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

How is power saved if I turn off appliance?

  1. Aug 10, 2014 #1
    I've heard of Green Day, where the people of a city/town turn off the appliances for an hour in the evening.

    How does this help? Like you can't tell the generators to turn off for an hour, all of a sudden.

    Suppose a city has load of X MW, then if .5XMW appliance is turned off, then where does the other .5MW current go?

    Also if i turn off the bulb of my house, where does the current that would otherwise light the bulb go?
     
  2. jcsd
  3. Aug 10, 2014 #2

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Power plants must have a means whereby the amount of current generated matches the electrical load on the grid. Loads on the grid fluctuate during the day and throughout the year.

    Most power stations do not run at their rated generating capacity 24 hrs a day 365 days a year. There may be times when electrical demand is high, like during the day in the summer when a lot of people are running air conditioners, and times when the demand is relatively low, like at night during winter when appliances aren't in use and non-electrical heating is used. There may be several generators at one power station which can be started or shut down depending on the needs of the electrical grid, and at times where peak demand occurs, standby generators can be started and run for short periods until the demand stabilizes.
     
  4. Aug 10, 2014 #3

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The companies just need to put less oil / coal / gas into their boilers when the demand is less. You could look at it as their job is to keep the generators revolving at a given speed so a bigger load requires more fuel. This is the same in your car when, to keep your speed constant when you get to a hill, you need to put your right foot nearer the floor.

    There is a problem with Nuclear Power Stations, though. You can't instantly 'reduce the fuel input rate' and it takes significant time to reduce the reaction rate in the reactor. The surplus energy does need to be dumped - either by heating up a lot of water or I believe they sometimes use a pump (hydroelectric) storage scheme.

    When you turn your lamp on or off, there will be a minuscule change in generator speed which the governors will take care of. Of course, such small variations tend to be lost in the general trend at lighting up time or going to bed time or when the factories all wind down at the end of the day.

    PS If you look at the mains volts with a meter, they do actually vary quite a lot over the day but much of what you are seeing will be due to the resistance in your intermediate local supply cables and the loads that all your neighbours are switching in and out. The power station can't iron out variations as local as that.
     
    Last edited: Aug 10, 2014
  5. Aug 10, 2014 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Westinghouse nuke plants were originally designed to "load follow"
    so the machinery is there to do it.
    Increasingly conservative limitations on power distribution inside the reactor core have, as Sophie said, made it difficult to maneuver a nuke using the control rods - they pretty much stay full out..

    And economics of fuel cost dictate the nukes run full bore while you throttle your coal, gas, and oil burners to match generation to customer demand.

    "Smart Grid" will enable the opposite paradigm : customer demand can be managed from a central location by remote control of individual consumers' household appliances.

    You can't store AC electricity.
    In theory, when you switch off that bulb a governor someplace moves a throttle valve infinitesimally less open, admitting less fuel to an engine or less steam to a turbine.. so the current ceases to be produced.

    I used to watch from a fossil power plant control room as summer afternoon thunderstorms moved in over Miami from the Everglades just a few miles west.
    That causes airconditioners to cycle off because the big clouds shade everything and the rain cools down peoples' roofs. Old timers could predict from the size of the clouds how many megawatts we'd shed. Dropping a hundred megawatts in our three hundred megawatt plant wasn't unusual.


    old jim
     
    Last edited: Aug 10, 2014
  6. Aug 11, 2014 #5

    anorlunda

    User Avatar
    Science Advisor
    Gold Member

    The others who answered your question were correct. The grid and generators automatically adjust every time you turn a light on or off.

    Since energy is conserved, and since the power grid (transmission and distribution) has no way to store significant amounts of energy; then the electric power produced must match the power consumed second by second within small tolerances. The utility companies can and do regulate generation to make it balance. They have been doing so by various means since the 1880s; long before computers.
     
    Last edited by a moderator: Aug 11, 2014
  7. Aug 11, 2014 #6

    psparky

    User Avatar
    Gold Member

    I have visited a coal fired steam electric power plant and agree with all the comments above, but couldn't you store AC in batteries with a full wave rectifier?

