[Bldg-sim] Energy model calibration - normalizing the utility bills to month start-end

Mon Jun 22 07:40:34 PDT 2015

Hello Chris,

ASHRAE Guideline 14-2014 has lots to say about this. So does the ASHRAE PMP, including lots and lots of references. In general, there are several things going on that can add noise to the answer.

First, for best results you should use measured weather data that has been recorded for the same period as the utility bills. Unfortunately, although this is easy to say in principle it can be difficult to apply. For example, not all weather sources have measured data for all channels, and worse, don't tell you when they have used an algorithm to "fill in" for missing data or to synthesize a channel that was never there. In addition, if your utility billing period runs over two different years, then you have to split the simulation since most simulations run from Jan to Dec, don't run contiguously for non-calendar periods (i.e., September to August). Weather data files also need to be consistent with the billing periods for non-contiguous periods. Use of average weather files to match actual utility bills is not recommended as this can introduce more error that you're trying to measure.

Second, since you have the simulation, I'd suggest you extract the hourly data from the simulation to match the calendar periods of the measured data. Then both can be "day normalized", or expressed as monthly average daily energy use (i.e., divide by the days in the month), and then weight the average by multiplying back by the number of days....since a month with 32 days should carry more weight than a month with 28 days when regressing...etc.

Third, don't use HDD or CDD to do anything. This is because commercial buildings don't normally behave according to 65 F change points. Instead, use the ASHRAE Inverse Model Toolkit, which comes free with Guideline 14, including the FORTRAN source code (...yes...FORTRAN). The IMT performs (1P, 2P, 3p,4P 5P) change-point linear and variable-based degree day calculations, depending on the what works best. This will do the weather normalization for you and give results that are consistent with Guideline 14. In fact, there's even a tutorial on how to use it in the appendix to Guideline 14 and you can get the RP 1050 final report to read more, including looking at the test data sets.

If you do use someone's software to do this, look for "algorithms by ASHRAE" in the software manual to make sure they are consistent with ASHRAE Guideline 14. That's why ASHRAE publishes the algorithms in source code to encourage software developers to use them providing they include the "powered by ASHRAE" in the manual.

Stay away from polynomials, quadratics, bi-quadratics, etc. as these are hard to reproduce from one person to the next even though the results look good.

Hope this helps.

Jeff

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Jeff S. Haberl, Ph.D.,P.E.inactive,FASHRAE,FIBPSA,......jhaberl at tamu.edu<mailto:........jhaberl at tamu.edu>
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________________________________
From: Bldg-sim [bldg-sim-bounces at lists.onebuilding.org] on behalf of Jones, Christopher [Christopher.r.Jones at wspgroup.com]
Sent: Monday, June 22, 2015 8:09 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Energy model calibration - normalizing the utility bills to month start-end

When calibrating an energy model to utility bills the utility bills often don’t align with the month start and end.  I have reviewed a couple methods to calendar normalize the utility bills but find them somewhat unsatisfactory.

For example the method I am looking at does the following:
The April gas bill runs from March 25 – April 24.  The algorithm takes the average number of m3 per day from that bill, applies it to the days in April.  Then it takes the average number of days from the May bill which runs from April 24 – May 25 and applies that average to the remaining days in April.

The issue is that the March-April period has much higher HDD than the April-May period and the “normalized” gas usage is significantly lower than the simulation data for April.

I am wondering if there are any papers or other sources of information as to how others approach this problem.

[cid:image003.png at 01D09C46.E75BA0D0]
Christopher Jones, P.Eng.
Senior Engineer

WSP Canada Inc.
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Toronto, ON M4P 1E4
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