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Dear All,
I am modelling a heating system for which I have the thermal loads (kW). I have read many posts here and have decided to use type 941 for the HP and 682 to impose the load on the heat pump. However, I also need a storage tank. As there is no example, could you comment on my model?
1 - When I read the older questions, it seems that I do not need to set a signal control to the HP, as I already have the load. However, I can not understand because I have a storage in the system, which means heat will be provided even when the HP is off. Then, if the storage temperature falls below a certain value, the HP is turned on to raise the temperature again.
2 - What is the difference in using a pump between the storage outlet and the HP inlet? Type 941 example contains a pump between the HP and storage tank, but type 917 does not. I know that this is done to have control over the flow and the storage temperature, but I don't quite understand why some examples have and others don't.
3 - One thing I notice is that if I create a liquid flow loop between type941 and type682, I mean the flow from type682 entering type941, leaving it, and entering type682 again, it shows a duplicate error for type 682. The same happens when connecting type 682 with a storage tank. So, in the file I am sharing, you will see that the liquid leaves type 682, enters type 158, but does not return to type 682.
I am sharing a .tpf file, and if you can comment with suggestions and recommendations, that would help a lot. I appreciate your time. Thanks in advance.
Regards
Humberto
HumbertoSantos Topic starter
22/05/2025 11:47 am
Here is what I would suggest, based on what I can infer from your picture, description, and questions:
1) I am not sure why you concluded that you do not need to set a control signal to the HP if you are imposing a load with Type 682 – seems like you might have read something misleading or that needed further clarification. You are correct, you still want to connect an on/off control signal from a thermostat Type (such as Type 106) to turn the heat pump ON when the temperature falls below a certain value and OFF when the temperature is above the desired value.
2) Good observation! I agree, the difference between the two examples is not obvious. You have to read the documentation of Types 917 and 941 to understand why they differ in this respect. Both Types 917 and 941 will set the air flow rate when they are ON, but how they handle the liquid flow rate is a little different. Type 917 sets the flow rate on the liquid side of the heat pump, so whenever the heat pump is ON, the liquid side automatically runs at whatever flow rate is set by Parameter 26. Type 941 does not have a parameter that sets the liquid flow rate; when it runs, it uses whatever incoming flow rate is read from Input 2. Type 941 will not run if the flow rate is 0, no matter what other control signals are ON, so that is why there is a pump in the loop that receives the same conditioning signal from the thermostat.
3) I’m not sure exactly what you mean by “duplicate error”, but if you tried connecting a loop from Type 682 to Type 941 and back to Type 682, I am not surprised that that would generate a convergence error. That connection would result in an uncontrolled flow loop with no thermal capacitance (thermal storage), which is not usually possible for TRNSYS to solve. There are two ways you could resolve the flow rate problems in your model. One way is to connect Type 682 in between the outlet of the storage tank and the inlet of the heat pump. I would do this if the system you are modeling contains the heat pump, storage tank, and loads in series on one loop. You’ll want to add a thermostat/control signal and liquid pump to that loop as well, as discussed in answers 1) and 2) above. The other solution would be to connect a flow rate leaving Type 158 and entering Type 682, so you have one loop between the storage tank and the heat pump and a second loop between the storage tank and the loads. I would do this if the flows to your heat pump and your loads are separate loops that draw from and feed into the storage tank in parallel, potentially at different times or different flow rates. In this case, I would still add the thermostat/control signal and liquid pump to the HP loop and also add a liquid pump to the load loop, but whether the flow in the load loop is continuous (liquid pump signal is always 1) or varies with the load is up to you.
You are asking good questions and thinking through your model well. Good luck!
Dear @a_weiss, thank you so much for the answer.
1 - I read that in a previous posting here, but maybe I interpreted the someone else's comment in the wrong way.
2 - Now it seems clearer to me, anyways I want to read the material as you have recommended.
3 - Unfortunately, I do not have an image of the error message, but you pointed it right, it was a convergence error. After studying more and trying different option over the weekend, I improved the model, and coincidentally I implemented it as you mentioned in the second solution you propose for this third answer. Including also the control type to keep the temperature in the tank around 65 degrees Celsius to be more realistics, and the results seem to be coherent, but I will continue trying to improve it.
Thank you so much for the support as always.
Regards
Humberto
