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Trouble in CFL land

Posted: 20 Jan 2009 ?? ?Print Version ?Bookmark and Share

Keywords:land CFL bulb? UV incandescent?

By Rick Zarr
The Energy Zarr Blog

Zarr: think about how you plan to use your lighting before running out and replacing every light in your house with CFL bulbs.

The symbol has become ubiquitous with the efficient use of energyit is the squiggly shaped compact florescent lamp or CFL that screws into an Edison base light fixture. These little wonders of engineering have improved the efficacy of lighting over the incandescent bulb by quite a bit, but are still relatively expensive and have serious issues with lighting systems and dimmers.

Let's take a look at the basics first. A Compact Florescent Lamp or CFL has an evacuated tube filled with a noble gas and a very minute amount of mercury. The mercury vaporizes and forms a medium which when excited by an electrical current emits ultraviolet light (very energetic short wave invisible light - the kind that gives you a sunburn at the beach). The inside of the tube is coated with a chemical that when hit by UV light, emits visible, broad spectrum (white-ish) light. This conversion is very efficient and provides a much higher efficiency in converting electricity to visible light than traditional incandescent bulbs. This technique is also used in sign lighting tubes to make colors not easily produced by the various gasses available to the manufacturersuch as white light.

Florescent bulb technology has been around for almost 100 years, but the small or "compact" electronics required to drive the tube have not. It was only last decade or so that CFL bulbs showed up commercially. Now you can buy a 4 pack at a reasonable price at your local hardware storehowever, buyers should be aware of the implications of how the bulbs work in order to make correct choices when purchasing.

CFL bulbs have serious issues with dimming and here's why. An incandescent bulb (Edison's invention) uses electricity to heat a filament causing it to emit light and is unaffected by the frequency or quality of electrical power used. The spectral quality of the light is a function of the material used in the filament along with any colorings mixed with the glass or coatings placed on the bulb. There is a direct correlation between the power provided to the bulb and the optical power that is emitted which also includes a great amount of infrared light (which makes your skin feel warm when placed under a bright light). This made dimming an incandescent bulb as simple as controlling the amount of power provided to the bulb or a group of bulbs (such as found in a chandelier).

Modern dimmers use electronic circuitry to cut off pieces of the alternating current (AC) found in modern society (thanks to Tesla and Westinghouse) to control the energy reaching the bulb. These devices use the alternating nature of the energy which passes a zero point every 120th of a second (100th of a second in other parts of the world such as Europe). At this point, no energy is flowing to the bulb, so a switch (a device called a TRIAC or TRIode for Alternating Current) is automatically reset to prevent current from flowing. The dimmer circuitry waits while the voltage rises in the cycle until it passes a preset level set by a user control. The switch is then turned on and the energy in the remaining part of the cycle is sent to the bulb. This controls the total energy content reaching the bulb and the thermal delay of the filament eliminates any "flicker" that would otherwise be visible. This technique provides a full range of dimming and is very inexpensive to implement.

The problem shows up when people replace an incandescent bulb with a CFL that is driven by a dimmer. The circuitry in the base of the CFL converts the conventional AC power to DC first. It then boosts the voltage high enough (400+ volts) to ionize the gas in the CFL bulb's tube by using a high frequency switching converter. There is a very narrow range in which the voltage can vary to affect the output level of the fluorescing phosphor chemical so this voltage is tightly controlled. By "chopping" the AC power and reducing the overall energy, the input DC voltage "sags" to the CFL bulb's control circuitry. This limits the circuitry's ability to provide the correct energy to the tube which can cause early failures in the bulb due to higher currents in the switching circuitry. I'm sure there are warnings on most of these bulbs against using a dimmer.

There are several CFL bulb manufacturers now producing a "dimmable" version however their light quality is often poor at low levels when compared to incandescent lighting. It's a problem with the physics of the technology. The light emitting phosphor is a combination of many chemicals - some of which radiate visible light at one energy level and others at higher levels of UV radiation. If you reduce the levels of UV light hitting the phosphor, certain chemicals will drop out before others causing a shift in the color of the bulb.

Another problem is the cost of CFL bulbs. If you consider you can buy a standard 60 watt light bulb for around 50 cents (U.S.) or less, paying US$8.00 for a CFL bulb makes you take a step back. If the CFL bulb provides the same light for one third of the energy (20 watts), then you can compute your payback period. If electricity costs around 15 cents per kilowatt hour (1000 watts consumed for one hour), the incandescent bulb costs you about US$79 per year to operate if on 24/7/365. The CFL bulb will cost you roughly US$26 for the same period - a significant savings. The bulb will pay for itself in roughly 2 months.

That's fine for bulbs that stay on all the time, but what about lights that are used periodically. A hallway light for example may only be used once or twice per day and contain a series of bulbs - let's say 4 to light a long corridor. The incandescent bulbs cost would be roughly US$2.00 and the CFL bulbs around US$12.00 (on sale) for a 4 pack. That's a cost differential of US$10 that needs to be made up by the energy savings. The incandescent bulbs use 240 watts and the CFL bulbs only use 80 watts. That's a total energy savings of 160 watts. Now, the hallway light is used when coming home late or early in the morning as well as when looking through the hall closet. Let's say 15 minutes of a 24 hour day. That's only about 1% of the time. So, looking at our electrical cost of US$0.15 per kilowatt hour, we have a payback period of 417 hours of "on" time. Since the hall light is only on 15 minutes per day, that period would span about 4.5 years. This also assumes that the bulbs don't fail and need to be replaced before the end of the payback period.

The Good News
Most people use task lighting at full brightness such as kitchen lighting during cooking, but tend to completely turn it off when done. This is a good area for replacing incandescent bulbs with CFL bulbs since dimmers are rarely used. Other areas to look for replacement are outside security lighting which tends to be on all night long or work areas that are rarely dimmed. A mix of CFL and incandescent bulbs on dimmers can provide both energy savings and mood lighting - an often important factor to the ambiance of a home. See my previous post on CFL bulbs versus incandescent bulbs with dimmers.

On the horizon is LED (Light Emitting Diode) based lighting. LEDs use direct current in operation. By "chopping" the direct current at very high frequencies, LEDs can be accurately dimmedand they have one more trick. These bulbs can contain red, blue and green LEDs that can be mixed to provide ANY color the user can imagine. Imagine not only setting the light level, but the color as well. Feeling a bit blue, then with a few taps of the light switch you could reprogram the bulbs to change to a pleasant light blue color. This is already being used in architectural lighting for commercial buildings and it won't be very long before bulbs like this start showing up on your local store's shelves.

So, think about how you plan to use your lighting before running out and replacing every light in your house with CFL bulbs. They can save you money on your monthly energy bill if applied correctly and in the right areas. Till next time

- Rick Zarr is the chief technologist of PowerWise Solutions at National Semiconductor.

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