I agree that measured weather data is the best way to go, but it is also difficult to get everything you need for a complete weather file. However, there are middle of the road approaches that work very well. Typically, the most important variables are the temperature and RH, which are also the easiest to measure or get from local weather stations. An easy approach is to replace the temperature and RH (or dew point temperature) data in the TMY file with your measured data. If you don’t have hourly measured data, you can also adjust the temperatures up or down based on daily or monthly measured data. I have done this quite successfully on several projects.
Michael
Michael Deru, PhD
Manager Systems Performance
Commercial Buildings Research
National Renewable Energy Laboratory
303-384-7503 (o)
303-725-3528 (c)
From: bldg-sim-bounces@xxxxxxxxxxxxxxxxxxxxx [mailto:bldg-sim-bounces@xxxxxxxxxxxxxxxxxxxxx] On Behalf Of R B
Sent: Thursday, June 28, 2012 3:10 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Cc: bldg-sim@xxxxxxxxxxxxxxxxxxxxx
Subject: Re: [Bldg-sim] [EnergyPlus_Support] Food for thought....
I have a different take on this. I think a TMY is good enough for doing model calibration that is used for ECM savings analysis on a whole building level and yearly basis - I think this is the type of project Jim is alluding to. In this scheme of things, the most important factors are the scheduling of the various equipment/loads and the control logic and gathering relevant data and making sure that the bills actually represent the building in question properly. If you are going for hourly calibrations etc, or calibrating to the few weeks of trend data that is collected, then may be actual weather data is useful, although not 100% convinced on this - depends on how much data one is able to collect.
If we go for a retrofit of a typical commercial building which is mostly internal loads dominated (unless the perimeter to sqft is a lot more-I am sure you can find papers related to this), I am not sure if weather related issues are as important. Also, while doing retrofit, you have access to three or more years of utility bill. Averaging out the utility bills (removing obviously outlier data) and calibrating to it is a reasonable way of proceeding to decide the savings due to ECMs. This takes care of the weather issues as well as all the operating/internal load issues that are also changing over time. Now you are no longer dealing with one year.
It obviously depends on the building type - OA dominated buildings get calibrated better with actual data - even here the more important data is the amount of OA coming into the building and the control logic.
Residential and small buildings are a different category - they will get calibrated better with actual data.
I agree with Jim that only recently a easy/non-time consuming way of using actual data has become available. Has anyone tried it (the files from weather analytics or equivalent) - is it as easy as using TMY without having to worry if the results discrepancy is due to issues in the weather file?
-Rohini
On Thu, Jun 28, 2012 at 1:48 PM, Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Dear Joe,
No fair! You and Dru have been at the forefront energy modeling research for most of my adult life, and have a big head start.
My guess is that you spent a lot of time preparing the actual weather files for the research, however. Unless I’m missing something, the ready availability of high quality (e.g., no big hunks of missing data) actual weather data has been pretty limited until recently. With folk like Weather Analytics getting on board and making it pretty easy to get and inexpensive, it becomes a lot faster and lower cost than trying to clean some of the NOAA / NCDC data, not to mention getting good data for sites not in or near a major city.
Kudos for being way ahead of the industry curve (at least my own curve)! It’s getting easier to catch up!
p.s., Dru sent me that paper and I’ll be reading it with interest very soon.
From: Joe Huang [mailto:yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx]
Sent: Thursday, June 28, 2012 1:53 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Cc: Jim Dirkes; bldg-sim@xxxxxxxxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Food for thought....
I've always thought it was a "no-brainer" to use actual weather data whenever you're comparing simulation results to actual consumption data. Even with the earliest degree-day software such as
PRISM (Princeton Scorekeeping Method) in the 1980's, it was stressed to use the degree days
from the period of record, and not the long-term average, so I'm not sure why this (using actual
year weather data) is such a revelation.
The variation in total energy consumption of course depends a lot on the building characteristics.
Back in 1996, Dru Crawley and I wrote a paper on "Does it matter which weather data you use in energy simulations?", for the ACEEE Summer Study on Energy Efficiency in Buildings (it also appeared as two
separate ASHRAE papers at around the same time) where we took some prototypical building models (Dru did commercial, I did residential) and ran them with various "typical year" weather files and also 25 years of historical data in 10-12 US locations.
Joe
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
yjhuang@xxxxxxxxxxxxxxxxxxxxxxxx
www.whiteboxtechnologies.com
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"
On 6/28/2012 8:49 AM, Jim Dirkes wrote:
Dear Forums,
I am busy preparing a short talk for the Fall ASHRAE Energy Modeling Conference. The topic is “An Approach for Calibrating Existing Building Energy Models to their Utility Consumption”.
As part of the preparation, I will address the issue of how much difference might result in energy conservation measure savings predictions if you use actual weather data for the billing period versus TMY data.
To get a rough idea how much variation there might be, I looked at Degree Days for a span of years. What a variation! (for the city I’m studying at least)
I am not yet sure how that affects total energy consumption – you’ll have to attend my presentation in Atlanta to find out J.
In the meantime, I am starting to think that existing building energy models should use actual weather, not TMY data. Have any of you run similar comparisons for existing building models?
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