Not Cool to Be Hot
Not Cool to Be Hot
Heat generated in fluorescent luminaires can result in 15% to 20% less light output and reduced efficacy. To overcome the problem, manufacturers are adopting design solutions developed by researchers.
The actual light output and energy use of fluorescent lighting systems can be significantly different from labeled (optimal) values. The key is temperature.
The notion of thermal efficiency has long been understood, but often neglected in luminaire design. Temperatures inside fluorescent fixtures are important because excess mercury condenses at the coldest point on a lamp, regulating light output and (for long fluorescent lamps) power consumption. Sub-optimal temperatures lead to losses in light output. Lamp temperature depends on lamp and fixture geometry, wattage, ballasting, ambient temperature, lamp orientation, and air circulation.
A frequent criticism of CFLs-not enough light-can be traced partly to overheating. This arises because of lamp orientation or that CFLs are often used in fixtures designed for incandescent lamps, where temperature does not affect light output. Other unwanted side effects may include color shifts towards blue-green, reduced color rendering, and shortened system life.
Because power consumption decreases as temperature increases, thermal factors make it hard to design and calculate energy savings from changes in long-tube fluorescent systems. A pre-retrofit system may operate far from the optimal temperature while the post-retrofit system may run at or near the optimum. Reasons for this include fewer or lower wattage lamps and ballasts in the post-retrofit fixture and better ventilation characteristics. (CFLs do not exhibit the same reduction in power as do long fluorescents and, as a result, efficacy (lumens/watt) diminishes dramatically as temperature increases.)
Consider the change from two 40-watt T12 cool white lamps with a standard core-coil (magnetic) ballast to two 34-watt T12 cool white lamps with an electronic ballast shown in the diagram. According to the "rated" light output (defined as the peak of each curve), the new option produces ~20% less light than the original one (5800 lumens). However, if the original system in fact operates at an ambient temperature of ~45°C and the new one operates at the optimal temperature of ~27°C, then light production is identical. According to the rated situation, there would be a reduction in power of 35 watts (105 kWh/year for 3 000 hours of operation), but the actual reduction is only 25 watts (75 kWh/year).
BEATING THE HEAT
A variety of clever strategies have been devised to optimize the thermal performance of fluorescent lighting systems. These include modifications of lamps (conductive cooling) and fixtures (convective cooling). Another approach is to use mercury amalgams. These are not widely adopted by lamp manufacturers because they can complicate the manufacturing process and be more costly than thermal management strategies.
For long-tube fluorescent systems, one of the technologies developed at Lawrence Berkeley Laboratory (LBL) is a "spot cooler" that can be inserted so that it rests lightly against the top of the lamp (or beneath the lamp in an indirect pendant fixture). When applied to F40 lamps in enclosed wrap-around fixtures, this strategy achieves a 15% increase in light output and an 8% increase in efficacy. A number of manufacturers are now planning to integrate spot coolers with their luminaires.
For CFLs, one strategy involves attaching a simple copper fin through the lens of an enclosed fixture (the worst fixture from a thermal point of view). By touching the lamp tip, the fin cools it down to the ideal temperature, or near it, with less than 1% optical light loss from shading caused by the fins. This strategy achieves nearly 100% of the rated light output and a 15% increase in efficacy. Lumatech is planning a production run for such a system.
Thermal factors are also responsible for the fact that CFLs operated in a base-down position produce 15% to 20% less light than when used in a base-up position. This is because excess mercury drips from the cold spot at the top of the lamp into the hot glass tubulation in the base where it is revaporized. Work at LBL shows that small copper strips fitted around the tubulation rapidly conduct heat away from the lamp, achieving up to 99% of rated light output (see photo).
Another approach to thermal management is to increase air movement around the lamps. One solution is fan-based systems, but these are relatively costly, use extra energy, and require maintenance. A simpler strategy is to modify fixtures so that they are passively cooled.
Prototype recessed CFL downlights developed at LBL have increased light output by up to 20% simply by adding ventilation slots. Proper size and positioning of slots is critical because excessively large openings result in optical losses that offset part of the benefits related to lower temperatures. The optimized vented fixtures have only 1% to 2% optical losses and are now manufactured by Delray, Edison Price, Indy Lighting, Lightolier, Lithonia, Microflect, Prescolite, Reggiani, Staff, and Zumtobel.
The most effective solution yet identified requires tilting the lamp downward by 5° to 10°, placing the tip in a cooler part of the fixture, which allows the mercury to settle to the tip more easily and improves the air flow. The result is a luminaire that achieves 98% to 99% of rated light output.
One concern about convective ventilation is the potential for increased dirt build-up on the lamp and fixture leading to reduced light output over time. In experiments conducted at LBL, dust was injected into a controlled chamber containing vented and unvented fixtures. Candlepower readings taken one meter below the fixtures revealed that vented fixtures had consistently less lumen depreciation over time than unvented ones. This may be because dust leaves the fixture before it has a chance to settle.
The various measures described above are expected to add very little, if anything, to the production cost of lamps or fixtures. Convective venting adds virtually nothing to the cost of CFL downlights; conductive cooling in the lamp base costs ~10¢ per lamp; and spot coolers add ~$5 per fixture. The resulting cost-performance tradeoff is increasingly viewed by lamp and luminaire manufacturers as an attractive opportunity for increasing the competitiveness of their products in markets where achieving higher lumen output is the name of the game. (See also Dedicated CFL Fixtures Bring Savings Home, IAEEL 1/95)
Evan Mills
For more information, contact:
Michael Siminovitch
Lighting Systems Research Group
Lawrence Berkeley Laboratory, USA
Tel: +1 510 486 5863
Fax: +1 510 486 6940.
Light output for three 15-W CFL alternatives
| The base-up alternative produces almost 100% of the rated light output. The light output from the base-down CFL decreases by -25% after a few minutes of operation. However, by adding a thermal bridge, the performance of the base-down CFL improves dramatically. (In all three cases, light output rises to 1.00 during the first few minutes.) |