Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Do light bulbs or convection heaters heat a room with the highest efficiency?

  1. Mar 23, 2007 #1
    There's a debate these days related to the australian ban of traditional light bulbs, because of the loss of heat energy:

    http://www.ens-newswire.com/ens/feb2007/2007-02-21-01.asp

    But in a colder place than Australia (like Norway), it think this number would be lower than 66% because some of the heat from the light bulbs will just compansate the output from the thermostat convection heater . But do for instance 1000W of traditional light bulbs (or about 17 60W light bulbs...) heat the air in a room to the same temperature as a 1000W convection heater?

    Wikipedia says this about radiative heaters:
    and convection heaters:
    Do light bulbs heat a room in the same manner as radiative heaters, like "The element emits infrared radiation that travels through air or space until it hits an absorbing surface, where it is partially converted to heat and partially reflected"? Or do light bulbs heat the air directly as convection heaters?

    If light bulbs are more like radiative heaters, would they heat the room with less energy efficiency than convection heaters, because more of the heat energy would be transfered directly into, for instance, the walls and the roof where the heat loss is greater (when it is colder outside than inside)?

    About 95% of the energy input to traditional light bulbs produce heat.

    Some people wants to keep the traditional light bulb because of its alleged comfort advantage.
     
    Last edited: Mar 23, 2007
  2. jcsd
  3. Mar 23, 2007 #2

    russ_watters

    User Avatar

    Staff: Mentor

    With a few small caveats, yes, if a place is always using electric resistance heat, there is no benefit to more efficient lighting.
     
  4. Mar 24, 2007 #3
    Main caveat being that electric resistance heating is less energy efficient overall than other heating methods.
     
  5. Mar 24, 2007 #4
    Have you ever been inside a computer lab with the windows shut on a warm summer day? Its hell.
     
  6. Mar 24, 2007 #5

    rbj

    User Avatar

    but, to be clear, the inefficiency in electric resistance heating is at somewhere other than the home where the energy is "consumed". it is the natural inefficiency that you get when you convert unordered energy (what we called in an engineering Thermodynamics class, "low-grade energy") a.k.a. heat to ordered mechanical energy involving the twist of a shaft. the best efficiency that can be hoped for is what comes from the Carnot cycle.

    this minimum theoretical loss of efficiency can sorta be recovered with the use of a heat pump which, if the temperature difference is not super great, is practically useful. one of the cheapest places i lived, in terms of energy consumption (less than $75/month in the winter in 1992) was a townhouse condo i lived in Hackensack NJ. was all electric: had a heat pump on each floor that doubled as an air conditioner in the summer. anyway, at the home, resistance heating can be thought of as 100% efficient since no heat need go up a smokestack or be lost in venting. but heat pumps are more than 100% efficient. how do you think that happens?
     
    Last edited: Mar 24, 2007
  7. Mar 24, 2007 #6

    vanesch

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    1000W of heat is 1000W of heat, no matter how it is produced. So indeed, a light bulb of 100W is exactly as efficient in heating as any other 100W heating device.

    Now, there can be small caveats here, because of the way the heat is distributed afterwards. For instance, having a very hot, small spot might induce thermal conduction away from the place where you want the heat to be (hot spot on a wall, conduction through the wall, increasing heat loss of the room), things like the uniformity of the distribution and so might make one prefer certain heat sources over other. But thermodynamically, both sources are strictly equivalent. The reason is that "heat at low temperature" is already the lowest form of energy you can have, and there are no "more efficient" ways to produce that lowest form, given that "efficiency" usually means "avoiding the production of that lowest form of energy".

    Now, I know that people do not accept this, and that's why erroneous publicity of "efficient heaters" has so much success. My brother in law, who is a MD, doesn't believe this for instance.
     
  8. Mar 24, 2007 #7

    vanesch

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Usually, the computer lab is one of the few places equipped with an over-powerfull AC, so often, the computer lab is icy-cold in the summer :biggrin:
     
  9. Mar 24, 2007 #8
    When I took my Matlab final it was when the weather was nice and hot. The room had almost no AC and the professor shut the windows because the air was making the blinds rattle and blowing papers around. So for 2 hours I was stuck in that damn lab with 30 computers all running and the windows closed. Adding insult to injury, he would come around and turn the printers off 15 mins before the exam was over and would say "hurry up and print, im turning the printers off now".....:rolleyes: :mad: Man that day sucked.
     
