Energy-efficient SSL failure modes spurs DOE funding

The Obama Administration announced earlier this month a multi-agency initiative to spur regional economic growth while making buildings more energy efficient.  Seven federal agencies issued a combined Funding Opportunity Announcement of up to $129.7 million over five years to create a regional research center that will develop new building efficiency technologies and work with local partners to implement the technologies in area buildings. 

According to the U.S. Department of Energy (DOE), buildings account for nearly 40 percent of U.S. energy consumption and carbon emissions.  Improvements in building efficiency will provide significant benefits such as lowering utility bills and decreasing carbon emissions. According to the Buildings Energy Data Book referenced by the DOE, energy consumption for all lighting in the U.S. is estimated to be about 18 percent of the total electricity generated in the country. More than half of the energy is consumed in the commercial sector, where lighting coincides with peak electrical demand and contributes to a building's internal heat generation, increasing air-conditioning load. One option for meeting these green building needs is high-brightness LED (HB-LED) solid-state lighting (SSL), which can be up to 80 percent more energy efficient than incandescent bulbs. A recent article highlighted the costs of SSL types and alluded to reliability and lifetime issues associated with LED components being addressed by the industry.

Many industry experts see capacitors as being a bottleneck in SSL product development. One of the most important factors affecting the lifetime of electronic products, in general, and capacitors specifically, is heat, including both the device's operating temperature and ambient surroundings.

An HB-LED and its driver circuit are critical in determining the lifetime of a SSL fixture. HB-LEDs, which are the base component of SSL fixtures, have two major failure mechanisms. First, the outer packaging encapsulant discolors over time due to heat generated degradation, affecting light output. Second, the amount of current through the diode over time eventually becomes less able to convert electrons to photons, causing the LED to grow dimmer rather than simply burn out, as in the case for incandescent bulbs. Lumen maintenance is a number that defines end of life for an LED and generally is L70, which means that the LED is emitting 70 percent of the light of its initial state.

The main failure modes for driver circuits are often associated with the degradation of the electrolytic component, solder joints, and the optional optoisolator. The electrolytic in the electrolytic capacitor ages due to the chemical reaction that takes place as the capacitor charges and discharges, which accelerates with heat. Although the chemical reaction in a driver differs from that of a battery, their aging processes are similar. Since overheating is destructive in HB-LEDs, they are typically mounted on a heat sink to allow for effective heat dissipation.

The Recovery Act has aided product development for LED lighting, as numerous companies with facilities in the U.S. already dealing with these issues, have been awarded numerous stimulus grants, as noted in prior articles, to ultimately accelerate deployment of SSL for green building.

"Photo credit- www.howstuffworks.com: Inside an LED"