Naked Science Forum
Non Life Sciences => Technology => Topic started by: cclarke on 26/09/2011 15:01:03
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Cathy Clarke asked the Naked Scientists:
We are thinking of installing a capacitor to save electricity in our house - apparently it works in industry. Â Does a capacitor installed in a domestic house as a way of saving electricity in fact fulfill this function, or does it simply prevent spikes and surges in electricity? Â Could you explain how it works?
Thanks
Cathy
What do you think?
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Roy Clarke asked the Naked Scientists:
Hi Chris
Please can you help me. I have been trying to find out if the plug in powersaving devices actually reduce the amount I will pay for electricity in my home ie the electricity consumption. The price of electricity has increased by 25% and will do so again next year and the year after. As far as I understand it, they are capacitors and and are supposed to work on the basis of reducing the power consumed by appliances with electric motors ie fridge, aircon, washing machine etc by power factor correction. They are promoted as saving 10 - 15%. There is a huge amount of online information both for and against.
Thanks
Roy Clarke
Cape Town
South Africa
What do you think?
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My first reaction is that such a device would likely consume power, and would be a scam, as discussed by Straight Dope. (http://www.straightdope.com/columns/read/2851/am-i-really-wasting-money-leaving-appliances-plugged-in)
However, further reading found this Robert Fletcher article Summary (http://www.allbusiness.com/utilities/electric-power-generation/581684-1.html), and full article PDF. (http://www.microplanetltd.com/upload/pdf/IEEE%20article%20draft%20Fletcher%20circa%202002.pdf)
I think the idea is this.
Your electricity delivered to your house actually varies from about 110V to 126V which may be dependent on where your house is, and can also vary somewhat during the day.
You used to be able to buy a resistor to use with incandescent lights, say to convert a 100W light bulb to 60W output.
This device would do the same thing. Instead of just passing through the 126V, it drops the power down to 110V, and thus provide you with about 10% less voltage, but still within the acceptable range for household appliances.
What happens next depends on the appliance.
Your lightbulbs in the house would all dim slightly, but perhaps not enough to be noticeable.
Your toaster would run cooler, so you would likely compensate by toasting your toast longer.
Your hair dryer would also be slightly cooler, and would also require slightly more time to dry the hair.
Motor driven devices such as the refrigerator would likely use similar wattage and not show a savings.
Your TV and computer with regulated power will likely be unaffected.
Overall, in an artificial test situation like in the Fletcher article, it is likely that a small benefit shows up. In the Real World, it is likely that this benefit goes away.
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In industry they are used for Power_factor correction (http://en.wikipedia.org/wiki/Power_factor#Active_PFC).
This is normally in cases where large electric motors are used and cause a large inductive loading on the supply. As the electricity companies don't fair well with having to deal with these phase-shifts, they charge the consumer for the trouble.
Home installations are unlikely to have enough effects such as these to make it economically viable IMO.
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In industry they are used for Power_factor correction (http://en.wikipedia.org/wiki/Power_factor#Active_PFC).
This is normally in cases where large electric motors are used and cause a large inductive loading on the supply. As the electricity companies don't fair well with having to deal with these phase-shifts, they charge the consumer for the trouble.
Home installations are unlikely to have enough effects such as these to make it economically viable IMO.
PC is correct. As he points out it will only adjust the phase of the current in the supply which will reduce the power lost in the transmission system. It won't make any difference to the power consumed in the home.
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PC is correct. As he points out it will only adjust the phase of the current in the supply which will reduce the power lost in the transmission system. It won't make any difference to the power consumed in the home.
Except, as I mentioned, if you run your lights at 110V rather than 120V, then you might save about 10% of the energy usage. However, you could accomplish the same thing by buying 10% lower wattage bulbs.
Many other devices in the house are likely less dependent on the voltage, especially if the goal is to convert a certain amount of electrical energy to heat, or something similar.
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Except, as I mentioned, if you run your lights at 110V rather than 120V, then you might save about 10% of the energy usage.
That's true, but you can't get there with a capacitor. You might drop the voltage a bit with a resistor, but the resistor will get hot and dissipate heat that is not producing any light. You could use a transformer to drop the voltage. That would be more efficient than a resistor, but it would still produce a certain amount of waste heat.
The best way to reduce light output without wasting a lot of energy is with some sort of switching regulator (dimmer), although that might not be compatible with a CFL.
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well if i remember right from my ham radio thing a capacitor temporally stores energy making less available there for making (only with batteries) the the object work at the lowest power lose possible
if i remember right tho
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PC is correct. As he points out it will only adjust the phase of the current in the supply which will reduce the power lost in the transmission system. It won't make any difference to the power consumed in the home.
Geezer.
