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