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Monday, April 13, 2009

LED inefficiencies: 82% of lighting energy lost as heat

Apr 13 2009 10:34AM | Permalink |Comments (27) |

Optek sent me a DVD they’ve produced on heat management for HB LEDs. The Optek folks are hoping that Mike King can hand it out to LED Workshop attendees at his morning paper, “Thermal management of visible LEDs.” I said, well, maybe, let’s have a look at it first. Mike’s paper, btw, is excellent, full of good stuff while avoiding product promotion.

It turns out that the DVD is a 3-D animation showing all of the losses (listed in the table below) that can occur in LEDs. Those losses show up as heat, and the challenge is to remove the heat, or preferably, to design the HB LED so that the losses are minimized and never show up as heat in the first place.

In our current love-affair with solid-state lighting, it’s easy to forget that LEDs are only about 18% efficient – the other 82% of the energy put into an LED dissipates as heat. From the Optek DVD the breakdown by type of loss is:

Type of loss	Reduce by improving:	% of energy lost
Electrical	P-doping, P-contact	15%
Quantum	Lattice vibrations due to imperfections, Auger recombination	17%
Extraction	Texturing layer surfaces	20%
White light conversion process	Phosphor chemistry	30%

There’s a lot of room for improvement in each of these areas of loss – especially the white light conversion process. As LED technology improves, each becomes more efficient and the amount of heat drops correspondingly. But there will always be losses associated with each step in the light conversion process, so thermal management will continue to be one of the areas of HB LED design that’s all too easily overlooked.

You can attend Mike King’s presentation on thermal management for HB LEDS – and snag one of those DVDs -- at EDN’s free LED Workshop on April 30 at the Hyatt Regency in Santa Clara, CA.

Related entries in: Components, Hardware, Interconnect | LED |

Reader Comments

at 4/13/2009 2:16:06 PM, W17053 said:
How do these losses compare to incandescent?
How do these losses compare to CFL?

at 4/13/2009 2:18:43 PM, Art F said:
For comparison, what is the efficiency of tungsten-halogen or CFL lamps??

at 4/13/2009 2:22:32 PM, Ron Bauerle said:
And are LED drivers going to be as heat-sensitive as CFLs? Most of my lighting applications are either enclose or high-power 3-way (50/200/250 W) - CFLs don't last in the former, and don't exist for the latter :^(

at 4/13/2009 2:34:16 PM, TMcD said:
The first three writers asked exactly the first questions that came to my mind as well. I hope EDN will address these definitively soon.

at 4/13/2009 2:38:03 PM, Jay said:
I have wondered for some time now if anyone anywhere in the last 50 years has worked on finding a more efficient emitter for an incandescent bulb

at 4/13/2009 2:54:37 PM, awc said:
The way I understand this is if LEDs were 99% efficient, the 1% of the energy lost would be broken down into these percentages. Fairly misleading..what is the absolute, not relative number?

at 4/13/2009 3:02:08 PM, J. WIlliams said:
Perhaps a more meaningful comparison would be lumens/watt. A typical incandescent bulb is about 14-16 lumens/watt. A CFL is about 60-72 lumens/watt. The newest HP LED''s are running at about 95 lumens/watt. Perhaps another measurement tool would look at the total life cycle cost to include the cost of the luminary.

at 4/13/2009 3:06:53 PM, Ted said:
Or, if the source you quote is giving the efficiencies of the individual, cascaded, subsystems then the total efficiency is the product, and the loss is 60% rather than 82%.

Can you clarify?

The idea that LED's are "cold" is a futurist myth.

at 4/13/2009 3:07:25 PM, Bob said:
LED 18%
CFL 9-11%
Tungsten 2.6%

see wikipedia - luminous efficiency

at 4/13/2009 4:24:19 PM, ToocooLEDs said:
lumens/Watt is a useful starting point for efficiency comparisons between technologies, but for most lighting applications an even better number for comparison is efficacy - what is the amount of light reaching the target for a given amount of energy input?
For an equivalent number of lumens per Watt, an LED source will provide more useful light than a fluorescent in many applications because the light usually needs to be directional. A lumen is a measure of total light flux emitted in all directions, much of which gets lost if you put a CFL in a floodlight reflector.

at 4/13/2009 5:23:41 PM, stan-2 said:
Does this include the power in the 'special drive electronics' or just the LED

at 4/13/2009 6:01:58 PM, DGI said:
I''m not a power company type but if the load from LED drivers is anything like CFL''s, I''d be concerned.

Using a Tektronix current probe and a Tek scope I measured the current into a 23 W CFL. I was interested in the power factor of the lamp.

What I measured was a 1.3 amp peak pulse every half cycle. It was a 2.3 ms wide pulse with an irregular shape. The CFL was a Feit brand.

IR loses in the supply lines are very much in play here.

