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

Electric heat efficiency

  1. Jan 8, 2009 #1
    It appears everyday electric space heaters, that is resistance heaters, are virtually 100% efficient. No matter what resistance material is used, there is only 3.413 BTU per watt hour of heat output; in other words, all 1500 watt heaters consume the same amount of electricity and produce the same thermal heat output: about 5,120 BTU....All claims of greater "efficiency" are silly. (I'm ignoring that some use just convection, some radiant and some fans.)

    So what prevents us from extracting more heat from electricity? How do we know it's really 100% efficient and if it is, is there some theoretical limit to this conversion and if so what is it??

    A Wikipedia reference: http://en.wikipedia.org/wiki/Electric_heat
     
  2. jcsd
  3. Jan 8, 2009 #2
    BTU. That is an old unit.

    3.413 BTU = 1 watt hour.. Near enough. So that is as far as you can go.

    Resistance heaters are pretty well 100% efficient. If you are feeding in X Watts of electric power you have to ask in what form the power is coming out. If it is not heat what is it? Bit of electromagnetic radiation, light if the element glows, other wavelenths, sound if there is a slight hum. All these will be small so most is converted to heat.... nearly the full X. A fan heater will use some elec running the motor.
     
  4. Jan 8, 2009 #3

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    It's just simple conservation of energy that tells us that 100% is the maximum efficiency possible. That is the theoretical as well as practical limit. We "know" this because in the 150+ years that the conservation-of-energy principle has included heat as a valid form of energy, we have yet to observe a violation of it.

    http://en.wikipedia.org/wiki/Conservation_of_energy
     
  5. Jan 8, 2009 #4
    Maybe there is confusion over units. kW and kWh are not just electrical units. There is a slow move to using them for all power and energy whether mechanical, heat or anything.

    Gas fires and boliers are rated in kW (in many countires) now and even car engines.

    Calories, joules, BTUs have a fixed numerical relationship to kWh.
     
  6. Jan 8, 2009 #5

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Hmmm, that didn't occur to me. It would be akin to asking "why do I get 2.54 centimeters per inch whenever I convert a length measurement from inches to centimeters?"
     
  7. Jan 8, 2009 #6

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Note that a heat pump can give you more thermal energy than is used in the device, but it must take the excess energy from another location (thus, energy conservation is satisfied). In contrast, a resistive heater can operate as a closed system but its output rate of thermal energy is limited to the input power.
     
  8. Jan 8, 2009 #7
    the "100% efficiency" of a resistance heater is small potatoes compared to the overunity efficiency of electric heat pumps :wink:
     
  9. Jan 9, 2009 #8

    stewartcs

    User Avatar
    Science Advisor

    To be correct is not really the efficiency, but rather the COP that is over unity (this convention avoids the confusion with PMM's).

    CS
     
  10. Jan 9, 2009 #9

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Absolutely, and that's a key clarification. COP's of 3 or 4 are typically achieved in practice.
    http://en.wikipedia.org/wiki/Heat_pump#Efficiency
     
  11. Jan 11, 2009 #10
    Ok I'm not impressed with any of the answers so far. Maybe my question is poorly phrased. I am not asking about the different varieties of heat pumps....they typically switch from heat extraction (as explained in posts above) to resistance heating when temps fall enough...at least some even have adjustable settings for that change...

    How do we KNOW a plain old electric heater is "100% efficient"?
    According to the superficial logic of "energy conservation" posted above seems like one would also answer the same for incandescent and fluorescent bulbs as for resistors: all use electricity, all produce electromagnetic waves, all produce some visible light along with heat.

    Yet I believe fluorescent bulbs produce significantly more light than heat....so I think something is going regarding the conversion efficiency of electricity to electromagnetic radiation at different frequencies....seems maybe its more efficient at lower (infrared) frequencies??? This is perhaps more a question of thermodynamics than EE...
     
  12. Jan 11, 2009 #11
    You seem to want to complicate what is quite simple.

    A resistance heater is efficient because there is essentially only one process going on. The heating element is getting hot because of current flow. There isn't much light (unless it glows very brightly) or any other significant form of energy output. Watts = current x voltage = electrical power input = heat output in watts.

    You can actually use the heating effect (rate at which water is heated) to measure the electrical power.

    An incandescent bulb and a fluorescent use different processes. One produces far more waste heat than the other.
     
  13. Jan 11, 2009 #12
    I'm reading into this but I think that the OP wants a reason why we can't find or haven't found a substance that yields more heat than the current carbon, nichrome, etc. devices yield when the same current runs through said device with the same voltage across said device. If we were to find such a device (which we won't) then we could assume that all of this time our carbon resistors on circuit boards and nichrome wires in toasters have been generating a given amount of heat along with some other form of undiscovered energy.
     
  14. Jan 12, 2009 #13
    yes,in a sense, because that's how all improvements and new insights are made: simple questions are the best ones. But the key insight is that it is NOT a simple question.


    Post 12...
    I haven't gotten that far, but YES, that's the kind of thinking I am looking to elicit....This perhaps involves not only thermodynamics but materials science so maybe I posted this in
    the wrong forum.

    I have no idea why current materials would be 100% efficient as Wikipedia claims....
    Why should tungston and nichrome, for example, be equally efficient?? I don't know because I don't know the exact process for converting electron flow into radiant energy.

