Integrated Reflector Could Lead to Ubiquitous LEDs

Andreas writes "Professor Schubert says he has found a way to raise the efficiency of LEDs to 99%. From an article on Advanced Technology: "Until now, all lighting systems, especially incandescent bulbs, generated more heat than light. But our 99-percent efficient reflectors for LEDs makes them the first candidate for light-bulb replacement that generates more light than heat," said Schubert."

Comments

Not so fast, Sparky!

[rco3]

"Professor Schubert says he has found a way to raise the efficiency of LEDs to 99%.
No, that's not what he says. The reflector is 99% efficient, not the LED. This reflector just means that some of the light emitted by the LED, which otherwise might be absorbed by the LED substrate or other structure and converted to waste heat, is now being reflected back out as usable light.
This new technology does nothing to improve the quantum efficiency of the LED itself. It's an important and useful technology, sure, but it is NOT a 99% efficient LED.

Costs have to be considered in toto

[Engineer-Poet]

Quoth the poster:

There is no point making a bright LED if the total system cost is still ten to twenty times that of an incandescent source.

As a counterexample, consider the compact-fluorescent bulb. Most cost around ten times as much as an incandescent of equivalent brightness, but the savings in replacement costs and power will pay back the difference in as little as a few months.

LED technology has the advantage of longer life than fluorescent. With the increase in efficiency from reflectors, they could cut power costs below fluorescent and become the TCO winners.

Re:Costs have to be considered in toto

[Elledan]

"LED technology has the advantage of longer life than fluorescent. With the increase in efficiency from reflectors, they could cut power costs below fluorescent and become the TCO winners."

Also something to keep in mind is that LEDs are far more robust than fluorescent and incandescent lights. Incandescent lights can't take shocks very well, and the huge temperature delta between an on/off state reduces its lifespan significantly. Fluorescent lights are also relative fragile (ever tried replacing fifty or so of those tubes without shattering at least one of them?), and the ballast used to generate the required high voltage (most types of ballast) create quite a lot of EMI, which is bad for sensitive equipment and cables like Cat-5 etc.

LEDs generate very little heat, require only a very small current (tens of mA!) at equally low voltages, produce no EMI, are unaffected by all but the most severe shocks and last virtually forever (100,000 hours for red, green, etc. with ease).

If LEDs are made brighter, even if this makes them more expensive than other technologies, there are always some (less friendly) places where they would work very well and would be cheaper in the long term.

110/230V AC

[EnglishTim]

Okay, so let's assume all our lightbulbs start being made from LEDs... At some point soon we're going to have to start changing our lighting circuits to 5V, or something like that. It's madness that each lightbulb will have to contain it's own little transformer - it'll make the bulbs vastly more expensive and wasteful.

There are a selection of appliances that work well with 110/230V AC - things that require a lot of power like kettles, hoovers, heaters, washing machines, hobs, tumble driers and the like. However, there's an increasing number of appliances in a modern household that would be much better served by a 12V DC supply.

How long do you think it'll be before we start changing over?

Re:110/230V AC

[Smidge204]

Unfortunately DC power doesn't transmit over any kind of distance very well. AC is much more efficient for that. (Esp. at high voltages... 20,000V+)

Also, all flourecent lights have transformers in them, so suddenly it's not too unreasonable for each light fixture to have its own little transformer in it!
=Smidge=

Re:110/230V AC

[bluGill]

Electronics 101: when you connect 2 circuits in series each sees half the total voltage. Connect 24 LEDs in series to a 120 volt line and each sees 5 volts! By definition there is no need to transform the AC into DC - the D in LED stands for diode, which is what you use to turn AC into DC! In the real world you are likely to use 48 LEDs, in two different strings, so that you get light from both sides of the wave.

Re:110/230V AC

[SuperBanana]

It's madness that each lightbulb will have to contain it's own little transformer - it'll make the bulbs vastly more expensive and wasteful.

If you have 2v LEDs, you only need wire about 60 of them in series and you've taken care of the voltage problem. Well, except they'll blink at 60hz, quite strongly...and if one failed, they'd all go out. But in any case, it's hardly rocket science to make use of the higher voltage level, especially since LEDs will tolerate A/C. Incidentally, look at a screw-in fluorescent bulb some time- they've gotten the whole thing down to $10 or so, and that includes a transformer and electronics to raise the voltage. Transformers etc are very cheap.

However, there's an increasing number of appliances in a modern household that would be much better served by a 12V DC supply. How long do you think it'll be before we start changing over?

Never. The whole point behind A/C is that it is very easy to step up/down, and as a result, you can use a higher voltage for transmission and distribution. Higher voltages mean less current flow for the same amount of energy, which means reasonably sized wiring and such.

Even in the short distances involved in a house, losses from wiring can be substantial at such a low voltages as 12v. 48v might be a better choice, but I can't see it ever taking off.

