efficiency improvement by local cos phi compensation 1

[pbrod]

Hi,
An energy study indicated that we could save 3-4% active power consumption by installing condensator banks near some of our biggest electric motors (range 35-110kWe).
the local cos phi is in the range of 0.60 - 0.85.
A company offerd us a capacitor banks to have a local cos phi of around 1. This could improve the engine efficency by about 4-5% in local active power consumption.

My question is:
- is this a realistic offer?
- does someone has experience with this?
- if anyone has background literature on this, always welcome!

Thanks for the help!

 

[X49]

Active Power used should remain virtually unchanged with the addition of power factor correction capacitors (there will typically be a minor reduction in losses in cables due to lower current, but I would think this would be less than 3%). The majority of your savings will be in reducing Reactive Power. Also, it's usually not economical to correct to a power factor greater than about 0.95. I would be wary of this company, especially if they are the ones who did the study and they are also trying to sell you their equipment. That being said, power factor correction is usually economical for systems like yours.

You need to find out how your utility company bills you. Some have demand charges based on kVA. Others have surcharges for low power factor (below 0.9 for instance.) Then you need to estimate your annual cost savings, and compare this to the cost of the capacitor banks. If you cannot do this, you should hire a consultant who can.

 

[jraef]

This could improve the engine efficency by about 4-5% in local active power consumption.

So from this can we imply that you generate your own power on-site with a diesel generator?

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[pbrod]

For the moment we do not generate or own power. With engine efficiency I meant that they claim an active power consumption reduction of 4-5% on the electrical motor.
They say this is realised by the reactive power compensation and the higher voltage at the motor connection.

I also contacted some academic people to find out more. I keep you informed.

The company is

http://www.pentaq-energy.nl/

 

[waross]

Most utilities charge large users a penalty for a poor power factor. Power factor correction is typically used to avoid PF penalties. PF penalties are typically quite onerous and the ROI may be less than one year.
Your motors draw reactive power as well as real power. The power factor is the ratio between real power and reactive power. The motors draw the same current regardless, from the grid, or from power correction caps if caps are installed. There is negligible change in motor current with the installation of PF caps.
There may be some reduction in losses in your conductors from the reduced current in the conductors to the motors.
In a code compliant installation, there should be no more than 3% voltage drop between the service (metering) and the motors.
If we improve the PF from .65 to 1.0 we may reduce the voltage drop by 24%. This may look like a big saving in feeder losses, but wait; Motor PF typically runs around 85% to 90%. A PF of 65% is a strong implication of a lightly loaded motor. With a light load the voltage drop to the motor will be less than 3% and the actual savings will be much less than indicated by a simple calculation.
Further, I have done the calculations once and never again on the cost of individual capacitor banks as opposed to correcting at the service.
Now when penalties are charged, the penalty may be 10% or more of the total consumption charges and the ROI would be about one year. Individual correction cost several times as much as correcting at the service, and you are saving much less cost. I expect a very long ROI to save a few percent with individual correction.
There are exceptions, but those of us who have done the numbers and seen the snake oil schemes that come around are quite skeptical about the anticipated savings. I would not bother to investigate further unless someone was paying me for the survey.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

I should add that there may be bigger savings by avoiding distortion power factor but that must be addressed by filters, not capacitors. Capacitors may make the losses from poor distortion power factor even worse.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[pbrod]

Thanks for the replies!!
I can add that we have a general cos phi improvement just before the transformator, to improve the cos phi to avoid a penalty from or transmission system operator.

 

[electricpete]

Even more reason to consider the claim snake oil

I meant that they claim an active power consumption reduction of 4-5% on the electrical motor.

jraef led you to this for a reason. Think about how power factor correction can reduce active power consumption of a motor by 4 – 5%!

Typical large motor efficiency may be near 95% to begin with. That means 5% losses. In that case, the only way to reduce active power consumption by 5% (without changing the mechanical load) is to eliminate the losses. You may reduce those losses a tiny bit by increasing motor terminal voltage (within a limited range of load near full load, voltage near full voltage, depending on motor characteristics), but that reduces the losses by a tiny fraction, it doesn’t come close to eliminating them.

Not close to realistic.


=====================================
(2B)+(2B)' ?

 

[LionelHutz]

The only reason to install power factor caps which has a realistic payback is to save on the utility penalties for poor power factor. You are already doing this.

Did they give a useful explaination on how these savings are possible? A simple explanation of how should be readily available if the savings are possible. It's not possible to lower a motor's losses by 3% to 5% just by installing a capacitor on it's line terminals so they must know where these savings are coming from.

 

[DRWeig]

Agree with Lionel. I've been through this many times, including in a courtroom when I had to argue against a capacitor-seller for deceptive practices. He was selling to churches and other unsophisticated end users, and he used a clamp-on ammeter to make the sale. Bad guy. When they called me to explain why the utility bill was still so high, even though the current had dropped as shown on the ammeter, it turned sort of war-like.

