[TRNSYS-users] Type 557b: Question about simulating TRT and step response for 20 years

[Michaël Kummert]

Clara,

I have used Type 557 to reproduce Thermal response tests in the past and I
have not experienced the problems you describe. I did get a straight line
vs. ln(time). Type 557 does neglect the short-term transient behaviour of
the fluid loop and grout, which is important when you consider short-time
dynamics (i.e. one hour or less) but this should not have a significant
impact over a few days. However, Type 557 "history" can be tricky to setup
and the model can be sensitive to the ambient temperature that you connect
to it (especially if your boreholes are not very deep). Perhaps what you are
seeing is a combination of the response to a constant heat input AND the
effect of history / ambient temperature? To check that you could just set
the heat injection to zero and see what happens. Or increase the heat
injection so that these effects get smaller proportionally.

Also I am not sure I understand you correctly but it seems that you are
simulating the Thermal Response Test with the whole borefield, not just one
borehole. Usually the test is performed on one borehole so I am not sure how
you are doing the comparison. Are you scaling up the measured flowrate and
heat input? 

Regarding the second problem, the only time I got that "unspecified error"
in Type 557 is when I cooled the ground to an extremely low temperature
because of an error in the input file. Are you by any chance cooling the
ground (or rather warming it up) to unrealistic temperatures? Type557 has a
simple physical model (e.g. it does not know about freezing the ground or
boiling the heat transfer fluid) but still "unexpected errors" are easier to
forgive when the model is operating well outside the intended (and in this
case physical) range.

Hope this helps,

Michaël Kummert
ESRU - University of Strathclyde




From: Clara Verhelst [mailto:clara.verhelst at mech.kuleuven.be] 
Sent: 17 November 2008 18:34
To: trnsys-users at cae.wisc.edu
Subject: [TRNSYS-users] Type 557b: Question about simulating TRT and step
response for 20 years

Dear all,
 
I am using the borefield storage model Trnsys Type557b to learn more about
the borefield dynamics. I have 2 problems: The first concerning the model
accuracy for short time periods (at start-up), the second concerning very
long simulation times. 
 
1. Simulation of short time periods --> determination of borehole resistance
First, I would like to simulate the Thermal Response Test, which is the
evolution of the mean fluid temperature as response to a step heat input.
There exists an analytical solution to this problem in case of a single
borehole. This analytical solution shows that the temperature rise versus
the logarithmic (ln) of time, should be a straight line. The slope of that
line is the borehole resistance (see eg.PhD thesis of Gehlin 2002). 
 
For short time periods, eg. 3 days, there is no interference between the
boreholes and therefore I suppose that the response should correspond to the
analytical solution of a single borehole (after scaling the power by the
number of boreholes, in this case 100). However, if I apply a step heat
input to the borefield, the temperature rise versus the natural logarithm of
time yields not a straight line, but looks quadratic.
 
Is there a physical reason for this? 
 
2. Simulation of long time periods --> determination of borefield time
constant
Second, I would like to determine the largest time constant of a borefield,
by applying a step input during 20 years. The simulation however stops
somewhere in the middle. The program returns:
            
              TRNSYS message 103: The TRNSYS TYPE checking routine has found
an inconsistency in the specified input file and the information expected by
the Type
Reported information: The component model has reported an unspecified error.
Please check the input file for possible sources of error. 
an unspecified error from Type557b. 
This error seems to depend on the simulation length only. There is no error
if I apply the step input for only a couple of years. I don't expect that
the error has a physical reason neither, because the temperature at which it
'crashes' depends on the heat power applied. The lower the heat power level,
the longer the simulation continuous, but it eventually chrashes too. So It
is not possible to determine the time after which the borefield is in
'steady state'. 
 
 
I would be very grateful if someone could give an answer to this
physical/numerical (?) problems!
 
Thank you very much in advance, 
Clara Verhelst
 
University of Leuven, Belgium
 
 


Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm for more
information.