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GBlist: Fluorescent lights - to switch off or not? That is the question
BGlisters:
I forwarded the question about switching off fluorescent lights to
my "go-to" guy on illumination, lighting guru Steve Selkowitz at Lawrence
Berkeley National Lab, UC Berkeley. He sent me the following message. Some
of the terminology and utility rates are a bit strange because the response
comes from New Zealand. The answer appears to be: Yes, switch 'em off when
they will be off for more than 20 minutes, even at extremely low energy unit
cost..
Hal Levin
Hal,
The simple answer is "turn off the lights"- just so happens there was a
recent more detailed discussion of this on a bulletin board- here it is.
Hope all is well.
Steve
---------
From: Lighting@garnet.nist.gov (5/15/97)
To: Lighting@garnet.nist.gov
RE>Effect of short cycling on fluorescent l
> I concurr with the practical effects of frequent switching set out in
>Ken Berg's posting however this does raise a question to which I have
>previously failed to find an authoritative answer:
>
>The object of occupancy switching is given as "energy saving" The
>question raised by this thread is that does it provide an economic saving
>as well? I don't think that we have fully answered the first point. I
>have been unable to find good figures for the energy required to
>manufacture any kind of lamp. This figure would include the energy
>required to refine or recycle base materials as well as that used in the
>manufacture of the specific lamp, from work I have seen relating to
>automobiles where it has been shown that 25 percent of all the energy
>they consume during their life is that taken to manufacture them, I am
>suspicios that , particularly "energy saving" lamps may not be as
>effective as they seem taking production energy into account, and that is
>before we start looking at other environmental impacts of materials used
>such as mercury.
>
[snip]
>
I hope the following answers some questions about cost savings from turning
off lights. Someone at the Minnesota Dept of health has put together a good
article on net mercury emissions from using fluorescent lamps - the numbers
showed fluoresent lamps providing net mercury benefit - burning coal
releases mercury, but the numbers were not as optimistic for the benefits of
fluorescent lamps as the IAEEL article on net mercury emissions from using
fluorescent lamps.
The following is a long winded discussion of the myths surrounding switching
off fluorescent lamps.
Fluorescent Lighting - Should I turn the lights off?
There have been two very resilient energy myths that have dissuaded people
from turning off fluorescent lamps. These myths are hard to dispel because
inside each myth is a kernel of fact.
Inrush myth: I don't turn off fluorescent lights because the inrush energy
when I turn the lights on is more than the energy I saved by turning them off.
Inrush fact: When fluorescent fixtures are turned on there is a MOMENTARY
inrush of current. This inrush is 5 times greater than normal operating
current for magnetic ballasts and can be as high as 40 times greater than
normal operating current for electronic ballasts. This inrush lasts for
1/10th of a second or less. Thus the energy content of the inrush event is
comparable 5 seconds or less of normal operation. Thus, turning off the
lights for more than 5 seconds will save more energy than leaving them on
even when including the inrush current.
Lamp life myth: I don't turn off fluorescent lamps at all because turning
them on and off reduces the lamp life and I would rather replace the lamps
less often.
Lamp life fact: Each time the lamp is started some of the emissive material
on the electrodes is eroded. This does reduce the BURNING HOURS of the lamp.
The burning hours are how many hours of OPERATION the lamps will last. By
turning lamps off at night, the fraction of operating hours per year can be
reduced by much more than the reduction in the lamp life in terms of burning
hours. Thus turning lamps off at night will make the lamps last longer
(longer period between lamp replacement).
In "Economics of Switching Fluorescent Lamps" IEEE Transactions on Industry
Applications Vol 24, No 3, May/June 1988, Carriere & Rea provide a function
f(u), that describes the lamp life in burning hours relative the rated lamp life
under that standard burning cycle of 3 hours lamp operation per start.
Actual Lamp Life = Rated Lamp Life x f(u)
where,
f(u) = 1.71 (1-exp[-(u/3.89)^0.505])
u = burning cycle, hours of operation per start
A continuously burning lamp has a large value of u and the exponential term
goes to 0,
thus f(u) = 1.71 and the continuously burning lamp has an average life 171%
of the rated life.