    Then use an inverter to feed loads from the batteries?

    And yes, the amount of burning coal boiling the "tea pot" is the way to control speed/power output.

    Kinda similiar to the Titanic....more coal, more speed. Until you see an iceberg...."shut all the dampers!!!".
    Not the best braking system.
     
    Last edited: Aug 11, 2014
  8. Aug 11, 2014 #7

    anorlunda

    User Avatar
    Science Advisor
    Gold Member

    Yes indeed. There is a rectifier/battery/inverter installation just like that in Fairbanks, Alaska. It can power parts of the rural Alaska net for up to half an hour.

    To do it for the whole USA grid is a problem of scale. The rectifier/intverter would cost about $750 billion. The battery bank would need to be 30 feet high with an area about the size of the state of Connecticut. [I didn't really calculate the actual size. The point is too big to be practical.]


    General Electric is building a new factory to make special batteries for electric utilities to do what you suggest. Flywheels are another way to store energy. Pumped storage hydro is the largest, most established way of storing energy. But even with all these, the output of generators still needs to be adjust continuously throughout the day.
     
  9. Aug 11, 2014 #8

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Thomas Edison is generally given credit for setting up the first central generating station at Pearl Street in NYC in 1882. The idea of using a steam engine to turn a dynamo was not new with Edison, but what Edison's innovation involved was a means to regulate the generating capacity of the central station to match the load put on it by his subscribers, so that the system did not see sudden surges of current, or conversely, brown outs due to insufficient current generation. Edison was also responsible for another less attractive invention: the electricity bill.

    http://www.ieeeghn.org/wiki/index.php/Pearl_Street_Station
     
  10. Aug 12, 2014 #9
    I read somewhere after the last earth hour someone arguing that having lots of people shut off their appliances for an hour actually results in a next increase in the amount of green house gas emitted.
    He argued that the energy saved over that hour were overwritten by the extra energy it takes to ramp up the generators again when everyone turns back on their electric flim-flams and doo-dads.

    I'm not sure if this would be true or not, but it seems somewhat plausible
     
  11. Aug 12, 2014 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I can't think that the startup energy is that much. It's not as if the generators actually stop turning and the total load can't have gone to zero. The statement sounds a bit like the daft opinion that you should leave your heating on all the time because of the energy used to warm up a cold house.
     
  12. Aug 12, 2014 #11
    Well the only generators I've worked around were co-gens at a industrial plant I worked at last year. When the plant went down or they shut off one of the TG's they would reduce the pressure in the steam vessels at the same time (granted this could have had more to do with the plant process than the TG's).
    As they had reduced the pressure when they would ramp back up the TG's that pressure would have to be restored in the boilers taking additional energy. Hence why the argument seems plausible

    But I'm still with you, it reeks of "Leave your lights on when you leave the room cause turning them on takes more energy"
     
  13. Aug 13, 2014 #12
    Don't generators and motors work at maximum efficiency when they are working at full load?

    Whether turning the appliance on takes more energy depends on the appliance. If it's a fridge, it takes more energy. If it's fridge to be kept off for a week, then let it remain off. If it's a fan then you can turn it off.
     
  14. Aug 13, 2014 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The basics of heat flow and temperature difference don't really change across the board. Certain things - like fridge compressors and flou tubes have a slightly higher start current. At that extreme there is always the Blast Furnace. They take hundreds of thousands ( tens of ??) of pounds to turn off and get going again.
     
  15. Aug 13, 2014 #14

    anorlunda

    User Avatar
    Science Advisor
    Gold Member

    Sorry cpscdave, that is completely wrong in most circumstances. An exception could be turning off the air conditioner for an hour. It will just run double in the next hour if you turn it back on. However, if you leave it off for 4 or 8 hours the energy savings will be big. That's basically what jaus tail said.

