Lighting : Lighting October 2016 - Vol 36 Issue 5
28 LIGHTING MAGAZINE | October/November 2016 Currently, given the same CRI, cool white LEDs have higher efficacy than warm white LEDs since red LED diodes are less efficient than blue ones. However, in the future the situation may be reversed because the maximum LERs of warm white LEDs are generally higher than cool white LEDs11. Besides LED packages, related to the previous discussions on luminaire efficacy, the efficiency breakdown and efficacy projection on luminaire level is shown in Table 4. The final luminaire efficacy for warm white phosphor-converted and colour- mixing LEDs are both projected to be around 230lm/W. This seems to be the final feasible efficacy ceiling for LEDs – but will the industry be feverishly chasing after this? CONCLUSION There are many factors influencing the real efficacy of LEDs. The LED architecture, which suggests a potential LER, the system level, which indicates which efficiency-loss channels should be applied, and the definition of good white light, which determines what CRI, CCT, Duv etc. should be expected. And based on that, we have seen different efficacy predictions since the beginning of the LED time. Thanks to Haitz’s law and work by other researchers, people realised the easy energy savings that LEDs could deliver and governments became interested in the corresponding reduction in power plants construction costs and greenhouse gas emissions. Then funds and incentives were injected, which stimulated the industry R&D. Several years later, LED products landed on the market and consumers have been told that switching to LEDs is both ecological and lucrative. Meanwhile, consumers’ increasing trust in LEDs in return supports further R&D and thus accelerates the market penetration. Eventually, solid state lighting technology will reach the efficacy threshold and to stand out in the harshly competitive markets, lighting companies will have to explore new applications based on the market demand for their innovations. In the foreseeable future, the penetration rate of LEDs will increase further and LEDs will dominate the lighting market. Hence, when it occurs, people will have to ask themselves, should we explore an improved technology or stick to the LEDs? This may be answered by laser diodes12. REFERENCES 1 Susan Callahan et al.(July 2015). Adoption of Light-Emitting Diodes in Common Lighting Applications (http://energy.gov/sites/prod/files/2015/07/f24/led-adoption-report_2015. pdf) 2 CIE. CIE (1931) 2-deg color matching functions. (https://web.archive.org/ web/20081228084047/http://www.cvrl.org/database/text/cmfs/ciexyz31.htm) 3 Ohno, Yoshi. Spectral design considerations for white LED color rendering. Optical Engineering 44.11 (2005): 111302-111302. 4 Norman Bradsley et al.(May 2015). Solid-State Lighting R&D Plan (http://energy.gov/ sites/prod/files/2015/06/f22/ssl_rd-plan_may2015_0.pdf) 5 Haitz R, (2003) Another Semiconductor Revolution: This Time It’s Lighting! Advances in Solid State Physics, Vol. 43, pp. 35 –50. 6 Norman Bardsley et al.(April 2014). Solid-State Lighting Research and Development Multi-Year Program Plan, U.S. Department of Energy (http://apps1.eere.energy.gov/ buildings/publications/pdfs/ssl/ssl_mypp2014_web.pdf) 7 Philips. Philips debuts blue LED for remote phosphor use and the new Luxeon A (http:// www.ledsmagazine.com/articles/2011/05/philips-debuts-blue-led-for-remote-phosphor- use-and-the-new-luxeon-a.html) 8 Mike Wood. (Win 2011). How do white LEDs work? (http://www.mikewoodconsulting.com/articles/Protocol%20Winter%202011%20-%20 White%20LEDs.pdf) 9 Department of Energy, U.S. (March 2013). Energy Efficiency of LEDs (http://www.hi-led. eu/wp-content/themes/hiled/pdf/led_energy_efficiency.pdf) 10 Dave Bartine et al (April 2012). Solid-State Lighting Research and Development: Multi- Year Program Plan (http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/ssl_ mypp2012_web.pdf) 11 Norman Bardsley et al (June 2016). Solid-State Lighting R&D Plan (http://energy.gov/ sites/prod/files/2016/06/f32/ssl_rd-plan_%20jun2016_2 .pdf) 12 Ray Molony (24th Feb, 2016). LED inventor bets on lasers to replace LEDs (http:// luxreview.com/article/2016/02/led-inventor-bets-on-lasers-to-replace-leds?nocache=1) Table 4. Breakdown of pc-LED luminaire efficiency projection6 Efficiency channel 2013 2015 2020 Goal Package efficacy projection (lm/W) 135 165 225 250 Thermal efficiency (%) 86 88 93 95 Driver efficiency (%) 85 87 93 96 Luminaire/Optical efficiency (%) 85 89 94 96 Electrical efficiency* (%) 115 113 109 105 Overall luminaire efficiency (%) 71 77 89 92 Luminaire efficacy (lm/W) 96 130 200 230 *The gain obtained by under-running the LED, reducing “droop” ACKNOWLEDGEMENTS The author is immensely grateful to Mr Emrah Baki Ulas (Steensen Varming) and Mr Mike Wood (Mike Wood Consulting) for their valuable comments and help. He also gives his special thanks to Prof Barry Webb AM, who supervised the work, for his inspirational support.
Lighting August 2016 - Vol 36 Issue 4
Lighting December 2016