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Forwarded question from ARCHIT MAHAWAR:
Here the TRNSYS model and simulation results have been attached. The results needed should be temperature within 55 to 60 deg C. The blue line denotes the AVG TANK TEMP and red line shows TANK OUTLET TEMP. The red line has max temp of 48 deg C and is not falling within the required temp range.
This model is for a solar assisted heat pump for hot water generation. Can anyone suggest whether a thermostat should be used and how to connect it.
Thank You.
Archit,
There is not enough information to say for sure what is happening, but I can offer a couple suggestions.
Just like a real system, both the solar loop and the heat pump loop need a pump to set and control the flowrate. When there's no pump, TRNSYS sets the flowrate in the loop using the initial flowrate (the value on the Inputs tab of the proforma) of whichever loop component it happens to call first. Leaving the flowrate up to the calling order like this is hard to see/understand, hard to control, and may change without warning. In this case, I’m guessing you got lucky and got a nonzero flowrate on the solar loop (which is why the system is doing anything at all), but zero flow on the heat pump loop, thus your heat pump does nothing; you can (and should!) plot flowrates out of the collector, the heat pump, and both tank outlets to confirm this.
I'd start with a basic on/off pump, like Type 114 in the standard Hydronics/Pump library. Be sure to set the flowrate and the pump power for each pump to something that makes sense for your model. You can set the pump power to 0 if you don't want to model its power consumption or any thermal effects from the pump on your system.
Once you have pumps on your loops, you can then add an aquastat to monitor a temperature in the tank and send a control signal to the heat pump and/or the loop pump for the heat pump based on that temperature, just as it would in a real system. You may also want to connect a differential controller to your solar loop pump so it only runs when the collector is hotter than the tank. There are examples of both an aquastat and a differential controller in the SDHW example, located in the Examples/SDHW directory of your main TRNSYS18 directory.
We go over pumps, aquastats, equation blocks, Types with data files (like the heat pump), connecting weather data, handling warning/error messages, and much more in the TRNSYS Academy learning course. You can find more information about it at https://tess-inc.com/trnsys/training.html , or send an email to training@tess-inc.com to initiate payment for and enrollment in the course.