  10. Mar 24, 2007 #9
    Huh? Say you burn enough gas to produce 1000W of heat. Sure, you could use that to warm your room. Or you could try running a steam turbine at the 'station, transmitting electricty to your room, and running that through a resistor to produce (at "100% efficiency") maybe 700W of heat there, 300W of heat lost along the way. Or instead of dissipating the power with a resistor, you could instead dissipate it with some device that pumps an additional X watts of heat (from outside) into your room.

    I don't have a clue what the actual total efficiency of each is, only that if you care about the total amount of fuel that gets burned, you'll want to avoid heating with resistors and incandescent lightbulbs, since putting the energy through a heat pump plus CCF will make your room warmer.
     
  11. Mar 24, 2007 #10

    russ_watters

    User Avatar

    Staff: Mentor

    No, that isn't the main caveat. The way the question was asked, the heat produced by a light bulb is exactly the same as that produced by an electric heater.

    The main caveat would be where the lights are located. Electric heaters are typically located under windows to most effectively deal with the main load of a building (the walls/windows). Some lighting, such as recessed lighting, might not even be putting all of its heat into the building envelope (as someone pointed out in the similar thread in the engineering section).

    Another caveat would be that light that is transmitted through the window is lost to the environment and doesn't go to heating the building.

    However, these are both small effects - perhaps a couple of percent.
     
  12. Mar 24, 2007 #11

    russ_watters

    User Avatar

    Staff: Mentor

    The question you are answering here is not the question that the OP is asking. The OP is only comparing an electric radiator to a light bulb. That a gas furnace is more efficient thermodynamically than a gas turbine in heating your house is irrelevant to the OP's question.

    I once had an apartment that had electric resistance heat. I left my computers on 24/7 in the winter. The difference to my electric bill in doing so was precisely zero. That's the concep the OP is asking about.

    [late edit] Just because it is an interesting side issue, the difference between heating your house with an electric heater on a mostly gas-turbine electrical grid and gas furnace goes something like this:

    -A good gas furnace is typically about 95% efficient.
    -A gas turbine is around 45% efficient and the generator hooked-up to it about 95% efficient. Many gas turbines are combined-cycle, which means they use the waste heat from the GTE to heat water to power a steam turbine. They may recover 95% of the waste heat and run it through a 40% efficient steam cycle, connected to another 95% efficient generator. The generated power is connected to transformers, a transmission system, more transformers, a distribution system, and more transformers, before getting to your house, losing perhaps another 15%. The total energy balance is: .85 (.45*.95+.55*.95*.40) = 53%

    If anything, my power generation efficiencies may be a little high.
     
    Last edited: Mar 24, 2007
  13. Mar 24, 2007 #12

    vanesch

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Russ already answered this, but look upon it this way:

    Put now the gas turbine, the wires transmitting the power (with losses) and the resistor all in the room. Effect is the same as the furnace.

    Of course a heat pump is more "efficient" but this is not in a closed system (in other words, there is a heat flux from the outside in).


    I understood the OP as follows: if I have a light bulb of 100W, that is not particularly designed to heat my room, or I have a superduper expensive "ultra-efficient" electrical radiator of 100W, which one is going to heat my room most efficiently ? Answer: they are equally efficient (up to some small details like the light that gets out of the window and so).
     
  14. Mar 24, 2007 #13
    The reason I think "alternate" (arguably more common) methods of heating are relevent, is that the OP refers to proposed legislation that is intended specifically to reduce total energy consumption. Though I do think the legislation is misguided*, it is nonetheless correct that efficient lighting will reduce energy consumption even in colder climates.

    *Personally, rather than outlaw any technology, I think individual products that are detrimental to society (including low efficiency lights, leading to greater greenhouse emissions) should only be able to be sold if (like cigarettes) the consumer is presented with advertising (to warn/inform about the greater TCO) and possibly a tax (since the human often fails to compare immediate vs. longer term benefits reasonably).
     
    Last edited: Mar 24, 2007
  15. Mar 24, 2007 #14
    In general, one should use the most suitable purpose-designed device for the purpose in question. The most economical approach is to use a good source of light to light and a good source of heat to heat. But it's not related to energy conversion as much as the way you use it. When you enter a storage room that you don't normally heat in the winter just to pick up a jar of pickles, using a more efficient source of light should save energy. You need to see, not to heat. Likewise if you want to stay warm in your office while you work, a space heater next to your chair will address your goal more efficiently than an incandescent bulb against the ceiling. And since you also turn on the lights in the summer, AC costs can be lowered by replacing incandescent bulbs. Using the right tools for your needs naturally works best.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Do light bulbs or convection heaters heat a room with the highest efficiency?
Loading...