    But the "standard answer" "clearly doesn't distinguish between fluorescent and incandescent bulbs...so as of this moment I don't accept it as a valid response.....if different processes yield different heat outputs (fluorescent vs incandescent, for example) how was it decided resistance is 100%???......why use the old resistance process?? When it was selected science did not know what we know today!

    what I was starting with was the idea that perhaps, for example, resistance heating materials are chosen for partly their longevity rather than only heat producing efficiency....tungesten filaments in a vacuum were chosen after other materials and fabrications burned up....hence I'm asking if there is an opportunity...and nichrome may be the toaster material because while more expnsive, it doesn't burn up in air. As another example, microwave ovens also provide heating: more or less efficient than resistance? And smooth top stoves...are they halogen?? How efficient and why??
     
  15. Jan 12, 2009 #14
    How many times do you want it???

    THERE IS ONE SIMPLE PROCESS GOING ON. Electric current flow through a conducctor and it heats up. The amount of energy which is released in any other form is neglible so you approach 100% efficiency. It doesn't matter what the conductor is made of. If there is voltage V across it and current I passing through it the dissipated power is V x I = Watts and this appears as nearly 100% heat. You have to pick a suitable resistance material for engineering reasons. The physics is the same for all.

    Lights of any kind produce heat as well so the light output power is usually well short of the electrical power output.

    A microwave oven has two energy conversion processes which are not 100% efficient.. AC to DC in the power unit and then DC to Radio Frequency energy in the magnetron. Most of the waste energy is heat but some of it could be blown into the microwave and add to the cooking process. Our 850 Watt microwave takes about 1400 Watts from the mains.
     
  16. Jan 12, 2009 #15

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Here's another way to think of it: energy can only be transferred by heat, work, and matter. Electric heaters and incandescent and fluorescent bulbs do a negligible amount of work on the environment and emit a negligible amount of matter during operation. That only leaves heat transfer. Thus, they are 100% efficient at transferring input energy into thermal energy. They are 0% efficient at doing work or transferring matter. Does this make sense?

    Despite this commonality, there are still significant differences in material properties. The resistive heaters contain filaments with different resistivity, thermal conductivity, specific heat, density, and emissivity. Fluorescent bulbs operate by a different mechanism entirely. If you want to talk about the operational and material property differences, let's do that. But heat efficiency isn't one of the differences.

    For example, tungsten and nichrome elements heat up to different maximum temperatures, emit photons of different wavelengths, emit different numbers of photons, weigh different amounts, have different resistances to failure by creep, cost different amounts, and consume different amounts of energy. Is this what you're asking about?
     
  17. Jan 12, 2009 #16

    russ_watters

    User Avatar

    Staff: Mentor

    I agree with the others that you are trying to complicate that which is simple.
    If the room is sealed and has no windows, all are 100% efficient at producing heat.
    All of the light emitted by a light bulb is absorbed by the walls and objects in the room and converted to plain, ordinary heat.

    But perhaps you are looking for a different type of efficiency. You could divide light output in watts by energy input in watts and get "light generating efficiency" (and you can, in fact, find light bulbs described this way), but that's not what you asked in the OP and not mutually exclusive with the fact that the heat generating efficiency is 100%.
    Energy has to go somewhere. If you take a box and keep putting energy into it, it will just get hotter and hotter and hotter without end. In order to avoid the paradox of a continuously increasing temeprature, the box must come to an equilibirum point where it releases - in whatever form - as much energy as is put into it.
    Again, this is just you overthinking it. Visible light, infrared, UV - these are all just different flavors of ice cream. They are all the same thing: radiant heat. As I said above, you can create another forumlua that describes the difference between visible light and ir light output and call it a different kind of efficiency, but that doesn't mean the heat generation efficiency of any of these devices is anything other than 100%.
    Correct: materials are chosen mostly for their durability (and that includes the operating temperature, since the operating temperature determines the light color and light producing efficiency).
    Less. You may notice that many (all?) microwaves have fans to cool the electronics. That's heat energy that doesn't go into the food. Of course, if you want to use your microwave to heat your room, then it is 100% efficient...
    Resistance heating elements and 100% efficient at producing heat, but not necessarily 100% efficient at transferring it to your food.
     
  18. Jan 13, 2009 #17
    I generally understand conservation of energy is the proper viewpoint and so I guess there is not something we can flesh out further.

    Beware: some really crazy assertions are posted here:
    (heat transfer is via conduction,convection and radiation,):


    Thanks for the inputs...
     
  19. Jan 13, 2009 #18

    Mapes

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    This comment is mine. Would you mind explaining what you find "crazy" about it? What non-negligible work would a power supply-bulb system do on its surroundings? (To be quantitative, let's assume a 120V supply connected to a 100W incandescent bulb.)
     
  20. Jan 13, 2009 #19
    nothing crazy going on. if you know the current and voltage, you can calculate the energy dissipation. next, measure the actual heat released from your load. from the difference, you can calculate energy lost via other mechanisms (light, radio waves, whatever).
     
  21. Jan 14, 2009 #20

    russ_watters

    User Avatar

    Staff: Mentor

    I don't see what's so crazy about saying you can't just keep putting oranges into a bushel basket forever.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Electric heat efficiency
Loading...