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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."
As the man says, a 99% reflector is not a 99% LED but this technology would certainly seem to be a useful advance.
Some current LEDs already use thin flim techniques and reflectors to collect the rear emitted light and throw it out the front but this is the first I have heard of combining the reflector with the wiring. This might also have the potential of moving heat out of the junction more efficiently would would be a real help in a lot of applications.
Still I'm curious about how much this will add to the cost of a white LED. There is no point making a bright LED if the total system cost is still ten to twenty times that of an incandescent source. I'm an illumination engineer in the automotive field and when we look at replacing a bulb with LEDs we have to add in not only the cost of the LEDs themselves but also the board, the heatsink, the drive module and associated cicuitry. So while it is true that more light and less heat would be better the real roadblock in my application is cost and we won't see the widespread replacement of filament sources with LEDs until that issue is resolved.
-Pinkoir
"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.
Site & blog: http://www.mayaposch.com
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
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=
Running 5V around the whole house would be much more inefficient than putting a transformer inside each bulb. It would also be expensive, and a fire hazard. Think about it: running 1kW of power through a 5V system would require 200 amps of current! That would require welding cable-sized wires.
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.
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.
Please help metamoderate.
> 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.
That's why it's used for the highest-power and longest-distance links, such as the lines from northern Quebec to New York.
AC lines radiate like antennas. 1/4 wavelength at 60 Hz is 776 miles at c (in practice, reduce by the appropriate velocity factor), so if a transmission line gets long enough, it can radiate a lot of power.
What AC is good for is *conversion*. For long distances, you want high voltages, and transformers are a simple, reliable, and cheap way to convert from a high transmission voltage to a low use voltage.
The only way to change DC voltages is to chop it to AC first. This may be done inside a motor-generator (commutator) or a switching power supply (switch), but either way, you're converting to AC.
Doing this at multi-megawatt power levels gets a bit tricky. It's only worth doing if the savings in power (reduced losses) and copper (you don't need to carry the current that isn't getting lost) make it worth the expense.
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
Exactly right. The real issue is that transmission lines are not perfect conductors; there is always some small resistance which causes power to be lost in heating up the wire. The heat loss is given by the square of the current times that resistance. Therefore, you want to transmit power with as low a current as possible to minimize the power lost along the wire itself. Lowering the current necessarily requires raising the voltage if you want the same power to come out the other end of the wire, thus power is most efficiently transmitted at high voltages and low currents. This is just as true for DC as it is for AC, but AC allows you to easily step the voltage back down again for use at the load.
If we had superconducting transmission lines--which could become commonplace this century--the whole issue would become moot because we'd be able to efficiently transmit at high current and low voltage and DC would be just as practical as AC (if not more so because of the lack of AC radiation effects mentioned by another poster).