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NREL / EnergyPlus

EnergyPlus™ is a whole building energy simulation program that engineers, architects, and researchers use to model both energy consumption and water use in buildings.
https://energyplus.net
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PsyTwbFnTdbWPb: humidity ratio calculation does not include case for TWetBulb < 0 #7187

Closed dmey closed 4 years ago

dmey commented 5 years ago

This may not be an issue per se but I just wanted to report it given that I have noticed differences between the results of wet-bulb temperature from dry-bulb temperature, humidity ratio, and pressure calculated by EnergyPlus and PsychroLib when running tests for a separate model I am working with. The following snippet:

https://github.com/NREL/EnergyPlus/blob/d830726d9ea59782c5f19a6ceab71219106e7713/src/EnergyPlus/Psychrometrics.cc#L661-L663

shows that the humidity ratio is calculated for cases where the temperature is below and above zero in the same way. The ASHRAE Handbook however also gives a formulae with different constants for cases where the wet-bulb temperature is below zero as also done in PsychroLib -- see this. This difference seem to amount to a max wet-bulb temperature difference of approximately 0.7 degree C close to zero, exponentially decreasing to approximately 0.1 at about -30 degree C dry bulb temperature. If you think that this may be worth addressing, I am happy to send you a PR for the additional case when the temperature is below zero.

rraustad commented 5 years ago

This does seem problematic especially for heat pumps operating in heating mode where the outdoor wet-bulb temperature is used to evaluate performance curves. Low temperature heat pumps can operate well below 0C (dry-bulb) where wet-bulb temperature would be negative.

From the NOAA handbook ftp://ftp.ncdc.noaa.gov/pub/data/EngineeringWeatherData_CDROM/engwx/handbook.pdf

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