Energy Efficiency and LEDs
Energy Efficiency and LEDs
Light-emitting diodes (LEDs) are already to be found everywhere, they`re
just becomingmore dominant. This is one-true-view of the LED market. But
there are some more fundamental developments about tohappen, where LEDs
may make inroads in the market shares for conventional light sources in
general lighting applications.
Everyone is familiar with light-emitting diodes (LEDs) from their use
as indicator lights and numeric displays on consumer electronic devices.
The low light output and lack of color options of LEDs limited the technology
to these uses for some time. Now, however, new LED materials and improved
production processes have produced bright LEDs in colors throughout the
visible spectrum, including white light, with efficacies greater than
incandescent lamps. These brighter, more efficacious, and colorful LEDs
may move LED technology into a wider range of lighting applications.
Already a leading light source for exit signs and developing as a popular
source for traffic signals, LEDs also appear in display, decorative, and
transportation applications, with plenty of opportunity for expansion.
Small, lightweight, durable, and with long life, LEDs have the long-term
potential to be the source of choice in many applications, from automotive
brake lights to task lights.
WHAT ARE LEDS, HOW DO THEY WORK?
LEDs are solid-state semiconductor devices that convert electrical energy
directly into light. LED “cold” generation of light leads to
high efficacy because most of the energy radiates in the visible spectrum.
Incandescent, and to a lesser extent fluorescent, lamps radiate much energy
in the non-visible spectrum, generating heat as well as light.
Light is generated inside the chip, a solid crystal material, when current
flows across the junctions of the different materials. The light-generating
chip is quite small, typically 0.25 millimeters square. The plastic encapsulant
and lead frame occupy most of the volume. Presently, the most commonly
used LEDs is the 5 mm LED package (or T 1 3/4). That is starting to change
as manufacturers are developing new package designs for specific applications.
Manufacturing LEDs involves a process known as epitaxy in which crystalline
layers of different semiconductor materials are grown on top of one another.
Advances in epitaxial crystal growth processes have enabled the use of
LED materials for colors that previously could not be made with high enough
purity and structural precision. The technique of chemical vapor deposition
from metal organic precursors enables the cost-effective production of
nitrides of the group-III metals from the periodic table, including aluminum
gallium indium nitrides. Highly efficient indium gallium nitride (InGaN)
blue LEDs result from this process.
LEDs emit energy in narrow wavelength bands of the electromagnetic spectrum.
The composition of the materials in the semiconductor chip determines
the wavelength and therefore the color of the light. A chip of aluminum
gallium indium phosphide (AlGaInP) produces light in the red to amber
range, while InGaN LEDs produce blue, green, and white light.
The first LEDs bright enough to use in outdoor applications were aluminum
gallium arsenide (AlGaAs). These red LEDs appeared as high-mount stop
lights on automobiles and in a limited number of traffic lights. They
were also used successfully in exit signs. Today the US exit sign market
has been almost completely transformed from incandescent sources to LEDs.
A 1998 Lighting Research Center survey of exit sign sales representatives
found that about 80 percent of exit signs being sold in the United States
use LEDs as the primary light source. (In Europe, exit signs are green,
which has made the transformation much more difficult, due to higher prices
and the more recent development of green LEDs.
Similar transformations have occurred in roadway work zone safety lights
used in the US and in some other countries, and variable message signs
when AlGaInP LEDs became available. AlGaInP and InGaN LEDs have succeeded
AlGaAs as the brightest available LEDs.
LEDS AND FUTURE APPLICATIONS
Traffic signaling is a growing market for LEDs because of their energy
efficiency and their color. “Energy savings can range between 88%
to 95% depending on the size and color of the traffic signal,” says
Gary Fernstrom, senior program manager, customer energy management department,
at Pacific Gas and Electric Company in California. Add to that the maintenance
cost savings, and traffic authorities can realize a good return on their
conversion investment, he says.
It should be noted, however, that where extremely efficient halogen lamps
already are used, the energy savings are much smaller.
