[Bldg-sim] Estimating system matrix A from simulation engine
Bryan Eisenhower
bryane at engr.ucsb.edu
Wed Oct 31 17:24:40 PDT 2012
Deuk-Woo,
I have thought of this a lot before and will outline my experience
below, in summary, if you are looking for accurate dynamical systems
matrices from a building energy model (other than a hand-built RC
network model), you will probably want to learn *Modelica*. Folks in the
Modelica community have been using this method to study dynamics and
controls for over a decade in the automotive, aerospace, and other
industries.
As for other simulation environments, its really up to the one you
pick. Remember that the equations in an energy model are an
approximation to reality. The numerical solution of these equations are
yet another approximation to the dynamics in the equations. So, if you
are going to do system identification on the model, you are going to get
artifacts of both approximations. In my experience, the numerical
solvers in programs like E+ and TRNSYS are not developed for accuracy in
dynamics that you would need for this approach. They are developed more
for long term analysis and numerical stability - both of which they do a
great job on.
I have been involved with a project to try and obtain ABCD matrices from
*TRNSYS*. In this project, we went into the fortran code and wrote
extra code to perturb state variables to observe how they relax and
obtain matrices from this. We were successful in implementing this but
did not trust the results because we felt the solver was overwriting the
short term dynamics in the equations.
I also worked on ABCD matrices using *EnergyPlus*, this time using a
different approach. In this approach, I wrote an analytical model that
can 'talk' to the EnergyPlus input file and .sql database files. In the
sql file, there is most of the information about materials (e.g.
capacitance), as well as all the bookkeeping on view factors and which
constructions pertain to which zones. I wrote some scripts that read
this file and automatically generate a linearized model. The linearized
model does not take any time trace information (it does take some
operating point information). It was a lot of work and although I trust
the results, I wouldn't pass on the code to anyone else because it was a
bit of a mess in the end. I have published this work this year and if
you would like the papers, please write me personally.
A fourth data point: *IDA-ICE* is another modeling tool that has a nice
solver should capture short term dynamics. I asked Per Sahlin at IBPSA
London this September if there was a way to produce ABCD matrices from
the model and he said his customers were not interested (my paraphrasing
- hopefully correct) - which is completely fair. My understanding is
that the historical intersection of control analysis and building energy
models has been on longer time scales (e.g. setback / setpoint analysis)
or just time trace analysis - very little stability, frequency domain,
or other advanced traditional techniques.
Good luck! Since I have thought of this for some time, I'd be
interested to discuss further with you, especially if you obtain any
good results.
Bryan
On 10/30/2012 6:32 PM, 김덕우 wrote:
>
> Thank you for your reply.
>
> Unfortunately, I’m not familiar with Modelica simulation environments.
>
> If you don’t mind, could you please let me know any other ways?
>
> Best Regards,
> Deuk-Woo Kim
>
> *From:*Michael Wetter [mailto:mwetter at lbl.gov]
> *Sent:* Wednesday, October 31, 2012 1:49 AM
> *To:* 김덕우
> *Cc:* bldg-sim at lists.onebuilding.org
> *Subject:* Re: [Bldg-sim] Estimating system matrix A from simulation
> engine
>
> Most Modelica simulation environments allow you to linearize a model
> at the current state and save the A,B,C and D matrix that are its
> state-space representation at the point of linearization. See
>
> Michael Wetter.
>
> Modelica-based Modeling and Simulation to Support Research and
> Development in Building Energy and Control Systems.
> <http://dx.doi.org/10.1080/19401490902818259>
>
> /Journal of Building Performance Simulation/, 2(2):143-161, 2009.
>
> Also available as LBNL Technical Report LBNL-2740E.pdf
> <http://simulationresearch.lbl.gov/wetter/download/LBNL-2740E.pdf>.
>
> where this is demonstrated for a nonlinear heat exchanger model. The
> Modelica Buildings library
> (http://simulationresearch.lbl.gov/modelica/) contains a room model
> which you should be able to use for the same process to get the A,B,C
> and D matrices for a building model.
>
> Best,
>
> Michael
>
> On Sun, Oct 28, 2012 at 6:54 AM, 김덕우<deukwookim at skku.edu
> <mailto:deukwookim at skku.edu>> wrote:
>
> Dear all,
>
> Is there a method to estimate system matrix *A* (of zone or building)
> from simulation engines such as E+, ESP-r??
>
> (The*A* is commonly found in state-space equation, e.g. x’ = *A*x + Bu.)
>
> As far as I know, only system identification method can be applied in
> this situation.
>
> (but, I not sure that it is reliable)
>
> If there is more advanced way or efficient way to estimate *A, *please
> let me know.
>
> Any comments would be appreciated.
>
> Thanks in advance.
>
> Best Regards,
> Deuk-Woo Kim
>
>
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