IAEEL newsletter 3/94

Sun on Earth

In a small, electrodeless bulb powered by microwave energy, the sulfur atom is the star of the show, promising artificial sunlight in a highly efficient way.

In recent years, new electrodeless light sources have attempted to overcome some of the problems inherent in conventional lamps. The Philips QL lamp and General Electric's Genura lamp are both examples of induction technology which does not rely on filaments or electrodes. Nevertheless, induction lamps still have much in common with fluorescent lighting since both also use fluorescent powders bombarded with UV radiation to produce visible light.

The main advantage of induction lamps is their longer life compared with other fluorescent technologies. In terms of efficacy, there is not much difference. Although high-intensity discharge (HID) sources such as metal-halide lamps and high-pressure sodium lamps are very efficient, they still rely on electrodes which eventually burn out. The presence of electrodes also drastically limits the types of substances that can be used to generate light since many substances will attack the electrodes.

On October 20, 1994, the US Department of Energy (DOE) and the US manufacturer Fusion Lighting presented a new light source with great fanfare. This so-called sulfur microwave lamp represents a totally different approach in lamp technology. The principle behind the lamp could, in very simplified terms, be described as follows: Sulfur and argon in a small, electrodeless glass bulb are turned into a plasma by microwave energy at 2.45 GHz. The physical properties of the excited sulfur atoms ensure that most of the microwave energy is converted into light while little energy is emitted as ultraviolet or infrared radiation.

The first lamps are prototypes with an input power of 5.9 kW. Fusion is developing a lower power version under the project name Solar 1000, anticipated to be commercialized in 1995. Fusion is presently not willing to provide any data on the Solar 1000 lamp, but say that the lamp is expected to be the most efficient light source on the market (excluding monochromatic light sources such as low-pressure sodium lamps). If this holds true, the system efficacy (including power supply and magnetron) will have to be well above 110 lumens/watt. (Good high-pressure sodium lamps have a system efficacy of ~110 lumens/watt and the best metal-halide lamps a system efficacy of ~100 lumens/watt.)

THE CRUCIAL MAGNETRON
According to Fusion, the major opportunities for improving efficacy and lifetime are to be sought at the magnetron, which presently has an efficiency of ~70% (excl. power supply). Whereas the bulb appears to have almost infinite life with a very stable spectrum-the materials making up the fill don't react with each other or with the glass-the magnetron of the present experimental lamps is not rated for more than 10 000 hours. However, Fusion expects the Solar 1000 lamp to be equipped with magnetrons that last much longer.

The US DOE also supports research at LBL and Fusion on a miniature sulfur lamp powered by a solid-state power supply (without a magnetron), which should extend the life of the system and provide even higher efficacy. However, this small lamp is not expected to reach the market for several years.

Present test installations all use the 5.9-kW unit with a system efficacy of ~80 lumens/W. One reason for the lower efficacy of the present version is that the small bulb needs to be cooled with compressed air, a process that demands extra energy. To reach high system efficacy, Fusion will likely have to eliminate the need for compressed air. This should also make the lamp more silent.

Optical qualities: The small size of the bulb makes it easier to design optically efficient reflectors, and the stable plasma makes it easier to predict the outcome of a given design. (The purpose of rotating the bulb is to stabilize the plasma.) In a metal-halide lamp, for instance, the arch between the electrodes moves continuously which may pose a problem when using advanced reflectors with low optical tolerance.
Spectral qualities: Almost 75% of the energy emitted from the bulb is emitted as light in a full-color continuous spectrum. (About 50% of the energy emitted by a metal-halide lamp is in the form of light, whereas the corresponding value for an incandescent lamp is about 10%.) Fusion says that the technology allows for lamps with a correlated color temperature in the range of 4000-9000 K. The lower temperature will be achieved at the cost of reduced efficiency.
Other features: The lamp starts within seconds, even at low ambient temperatures. The restrike time is also just a few seconds, and the lamp can be dimmed. Since there is only sulfur and argon in the fill-both non-toxic materials-their disposal does not pose an environmental problem.
Other manufacturers: The best known of other manufacturers' electrodeless, high-power lamps is Philips Cluster Lamp, which also uses microwaves. Here, the fill consists of mainly wolfram and argon instead of sulfur and argon, and the lamp is not expected to be as efficient as the sulfur lamp. According to Philips, the lamp is still a laboratory prototype. General Electric is working on the Multilox lamp, partly based on metal halide lamp technology, but without electrodes. According to GE, the lamp has a very high potential efficacy, but GE is still still struggling with the power supply.

APPLICATIONS
Some of the potential applications can be viewed at present demonstration lamp installations. Two installations in Washington DC both use hollow light guides (or "light pipes") to distribute the light: One is an outdoor project and the other an indoor museum project. A solid-core light pipe has been used to illuminate a 23-meter-high weather beacon in Toronto, Canada. The lamp is also being used in a demonstration project in Swedish hospitals where their potential efficiency and sunlight-like spectrum were desired qualities. (See IAEEL 4/94: A Systems Approch to Remote Light Sources
Possible applications also include warehouses, shopping malls, factories, arenas, and street lighting. Given the lamp's spectral qualities, they also hold great promise for use in greenhouses.

Nils Borg

IAEEL Newsletter 4/94 describes some of the above demonstration projects in more detail. See also 1000-watt sulfur lamp now ready in IAEEL 1/96

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Proportions of visible and UV/IR radiation emitted from various light sources. NB: The numbers above are not proportional to the system input power. (Source: Fusion)

Spectral irradiance of the sulfur lamp and direct normal solar radiation at sea level. The sulfur lamp spectrum is normalized so that the area under both curvecs between 400 and 700 mm is equal. (The solar spectrum is from the CIE technical report "Solar spectral irradiance.")

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