[TRNSYS-users] "Average value over time step" in TRNSYS Types

[knut.erik.enerstvedt at niva.no]

David,

Thanks for replying.

My component models a thermal storage and uses an explicit finite 
difference method to solve the model equation for the temperature T. I do 
indeed need T from the end of the last timestep to solve the eqation for T 
at the end of the current time step, and I use "getStorageVars()" and 
"setStorageVars()" to save and retrieve T between time steps. My question 
was really about which value of T I should use as output from this 
component, to be received as inputs by the other components. Would it be 
correct to use an average of T at the end of the previous time step n-1 
and T at the end of the current time step n, simply computed by (T_n-1 + 
T_n)/2?

Kind regards,
Knut Erik 



From:
David Bradley <bradley at tess-inc.com>
To:
knut.erik.enerstvedt at niva.no
Cc:
trnsys-users at engr.wisc.edu
Date:
08/26/2009 12:54 AM
Subject:
Re: [TRNSYS-users] "Average value over time step" in TRNSYS Types



Knut Erik,
 Sorry for the delay in getting back to you. Since almost all of the 
outputs that TRNSYS deals with are averages over the timestep, the input 
values that are passed to your components are the averages at the previous 
timestep and so your component doesn't really have to compute what those 
values are. If your component has some capacitance (thermal or electrical 
storage) to it then you do need to get what the value of certain inputs at 
the end of the last timestep were in order to calculate what the outputs 
at the end of the current timestep are. One typical way of handling the 
job of keeping track of those is to use the "getStorageVars()" and 
"setStorageVars()" subroutines. If you have to call the Differential 
Equation subroutine from within your Type, it will return the average and 
final value for the time step. You can then store that final value and use 
it as the initial value for the next time step.
Kind regards,
 David


knut.erik.enerstvedt at niva.no wrote: 
Hello TRNSYS users, 
 
As I understand, TRNSYS outputs are averaged values over a time step, 
meaning that the output value of T at timestep n is an average of T 
between n-1 and n. When creating a new Type, would it be correct of me to 
have T_output(n) = (T(n) + T(n-1))/2, where T(n) is the newly computed 
value in the current iteration and time step and T(n-1) is the converged 
value from the previous time step? If not, how is this average normally 
computed in TRNSYS Types?
 
Best regards,
Knut Erik Enerstvedt


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