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Energy-efficient lighting: Challenges for the future
Dr Michelle Moram
Centre for Gallium NitrideUniversity of Cambridge
19th century 21st century
19th century 21st century
21st century19th century
Thomas Edison (1879)
We need to move on!
Lighting in the developed world
• Electricity generation is currently the single biggest source of CO2 – 2,000,000,000 tons produced worldwide per year1
• Emissions from lighting have reached 70% of CO2 emissions from all cars1
• Approximately 3 times more than all aviation emissions1
• Consumption is evenly split between domestic and commercial use
• Domestic consumption is significantly higher in the USA2
Lighting consumes about 20% of our electricity supply2
1International Energy Agency Report, 20062US Household Electricity Report, DOE, 2005
Lighting in the developing world
Demand for lighting is rising, but we cannot limit usage
• 1.6 billion people have no access to electric lighting
• The USA uses 30× more energy for lighting per person than India
• Lighting is essential for development (education, work, health) and will be a primary driver of electricity usage
• We cannot limit this!
We expect that demand for lighting will be 3× higher in 2030
Facts and figures
• Efficiency (%)
Total light output (W)
Electrical input (W)
Our eyes can’t detect every wavelength!
• Luminous efficacy (lm/W)
Visible light output (lm)
Electrical power input (W)
cones
rods
Maximum for white light: 240 lm/W
• Colour rendering index (0 – 100)
Ability of a light source to reproduce colours faithfully
Current technology: incandescent bulbs
About 95% of the energy supplied is wasted
Resistively heated tungsten wire (up to 3200 oC) surrounded by inert gas to prevent oxidation
Failure mostly through thin ‘hot- spots’, leading to filament evaporation and breakage
Luminous efficacy of 12 lm/W
Current technology: incandescent bulbs
80% of lamp sales by volume!
Lifetime of 1000 – 2000 hours (up to a year of intermittent use)
The price of a bar of chocolate
Warm, cosy light
Resistance to change…
Current technology: compact fluorescent lamps
80% of the energy supplied may be wasted
Electrodes are placed at either end of a tube full of mercury vapour and a plasma is ignited
Electrons collide with mercury atoms, producing 185 nm and 254 nm UV light
A phosphor coating converts UV light into visible light
Luminous efficacy of 60 - 80 lm/W
Electronic ‘ballast’
• • • ••Ballast
Daily Express, Saturday 14th March 2009
Current technology: compact fluorescent lamps
CFLs do contain mercury (but they also reduce mercury emissions from power stations)
CFLs do emit UV light (but so do incandescent bulbs; also, the UV is absorbed by air within a foot or so)
BBC News Online, Thursday 9th October 2008
Current technology: compact fluorescent lamps
High failure rates and poor quality light
Lifetimes of 6000 – 15000 hours (continuous use), but can be as low as 1000 hours when frequently switched
Catastrophic failure may occur
Phosphor wears out and mercury leaks away
Can’t be used with dimming circuits (severe fire risk!)
Poor colour rendering
Slow turn-on times
Challenges faced by current lighting technology
High luminous efficacy and good “fixture efficacy”
Acceptable light quality (good colour rendering)
Long lifetimes
Cheap initial price
Trustworthy, reliable devices with a good public image
All vital to ensure energy savings – consumer acceptance is just as important as energy efficiency!
Either popular (but very inefficient), or unpopular (mostly for good reasons, and still not efficient enough)!
We need:
How many scientists does it take to change a light bulb?
“We believe there is a strong possibility of developing the LED as a practical white light source. If these plans work out, the lamp of the future may be a piece of metal the size of a pencil point, which will be practically indestructible, will never burn out, and will convert at least ten times as much current into light as does today’s bulb”
Nick Holonyak, Reader’s Digest, February 1963
Light-emitting diodes (LEDs) first developed in the early 1960s and based on GaAs – but could only emit red light efficiently
Now use the AlInGaP family of semiconductors
New technology: solid-state lighting
Single-colour LEDs can be very, very efficient!
Red incandescent: 4 lm/W
Red LED: 100 lm/W
The first GaN-based blue LED was developed in 1992
GaN-based green LEDs were developed later and are much less efficient than blue LEDs
Other LED colours
Group III-nitrides are the only semiconductors that emit light efficiently at short wavelengths
Blue + phosphor
How to make white LEDs
Trade-off between high CRI and high efficiency!
