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Does EnergyPlus calculate TDV in units of J TDV or kBtu TDV?

asked 2020-07-07 09:55:25 -0500

mldichter gravatar image

In the documentation for the FuelFactors object Source Energy Factor field, there are some instructions on how to calculate TDV. In particular, there's a value of 0.293 mentioned for electricity and 0.01 for natural gas. These are also present in the 5ZoneTDV.idf example file, along with some schedule objects to reference the CSV files where the TDV factors are stored. My question is what these numbers actually mean and how they were derived? These values only seem to be mentioned in relation to EnergyPlus and Energyplus is the original source of these two numbers.

For a little background, I'm trying to reproduce some TDV calculation results and I have failed repeatedly trying all sorts of plausible variations to the calculation. I'm not sure if the person who made the calculation made a mistake, if I am making a mistake, if there is a difference in TDV units, if I'm not working with the correct IDF, etc. I've used a procedure to calculate TDV in the past, but I've never known for sure what each part of the calculation is intuitively doing and where all the constants come from.

The calculation method for TDV has never been super clear and I'd like to be able to say something other than "I don't know how the TDV calculation works, but this is the formula." if I'm going to be using TDV in published research. I'm pretty sure the 0.293 and 0.01 in EnergyPlus are for converting the TDV factors from their units of kBtu/kWh and kBtu/therm in the CSV files to Joules/Joules, which would mean that the annual TDV calculation would be in units of J TDV rather than kBtu TDV, but I'm not sure. I thought I would ask the creators of the 5ZoneTDV.idf example file and clear up one of the many opaque portions of the TDV calculation that I don't fully understand yet.

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Ha ha. I was using 2019 TDV factors, but the other source was using 2022 TDV factors, which got released last month. Phew! The results I've been giving people for years aren't wrong. : )

mldichter gravatar image mldichter  ( 2020-07-07 14:35:08 -0500 )edit

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answered 2020-07-07 14:36:59 -0500

mldichter gravatar image

As usual, this is really simple after you know the answer. The constants are from kBtu/kWh = 0.2931 and kBtu/therm = 0.01, (I kept trying to figure out how [J source/J site] fit in from the FuelFactors object) which makes the TDV factors lose their energy units, then multiplied by output variables [J] from EnergyPlus, so the resulting unit is J TDV. At least, I'm pretty confident the output unit is J TDV, not kBtu TDV.

I'll wait for some confirmation before accepting this answer though.

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Asked: 2020-07-07 09:55:25 -0500

Seen: 162 times

Last updated: Jul 07 '20