of Sternberg Lighting offered city officials, college campus officials, utilities and developers seven essential tips for navigating today’s LED landscape

  1. Why Consider LED? Three primary benefits of LED over traditional light sources are the immediate energy savings, the long-term maintenance savings and the improved quality of light, particularly if you’re converting from HPS. Some funding opportunities are tied to energy savings, but the truth is that maintenance savings is generally what really provides the payback, particularly when/where energy is cheap. Some customers produce their own power, or have access to relatively cheap hydro power, so their potential energy savings are minimal. For them, the only real payback is the long-term maintenance savings. Many people have already found that the ROI on LED, when based solely on energy savings, just isn’t cost effective; it’s too long-term. Still, LED makes sense to many cities and universities because they’re going to reap not just the energy savings, but the long-term maintenance benefits.
  2. Say Watt? Further mitigating the importance of energy savings, watts aren’t the primary measuring stick anymore–lumens are. People generally know what lumen output they can expect from a 150-W HPS lamp, but 150 watts can produce entirely different lumen output in different LED luminaires. Watts are certainly important in terms of calculating energy savings, but they’re not the key metric to use when comparing LED to HID, or even LED to LED. Wattage will vary based on several factors, such as the drive current of the LED array and the number of diodes used. What one fixture produces using 90 watts may be vastly different from what another fixture produces using similar wattage. Generally, you’re going to significantly cut your wattage when going from traditional light sources to LED. But saving 10-15 watts may not be the way to go if the light output is significantly lower than another luminaire with slightly higher wattage, especially if the lower wattage luminaire requires you to use more fixtures in your overall site plan, increasing your long term energy and maintenance costs, as well as your upfront material and installation costs.
  3. Please Keep it Down. While lumen output is important, the most emphasis should be placed on a luminaire’s downward delivered lumens, not its total lumen output. With HID fixtures, the difference in manufacturing cost between a 70-W version and a 250-W version of the same fixture is negligible (unless you’re going to a larger housing, just for aesthetic purposes). With LED, the difference between a 70-W luminaire and the same fixture using 250 watts can be significant. You’re probably talking about more diodes and micro optics, more circuit boards and possibly more heat sink required in the higher wattage version.

    You don’t want to be paying for lumens that are going up and out, but rather the useful lumens that are being directed down into/onto your target area. That’s not to recommend narrow beam patterns that produce hot spots and significant reflective light off the ground. You can still have a responsibly wide beam pattern, producing improved uniformity and reducing material and installation costs. But light that is uncontrolled, going out sideways and up is not what you want to be paying for. Remember that the increased directionality and control offered by today’s LED fixtures over older HID lamp and optic combinations is part of what allows them to produce equal or better light using less total lumens, and significantly less energy. This is where BUG ratings and protection of the night sky should be considered.
  4. Get a Life. The proper way to evaluate the ROI when making the move from traditional lighting to LED is through life cycle analysis, not unit price on bid day. Look at the life cycle of the LED system, say 100,000 hours for the sake of discussion. Then look at the comparative life cycles of other light sources being proposed, such as HID or induction. It is critical to note that these life cycles are based on different metrics. The “useful life” of an LED has been defined as its hours of operation to L70, (a 30 percent depreciation in lumen output), whereas the life of HID lamps and induction systems are based on their failure rates. Metal halide lamps are generally rated to a 50 percent failure rate, whereas HPS lamps are generally rated to a 33 percent failure rate. Induction generally has a 100,000-hour life rating, but that’s to a 50 percent failure rate. So using that example, compare the energy, material and maintenance cost of an induction system where 50 percent of your units will have to be replaced in the same time that it will take the LEDs to depreciate to L70 with little/no projected failures.

    For outdoor applications, such as street and roadway lighting, the cost of changing a lamp is generally several hundreds of dollars, per fixture, per event. Over a single “useful life” span of an LED luminaire, you may need six to eight visits to that fixture to relamp an HID luminaire, each time incurring not only the cost to send a couple of guys with a bucket truck, but the cost to replace the failed component(s), divert traffic, do the initial trouble-shooting, etc. These additional costs need to be considered on top of the energy savings that the LED unit will provide over that same life span, and most likely none of us expect energy costs to go anywhere but up.
  5. Get a Real Life. While a fixture’s L70 rating is an important metric of consideration, you must also consider the life of the driver. Because LEDs last so long and depreciate so slowly, one of the reasons L70 was derived was to answer the question, “If LEDs don’t fail for such a long time, how do we know when it’s time to change the lights?” While the actual LED array may not “fail” on its own, the array will still go out when the driver eventually fails. Some of the better LED drivers on the market today have a life expectancy of 100,000 hours, if engineered into the luminaire in a way that maintains the proper case temperature. That’s really the life span that you want to use in your ROI calculations, because regardless of the condition or depreciation of the diodes, that’s when you’ll actually need to make a maintenance visit to the fixture.

    Consider also that when the driver fails, it might be prudent to consider changing/upgrading your LED array. If the projected life of the driver is 23 or so years, think what we’ll be doing with LEDs by then. With the speed at which efficacy is improving, you’ll be able to dramatically reduce your energy consumption and still maintain the light levels you originally designed to.

    Keep in mind also that there are other components in the fixture, such as gaskets, lensing, photocells or controls components, which will eventually need to be repaired or replaced. Related to this, a shift that’s taking place in LED site design is the cessation of relying on an individual button-style photocell to operate each fixture, a common practice in HID site design. If you’re going to invest in LED, then it’s worthwhile to explore the expanding offering of wireless controls. If a PEC is absolutely necessary, a utility grade twist-lock PEC is much more reliable than the button style. If you must use a button style, then consider controlling a number of fixtures from one PEC, rather than having one in each fixture. Either way, recognize that the button approach puts the burden of your entire lighting system–and all of your projected maintenance savings–on the shoulders of a $5 component.
  6. Got Protection? Tool-less access is still an important part of good luminaire design, but equally if not more important is a good IP rating. Because LEDs are expected to stand on their own for such a long period of time, without any maintenance, they must have their own defense against both water and particulates, such as dust and dirt. Otherwise, your diodes could be shining (and corroding) under water, or trying to emit light through a filthy lens. An IP rating of 65 is a good base for most outdoor street and area applications. The ‘6’ indicates complete protection against the entry of dust, and the ‘5’ indicates protection against low pressure jets of water from all practicable directions. A well-engineered luminaire, regardless of the light source, should probably have an IP rating of 65 or higher for at least the optical chamber, if not the entire luminaire.
  7. Get Used to Change. In the world of LEDs, lumen output, available CCTs, optional drive currents, etc. change quickly as new diodes become available and get adopted. The same is true of some of the newer HID lamp technologies. You can’t rely on printed brochures; the technology and product offering changes too quickly now. Most updates are provided via PDF or posted online at the manufacturer’s website. However, you also shouldn’t rely on info from the website if you’ve not checked it in a while. Before making any final decisions, return to the manufacturer’s website to check for updated information. There may be newer options available to make your project even more energy-efficient, or to reduce your up front and/or long term costs.



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