I never did figure out the power factor.

at 4/13/2009 6:55:18 PM, Steve said:
Hey, cite your bad bulb, DGI; nice description! People watch bad news though, so maybe this will bring some love for makers, who after licensing enough IP from designers will eventually sell direct to market for some of their output. Hopefully they'll build in enough antagonism that when efficiency hits 100% or better (hey, they can suck up the harmonics from the evil CFL ballasts around) they don't get hit for longer lifetime guarantees.

at 4/14/2009 10:47:46 AM, Timbalionguy said:
In all three of these devices, you are taking electrical energy (moving electrons) and transforming the energy in that movment into electromagentic waves (pure energy). This step in the process has always been the lossy one, and there are physics limits to how much it can be improved. I think, but I can't prove, that the losses in the driver electronics, especially for LEDs, can be reduced to a negligible level compared to the losses in the LED. CFL's will be more of a challenge, as they are essentially high voltage devices. Incandescent lighting really doesn't need electronics, but it probably has been improved about as much as it can be at this time. (2.8 percent was quoted above, but I have seen 1 percent more commonly given for conversion efficiency.) One thing to keep in mind, especially for incandescents and maybe LEDs-- heat is also electromagentic energy. Maybe there is a way to convert this into light with a phosphor, etc.

at 4/14/2009 9:25:31 PM, wridot said:
An interesting comparison on LED versus incandescent efficiency is found by
GOOGLEing the terms "Comparison of Power Conversion of White Light Sources" and "energy" to find a USgovt DeptOfEnergy website

LED has 15% radiant emissions versus 80% radiant emissions for incandescent lamp -- the opposite of what you'd expect. An LED creates useless heat that needs to be dissipated by conduction, whereas an incandescent lamp's huge radiant emissions are largely infrared, and not visible light. So an LED wins efficiency for visible emissions, whereas an incandescent lamp wins for total radiant emissions.

at 4/17/2009 9:15:27 AM, SteveP said:
wridot,
You need to look at the numbers that make up the "radiant emissions".
For the incandescent only 8% ends up as visible light, the desired form of radiation. For LEDs 15%-25% end up as visible light, two to three times more visible light per unit of energy.
Also, the "LED's useless heat" is the same wasted energy as the "incandescent's infrared" The LED doesn't operate at high temperature like the incandescent so most of the LEDs wasted energy is removed through conduction and convection instead of radiation.

at 4/17/2009 4:50:42 PM, wridot said:
SteveP''s comments on visible emissions are totally correct but depending on the context may be misguiding the public into demanding LEDs.
An LED automotive taillamp assembly (not a resistor + LED) is a complex device due to a design requirement to limit LED temperature over a broad operating temperature range -- lots of LED heatsinking too. It is misleading to think that LEDs do not generate heat. An incandescent taillamp assembly is simple despite the hot bulb.
As for being ''green'', in Quebec the power is hydroelectric and the utility has admitted that using incandescent lamps in the winter (the IR warms the entire room like a heatlamp) is cheaper and less polluting than heating with expensive oil or natural gas (it may be different elsewhere where you need airconditioning) and using fancy lamps.
So LEDs (3 times more efficient for visible light than incandescent, but they heat the ceiling fixture only) aren''t always better.

at 4/21/2009 1:00:06 PM, gmorita said:
There is another thing to consider when talking about using LEDs for lighting. Much of the heat load in a building is the result of the heat generated by the lighting fixtures. In the summer this means more energy has to go into air conditioning so it would seem that LEDs win. On the other hand in the winter, the heat generated by the lighting actual reduces the demands on teh HVAC system, especially in northern tier cities. Has anyone calculated the energy savings over the course of the whole year in places like Minneapolis compared to Las Angeles?

at 4/21/2009 1:01:27 PM, Ampman said:
wridot, there's no magical device that is good or best for everything. LED is currently the most efficient light emitter which means it converter electrical power into light most efficiently, therefore it can't possibly waste electrical energy as heat, otherwise it wouldn't be efficient light emitter.

Heat is the lowest form of energy. Any other form of energy can be turned into heat at 100% efficiency, but to turn heat into another form of energy has low efficiency, that of Carnot or Rankine efficiency.

at 4/21/2009 8:20:26 PM, wridot said:
There''s absolutely no argument that LED is more efficient at producing visible light than lowly incandescent bulb -- just what is needed in California.
But in certain circumstances the huge infrared radiation emitted by the incandescent lamp is very beneficial -- such as melting snow/ice forming on a car headlamp during a storm, or evenly heating a cold room, being a mini heatlamp. That''s something an LED conduction fixture''s heat dissipated in the ceiling cannot do since the dissipation isn''t radiated -- but I guess you can add a fan into the LED assembly (it''s done). I haven''t seen an LED heatlamp yet...
What is best depends on the application. Personally I''m waiting for LEDs to replace compact fluorescents once smart power meters used by utilities start dinging consumer wallets due to CF''s poor power factor...

