Would like to Understand Demand Charge and Energy Charge 3

[CyDog]

We have a 150 hp motor that I am thinking of putting a
Frequency Drive on. My goal is to save money on our demand
portion of our Electic Bill. Could explain the following.

1. What is the First Demand Block and the Second Demand
Block ?
2. Are these 1 time blocks during the billing cycle ,or
do they create a new block according to a higher demand?
3. During these blocks,do they sample a few times during,
lets say a 15 min. or a 30 min. block? Is there a pulse?
4. Lets say if I start this 150 hp (Across the line)and I
have the following: (For example 15 sec.)
Start up 784.0 kw for 5 sec.
99.2 kw for 5 sec.
99.2 kw for 5 sec.
784.0 kw + 99.2 kw + 99.2 kw = 982.4 kw
982.4 / 3 = 327.46 kw
Is 327.46 kw what the demand charge would be based on?

Below is our Demand portion of our bill.

Billing Units Charge Rate Charge
First Demand Block 500.0 1.72 $860.00
Second Demand Block 630.0 1.69 $1,064.70
Total Demand Charge 1,130.0 $1,924.70

I may be off track , I would appreiate any help.

Thanks in advance,

 

[waross]

Demand charges are relatad, among other things to transformer heating. A demand meter typically responds to a step increase in the load in a manner similar to the heating of a transformer.
It may take the meter 15 minutes to register completely an increase in demand. It may take 3 minutes to respond to 67% of the demand.
Motor starting is typically ignored by a demand meter.
As to your blocks, your tariff may state:
The first 500 kW of demand will be billed at $1.72 per kilowatt.
The following xxx kW of demand will be billed at $1.69

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

 

[jraef]

Excellent answer. To further that, adding a VFD will only help in your demand charges if you can trim your total energy at any one point in time use by slowing down your process, whatever that is. That of course usually means whatever you do may take longer, so your total energy consumption may not change, but you may be able to soften the blow in terms of demand charges.

But it all depends upon what you are doing with this motor.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[dpc]

Your utility can(and must) provide to you the method used to compute the demand charge. These days, it is often available on-line. It may vary depending on your rate schedule or tariff.

As Bill mentioned, the demand charge is based on the peak average kW over a specified time period used during the billing period. This can be based on a 15 minute, 30 minute, or even 60 minute average.

For most modern billing meters, the demand interval is a rolling window, and the meter will store the maximum average kW demand it saw over any period (15, 30 min or whatever).

But you really need to get the specific method used by your utility, because many utilities have more than one component to the demand charge. In many cases, there is a charge for the peak demand during the previous month, and a second charge for the peak demand for the previous YEAR.

Having said all that, motor starting is generally not as big a culprit in demand charges as you might think at first. The demand kW is an average kW over a fairly long time interval. A motor draws high current during starting, but it doesn't last very long and the current is highly reactive, so it draws a lot more kVARs than kW until it gets up to speed.

Your utility may have time of use data that could help you determine when the peak demands are occuring. If not, you might consider monitor power demand with some portable metering for a month or two.

Good luck,

Dave

 

[waross]

Hi Dave. I am more familiar with older electromechanical meters. The old meters did not use average kW, they used peak kW.
However, the peak reading was generated by a damped thermal element with about a 3 minute time constant. 5 time constants to reach 100% of the step increase.
I saw one tariff that used either the actual demand or 92% of the previous months demend charge. Even if a plant closed and used zero energy, it took a year to pay out the demand charges.

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

 

[dpc]

Bill,

Yes, that "ratchet" on the demand charges can be a killer. One bad hour can cost you for the next year.

Lots of creative ways of billing for demand. I've also seen kVA used instead of kW.

A lot of the digital meters have a "thermal" demand option, in addition to the rolling window.

Now there are also ratchets on power factor in some cases.

Dave

 

[jghrist]

4. Lets say if I start this 150 hp (Across the line)and I
have the following: (For example 15 sec.)
Start up 784.0 kw for 5 sec.
99.2 kw for 5 sec.
99.2 kw for 5 sec.
784.0 kw + 99.2 kw + 99.2 kw = 982.4 kw
982.4 / 3 = 327.46 kw
Is 327.46 kw what the demand charge would be based on?

Below is our Demand portion of our bill.

Billing Units Charge Rate Charge
First Demand Block 500.0 1.72 $860.00
Second Demand Block 630.0 1.69 $1,064.70

Assuming a 15 minute demand interval and that the demand stays at 99.2 kW for the rest of the 15 minutes (900 sec), the demand charge would be based on:

[784 kW x 5 sec + 99.2 kW x (900 sec - 5 sec)] / 900 sec = 103 kW

This would all be in the first demand block, so the demand charge would be $177.16 if the average load was less than or equal to 103 kW in every other 15 minute period during the month.

It is questionable that the starting kW would be 784, however, because as Dave mentioned, it is largely reactive.

If you had other loads, they might contribute to the demand. On the other hand, if the 150 hp motor was not running during the whole 15 minute peak load period, the motor load would not contribute as much to the demand charge.

If the total demand was over 500 kW, then the marginal rate would be 1.69 instead of 1.72.

 

[ykee]

Most of the product manuals for meters will tell you all about the different ways demand charges can be calculated, but as others stated, you need to contact your utility to find out what yours does. This is a link to a multilin meter manual:

http://www.geindustrial.com/products/manuals/epm/GEK-113198A.pdf

 

[roydm]

If your utility can provide a printout from the demand meter it's worthwhile plotting it on a graph, you will then be able to see if it's worth persuing. Another quick way would be to compare the maximum demand to your average demand e.g. KWH / operating hours.
There are huge savings to be made if you have a fluctuating demand and some load you can shed or re-schedule.
Roy

 

[waross]

Hello ykee;
Thanks for the link. I have a metering project coming up and you have saved my quite a bit of google time.
Little purple star for you.

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

 

[slavag]

Hi Bill.
I would like recommend you see great Canadien company
PML, today it's part of Shnaider.

http://www.pwrm.com

Good product and good team.
Regards.
slava

 

[waross]

Thanks Slava.

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