    Sorry jaus tail, that's also off target. The efficiency of the generators and motors are small stuff. The big effect is the efficiency of the whole power plant. Think boilers, turbines, pumps, jet engines and the like.

    Some plants are designed to run at 100% capacity all the time. We call those base load plants, and they tend to be the most efficient plants. Some are called peaking plants and their designs are optimized to turn off and on on short notice to meet demand. Still others, like diesel generators, run only in emergencies. Most power plants lie somewhere between those extremes.

    Grid operators in deregulated regions not only pay power plants to provide energy, they also pay separately for some of the plants to participate in up/down regulation, and they pay a third category of power plants to stand by in reserve not making power just in case they are needed. All these payments, are adjusted continuously to minimize the total cost to consumers.

    It may sound complicated but it makes good sense. Why shouldn't we have diverse kinds of power plants, with each kind optimized to best serve a particular need? Working together, they provide the least cost solution.

    Compare it to automobiles. We have cars, pickups, SUVs, big trucks, vans, buses, scooters, and many other kinds. All together they serve the nation's transportation system.
     
  16. Aug 14, 2014 #15

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    That doesn't really seem likely. The same principle must apply for all times and temperature differences. Temperature drop will be more or less exponential and the greatest heat loss / gain will be in the first hour. Refrigeration units take higher current whilst they are starting up but apart from that, what difference could there be, compared with the principle involved in heating systems?
     
  17. Aug 14, 2014 #16
    If you keep a refrigerator off for a week the running cost that you save is more than the starting cost of starting it after the week.

    But if you keep it off for an hour, then the running cost saved in that one hour, is less than the starting cost after the hour.

    The starting cost is the same but the running cost is important here. It depends on the appliance. A fan or electric bulb doesn't have much starting cost thus turning it off for half hour is better than keeping it on.
     
  18. Aug 14, 2014 #17

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    You would need to substantiate that. What is the total startup energy for a refrigerator motor? The startup power cannot be more than 3kW or the fuse would blow and the compressor takes a matter of seconds to reach its normal running speed. How can that total energy compare with running the fridge for an hour? Is there an additional factor in starting the unit that I have not included? The actual numbers are important in this sort of discussion
     
  19. Aug 14, 2014 #18

    anorlunda

    User Avatar
    Science Advisor
    Gold Member


    The start surge and inefficiency of short refrigeration cycles are real, but not the major factor.

    If you turn off AC for a short time (say an hour or less), heat capacity in the building will keep the inside temperature almost unchanged for a while. When it begins to get noticeably hotter and the human turns the AC back on, he/she feels cooler immediately because of the circulating air, but the building itself has to be re-cooled. Therefore, most of the energy savings are wiped out. It has nothing to do with motor/refrigerator efficiency.

    When the AC is off for a longer period (say four hours or more) the in-out temperature difference averaged over the whole period is less, and energy is saved. The occupants save energy by tolerating less comfort.

    Note that this is part of the emergency demand reduction plans of some utilities. They know that AC can be shut off for shorter periods to temporarily reduce load, without the public becoming aware of it. The load in the following hour will be increased, so the purpose is not energy saving, but rather overload prevention.

    So the energy savings is more question of human perception and tolerance than one of physics and engineering.

    My boat has a 12 volt refrigeration system. The manufacturer says the ultimate efficiency is gained by slowing down the motor so that it runs 24/7 and never cycles on/off. They don't do that because customers insist that it must be broken because it doesn't cycle on/off. Once again, human factors dominate actual efficiency.

    Keep up your skepticism sophiecentaur, that's very healthy.
     
  20. Aug 14, 2014 #19
    Possibly a more efficient method for short term storage would be a very large flywheel. While long term losses would certainly be higher than for the system you describe, for short term storage a flywheel would avoid rectifying to DC and then inverting back to AC. A simple motor/generator would suffice.
     
  21. Aug 14, 2014 #20

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Why doesn't the same principle that applies to heating apply to cooling? Yes, I am skeptical about this and you have only made assertions - not given data.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: How is power saved if I turn off appliance?
Loading...