[TRNSYS-users] Problem with high temperature peaks in model

Fri Dec 9 09:24:33 PST 2022

Jan,

   I cannot count how many times I have been in similar situations!

   A couple of thoughts about how I would debug the issue. Probably you 
have already tried most of these things but hopefully there will be some 
little trick in there.

- I would first put in a Type65 that shows the temperature and flow rate 
leaving every component in the system.

- force the system to be only in mode 1 or in mode 2 and see if the 
problem is only happening in one mode or another. If the problem goes 
away when you force the mode then the issue is likely in the mode decision.

- make sure that you have some thermostat models in the system to 
determine control signals. Thermostat models have hysteresis built into 
them where equations do not and the problem might be hiding a 
convergence issue.

- when your system is running in direct heating mode you don't have any 
thermal capacitance. The heat pump could be supplying hot to the load, 
which isn't removing enough energy from the stream, which causes the 
return water temperature to go up at the heat pump inlet, which then 
adds more heat to the stream and it runs away on you. Part of the 
solution might be to modify your heat pump performance data file so that 
the heat pump's capacity drops off if the inlet temperature gets too 
high. As a side note, this is a problem in real life too! We had a heat 
pump directly feeding some radiators and when the room was near its 
setpoint, the radiators weren't taking much heat out of the loop, the 
return water temperature went up and tripped a safety on the heat pump!

- try breaking the loop somewhere and watching inlet and outlet 
temperatures and flow rates. There is a very good chance that the 
problem will go away when you do this but it might give you a hint about 
the source of the problem.

kind regards,

  David

On 12/9/2022 7:09 AM, Böttner, Jan via TRNSYS-users wrote:
>
> Hi everyone,
>
> I have built a model with a heat pump that can operate in two 
> different modes:
>
> Mode 1: The heat pump directly supplies the heating cycle
>
> Mode 2: The heat pump supplies a storage tank and the heating cycle is 
> supplied through the tank.
>
> The supply temperature for my heating cycle, as well as the return 
> temperature and the mass flow are calculated in advance and supplied 
> through the Input file.
>
> The system looks like this:
>
> _So what does the heat pump do?_
>
> Mode 1: Incoming and outgoing mass flow are the same, determined by 
> the input file. The incoming fluid temperature is the same as the 
> outgoing temperature of the Heating cycle. This temperature Is raised 
> by 5K and the outgoing temperature of the heat pump is the same as the 
> incoming temperature of the Heating cycle. These temperatures are also 
> determined by the input file, calculated in advance.
>
> The diverter and mixer control signals are set by the heat pump. So 
> this means 0 for the diverter and 1 for the mixer on the heat pump 
> site, 0 for the mixer and 1 for the diverter on the heating cycle 
> site. The bypass flowrate on the heating cycle site should be 0, as 
> the supply temperature is already correct. So the control signal 
> “Fraction to heatsource” from the valve to the diverter should be 1.
>
> Mode 2: The storage tank should be supplied until it reaches a 
> setpoint (55°C). Therefore the maximal mass flow (2000 kg/hr) runs 
> through the heat pump and is heated with the maximum Power. Meanwhile 
> the heating cycle needs to be supplied with water from the storage 
> tank, Supply temperature, mass flow and return temperature are 
> determined by the input file. Delta T is again 5 K.
> The water from the storage tank has a higher temperature than the 
> target for the heating cycle and needs to be cooled with the bypass 
> flow. The control signals are set opposite as in mode 1 and the 
> control signal for the bypass diverter is set by the tempering valve 
> (Fraction to heat source = 0…1).
>
> _What is my problem?_
> It doesn’t work. I have huge temperature spikes (up to 20.000°C) in my 
> streams and I don’t know where they come from.
> In mode 1 this results in too high temperatures coming in to the heat 
> pump, 5K are added and the temperature arriving on the heating cycle 
> site are too high, so that the bypass starts mixing in “cold water” 
> (which is already higher than the target temperature). But in mode 1, 
> the bypass should never be letting through fluid and it should be one 
> large outer circuit.
>
> I have tried to calculate the control signal for the bypass diverter 
> within an equation as follows:
>
> ctrl_signl_bypass = 
> GE(fraction_to_heatsource,heating_cylce_Diverter_ctrl_signl)* 
> fraction_to_heatsource + GT(heating_cylce_Diverter_ctrl_signl, 
> fraction_to_heatsource)
>
> This might be incorrect as well. But I have tried a lot of different 
> combinations and the result is always wrong (only in different ways 😃)
>
> I would be so grateful if anyone could help me, as I’m stuck here 
> since days, tried a lot of stuff and nothing seems to help.
> I have the feeling, the system can’t cope with the two different 
> circuits and the inconsistent mass flows (which varies, as the 
> delivered energy isn’t constant but delta T is always 5K).
>
> If you have any questions feel free to ask, the whole system is even 
> more complex and I tried to break it down as good as I could.
>
> Best regards,
> Jan
>
>
> _______________________________________________
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> TRNSYS-users at lists.onebuilding.org
> http://lists.onebuilding.org/listinfo.cgi/trnsys-users-onebuilding.org

-- 
***************************
David BRADLEY
Principal
Thermal Energy System Specialists, LLC
3 North Pinckney Street - suite 202
Madison, WI  53703 USA

P:+1.608.274.2577
d.bradley at tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com
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