Let's assume that adding the capacitor does not change the operation of the motor at all (when in fact, it will slightly raise the motor terminal voltage due to lowered line losses). If there are energy-saving effects from the slightly-higher voltage, I don't know how to compute them -- seems it would depend on the type of load. If the load were centrifugal (pump), a higher voltage might speed the motor up a tad and cause more work to be done? Anyway, consider in this case that all we're doing with the capacitor is lowering I-squared-R losses in the feeder to the motor:

100 kW load at poor power factor, say 50%, 480/3: Line current = 240A
Improve power factor all the way to 100%: Line current = 120A

350M three-wire (at 0.037Ω per 1000 feet, 100-foot distance) has total 0.0111Ω

Total active power at 50% power factor = 100 kW + 240A^2 * 0.0111Ω = 100.639 kW

Total active power at 100% power factor = 100 kW + 120A^2 * 0.0111Ω = 100.160 kW

So we save almost 75% of the line losses, but just a tad less than 0.5% of the total load.

Assuming 8760 hours/year and USD$0.09 per kWh, our bill goes from $79,343.79 per year to $78,966.14 per year. That’s $377.65 in our pocket, saved.

Now, how much is that 100 kVAR capacitor bank to purchase and install? I haven't priced that kind of stuff in decades, but I'm betting it would be over $5000. If so, that's a 13+ year payback. I wouldn't buy it.

I gave a very-best case above. If the motor runs 12 hours per day instead of 24, that's a 26+ year payback. If the original power factor was better than 50%, there's less to be saved (but smaller capacitors to buy).

It all boils down to economics. If you have a power factor penalty, it can pay quickly. If not, do the math. Energy savings won't be big compared to the cost of installation.



Good on ya,

Goober Dave

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[jraef]

Given that your supplier provides NO details on their website, it smells of a scam to me, as if they have something to hide. This is a very big scam going on all over the place, DRWeig has provided you with an excellent case study of the reality behind much of the arguments they try to make, although I think he is being a bit too generous in many of his assumptions.

In addition, it appears as though they are claiming to improve the power factor of the motor just by increasing the terminal voltage? Here is a reality check on that idea, taken from some very tried and true motor studies.

Bottom line, this is not going to provide the kind of benefits they claim, it never does.


"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[DRWeig]

Thanks, jraef. I agree that I was very generous.

That was a horribly over-sized motor in my example, and the feeder was sized for the nameplate. It just isn't a good investment unless PF penalties are involved.

Good on ya,

Goober Dave

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[LionelHutz]

You should be asking - Saving 5% of what value?

I agree with the numbers Dave posted and they are most likely optimistic or best case. The only comment on the numbers is that with a motor you usually buy the same capacitor regardless of it's running power factor.

Just for fun, consider this. A 100kW motor could be 96% efficient or have about 4kW of losses. So, the total losses associated with the motor could change from 4639W to 4160W or be reduced about 12%. Now, you have 12% reduction in the losses for that motor. You still will have a terrible payback on the new capacitor.

 

[jraef]

Sometimes I think they play a game with numbers and semantics.

"4-5% improvement in efficiency" could (in their minds) be interpreted that if you have 96% efficiency, that's 4% INefficiency and you improve it 5%, that is .05 * .04 = .002 or 0.2% improvement, so your motor efficiency goes from 96% eff to 96.2% eff.

I know it's pretzel logic, but I really think that's their game. I've locked horns with some of those "motor energy saver" people before and they would claim "25-40% savings". When it all came down to it, they were (maybe) saving 40% of ONE of the sub-types of LOSSES in the motor (voltage dependant iron losses), which represented 50% of the next level up (magnetic losses), which represented 25% of the total losses, which themselves were only 5% of the total absorbed power. So what they REALLY were saving was 40% of 50% of 25% of 5%, so it was only 0.25% of the total absorbed power, a far cry from the 40% claim.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[pbrod]

I thank you all for the information and the help posted.

I think there must be something in the way they measure and then calculate the "efficiency" or power consumption reduction. They compare the "local (read motor efficiency improvement)" by a local measurement of the voltage and current. So with the condensator bank in service it is something like cos phi 1 and with the bank switched of it is something like 0.8 and then they indicate the power difference as savings.
But I will try to figure and clear this out with some calculations.

they also do a central measurement with and without the condensator banks and then these are the total savings of the project.

regards

 

[LionelHutz]

Make sure they aren't using the current measurements or the VA measurements to prove the energy savings. It amazing to me how many examples of this I've run across. You should not be making any decisions on power saving equipment if you don't understand the basic AC power principles.

 

[DRWeig]

Yes, do what Lionel says. You pay for watts, not VA, unless you have a power factor penalty being imposed. If they're measuring with a watt meter, question it. Rent your own, get a good one.


My final added advice is to have ALL the economic facts in one place before making a decision. The folks who preyed on the preachers and priests with their capacitor sales were trained to NEVER allow their savings calculations to be shown at the same time as the installed cost estimate. They didn't want anyone to compute actual economic performance. Most of their victims simply didn't look beyond the claims of massive savings.

Good on ya,

Goober Dave

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[stevenal]

I suggest you have your energy study redone by someone who has no products to sell.

 

[electricuwe]

It's quite simple from my point of view:

By local power facor correction you can only save on the losses in your downstream distribution between transformer secondary (assuming PF correction located there) and motor. Unless there are tremedous losses due to long lines at low voltage the the total saving to expect is quite small.

 

[waross]

The savings are between the meter and the capacitor connection. No change in the current in the motor conductors from the motor back to the point of connection of the capacitors.

Bill
--------------------
"Why not the best?"
Jimmy Carter