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

[knut.erik.enerstvedt@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@tess-inc.com>
To:	knut.erik.enerstvedt@niva.no
Cc:	trnsys-users@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@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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