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

[Clara Verhelst]

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



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