Wednesday, November 14, 2007

Are flourescent lamps efficient

I have been asked a few times lately if it is better to leave fluorescent lights switched on as they use so much energy to start up? And, are fluorescent tubes (and by extension compact fluorescent lamps – low energy lamps)really efficient?

Fluorescent lamps, be they standard strip lights or compact fluorescents, use gas discharge technology and it takes 3 to 15 minutes (depending on the type of lamp) to vapourise the gas, get up to temperature and reach full luminosity. It takes no extra power to do this, just a bit of time. So the argument for leaving lights on relates to time rather than energy. If you are popping in and out of a room all day you probably want to leave it on. Otherwise turn it off. To put it simply a light left on uses more energy than a light turned off.

Fluorescents are more efficient than incandescent lamps by a factor of about 4. To be technical, they produce around 90 lumens per watt compared to about 20 watts for incandescents (these figures vary with the type of lamp but are broadly accurate). It is why incandescents have been banned in Australia and are being phased out here. They give a different quality of light, which is what leads to the idea that they are not as bright and therefore you need more of them. My Grandma said the same when her gas lamps were replaced with nasty electric bulbs, but she got over it.

While we are on the subject, I have also been asked if fluorescents have nasty chemicals and gases in them, and the answer is yes! They have 5mg of mercury in a 40w tube. It is a tiny amount, less than the size of a pin head – but poisonous nonetheless. They also have a phosphor coating to the glass (which is what fluoresces and produces the light) which is not pleasant, but not poisonous. The gas in the tube is usually argon which is inert and harmless.

There are new lamps about to hit the market that use xenon gas – no mercury and no phosphors – which are even more efficient. These will produce over 120 lumens per watt.

6 Comments:

Anonymous Jim said...

Thanks for the interesting review of the situation. I wanted to raise a few queries in response:

* Production efficiency - Do you know of any data on whether fluorescent bulbs take more energy to produce?

* Life spans - Compact fluorescents often seem to come with claims that they last ten times as long as incandescents, but in my (totally unscientific) observations they seem to die at about the same rate as I would expect if I had incandescents. Are you aware of any data on this?

* Disposal requirements - You mention the nasties in fluorescents. Presumably disposing of these properly would take some amount of energy.

The lifetime energy cost of compact fluorescents would need to factor in all of these things.

Thanks,
Jim

1:08 pm  
Blogger Tim Pullen said...

The DTI, in conjunction with National Physical Laboratories did some comparative studies on CFL's and incandescents a couple of years ago. Production efficiency relates to size rather than output so they are fairly like-for-like. NPL found that manufacturers tend to under-estimate the life of a CFL. Lamps with an advertised lfe of 8,000 hours commonly reached 10,000 where your typical incandescent struggles to reach 1,000 hours. My own wholly unscientific tests confirm this, in as much as I have not chnaged a CFL in 3 years.

As to disposal, the only real nasty is the mercury and to be honest I have no idea what happens to this as I have yet to try and dispose of one. 5mg of mercury is not much, but multiplied by 30 million houses and it starts to mount up.

The dust-to-dust analysis of CFL gives an energy saving factor of 4 (so says Energy Saving Trust)over incandescants.

5:04 pm  
Anonymous Anonymous said...

unscientific tests dont say much - I've been lucky and not had to change any of the standard incadecent bulbs in my house since we moved in 2 years ago. interested in the idea of using xenon though - any idea how the comparitive gain in terms of lums/watt compares to the increased embodied energy of producing the xenon over argon?

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