In the US, linear fluorescent lamps are rated for 20,000 hours on a magnetic
ballast. Thus a continuously burning lamp would be expected to last 34,200
burning hours. The same lamp operated on a 3 hour burning cycle (3 hours on,
and indeterminate time off, another 3 hours on etc) will last 20,000 BURNING
hours i.e. f(u) = 1.0. Since there are 3 hours of burning time per switching
on operation, the lamp will be switched on (20,000 operating hr per lamp)/
(3 operating hr per start) = 6,666 starts per lamp.
On average from continuous operation to 3 burning hours per start, there is
a loss of 2.13 hours of burning lamp life per switching operation.
[34,200 - 20,000] burning hours / 6,666 starts = 2.13 burning hours
reduction per start.
If, ON AVERAGE, the lamps are off longer than 2.13 hours, the mean time
between failure for the switched lamps is longer than the continuously
burning lamps. Thus the switched lamps will not be changed as frequently as
the continuously burning
lamps.
If, ON AVERAGE, when lamps are switched off it is for less than 2.13 hours,
the continuously burning lamps will last longer - they will not be changed
as frequently as the switched lamps.
Switching off lamps saves energy and saves on ones electrical bill, thus the
break even point for turning off lamps is somewhat less than having the
lamps off for 2.13 hours.
In general, switching off lamps saves energy not demand. Assuming an energy
cost of $0.03/kWh, switching off a 34 W lamp (36 W with ballast losses)
saves $0.00108/lamp-hr.
36 Watts x 0.001 kW/Watt x $0.03/kWh = $0.00108/lamp-hr
If y, is the average number of hours the lamp is off per 3 hour operating
cycle, the annual hours per year the lamps are off, HO, is:
HO = (y)(8760)/(3+y)
The Annual Electrical Savings, AES, from switching the lamp off is:
AES = [(y)(8760)/(3+y)]( $0.00108/lamp-hr)
Assumptions for replacement costs for fluorescent lamps:
Lamp Cost = $2.00/lamp
Disposal Cost = $0.60/lamp (recycling)
Labour Cost = $4.00/lamp
Total Replacement Cost = $6.60/lamp
Annual Burning Hours, ABH, per year for lamp with 3 hours on and y hours off
per cycle is:
ABH = (3)/(3+y)(8,760)
Given that the burning hours for a lamp with a 3 hr burning cycle is 20,000
hours and the burning hours for a lamp that is continuously burning is
34,200 hours, and given the cost of replacing a lamp is $6.60/lamp, the
Additional Annual Lamp
Costs, ALC, of switching lamps is given by:
ALC = {(3/(3+y))(8760/20,000) - [8760/34,200]}x $6.60
If y = 2.13 hours, Add. Annual Lamp Costs = 0
If y < 2.13 hours then additional lamp costs are positive. There are more
lamps to replace per year if the lights are off, on average, less than 2.13
hours.
The Overall Cost Savings, OCS, of switching lamps off is simply the Annual
Electrical Savings, AES, net the Additional Annual Lamp Costs, ALC.
OCS = AES - ALC
For the three hour operating cycle with y hours off between burning cycles,
the Overall Cost Savings, OCS are:
OCS = {[(y)(8760)/(3+y)]($0.00108/lamp-hr)} -
{(3/(3+y))(8760/20,000) - [8760/34,200]}x $6.60}
The break even point where the additional energy savings just pays for the
additional lamp replacements is found by setting OCS to zero and solving for
y. In this case y = 0.323 hr or 19 minutes. Thus if the lights are turned
off for longer than 19
minutes, there is a net savings to turning the lights off.
To summarise:
Turning fluorescent lights off:
1. Inrush energy when turning fluorescent fixtures on is equivalent to 5
seconds or less of normal operating energy.
2. If lamps are turned off on average longer than 2 hours, they will last
longer than not turning them off.
3. Using conservative values of 3 cents/kWh and $6.60/lamp for lamp
replacement costs, money is saved by turning lamps off longer than 20 minutes.
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<><><><><><><><><><><><><><><><><><><>
Hal Levin <hlevin@cruzio.com>
Hal Levin & Associates
2548 Empire Grade, Santa Cruz, CA 95060-9748
408.425.3946 fax 408.426.6522
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