Blue + yellow-green + red
White LEDs: 160 lm/W demonstrated in the lab
Comparing technologies
LEDs already offer significant energy savings
100 W incandescent:
25 W CFL: 20 W wasted
5 W emitted as light
10 W LED: 5 W wasted
5 W emitted as light
95 W wasted
5 W emitted as light
1,000 hours
10,000 hours
100,000 hours
Extremely physically robust
Gradual failure
Will work in a dimmer circuit
Excellent “fixture efficacy”
Colour can be tuned easily
Extremely long lifetimes (60,000 hours+) 60 years of intermittent use never need replacing!
Advantages of white light-emitting diodes
Performance problems solved – plus more efficient!
No flicker
No mercury
Easily retrofitted
Increase the efficiency of electricity-to-light conversion at high operating currents – improving LEDs and luminaires
Better thermal management longer lifetimes and improved reliability run at even higher currents get more light out of single units reduce chip area needed per device remove heat sink reduce size & transport costs
Create designs that facilitate rapid, cheap productionThinner structures (higher throughput, easier processing, better uniformity) Large-area substrates (much less waste) Simple processing steps only
LEDs – key challenges
Costs MUST be reduced at least fourfold
1 10 100 10000
1
2
3
4
5
6
7
Effic
ienc
y (a
.u.)
Current Density (A/cm2)
Q-2T LED 2T LED
Cambridge work – improving efficiency
Efficiencies drop with increasing λ
and operating current
280 320 360 400 440 480 520 560 600 6400
20
40
60
80
Manchester- Cambridge
Literature
Inte
rnal
qua
ntum
effi
cien
cy (%
)
Wavelength (nm)
UV Green
AlGaN GaN InGaN
Best at blue wavelengths Best at low currents…
Blue
Cambridge work – improving efficiency
Improving the quality of the nanoscale light-emitting regions (new materials)
Developing alternative light-emitting nitride materials
HRTEM (0002) lattice fringe image
5 nm
InGaN
GaN
GaN (n)
1μm
Sapphire
GaN (p)
Bright field cross-sectional TEM image
(0001) Al2 O3
p-GaNn-contact
p-contact
n-GaN
Cambridge work – improving efficiencyDefects (109 cm-2) limit light output and increase heating
Reduce defect densities by 3 orders of magnitude
1 μm
Plan-view SEM-cathodoluminescence imageBF plan-view TEM image BF cross-sectional TEM image
Fluorescent lamps are already widely used in commercial settings, yet lighting still consumes 20% of our electricity supply
LEDs could reduce this to 10% by 2015 and 5% by 2030
Could save £1.7 billion per year in energy costs in 2015
Could close down 8 coal-fired power stations and prevent the emission of 30 million tons of CO2 per year
1. Impact of LEDs on energy reduction
CFLs already in use; new technology (LEDs) needed to reduce emissions further to the low levels required
A recent DEFRA report (May 2009) identified LEDs as the most promising new energy-saving lighting technology (full life-cycle analysis)
The largest LED screen in the world (Arkansas), with 2.5 million LEDs
1500 ft display in Fremont St, Las Vegas: 12.5 million LEDs
Solar-powered LED display in Beijing
2. Impact of LEDs on health
LED lighting can be tailored to increase Vitamin D production, reduce SAD and improve sleep
Vitamin D is produced by UV-B exposure; protects against breast and prostate cancer by preventing cell overproduction BMJ, 2003
Depression, eating disorders and immune deficiencies are linked to natural light deprivation The Times, September 15th 2007
3 million people in the UK suffer from SAD; LEDs are the most effective form of lighting for boosting serotonin levels Yale, University of British Columbia
Optimum λ: 308 nm
Cheap, efficient lighting will enable improved education (UN Millennium Development Goals)
Better visible LED technology leads to better UV-LEDs
Portable, robust, point-of-use water treatment devices
Distributed water treatment systems independent of centralised provision and poorly maintained infrastructure
Resilience to climate change – also relevant to First World!
3. Broader impact of LEDs
LED technology has a broad role to play in both limiting and adapting to climate change
Invest in applied research and create more incentives for industry to develop high-efficiency, low-cost LEDs
Set simple but high standards for the performance of energy-efficient lighting on sale in the UK
Encourage consumer acceptance (TV advertising, public outreach programmes, demonstrated safety record)
Consider legislation and/or subsidies to accelerate both household and commercial uptake of solid-state lighting
Lighting policy
Minimum efficiency and CRI, set colour temperatures
• No further step changes required - LED technology is already appropriate for our energy-saving needs
• Main barrier to widespread use is cost
• Improvements in efficiency and processing costs needed
• Consumer acceptance is likely
• Wider benefits: water treatment, public health improvement
• Enables development, mitigates climate change increases resilience in the face of global climate change
LEDs – lighting for the 21st century
Thank you to our funding bodies and collaborators!
LEDs – lighting for the 21st century