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Everything you wanted to know....

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  • Steve Stelzer
    Everything you ve always wanted to know about turning on or off fluorescent lights. Rob and Forum, from my files is the following-a couple of assumptions
    Message 1 of 5 , Feb 5, 2001
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      Everything you've always wanted to know about turning on or off fluorescent
      lights.




      Rob and Forum, from my files is the following-a couple of assumptions
      regarding cost is incorrect but the rest has the appearance of
      engineer-like accuracy. Readers can skip to the conclusions. SL
      - ----------------------

      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.
    • ChasMauch@aol.com
      Steve, Thanks for that info, which appears to answer the question pretty definitely. Since in nearly all cases we will be turning the lights off for more than
      Message 2 of 5 , Feb 5, 2001
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        Steve,
        Thanks for that info, which appears to answer the question pretty definitely.
        Since in nearly all cases we will be turning the lights off for more than 19
        minutes, looks like we ought to do it.
        So why do most of the big buildings in downtown Houston (and in the
        surrounding areas too - Greenway Plaza, Galleria, etc) seem to leave their
        lights on all night? Do they just not know any better? Should we tell them?
        Should this be a big press release by HREG to educate the building owners,
        conserve energy, and get us a lot of publicity (after we have double checked
        the numbers and found out what the owner's reasoning is, of course)?
        Charlie
      • Bridget Jensen
        The building owners will tell you that the lights are on during the night because the cleaning crews are working.
        Message 3 of 5 , Feb 5, 2001
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          The building owners will tell you that the lights are on during the
          night because the cleaning crews are working.


          ChasMauch@... wrote:
          >
          > Steve,
          > Thanks for that info, which appears to answer the question pretty
          > definitely.
          > Since in nearly all cases we will be turning the lights off for more
          > than 19
          > minutes, looks like we ought to do it.
          > So why do most of the big buildings in downtown Houston (and in the
          > surrounding areas too - Greenway Plaza, Galleria, etc) seem to leave
          > their
          > lights on all night? Do they just not know any better? Should we tell
          > them?
          > Should this be a big press release by HREG to educate the building
          > owners,
          > conserve energy, and get us a lot of publicity (after we have double
          > checked
          > the numbers and found out what the owner's reasoning is, of course)?
          > Charlie
          > Yahoo! Groups Sponsor
          > [Click Here]
          >
        • ChasMauch@aol.com
          I had heard the story about it being cheaper to leave the lights on than to turn them off, considering all costs, but Steve s research seems to settle that
          Message 4 of 5 , Feb 5, 2001
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            I had heard the story about it being cheaper to leave the lights on than to
            turn them off, considering all costs, but Steve's research seems to settle
            that one. Have also heard about the cleaning crews but find it hard to
            believe they are cleaning every room on every floor at the same time all
            night long. I could see how they might clean one whole floor at a time but
            not the whole building.
            I have heard similar questions about computers. They used to tell us to leave
            them running overnight during the week but turn them off over the weekend
            because of the wear and tear of turning them on and off. We had a discussion
            about that recently - I can't remember if it was on this listserv or another
            one. Anyway - strange how many "mysterious" things like that there are.
            Charlie
          • Marge Wood
            Wouldnt hurt to write some letters to the various news papers in town about it.....Marge
            Message 5 of 5 , Feb 6, 2001
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              Wouldnt hurt to write some letters to the various news papers in town
              about it.....Marge

              On Mon, 5 Feb 2001 16:10:26 EST ChasMauch@... writes:
              > Steve,
              > Thanks for that info, which appears to answer the question pretty
              > definitely.
              > Since in nearly all cases we will be turning the lights off for more
              > than 19
              > minutes, looks like we ought to do it.
              > So why do most of the big buildings in downtown Houston (and in the
              > surrounding areas too - Greenway Plaza, Galleria, etc) seem to leave
              > their
              > lights on all night? Do they just not know any better? Should we
              > tell them?
              > Should this be a big press release by HREG to educate the building
              > owners,
              > conserve energy, and get us a lot of publicity (after we have double
              > checked
              > the numbers and found out what the owner's reasoning is, of course)?
              >
              > Charlie
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