Posted on 12/17/2009 7:57:09 AM PST by decimon
If you've seen an Internet ad for capacitor-type power factor correction devices, you might be led to believe that using one can save you money on your residential electricity bill. However, a team including specialists at the National Institute of Standards and Technology (NIST) have recently explained* why the devices actually provide no savings by discussing the underlying physics.
The devices—sometimes referred to as Amp Reduction Units or KVARs**—are touted as good investments because they reduce the amount of current drawn from power lines while simultaneously providing the necessary amount of current to appliances inside the house. Though engineers elsewhere have discredited the devices for use in typical residences already, NIST physicist Martin Misakian and two of his colleagues decided to write a brief primer describing the devices' inner workings for readers who are not power engineers, but who still have some technical background.
"One of the important functions of our primer is to remove the mystery of how current from the power line can decrease while at the same time current going to an appliance remains the same," says Misakian. The nine-page Technical Note explains this result in terms that might interest readers with knowledge of college-level physical sciences. It shows that although the devices can indeed reduce current flow from the power line, but it is not just the current flowing from the power line that determines your electric bill, but the product of the power factor and the current. Though current decreases with a power factor correction device, the power factor increases correspondingly, meaning the product of the two remains the same—with or without the device. Because a residential electric bill is proportional to this product, the cost remains unchanged.
Power factor correction devices have some use, though. The authors point out that while they will not reduce the average homeowner's bill, they may benefit the environment. When electricity travels from a local transformer to a residence, some power is lost due to electrical resistance. But because a utility would need to supply less current to a residence that employs a power factor correction device, these losses would decrease—thus potentially reducing the amount of greenhouse gases a fossil fuel-burning utility would emit. But while the primer does provide a rough calculation of a utility's savings by considering the operation of a residential air conditioner, Misakian says readers must investigate the details of these options for themselves.
"If homeowners wanted to help reduce the amount of carbon dioxide produced, they could install a device," Misakian says, "but they would also have to consider the greenhouse gases generated during the fabrication of the device itself."
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* M. Misakian, T.L. Nelson and W.E. Feero. Regarding Electric Energy Savings, Power Factors, and Carbon Footprints: A Primer. NIST Technical Note 1654, online at www.nist.gov/cgi-bin//get_pdf.cgi?pub_id=903669.
** From "kilovolt ampere reactive," a unit used to measure reactive power.
HEHHEHE...if you ever read the brochure they send out with those things, it’d look to the layman like a bunch of randomly strung-together technical electrical terms...and that’s exactly what it was...
“If homeowners wanted to help reduce the amount of carbon dioxide produced, they could install a device,”
Ed will get one! Or maybe two!
This contradict everything I have learned about PF's. The closer to unity the less power you use meaning less cost.
The rates are determined by power usage not current draws.
This actually a pretty good illustration of the concept.
The closer the vectors are to horizontal (power factor of 1) the less energy is used to drive the load.
bump
The reduction in loss through a few tens of feet of pretty heavy gage wire is going to be way down in the noise.
OTOH, the power companies themselves use such devices to reduce the current, and thus the resistive losses, on tens to hundreds of *miles* of power line. They are not totally useless
Just consider the intended audience...
People who voted for "the great uniter!", and swallowed AGW hook line and sinker.
Based on fake data or not.
Proof once again that college doesn’t mean educated. The “technical” paper posted is so full of bullsh*t I can smell it from here. They made assumptions not in fact, used incomplete equations, and didn’t account for all factors, chosing to leave out those that didn’t support the conclusion they wanted. Typical illiterate paper in liberal form.
For instance, a motor made for alternating current doesn’t do so well with direct current, which is that this whole thing is about: Producing more of a flat line voltage like DC rather than an alternating current like the AC coming into the house. The device will save no energy and actually cause motors to eat up probably past their ability to dissipate that heat and then fail.
Also, they failed to account for the additional current draw required to produce the torque the motor is intended to provide for the load, meaning more current is used with that lowered voltage, not less current. They try to say less current is used by only showing one vector of energy consumption in the equation.
“OTOH, the power companies themselves use such devices to reduce the current”
They use higher voltages to reduce the current draw.
I will have to read the entire paper to find out what exactly they did but this still flies in the face of the engineering knowledge (albeit limited) that I have on this issue.
It's my understanding that PFC devices are useful where large amounts of power are used, like a factory or mill. Lots of inductive loads (motors, etc.) in places like that require lots of reactive current that will enter the building but also leave the building, the only power being consumed is resistive (or Real) power. The problem is that the power station still has to generate that unused reactive power, hence the usefulness of PFC devices.
If you have a large inductive motor(s) that have a significant contribution to your total load, like Air Conditioning, the will decrease the total current.
If you leave them turned on while you only have resistive loads, like an oven or incandescent lighting, they will add to your total current draw.
If you include switching devices to turn them on and off with your motors, you can save the electric bill but are more expensive to install.
Are you talking about the NIST paper? It wasn't about using DC instead of AC. It was about getting current and voltage in phase with each other, but still AC.
I also saw them being hawked at the flea market...if that is any indication of the target audience (reference Larry the Cable Guy)
That too, in fact that first and more importantly. But the power factor correcting devices (aka rotating capacitors) are also used, and help with system stability as well as reducing losses.
If you've seen an Internet ad for capacitor-type power factor correction devices, you might be led to believe that using one can save you money on your residential electricity bill. However, a team including specialists at the National Institute of Standards and Technology (NIST) have recently explained* why the devices actually provide no savings by discussing the underlying physics.These are in the category with those "use your house wiring as your antenna" boxes and those buttons to "change signal" at the crosswalks of busy intersections. Thanks decimon.
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