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Re: GBlist: more flourescent questions...

Dear Chris.

The following might be what you are looking for. It is copied from another
mailing list about lighting. If you are interested about this mailing list I can
forward you the subscription details.

Costas Vrouchos
Electrical Engineer - Energy Management Consultant
Rhodes - GREECE

" 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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