Re:110/230V AC

[falzer]

> Unfortunately DC power doesn't transmit over any kind of distance very well.

Looking at just the wire itself, transmission losses aren't worse for DC. There are a few HVDC transmission lines in operation now. Some are used for 50/60Hz conversion.

The reason AC is used because it's easier/cheaper to efficiently step up (and down) the voltage to useful levels, as per your power transmission example.

Re:110/230V AC

[Peepsalot]

It's madness that each lightbulb will have to contain it's own little transformer - it'll make the bulbs vastly more expensive and wasteful.

Sorry, I don't think you'll see a change(at least not to a lower voltage) in your wall outlet's voltage any time soon. Maybe a transformer in every light bulb seems wasteful, but take into consideration that led's can last roughly 100,000 hrs as opposed to 1,000 for incandescant's.

Still seems wasteful?
Transformers don't have to be huge, especially if they are powering an efficient device that doesn't draw much power. Disposable cameras contain transformers in them that step the voltage from 1.5 to 300 volts, and they are pretty damn small. In fact you could fit a transformer into just the metal base of a normal incandescant bulb. Not to mention the transformer's solid state cousins: voltage regulators, swithcing power supplies, and probably some others I don't know about (IANAElectrician.)

In fact, you could also scrap the idea of stepping down the voltage entirely. How about putting more led's in series. Think about it, led's emit a narrow spectrum of light, so to smoothen out the spectrum, you might want to add a bunch of led's from different parts of the spectrum. Just put 40 little 3v led's on a board and voila, you have a device that accepts 120 volts. Another reason to use a large number of leds: you might want to spread out the surface area where the light is coming from, a single point of light could seriously mess with your vision. Not to mention the fact that I doubt to see any single led in the near future that can output the equivalent of a 60 watt incandescant light bulb.

But the thing the I wonder about is if and when the standard plug model will be changed to a model where the socket and the plug contain coils to transfer the power, like an isolation tranformer that is split into two parts.(Have you ever seen how a sonicare toothbrush charges itself?) The main benefit I see in this is safety; never have any exposed wires to fry yourself with. Actually, if all your wall sockets had coils in them, the plug of your device could determine the voltage that device receives, simply by the nuymber of coils in it. Again, IANAE, so I don't know, maybe it would take the advent of room-temperature superconductors to make this idea feasible and efficient.

Re:110/230V AC

[rco3]

A few points to note, realizing that not many posters around here are EE's:

1) LED's are not voltage mode devices. There IS a typical voltage drop associated with an LED, but it can vary appreciably between devices. One sets the operational point of an LED by controlling the current through it, and allowing the voltage to settle to whatever value it wants. Typically, one would want to see around 30 mA through a normal T1-3/4 LED. Depending on the chemistry of the LED, this could result in a voltage anywhere between 1.5V and 3 or 4V. This, as I said, will vary somewhat between different LED's of the identical type. If you try to set the voltage, you'll get wildly varying currents and a lot of dead LEDs.

2) Stringing together LEDs in series to get something approaching 120V drop is a good idea, but you still have to limit the current. Leaving a few volts between the nominal operational voltage of your LED string and the nominal supply voltage is a good idea, because you can then use an active (or passive) current limitation scheme which operates within that voltage gap. The simplest way is with a single resistor, sized such that R= (Vsupply-VLEDS)/ILEDS. This is subject to variation due to device mismatch, temp variation, etc, and dissipates some power in the resistor. Another way to do this, which allows for the LED to be operated from a much higher voltage than it's rated for, is to use a series capacitor. The determination of proper capacitor size is a bit more tricky, but you can successfully run a single LED from a 120V supply. The indicator in my waterbed heater has run this way for a couple of years now. Nice part is that the capacitor does NOT dissipate any power as heat. Enough of them might screw up your power factor enough to piss off the power company, though :-)

Important messages to take home from this: you can't set LED operating point from the voltage across it, at least not safely and reliably; you can operate LEDs from 120VAC using a capacitor as the gain setting element, which is appreciably cheaper than using a transformer.

Re:How are narrow-angle traffic-signals done?

[zrail]

These lights use a Fresnel Lens to direct the light from the bulb or LED cluster to only the lane for which it is intended.

wikipedia link

Actually I do...

[Pinkoir]

If you read the article you will see that there is no claim on 99% efficiency of the LED. The claim is 99% efficiency on the reflector. No LED anywhere in the world comes even close to 220 lumens/watt. The best I've seen in the real world is about 80 lumens from the 3 watt Luxeon devices put out by Lumileds. These devices are very hot and need a lot of heat-sinking to avoid destroying themselves in any confined application. You have to remember that LEDs aren't magic. They are just full of inefficiencies and the back reflector issue is only one of them. I work with high intensity white LEDs every day and if you know of some that can give the output you talk about that don't need active cooling for God's sake tell me where I can buy them.

-Pinkoir