Commenting on the US situation, Fernstrom noted that “conversion
to red LEDs is proceeding rapidly as traffic authorities become comfortable
with national standards, equipment performance, and their confidence in
the return on their investment. While still very expensive and in limited
use, green and yellow replacements offer opportunity and will gain market
share as experience with their performance is gained and as their price
drops.”
In some European countries, the green and amber LEDs are becoming increasingly
common. The city of Stockholm will have replaced all its traffic signals
by three-color LED lamps by the year 2000. In the Netherlands, three-colour
LED lamps are increasingly used, as well as in the city of Moscow, which
has retrofitted some thousand traffic signals.
LEDs are increasingly being used as striplights for path marking and emergency
wayfinding systems. Their long life and cool operation allows them to
be embedded in plastic materials, which makes them perfect for these applications.
For example, a flexible plastic extrusion that contains LEDs can be implanted
in floors or on steps. LED modules can be fixed in roadways for lane markers
that remain highly visible in rain and fog or in crosswalks that light
up when a pedestrian steps onto a street.
In a paper for the Illuminating Engineering Society of North America,
“Will LEDs transform traffic signals as they did exit signs?”
Kathryn Conway and John Bullough of the Lighting Research Center note
that three factors are at issue in maximizing the performance of LEDs
for signs and signals. Cost of materials, active power demand, and visual
performance need to be balanced in product design.
Those design shifts can be seen in the development of LED traffic signals.
According to the paper, traffic signals appear to be following an optical
and physical design evolution similar to that of exit signs. Traffic signals
began with an incandescent light source with a reflector and evolved to
many LEDs arranged to form a signal face under a smooth lens. This has
been followed by LEDs arranged under an articulated lens display. The
next step is a design with high-output LEDs combined with a reflector
to distribute light.
Manufacturers have even grander plans for the tiny light sources, some
of which have already begun to gain acceptance, from pedestrian traffic
signals to full-color LED displays in stadiums.
Andrew Lipman, vice president for business development at LED manufacturer
Uniroyal Optoelectronics, describes a range of potential applications
that, if achieved, could make LEDs nearly as common as incandescent lamps:
signage, cove lighting, architectural lighting, more automotive interior
lighting, retail display lighting, reading lights in airplanes and cruise
ships, wing lights, and airport navigation lights.
“The question is, what is the application, and what are the trade
offs?” Lipman says. “You have to look at total lifetime and
system cost. Can you make the application lighter? How important is it
to have strong reliability characteristics?”
For example, current incandescent fixtures used on airplane wings require
that the entire expensive fixture be replaced when the lamp is spent.
If that happens on the runway when the plane is full, the cost is far
greater than equipment and labor, Lipman explains. “If you lose an
hour of operation, that can be a big issue,” he says.
LEDs have much longer life, requiring less frequent replacement. They
are also very durable, so that the vibrations of the plane will be unlikely
to disturb the LED, while incandescent filaments are more sensitive.
Michael Stewart, sales representative for LED manufacturer Nichia, says
his company is seeing LEDs used in applications where energy consumption
is critical, such as in solar panels. The solar panel charges during the
day, then LEDs draw on the power at night to illuminate, for example,
a billboard.
Another growing market is full-color LED displays, not only for large
stadium screens, but also for micro-displays. They are being incorporated
into the next generation of cell phone displays and automatic teller machines.
“The parts are getting brighter and will continue to do so,”
Stewart says. “LEDs are still pretty new, so we’re constantly
improving the parts.”
GELcore, a joint venture of GE Lighting and Emcore, has set as its goal
production of an image-making white LED, says Paul Southard, application
development engineer.
“We see within the next 10 years interior LED lighting taking over
fluorescent lighting,” Southard says. “We see boat lighting,
automatic breaking systems on tractor trailers, airport lighting, interior
lighting, dome lights.”
LEDs will be in back panel display lighting for computers, cell phones,
under-counter lighting, interior car lighting, he continued. “They’re
everywhere already,” he says. “They are just becoming more dominant.”
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Kathleen Daly
Andrew Bierman
The authors work with the US-based Lighting Research Center in Troy, N.Y.