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Hi,
I want to model a system which activates heat pumps (Type 793) sequentially in case the temperature of different zones drops below a certain amount and the need for heat exceeds the amount that a single heat pump can provide. How can I do it?
Thanks
Shams
@shamsoddinghiami Do you want to control the sequential activation off of a zone temperature, or off of the total desired load?
If you want multiple on/off signals from a monitored temperature (signal device 1 ON when the temperature drops below 20 C, device 2 ON when the temperature drops below 19 C, etc.), you can use any of the multi-stage differential controllers in the TESS Controllers library. Types 1502/1503 allow up to 100 sequential stages with different setpoints for each stage. The user can define whether prior stages should stay on or turn off when a new stage comes on. Types 970/971 are similar to Types 1502/1503, but allow the user to specify individual deadbands for each stage, as well as minimum run-time or time delays before starting/stopping each new stage. If you're monitoring multiple zone temperatures, you may need an equation block to calculate the minimum/maximum temperature of all zones first, then send that value to the controller.
If you want to control a central plant of devices based on the total desired load, you can use the staged device controller (Type 910) in the TESS Controllers library. The staged device controller is given a total desired value (total load to supply) as an input at each time step. It will run device 1 until its part-load ratio exceeds a user-specified threshold, then add a second device and share the load equally between the two until both device's maximum part-load ratio is exceeded and a third is brought online, and so forth.
ShamsoddinGhiami Topic starter
19/09/2024 11:14 pm
Thanks @a_weiss for your response.
I assume that the second approach is the one I need.
I checked the example in TESS model directory for Type 910 and found the example on pump and chiller activation. Their activation is based on the predefined loads and mass flow rates.
My problem, however, is different. I want to have multiple heat pumps. the outputs of the water-to-air heat pump (type793) to the diverter (type646) are "Inlet Air Temperature", "Inlet Air Humidity Ratio", "Inlet Air % Relative Humidity", "Inlet Air Flowrate", "Inlet Air Pressure". Should I connect the "Total heat transfer to air" output from the heat pump to the staged device controller and the air temperature and flow rate to the diverter for the model?
This is the snapshot of the model. I would appreciate it if you can elaborate.
Just one more comment to elucidate the problem. The maximum heating demand at peak period for all zones is 150000 KJ/hr and the heating capacity of heat pump is 37980 KJ/hr on the load side (air side). Therefore at peak period, 4 heat pumps are required to provide heat to the building.
@shamsoddinghiami to me it looks from your screen capture as though you might want the first approach that @a_weiss described. I can't entirely tell; I am basing my guess on there being a number of thermostats shown in the screen cap.
There are two ways of modeling a building. One way (called energy rate control) is to define heating and cooling in TRNBuild/Type56 so that the temperature in the building stays always between a heating and cooling setpoint and Type56 reports the load. In this case you can sense the magnitude of the load and turn on heat pumps accordingly. However, as you have correctly noted, the heat pump model that you are using takes inlet air conditions and sets outlet air conditions that are then normally connected back to the building. That type of model is not really appropriate when you already know the building load. If you know the building load then you need a heat pump model that takes load as an input. There aren't many such models in TRNSYS because usually we rely on the other way of modeling buildings called "temperature level control." In temperature level control you connect the building temperature to a thermostat model that then turns equipment on and off as needed.
If your building model is calculating loads you can either turn off the heating and cooling that you have defined in TRNBuild, decrease the time step to 1 minute, connect the building air temperature and RH to the heat pump models, the thermostats to the heat pump, and the heat pump outlets back to the building (via a VENTILATION type defined in TRNBuild). Alternatively, you can see if you can find (or write) a load-based heat pump model and leave the building set up to compute loads.
As a side note, there are some tutorials in the TRNSYS documentation that talk in more detail about energy rate and temperature level control.
kind regards,
david
ShamsoddinGhiami Topic starter
20/09/2024 5:32 pm
@davidbradley Thanks for your response and elucidating the temperature and energy level control.
My approach is temperature level control as I use the ventilation type for the model, defining Temperature, Relative humidity and mass flow rate to each zone of the building. However to have an initial guestimate of the energy demand of the building and having a rough estimation of the size of the heat pump, I use energy level control through activating the heating and cooling in TRNBuild. It gave me the total heating demand of 150000KJ/hr for the building at peak time.
Then I turned the heating and cooling tab off and activated the ventilation section. However, one single heat pump with 37980 KJ/hr of heating capacity is not capable of providing the whole building with the energy to keep the temperature within the comfort range. I therefore want to use 4 heat pumps that are activated sequentially as the heating demand exceeds the heating capacity of the first one. As @a_weiss just mentioned, it would be possible to use type 910 for the problem, however, it is very confusing to use it.
I would appreciate it if you can give me a hint on how to implement this type into the model.
With regards
Shams
A_Weiss 20/09/2024 5:54 pm
@shamsoddinghiami How would a real system solve this problem? To the extent I'm familiar, most real systems run off of temperature control. They will signal a unit ON when the temperature drops below a certain threshold; if that unit was insufficient to meet the load, the temperature will continue to drop. So, if/when the temperature drops below an even lower threshold, they will signal to turn on a second unit. (Some systems may use PID or other semi-predictive controls to switch units on sooner, based on the rate of change of the temperature signal, but they'll still ultimately be using a temperature signal to decide when and how many units to activate).
I think one of the multi-stage differential temperature controllers is what you want to use. Even if you tried to output the building load at each time step and use Type 910 to control the sequencing off of the building load, the actual capacity of each heat pump could vary substantially from its rated value based on the incoming temperatures and flow rates to the heat pump(s) at that time step, so trying to predict how much load each heat pump can handle at each time step could get messy.
I don't think there are any in-application examples of the multi-stage temperature controllers in the TESS Controllers library, but you can look at the example ControlsExample2_v2a to see a demonstrative example of how Types 970, 971, 973, and 974 can be set up to activate multiple signals off of a provided temperature.
