Welcome to the TRNSYS Users Forum.
The forum is a place where people can interact and have discussions about different topics involving the use of the TRNSYS software package. Here you can post topics for discussion or questions on using TRNSYS and get advice from other users or TRNSYS experts. This forum is not intended for detailed technical support. Users should contact their distributor’s hotline for such assistance.
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I am currently simulating a Type 938 water heater heat pump in TRNSYS. However, I’m struggling to understand the significance of its parameters. While I’ve searched for information on similar types, I’ve found limited details in the mathematical references. It seems that Type 938 is a specialized category, lacking the comprehensive explanations found in other types. As a new TRNSYS user, I’m particularly curious about the concept of “Rated Heat Rejection”, "Rated Total Cooling Capacity", and how it impacts the heat pump’s performance.
Type 938 is a performance map, or catalog data-based, water heater heat pump Type. As such, it is much more similar to other TRNSYS/TESS heat pump Types (like Types 919, 917, and similar, in the TESS HVAC Library) than it is to the other TRNSYS/TESS storage tank or conventional water heater Types. I'm not sure where you looked for documentation, but there is a description of the Type and its catalog data approach in the Storage Tank Library Mathematical Reference.pdf, found in the Tess Models/Documentation folder of your main TRNSYS directory. Since the Type is a modification of an air-to-water heat pump model for space cooling, it uses language like "cooling capacity" and "heat rejection" to describe the thermal energy removed from the air source and delivered to the liquid stream.
The basic approach of Type 938 and other catalog data-based Types is that the user specifies some performance values at "rated conditions", then specifies modifiers to the "rated" performance in an external data file for different combinations of inlet conditions. For example, the default "rated" parameters of Type 938 are to reject 7683 kJ/hr of thermal energy to the liquid stream, draw 6954 kJ/hr of total thermal energy (of which 6350 kJ/hr is sensible energy) from the air source, and consume 1224 kJ/hr of compressor power in the process. Open the sample data file Normalized_HPWH.dat, and look for the line that reads 1.0000 all the way across - for this file, that condition occurs on the first line of dependent data in the file. That means the rated values are the heat pump water heater's performance at an inlet water temperature of 10 C, an inlet air temperature of 21.6667 C, and an inlet air relative humidity of 30%. The next line in the data file gives modifiers for an inlet water temperature of 10 C, an inlet air temperature of 21.6667 C, and an inlet air relative humidity of 40%; at these conditions, the heat pump will draw 1.0338x its rated total cooling capacity (and 0.9616x its rated sensible cooling capacity) from the air source, reject 1.0322x its rated heat rejection to the liquid stream, and consume 1.0116x its rated compressor power in the process.
You can (and should!) read more about the format and syntax of TRNSYS data files in Section 7.4.4.6 of the ProgrammersGuide.pdf in the Documentation folder of your main TRNSYS directory. There is also a two-part video series on our YouTube page that explains how to read and create data files for TRNSYS in much more detail:
