Fluorescents-2: Lifetime

Compact fluorescent lights (CFLs) are mandated for the US market starting in January 2012. These bulbs certainly have good press; they are more efficient, last forever, give nice bright light, and modern CFLs can be dimmed and will turn on in cold areas. These are nice attributes.

Summary to here



Efficiencies were verified in Fluorescents-1; compared to ILBs, they show really good gains and long lifetimes.

FLs have primary illumination in two very narrow regions of UV wavelengths, then down convert to visible light. ILBs effeciency generate light at low energy wavelengths longer than 1 micron and only that 10% of the generated light (that in the visible range) is usable. Lumens measure the photonic power emitted by the bulb and we restrict this to the visible light region. Watts measure the electrical power provided to do this emission. Efficiencies of 67 Lm/W are appropriate for CFLs while 15 Lm/W is reasonable for ILBs.

click for a list of LastTechAge posts on illumination technology

Lifetime estimates need more discussion.  In Fluorescents-1, we discussed how lifetime could be reduced by the manufacturer taking shortcuts.  We also discussed that the local power grid could cause premature tube failure by delivering voltage spikes to the bulb.

How to shorten the rated lifetime

There is a third mechanism for shortening fluorescent bulb lifetime that should be discussed — frequent on/off cycles.  Review fluorescent light operation here (PDF).

A very good review of 12 effects that will shorten the lifetime of a fluorescent lamp was written by  R.V. Nesari  and available  here (PDF).  The lifetime listed in literature for any bulb is the result of a test where, typically, a bank of lamps are turned on for 3 hours, then switched off for 20 minutes (to cool down), cycle repeated repeated over and again.


Frequent power cycles damage CFLs

Rapid cycling.     The very best way to use a fluorescent lamp of any type is to have it operate for a very long time.  Some manufacturing plants never turn their lights off; offices will have them operating for 10 to 12 hours a day.

This is the most efficient use and the source of the justifiable claim that fluorescents are much cheaper than incandescents.  Fluorescents won the lighting wars in commerce, based on their own merits.   If you must leave lighting on for a very long time, use fluorescents.

Once generating light, the CFL has only small losses of its filament and associated coating.  The stress of the sudden very high voltages surge at turn-on causes highly enhanced sputtering and evaporation.  The rule for long lifetime:  do not cycle very often.

On the one hand:  The problem is partly due our basic training –  We learn in childhood to turn of lamps when we leave rooms.  It is a stressful time to get the child to switch off bathroom lights, closet lights, desk study lights, etc.  It is pretty clear that if you leave a light on for hours, you (or your parents) have to pay for the energy used.  You pay for 1 kWh if you leave ten 100 W lightbulbs running for an hour.   If you leave the fixture with two 40 W fluorescent tubes running all day, you obviously will have to pay for almost as much as the 100 W bulb, the internal urge to switch it off is there.

On the other hand:   if you turn your CFL on and off too often, you could be blowing the filament and find yourself changing bulbs after several hundred hours.  These things are expensive!  If you do find yourself changing a CFL about as often as a ILB, then you could pay 5 times the bulb price or more.  No win situation here.

How much lifetime reduction?   a 24 W CFL generates about as much lumens as a 100 W ILB.  About 1:4 in power use, and perhaps 5× in price per bulb. That means you may use a CFL bathroom light for 25 minutes for the same cost as a 100 W ILB for 5 minutes.  But you turn your lights off when you leave the room, right?  Is there a point when cost of CFLs vs ILBs rise above the incandescent cost of power?

These tables  summarize an attempt to look at the  condition where you are intent on minimizing the bill from your power company.

TestModels_TblUsage A: You walk into a room in your house and turn on the light for an average of 25 minutes, then turn it off.  5 minutes later someone comes and and does the same.  Usage B has be same thing but you just walk in then walk out again (closet model).  5 minutes lights on, 5 minuts lights off.    In A, the lab cycle is 5.7 times longer than your usage cycle; in B, lab is 20 times longer than usage.

Lifetimes_tblNow we need test data on the more realistic cycles A and B to measure the lifetimes.   Oops, these data are not available.    Sorry, we cannot figure out lifetime reduction  values  due switching!

I would guess that after the first several years (maybe by 2015), we will find self-righteous magazine articles telling us to leave the lights on all the time.

Overheating    This is another way to kill a CFL.  The fluorescent tube is only little bothered by temperature, but the built-in electronic ballast in each CFL will die quickly.  The failure rate of sensitive solid-state electronics goes up about 2 for every 5º C (9º F) increase in temperature over room temperature of 25ºC (77º F).  This effects outdoor lighting (where there are hot summers) and orientation of mounting.  The ballast is at the base of the bulb.  If the bulb is put upside down into a ceiling fixture, the (not insubstantial) heat generated by the bulb will cook the ballast.  This shortens the life well.  Nowadays, some CFLs with much more expensive ballasts exist and you can use these upside down without huge loss of lifetime, but you pay for this.  Temperature has other consequences, though, and we will return to this in another post.

CFLs are used to as intended – Energy impact

To reach the bulbs rated lifetime, you should not turn off the lamp very often.  This is why commercial interests are nearly 100% FL users.  Now consider use in a house with 30 lights.  Prior to change-over, all were ILB with an average of perhaps 75 watts (combination of 100, 75 and 60 watt bulbs).   By 2013, all lights will have been changed to CFL bulbs, with the same illumination characteristics (net lumens will be the same before and after change-over).  Will  you use less or more energy with CFLs?

AvPwr_tblUse data from Fluorescents-1:   75W  ILB generates about 1100 lumens, the equivalent CFL will need 20W of electricity.  Suppose your active time at home averages 8 hours.  When you used ILBs, they were ON about 20% of the time.   After change-over,  the bulbs are ON the whole time, as in an office.  The average energy used is the product of every cell along the row, as shown.   Yes there are quibbles to be made with these assumptions.  But the point is, CFLs are NOT the universal energy saver they are advertized as.  

ILB:  3.6 kWh  vs.  CFL:  4.8 kWh.  CFLs, used as intended, are not at all a bargain.  You pay increased power bills,  pay for expensive bulbs,  and potentially add to the hazardous waste load in the environment.  If you are ultra careful perhaps the effective use is 10% of the time,  then it is ILB 1.8 kWh to CFL 4.8 kWh.

This argument does not work if you do not turn of your incandescent lights when done.  But, since you are not concerned about energy costs or use, this discussion is moot.

One might argue that switching lights off is one of those utopian propositions, like saying everyone will send their spent CFL lights to special recycling centers.  No,  CFL ‘utopians’ must go out of their way (do things that increase cost a bit) to comply.    The ILB ‘utopian’ acts in monetary self-interest.  Do it right, save money.  The person doing the action receives tangible gain.  This is not a utopian assumption.

Summary of CFL considerations

Efficiency: the award for electricity into light goes to the fluorescent lamps, if no one cuts corners.

Lifetime:  The award for least energy used goes to incandescent lamps, if we try to reduce immediate out-of-pocket costs and switch them on and off frequently.  Frequent cycling of FL bulbs will shorten lifespan by (data unavailable)  hours.  If we leave every CFL on that we switch on, we can estimate that ILB bulbs use less energy, or in sloppy households, at least break even.

UPDATE, 2011 Jul 21.  My best critic and gentle wife ‘politely’ said that algebra done in a paragraph is hard to follow (words to that effect).  Well, yes.  So I turned the numbers into a table and clarified the text a bit.   Sorry for the late change, I hope this is easier to follow.

Charles J. Armentrout, Ann Arbor
2011 July 20
Listed under   Technology   …thread    Technology >  Fluorescents
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About LastTechAge

I am a physicist with years of work in fusion labs, industry labs, and teaching (physics and math). I have watched the tech scene, watched societal trends and am alarmed. My interest is to help us all improve or maintain that which we worked so hard to achieve.
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