General Photo-Electro-Thermal Theory for LED Systems & the LED Revolution for General Lighting

Transcription

1 General Photo-Electro-Thermal Theory for LED Systems & the LED Revolution for General Lighting Ron Hui Center for Power Electronics City University of Hong Kong 1

2 Content Introduction Lighting Systems and Power Systems The LED Revolution Problems & Solutions General Photo-Electro-Thermal Theory Conclusions 2

3 NASA Lighting consumes ~ 20% of total electrical power 2 Billion people have no electric lighting 3

4 Road Lighting systems today use High-Intensity- Discharge Lamps 4

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6 Energy saving of 30% 6

7 Examples of Saving Achieved by Outdoor System 鶴山 28% (10,000 street lamps in Sept. 2006) 高明 49% 東莞 30% 梧州 30% Hong Kong 25% (1500 street lamps in ) 7

8 City No. of street lamps Power e.g. 30% change Hong Kong 130, MW 9 MW Shenzhen 130, MW 9 MW Shanghai 240, MW 15 MW Public lighting systems in car parks, corridors and hallway can also be grouped together 8

9 Why LED? High-Intensity-Discharge (HID) lamps cannot be re-ignited immediately after turn-off unless hot-igniter of 20kV is available. HID lamps can only be dimmed over tens of minutes to avoid sudden breaking of discharge arc. LEDs are semiconductor devices capable of switching at Mega-Hertz (A Highly Dynamic Load micro-second response time) 9

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11 Content Introduction Lighting Systems and Power Systems The LED Revolution Problems & Solutions General Photo-Electro-Thermal Theory Conclusions 11

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13 candles Incandescent lamps Lumen/Watt 1 10 CFLs LEDs HID lamps

14 X e-cfl LED bulb Efficacy 60 lm/w 80 lm/w Lifetime 10,000 hrs 40,000 hrs Mercury Yes No Cost / unit US$ 7 US$ 40 Cost /10000 hrs US$ 7 US$ 10 E-waste / hrs 4 pieces 1 piece 14

15 LED Current Status Successful in Decorative Display Signaling Signage Not so successful in General/Public Lighting All major LED Road Lighting projects in China in failed. 15

16 3 remaining hurdles to General/Public Lighting High Temperature greatly affects the LED performance because Efficacy (Lumen/Watt) degradation with junction temperature rise 1. LED System Thermal Design not optimised. 2. LED Device structure not optimised. (junction thermal resistance is too high) 3. Lifetime of LEDs (<8 years) and LED Driver (<5 years) 16

17 Heat remains a limiting factor in LED Technology From 2008 IEEE APEC Tutorial Note 17

18 Simple methodology for heat measurement in an open system Photograph of the setup 18

19 Temperature data analysis and heat dissipation factor With the heat dissipation measurement obtained, one can define a heatdissipation factor k h (P lamp ) for a lighting device, where k h P lamp P P heat lamp P heat = Heat dissipation from the lamp in Watts P lamp = Total electrical input power of the lamp in Watts. This k h (P lamp ) factor is an indicator of the amount of heat energy emitted from a lighting device for a given electrical input power of the lamp. 19

20 Practical tests and heat dissipation comparison Thermal measurements have been conducted in (i) one 18W T8 fluorescent lamp, (ii) one 14W T5 fluorescent lamp, (iii) six Philips 3W white high-brightness LEDs (iv) eighteen Philips 1W white LEDs (v) six CREE 3W LEDs 20

21 Reference temperature-time curves & reference temperature-power curve based on a Reference Resistor Temperature Rise - Time curves of 12W, 14W, 16W, and 18W of the reference-resistor Temperature Rise - Power reference curve 21

22 Temperature measurements of the three types of lighting device i) One 18W T8 fluorescent lamp Temperature rise curves for one 18W T8 fluorescent lamp working at 12W, 15W & 18W Heat dissipation factor k h (P lamp ) of 18W T8 lamp as a function of lamp power. 22

23 Independent Confirmation 23

24 18W The Temperature Rise Time curves of the one 18W T8 lamp, Eighteen Philps Luxeon K2 1W LEDs, six Philps K2 3W LEDs and six CREE LEDs with all systems at 18W operation 24

25 At Full Power 18W T8 Fluorescent lamp (Osram) 14W T5 Fluorescent lamp (Philips) 1W LED (Philips Luxeon) 3W LED (Philips Luxoen) LED (CREE) Rated Efficacy Lumen/watt At 25 o C Junction temperature 40 At 25 o C Junction temperature 107 At 25 o C Junction temperature Measured Efficacy Lumen/Watt at 1W (heatsink temperature of 70 o C) 30 at 3W (heatsink temperature of 80 o C) 78.5 at 3W (heatsink temperature of 76 o C) Heat Dissipation Factor Comparison of luminous efficacy and heat dissipation of LEDs and fluorescent lamps 25

