In the residential sector in particular, lighting is still dominated by an old and inefficient technology, the incandescent lamps, which has been around for the last 100 years. These lamps have a very low efficiency wasting more than 95 % of the input energy to produce visible light. Typical efficiency from 10 lm/W up to 22 lm/W for the best halogen lamps.
Ban the bulb
Some policy makers have started to announce the phase-out of this obsolete technology. After Australia announced its planned phase-out by 2012, this was followed by some US states such as California. Recently at the last European Union Summit in March 2007, EU leaders asked the Commission to investigate a possible phase-out of incandescent lamps.
The "traditional" energy-efficient solution for lighting in the residential sector has been the compact fluorescent lamp (CFL). CFLs have improved over time in terms of size, performance, quality and at the same time prices have come down. However, CFLs are not accepted by all users as an incandescent replacement for performance and aesthetic characteristics (size, shape, colour rendering, light directionality). CFLs are currently promoted through a number of policy mechanisms, but cannot replace all incandescent lamps, for example reflector lamps and spot halogen lamps.
Recently, LEDs have become a real alternative light source for white light and LED manufacturers have claimed very high lumen efficacy.
The European Commission Joint Research Centre (JRC) is currently researching the most efficient technologies for the electricity end-use, together with their cost and environmental impact, in order to asses their possible implications for European energy-saving targets. In early May 2007, the JRC organised a workshop to discuss R&D, market information, and applications of LEDs for general lighting, in order to gather the latest information on this topic. LED researchers, market analysts, LED and luminaire manufactures and lighting experts were invited to air their views on the possible use of LEDs for general lighting.
The first part of the workshop was focused on the LED market development. Robert Steele of Strategies Unlimited presented an updated view of the LED market, which grew in 2006 by was 6% to reach $4.2 billion. The LED lighting segment was worth $205 million. This market is growing at around 37% per year - the highest of any HB-LED application - and is projected to reach about $1 billion in 2011. The majority of applications use RGB LEDs today, but white will increase to more than 60% of the total market by 2011 (see the recent LEDs Magazine article by Bob Steele).
The other speakers during the first day all indicated clearly that the LED efficacy continues to improve and it is nowadays approaching 100 lm/W in production in cold white (at about 350 mA, which is about 1 W input power). All the speakers highlighted that high-power LEDs (e.g. at 1000 mA) are less efficient, and that warm white LED have a lower efficacy.
Speakers also highlighted that there is a difference between manufacturers data sheet claimed values and real life applications values (at much higher junction temperature, and in steady state applications). It was highlighted that test methods and standards to measure LED performance and safety are currently under development.
Power LEDs are improving rapidly and continued improvement is expected. Performances of 100 lm/W are already available and about 150 lm/W is likely in the near future (for the time being has been reached only in the labs, and for low power). The theoretical efficacy limit being beyond 200 lm/W. As efficacy increases, thermal management, one of the major problems with LEDs, is simplified and system costs can be reduced. Key areas for future improvement are white colour uniformity, CCT and CRI.
Although LED efficacy is important, the optical (fixture) and electrical (driver) efficacy are also important.
Among the various example of LED light sources on the market for the residential sector, the following is illustrative of today's best available technology: a warm-white (2700 or 3500 K) downlight providing 650 lm at 11 W wall-plug power (this is equivalent to a 60 W GLS lamp) 60 lm/W efficacy (wall-plug), with a colour rendering index 92.
Efficiencies of higher than 85% have been achieved for both AC/DC and LED driver stages. For LEDs drivers, closed-loop feedback is also available to keep the performance stable over time. This will enable the LEDs to change from a basic light source to smart light processor.
When comparing LEDs to other light sources, many speakers indicated that the usable light is much more important than raw LED efficacy. LEDs may have very efficient drivers (80 to 90% efficiency) and also very efficient optics (again in the range of 80%), while CFLs usually have poor lighting distribution and the final efficacy can be as low as 20 lm/W.
Other key areas of current development include phosphors, a key material for solid-state lighting, to create different white lights from blue light LED, while preserving the optical efficiency. Another presentation described investigations to improve the performance attainable by fabricating photonic crystals and photonic quasicrystals (PQC) into the top emitting surface of LEDs.