at 4/23/2009 8:20:20 AM, herbster said:
years ago MIT and a lamp mfr. collaborated on an incandescent lamp whose bulb was close to a perfect sphere, coated with an infrared reflector and the filament was positioned at the focus (center). the i.r. was used to replace some of the electricity heating the filament, thus improving its efficency- must have had issues, didn''t make it to market

at 4/25/2009 2:12:55 PM, markem said:
herbster, the efficient incandescent lamp you mention did make it to market is and called an an IRC Halogen bulb (OSRAM make them for example). Such bulbs generate about 25lm/W.

at 7/2/2009 5:27:54 PM, Gerry said:
Not all of the phosphor conversion loss ends up as heat. Some of it just ends up as deep blue light radiating from the device which doesn't have a great deal of luminous efficacy and therefore contributes little to the lumens/watt result.

at 7/7/2009 2:09:22 PM, Jarhead said:
For bulbs there was a technology that was deep-sixed. It was developed by one of the National Laboratories, and it was called a Photonic Lattice. Google and you'll find it. The Lattice dimensions of the "filament", controlled the wavelength ranges of the electromagnetic spectrum emissions quite well.

at 7/8/2009 12:29:35 PM, Jarhead said:
Some companies blue LED products (not all are created equal), are over 50% efficient. Also, some white LEDs are now hitting 45% conversion, leaving only 55% which ends up as heat. Not all LEDs are created equal.

Optek is far from being a leader for efficient LEDs.

There has been further advances in the labs, such as some scientists putting a wavelength selective mirror between the die and phosphor (reflects "yellow" phosphor emissions to prevent them from being absorbed in the die), which the scientists tested and found a 1.64 to 1.95 gain in device output:
"Enhanced forward efficiency of Y3Al5O12:Ce3+
phosphor from white light-emitting diodes using
blue-pass yellow-reflection filter"

at 7/8/2009 2:00:30 PM, Dave said:
The 95Lume/watt values for white LED are misleading. Those only apply while the junction on the chip is still 25C. So these companies report peak power at <1 second operation. as soon as the lamp has been on for say 2 minutes, there is still a nearly 40% light output drop.
CONTINUOUS operation Best available technology from Nichia is ~70 LPW

at 8/3/2009 2:36:07 PM, Jarhead said:
Actually Dave, I've measured parts at a steady state condition that are doing 130 lm/W. One needs to pay careful attention to addressing thermal issues. And they particular ones were not Philips LumiLEDs devices.

Recently the bar has been raised and competition has resulting in a couple of LED makers who have exceeded the 100 lm/W number.

I did try to order through Future, some of the new Philips LumiLEDs parts which are specified to be greater than 100 lm/W, but they are still not shipping and no idea when they will have them, short some unofficial engineering samples.

And it definitely was not Nichia, one of their power LEDs did manage to do 105 lm/W at a steady state die temperature of 41.2ºC and 0.30002 Amps when under powering it a little. But yes, some of their SMT devices will hit 130 lm/W if pulsed, and then it drops quickly due to the packaging thermal resistance of the small edgelight and many keypad LEDs. Many of their package range from 200 to 300 for their ºC/W numbers. However, they do have a couple of ceramic "boat" parts (instead of plastic) that have a lower thermal resistance.

I believe it was CREE that was pushing to use the NIST method of steady state measurements, and there were some papers on the same, and CREE was submitting their parts to NIST for independent third party verification.

CREE has also been using ceramic (which is nice as they have electrically isolated thermal pads) which has their ceramic package XP-G ~3.45mm device that produces 132 to 139 lumens per watt at 350mA. For lab results, they have hit 161 lm/W. It will be interesting to see that part get on the market. They have a number of parts that exceed 100 lm/W that are available now.

OSRAM has been catching up, and certain new parts like OSLON SX are claimed at 73 lm/W, or their new Diamond Dragon which is claimed at 53 lm/W, and their new Golden Dragon Plus which is claimed at 93 lm/W.

Yes, it was CREE that was using NIST steady state measurements:
compoundsemiconductor.net/cws/article/news/31137

The was done with a 5 minute warm up to steady state, and it had a tiny heatsink, so there wasn't much thermal mass to heat up. They used to put the NIST reports up on their website, and there were pictures in the pdf.

I just looked on DigiKey, and even DigiKey has over 4,000 of the older CREE parts that are over 100 lm/W,
XPEWHT-L1-R250-00E01, minimum 114lm @ 350mA

I see Future has Philips LumiLEDs Rebel devices, LXML-PWC1-0100, which are specified at 100 lm/W minimum, which are probably in your 95lm/W range.

Sorry Dave, the best continuous lm/W is above your 70 lm/W numbers, including Nichia- which doesn't have the highest efficiency.

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