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

Carbon footprint! Turn your lights off! - Does it really make a difference?

  1. Jun 4, 2008 #1

    Got into a discussion at School today about reducing energy consumption at home in order to save the planet.

    I asked a question that stumped my Physics teacher.

    If I do turn off my lights and TV when i'm not in the room surely the fossils fuel would have already been burn and the carbon dioxide therefore already released into the atmosphere? I understand that Power Stations rely heavily on predicting the electricity demand and so if we all started being more energy efficient they would start burning less fossil fuels - but my question is this... Actually turning off a light switch does that have any 'immediate' benefit to the planet? Does it really make a difference? Surely all we are doing is not using electricity that has already been produced?

  2. jcsd
  3. Jun 4, 2008 #2
    There is an averaging process that goes into electricity demand. Your individual act will never be known by the power plant operators. If, however, 20% of the people on that grid turn out unused lights, the power plant sees that and adjusts it's output accordingly. But, keep in mind that almost all grids have many, many power plants, of differing types with differing response times.

    If you did not use the electricity, where would it go?
  4. Jun 4, 2008 #3


    User Avatar

    Staff: Mentor

    Though small changes like that are too small to measure, every change does have a real effect at the turbine.
  5. Jun 4, 2008 #4
    This is kind of like one person not voting because they think one vote wouldn't make a difference.
  6. Jun 4, 2008 #5
    I heard (on the radio?) that the methane generated by cattle has more greenhouse effect than all the carbon dioxide we produce from combustion. Is this accurate?
  7. Jun 5, 2008 #6
    I can only say that I have stood in a stable filled with farting cattle and I have stood at an open sampling port on a coal-fired power plant flue. I'll take the cows every time.
  8. Jun 5, 2008 #7


    User Avatar
    Science Advisor
    Gold Member

    No, but it is significant, mainly because methane is a much worse greenhouse gas than CO2.
    However, methane is not stable once it is in the atmosphere; it breaks down into water and CO2 (I don't remember the half-life, a few months?) meaning it does not accumulate in the atmosphere in the same way as CO2 from combustion.
  9. Jun 5, 2008 #8
    Back to physics and the original question - I don't really know much about this, but I think that the effect of the load reductioin on the generator is smoothed out by the changes in the fields induced around the power lines and around the coils in the generator itself. Also, the power grid has large capacitors to smooth out voltage changes. The effect of large load changes also depends on the type of load - resistive loads (like light bulbs, heaters, etc) have a different effect than inductive (or reactive) loads (large industrial - sized motors) that generate magnetic fields as they operate. Try Googling "megawatts" and "megavars."

    But even with this "smoothing" effect, there really isn't anywhere for the "power" to go except to the loads, since increasing magnetic fields eventually shrink (when your neighbor turns her light on...) and the energy is pumped back into the line, raising voltage.

    Turning off the lights will reduce your power bill and will reduce the amount of electric power produced by the plant. I will leave the morality of voting or not, and the effects of farting cows to other posters.
  10. Jun 5, 2008 #9
    It wouldn't have an immediate benefit because power plants need to keep electricity generation equal to demand. In most plants it takes hours to increase generation load, with the exception of hydroelectric, so they need to predict what the demand will be. This is why on usual hot days, you may experience power outages from people turning on their AC.
  11. Jun 6, 2008 #10
    An increase in the load reduces the system voltage, so if the generation rate is not increased, the new load will be satisfied by essentially stealing a little bit of the energy that was going to everyone else. If this goes on too long, it's a "brownout," if it goes too far the system protection features will trip the low voltage section off. That's when you see a "power outage" from high demand. In the normal case of small variations, the generator voltage regulator acts continuously to restore the voltage by adjusting the field strength in the generator. The plants have control systems to increase the flow of coal or gas to the boiler to increase the power output. Nuclear units do this "by themselves" due to the negative reactivity feedback (but in practice, the nukes are run continuously at full power because the fuel is the cheapest, and the controls systems won't let the demand drive power above the rating of the plant). Maybe a real EE out there can expand / correct this.
  12. Jun 6, 2008 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    As others have pointed out, in the grid, production must balance EXACTLY consumption to a few seconds precisely, because of conservation of energy. So, yes, if you turn off your lights, you will consume a little bit less, and hence, the produced power must also be a little bit less.

    However, the real question is: *what source* will adapt ? Will it be a coal-fired plant, that is not very flexible ? Or will it be a gas turbine, that is very flexible ? Or will it be hydropower that is very flexible ? Will it be a nuclear power plant that can or cannot be flexible depending on its steering mode ? Will it be wind energy ?

    The point is, for instance, that if one regulates the fast changes with hydro, then this has no effect at all on the overall CO2 production and fossil fuel combustion. Same if it is wind energy or nuclear that adapt to the changes.

    This is BTW one of the silly things with the wind energy in Denmark. Because wind energy is highly variable, the Danes use their neighbours the Swedes to act as a buffer. Now, Sweden is essentially 50% nuclear and 50% hydro, where they use their hydro to adapt to load changes and nuclear provides base load. So when the wind blows in Denmark, the Danes take a bit of this current, and sell a large fraction of it to Sweden, which buys it, and cranks down its hydro. When the wind drops in Denmark, the Danes buy back their electricity from the Swedes, which now boost up their hydro. So the Danish wind turbines trade essentially electricity with the Swedish hydro: the more it blows in Denmark, the less the Swedish use their hydro.

    Now, of course there is a net benefit in CO2 production, but it is far less than the total wind production in Denmark.
  13. Jun 7, 2008 #12
    Don't forget that some markets actually do store their excess power, e.g., by pumping water up a mountain. Of course, this can be problematic if the reservoir breaks.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?