26 Solutions Optimal LED Devices Optimal LED Integrated System Design LED Drivers with long (>15 years) lifetime 26

27 Content Introduction Lighting Systems and Power Systems The LED Revolution Problems & Solutions General Photo-Electro-Thermal Theory Conclusions 27

28 LED Characteristics 3-4 o C/W

29 Efficacy degradation with temperature I I 25 o C exp ( T j 25 T 1 o C) E E 1 o k e T j T o T j T o E 0 k e is negative 29

30 Basic General Photo-Electro-Thermal Theory Light and Electric Power N E v P d T j R jc T hs R hs Light and Heat o E E 1 k e T j T o Heat and Electric Power P heat 1 N N LED devices R jc P heat T a T T hs j T T a hs R hs R NP T R Nk P jc heat a hs P T R k P heat hs jc h h d d 30

31 Basic General Photo-Electro-Thermal Theory Light and Electric Power N E v P d T j R jc T hs R hs Light and Heat o E E 1 k e T j T o Heat and Electric Power P heat 1 N N LED devices R jc P heat T a T T hs j T T a hs R hs R NP T R Nk P jc heat a hs P T R k P heat hs jc h h d d 31

32 v NE o k T T P k k R NR 1 2 e a o d e h jc hs Pd v P v P 2 1 d 2 d Peak luminous output may not occur at the rated power!!! * P d P d 32

33 dv dp d NE o 1 k T T 2k k R NR 2 dke 2 dkh 2 dr jc T T P k R NR P k R NR P k k P a e o a d o h jc e h jc hs d hs dp d P d e jc hs d dp d e h d dp d 33

34 Simplified model with constant R jc v NE o k T T P k k R NR 1 2 e a o d e h jc hs Pd P * d 1 2 k k e h ke T R jc a To NR hs v Increasing sizes of heatsink k e is ve Rjc =10 o C/W Choices: Rhs= 3.9 o C/W N=? * P d P d 34

35 Vigorous model with temperaturedependent Rjc R jc jco R 1 k jc P d v NE o k T T P k k R NR P 2 k k k R 1 3 e a o d e h jco hs d e h jc jco Pd v P P P d 2 d 3 d Extra 3 rd term 35

36 Practical Verification Thermal resistance R hs o C/W The better the cooling effect, the lower the thermal resistance R hs becomes. 36

37 Tests on Luxeon 3W LEDs 37

38 Test-1: Mounting 8 Luxeon 3W White LEDs on a Small heatsink with thermal resistance of 6.3 o C/W 315 lm v P P 2 1 d 2 d Problem!!! P v P P d 2 d 3 d 38

39 Test-2: Mounting 8 Luxeon 3W White LEDs on a Medium heatsink with thermal resistance of 4.5 o C/W lm Problem!!! 39

40 Test-2: Mounting 8 Luexon 3W White LEDs on a Large heatsink with thermal resistance of 2.2 o C/W 560 lm Problem!!! 40

41 Effects of Thernal Design on Light Output 1400 Luexon 3W Total Luminous Flux (Lumens) measured light output of 6.3oC/W calculated light output of 6.3 degree/w measured light output of 4.5 degree/w calculated light output of 4.5 degree/w measured light output of 2.2 degree/w calculated light output of 2.2 degree/w calculated light output of 0 degree/w Power per LED (W) Rated LED Power = 3W 41

42 Tests on Cree 3W LEDs 42

43 Mounting 2 CREE 3W LEDs on heatsink with Rjc = 6.3 o C/W 43

44 Mounting 2 CREE 3W LEDs on heatsink with Rjc = 4.5 o C/W 44

45 Optimal Design Method Total Luminous flux/no. of LED devices (lumens) Theoretical limit: R hs =0 A d c b a B Rated P d C Per LED Power D Increasing cooling effect (e.g. thermal resistance R hs decreasing) P d (W) 45

46 Total Luminous flux/no. of LED devices (lumens) Theoretical limit: R hs =0 A d c b a B Rated P d C Per LED Power D Increasing cooling effect (e.g. thermal resistance R hs decreasing) P d (W) 46

47 Implications of the theory P * d 1 2 k k e h ke T R jc a To NR Device Level: for LED manufacturers [for the same rated power] Multi-chip LED structure is better than single-chip structure hs System Level: for the LED system designers [for the same system power] Distributed system with more low-power LEDs is better than concentrated system with less high-power LEDs 47

48 Device Level Multi-chip Multi-chip 30 x 0.1W = 3W Single-chip Single-chip 1 x 3W = 3W Multi-chip Single-chip 48

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50 Using Distributed five 1W LEDs or Concentrated one 5W LED? System Level Distributed Concentrated 50

51 Conclusions LED technology is at a critical point of entering the market of General/Public Lighting LED improvements continue. LED Systems with lifetime > 8 years to be ready in 2010 LED Drivers with lifetime of >15 years to be ready in Large-scale LED lighting systems are dynamic loads for emerging power grids. 51

52 Theoretically, LEDs are being promoted as energy-efficient lighting devices. Caution Practically, they have opened a door to widespread lighting energy abuses & light pollution problems. 52