The presentation by Wolfgang Budde of Philips addressed OLEDs for lighting, which have reached efficacies of 32-64 lm/W at 1000 cd/m2. The second part of the presentation was an analysis of power LEDs and of the efficiency improvement challenges of the power chain from "electricity in" to "light out". The overall performance of the power chain is influenced by many parameters, and optimisation is necessary to limit colour shift over temperature and lifetime and efficacy decay with higher driving current.
In conclusion, LEDs and OLEDs are already quite efficient but the overall system optimisation is very important, requiring a high level of control and automation. It remains a challenge to match the lifetime of the light source and electronics. Also, the importance of good luminaire design requires close cooperation between the light source manufacturer and electronics supplier.
Lighting designers have their say
The second day of the workshop was dedicated to presentation by lighting experts, luminare manufacturers, lighting designers and industrial designers.
It was highlighted by speakers that LEDs are still a niche technology in lighting, though LEDs could offer many advantages. First of all, the initial cost per 1000 lumens of white LED light is 2-50 times higher than conventional light sources (including the more expensive light sources such as T5 fluorescent). The lumen cost of LEDs has dropped by about 30% during the last year.
However long lifetime, energy savings and low maintenance costs can help to compensate the high initial costs of LEDs over lifetime, and thus the total cost of ownership (TCO) may be comparable to other light sources.
For the time being one of the most successful niche application is in portable lighting, such as torches, where LEDs offer superior performance. Another niche application to use LEDs with photovoltaics. This niche application, currently used widely for garden lighting in developed countries, can also offer a cost-effective solution for lighting in off-grid situations in developing countries. LEDs not only improve dramatically the quality of lighting, but also improve the indoor air quality compared to fuel based lighting, and this at a reduced cost.
In the residential lighting sector, LEDs are still a new light application, sold mainly as a fashionable and expensive light source. However, LEDs are already making a breakthrough in the retail sector (fashion and jewellery shops), and frozen food cabinets in supermarkets. Alberto Frazer, a lighting designer, said that LEDs offer to professional designers a lot of opportunity for creative design, particularly for architectural lighting, and the use of coloured light.
Advantages and challenges
Luminaire manufacturers also expressed great interest in LEDs. Among the design advantages offered by LEDs the following were mentioned:
• Very compact dimensions of lighting fixtures
• Can fit in conventional light fittings;
• Dimming is possible without change in colour temperature;
• Colour of light is adjustable with use of multi-coloured LEDs, enabling dynamic colour control and high colour saturation
• Large range of colour temperatures
• Long operating life
• Reduced or zero maintenance costs
• High luminous efficiency
• Absence of IR and UV light
• Safety, because of very low voltage
• Very reduced amount of heat (no risk of burns)
• Cold switch-on (down to -40°C) and higher efficacy at cold temperatures
• Precise directional emission without accessories or refractors
• Use of efficient optics made of polymers
It was also noted that LED technology is not yet able to replace traditional lighting. LED fittings have to be carefully planned before installations or they will lose their appeal and potential. It was also highlighted that:
• Realistic performance has to be declared by LED manufacturers
• There is a lack of standards to measure and describe LED performance Poor education of users for the acceptance and correct use of LED
• Poor CRI
• Different control and standards interfacing
• Lack of consistency (binning)
• Different approach to design (e.g. mechanical/electronic design fully integrated) in the luminaire
• Outsourcing design of strategic components (need particular attention due to the increased weight of electronics value) or companies need to acquire in-house new specific expertise
LEDs in luminaires
In addition, LEDs are not to be considered as a standard lighting source: a conventional lighting fixture is designed to host a standard light source (interchangeable), while an LED luminaire is built around the LEDs. Luminaire manufacturers should use their expertise to create more efficient and attractive final products, this includes looking at thermal management, electrical optimization (driver), optical optimization (secondary optics), optimized housing designs that make use of the LED's miniaturized size, and finally control versatility.
In summary, luminaire manufacturers and lighting designers expressed great interest in LED lighting, and indicated that LEDs offer several advantages beyond energy-efficient performance.
There was general agreement that LEDs will become an important technology to reduce lighting consumption in buildings and other applications, by offering better light quality than fluorescent lamps, and reaching the same level of efficacy (or even going beyond it). OLED technology, still in the R&D phase, will bring additional savings, and further enlarge the application of solid state lighting.
All the workshop presentations are available at: http://re.jrc.ec.europa.eu/energyefficiency/html/Workshop_LED_34052007.htm.