The supply company charges you for your consumption of apparent power, not true power. They simply use VxI.
So even though your true power consumption is the same, you will pay more for any large inductive load.
Single phase motors have phase shifting capacitors to generate a field, so this might cancel out the effect of the inductance of the motors.
Someone would have to check your power factor and advise accordingly, but i suspect your saving won't be much.
As others have said your lights are a resistive load only, and they would be your main cost.
If you have a large three phase motor running, then it's a different story.
In South Africa, consumers are not protected, so as Clifford says: beware.
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PC is correct. As he points out it will only adjust the phase of the current in the supply which will reduce the power lost in the transmission system. It won't make any difference to the power consumed in the home.
Geezer.
The supply company charges you for your consumption of apparent power, not true power. They simply use VxI.
Johann,
It's the other way around. Domestic electricity meters measure true power. A power company could hardly charge you for power you never consumed.
(If they charge based on VI in SA, that's fraud.)
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Johann,
It's the other way around. Domestic electricity meters measure true power. A power company could hardly charge you for power you never consumed.
Geezer,
The only way to force consumers to supply power factor correction is to charge them for the current they are using.
from wikipedia's power factor page:
Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor.
Electricity meters operate by continuously measuring the instantaneous voltage (volts) and current (amperes) and finding the product of these to give instantaneous electrical power (watts) which is then integrated against time to give energy used (joules, kilowatt-hours etc
No one is going to fit a true watt meter in your house, because that costs money. They simply measure the voltage and current and multiply them, then integrate over time.They are not going to simply let you get away with heating up their cables and transformers with the extra inductive (or capacitive) current for free.
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Johann,
As I said, domestic meters measure power. That's why they say kWh on them.
"The metallic disc is acted upon by two coils. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. The field of the voltage coil is delayed by 90 degrees using a lag coil.[17] This produces eddy currents in the disc and the effect is such that a force is exerted on the disc in proportion to the product of the instantaneous current and voltage. A permanent magnet exerts an opposing force proportional to the speed of rotation of the disc. The equilibrium between these two opposing forces results in the disc rotating at a speed proportional to the power being used. The disc drives a register mechanism which integrates the speed of the disc over time by counting revolutions, much like the odometer in a car, in order to render a measurement of the total energy used over a period of time."
http://en.wikipedia.org/wiki/Electricity_meter
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"They simply measure the voltage and current and multiply them, then integrate over time."
And that gives you the energy transferred.
It really is true power because sometimes the current and voltage are in opposite directions.
The real reason that the power companies don't charge domestic supplies for the power factor is that it's usually very close to one.
The big loads are heaters and they are resistive.
Tungsten lamps are resistive (at least in terms of power factors).
It's not worth measuring the power factor for domestic electricity.
There's also the fact that neighbouring houses might cancel out some of the "lost" power.
Why should they charge for that?
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BTW, I worked for the company that put the RF reading electronics in the GE electronic meter in the second photo in the Wikipedia article [;D]
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It really is true power because sometimes the current and voltage are in opposite directions.
Yes - that's the case when the "house" is is supplying power to the utility company, in which case the meter actually runs backwards, although the modern ones won't do that. You have to have a special arrangement with the utility if you supply them with power.
In the US, the meter sockets are "S" symmetric, so you can plug the older meters in upside down and they will count down instead of up. It's hardly a surprise that the power companies tend to take a very dim view of that.
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"Yes - that's the case when the "house" is is supplying power to the utility company, in which case the meter actually runs backwards, although the modern ones won't do that."
there are two possibilities here.
If you are selling real power back to the grid the current and voltage must still be in phase.
However, if you have a reactive load where, for example, the current leads the voltage, the current will go through zero and become negative while the voltage is still positive.
The speed of the rotor in the meter is slow enough that it averages these effects out.
If, the current and voltage were exactly 90 degrees out of phase the net effect on the rotor would be zero and the meter wouldn't register any power even though there was a current and a voltage. (Of course, in this case the real power would be zero and the meter would be reading correctly.)
With a capacitor, half the time you would be taking energy from the grid to charge the cap and the other half of the time you would be paying it back.
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If you are selling real power back to the grid the current and voltage must still be in phase.
No, it ain't! The current is flowing in the other direction, so it's in anti-phase. (It really depends on the convention you choose to indicate "in phase".)
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What's odd is that, whatever convention you choose, they swap places for part of the cycle if you have a reactive load.
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What's odd is that, whatever convention you choose, they swap places for part of the cycle if you have a reactive load.
Well, right. Otherwise it wouldn't be reactive.
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BTW, I worked for the company that put the RF reading electronics in the GE electronic meter in the second photo in the Wikipedia article
Well, then i stand corrected.
