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Fluorescent Lights and Efficiency
Name: Mary Pat R.
Status: educator
Grade: K-3
Location: MN
Country: N/A
Date: 1/26/2005
Question:
We are trying to save energy with our fluorescent
lighting in class. In the past we were told that it is not energy
efficient to flip them on and off, but to leave them on if they will be
turned back on in less than 30 minutes. Now we are told to turn them off
every time we leave the room, no matter how long it will be before we
turn them on again.
So... How do we best save energy with fluorescent lights?
Replies:
Mary,
Without a doubt fluorescent bulbs USE A LOT OF POWER DURING STARTUP (1-2
seconds). But that is about it. After they have been "activated" or fully
energized than the electrical power required to maintain the light is VERY
VERY LOW. This is something you probably already knew about FLUORESCENT vs.
INCANDESCENT BULBS.
To save energy just turn them off while not in the room. I do not know what
the magical number is as far as wait time. But I would say that 30 minutes
seems a little bit too long. I will tell you that the worst thing you can
do to BOTH lower the life of the bulb and WASTE the MOST electricity is to
RAPIDLY FLIP THEM OFF AND ON.
Hope this helps.
Darin Wagner
The reason for the 30 minutes specification was never energy savings.
Any extra energy used during 2 seconds of turn-on would be compensated by
less than 1 minute of being turned off.
The reason is simply that tube life _may_ be shortened if they are turned
on very often, something like ten times a day or more.
It is a trade-off between electric bills on one hand, and on the other hand
the price of new tubes plus the maintenance attention needed to change them.
In my opinion, it is a little difficult for most small institutions to
actually know their best position in that trade-off.
You never know how much real reduction of tube-life is occurring,
and the on/off patterns are often unstable, and changing one tube at a
time defies scheduling and cost-accounting.
Extreme micro-record-keeping might help, but I think it needs to be
extraordinarily convenient to use
and clearly distinguished from micro-management, which can be obstructive
and demoralizing.
Paraphrasing,
Nobody really knows the "best" answer yet. It probably varies.
If you are willing to ignore maintenance costs and manufacturing energy
consumption,
yes, your lowest electric bill is definitely to turn the lights off promptly.
Perhaps it is a good habit to teach.
It can be valid experiment to try the two strategies you mentioned, if
you can track the results of each.
Suppose your class got a ladder and changed your own bulbs for a year.
And got an "on-time meter" for the lights.
Then you could assign a cost to your time changing bulbs and a cost to the
presumed wattage the lamps draw.
I suppose that is probably an experiment for a higher grade than yours, if
only because of the math involved.
In about 8th grade, perhaps simply recording on paper each bulb that is
seen to fail, and later be replaced, would be enough.
Kids that age could probably estimate with you the hours of on-time in
your frequent-turn-off practice.
I am not sure what subject to put this experiment under. Maybe economics.
Jim Swenson
I had heard a limit less than your 30 minute criteria, but I am no expert
so I did a search and got this answer. The answer sounds reasonable, and
fits a theory of mine that the focus of most questions is too limited.
Here, the answer expands to the question of saving money. The repair of the
light. This aims at my pet criteria, time should be our ultimate criteria.
What tactics save us the most time.
The energy portion of your question is urban myth. The energy needed to start
a fluorescent lamp is not significantly higher than the energy needed to
operate it. Even if we assume that a fluorescent lamp uses twice the normal
amount of power during the starting phase, this phase lasts for only about 1
or 2 seconds. So, the crossover time to save energy would be less than 2
seconds.
But the situation regarding the true cost of operation, including not only
energy costs but also the cost of replacement lamps and the labor to change
them is more complex because frequent starting can reduce the life of
fluorescent lamps. The crossover time for lowest total cost depends upon your
labor costs, what types of fluorescent lamps you are using and, most
importantly, what types of ballasts they are operating on. Fluorescent lamps
operated on instant start ballasts will have the most significant decrease in
lamp life if started frequently - defined as more than one start per three
hours of operation. For T8 lamps operating on electronic instant start
ballasts the crossover point for turning off lamps to achieve lowest total
cost of lighting is about 10 to 20 minutes - though opinions on this issue
may differ. If you are using electronic programmed rapid start ballasts,
which are far more rare than instant start ballasts, there is virtually no
starting damage, so lamps should be shut off if the space is going to be
unoccupied for even less than 10 minutes.
James Przewoznik
Turning light bulbs on and off uses essentially no extra electricity. The
only disadvantage is that the bulb might wear out faster with many on/off
cycles.
Nowadays, light bulbs are inexpensive and reliable, and it is thought best
to turn them off when leaving the room.
Bob Erck
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