Greenbuilding Archive for August 2001

" 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 fluorescent 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 summarize:
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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