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Generally capacitors are used for industrial or manufacturing users to reduce the penalty on their electricity bill for having a small power factor. If you want to add a capacitor at your home, you may be slightly reducing transmission losses for the electricity supplier, but certainly you power consumption will not decrease by adding capacitors.
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People have been selling power back to the grid here in the U.S. for a very long time. At first the power companies used to just measure the power you created and gave you a discount on your bill. They did not actually let you effect the power grid with your equipment.
When people found out about this practice it came out that our power companies are creating a lot of power from very small generators that use a rather small amount of fuel, people started talking and demanding. So they started actually letting people apply power to the grid. Which is rather unsafe.
We have the power if anyone has the guts to be different.
The power lines either above your head or in the ground are capacitors. Air Capacitors, condensers. The power companies in most cases could not supply power to all their customers without those runs of wire, their length and the time it takes to bring those wires/capacitors up to voltage.
Do the math of 750 kc mil transmission wire at 140,000 volts it would be 56,000,000 watts. The wattage is not really that high. You can count the mega watts they actually supply these wires with on your hand. Nothing a couple medium sized diesel generators could not supply. If we got ridiculous and claimed the generator consumed 200 hundred gallons an hour, that would put the cost at $1,600 dollars an hour to run both generators.
The amount of wattage those lines can supply can be much greater than the input wattage. Because of the time it takes to charge them.
Sincerely,
William McCormick
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You can count the mega watts they actually supply these wires with on your hand.
Most utilities also use larger power stations, in the region of 400MW to 1 GW.
Compared to the typical residential solar photovoltaic installation of 0.005 to 0.015 MW.
The power companies in most cases could not supply power to all their customers without those runs of wire, their length
It's called a transmission network, or "grid", and there is still an advantage in some sort of transmission network, even if people generated enough power for themselves (on average), whether from diesels or renewables.
...and the time it takes to bring those wires/capacitors up to voltage.
Most countries run on 50Hz or 60Hz. It takes about 5ms to bring them "up to voltage" - and back down.
Although subtle phase differences on the order of 1 or 2ms do make a big difference to grid efficiency.
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110 volt house current if viewed on an oscilloscope, reveals that the peaks are actually 165 volt peak sine waves. 165 volts above neutral and 165 volts below neutral either 50 or 60 times a second. Which means that in one cycle you could get a 330 volt differential applied to your body, especially where air gaps, small capacitors, or standing on wet floors is a possibility.
Now if you look at square wave or modified square wave, you will see that often the peaks of the actually rather square waves are just 55 volt peaks or less. Yet they cause a RMS meter to read 120 volts.
There are ways to use a capacitor to power things and gain. But it is like building a radio from scratch it can be done but is it worth it? I would go with LED's myself.
Sincerely,
William McCormick
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Capacitors simply store and/or smooth out electricity and won't save you money for using. What is the OP here interpreting a 'capacitor' is though? It's not something a general consumer would be able to be concerned about nor could 'install' without a background in electronics. Perhaps he/she was referring to a backup supply for blackouts or storm surge? They certainly require capacitors as components but unless it is some brand name for some energy saving device, they are not something one would require using as a consumer. (And not to mention, dangerous if we're discussing general power supply.)
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What is the OP here interpreting a 'capacitor' is though? It's not something a general consumer would be able to be concerned about nor could 'install' without a background in electronics.
There were rumors years ago that by connecting a big capacitor across an electricity meter, you could make it run backwards (or at least slow it down). I once had a holiday job calibrating electricity meters, and if you adjust them to be optimized for an inductive load (like most houses), it is possible that an extreme capacitive load could make them run (slowly) backwards. But any savings disappears as soon as you turn on a real load like an electric jug or a refrigerator.
In Australia, all traditional-style fluorescent light fittings came with an inductor (a "ballast" to smooth the current and kick-start the glow discharge) and a capacitor (to improve the Power Factor: PF).
Newer light fittings still have the inductor, but not a capacitor.
Long ago, I saw one of these capacitors "smoke", and I imagine they could cause the occasional fire, so I guess the regulations were changed so they were no longer required. Residential users don't have a huge impact on the power factor of the grid.
The newer compact fluorescents have an electronic ballast, and I imagine that these (and the numerous electronic chargers we use today) could be designed to have a PF close to 1.
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There seems to some confusion about using a capacitor to reduce the voltage of your domestic supply , a capacitor in series with the supply will certainly do this but the reduction will vary with the load, what is required is a transformer the output voltage of which will vary little with the load but will in its self consume some power in iron losses.
In theory this would also require a capacitor across the line to correct for the magnetising current but domestic power supplies are metered in such a manner that that the power factor is of little consequence.
Apologies to Geezer I seem to have repeated largely what he had already said but his was the only mention of transformers in all the replies and I missed it