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Furnace Efficiency DOE-2/EnergyPLus

asked 2015-05-21 18:45:22 -0600

updated 2015-11-23 08:50:30 -0600

In the EnergyPlus documentation, for the description of the part-load fraction performance curve for the furnaces, it says: "This [...] performance curve [...] parameterizes the variation of gas consumption rate by the heating coil as a function of the part load ratio [...]. For any simulation timestep, the nominal gas consumption rate (heating load/burner efficiency) is divided by the part-load fraction (PLF) if a part-load curve has been defined."

DOE-2 calculates the HIR-op as the product of the rated HIR and the HIR-FPLR(PLR).

I compared the default (when using a template) EnergyPlus performance curve for the PLF and the default curve for HIR-FPLR(PLR) in eQUEST. They are pretty pretty similar! Which makes me thinks that we can consider that PLF = HIR-FPLR(PLR).

The HIR for a particular time step is ratio the of fuel used to the heating energy produced.

According to the DOE-2 definition we have: Conso-op = f(Conso-rated * HIR-FPLR(PLR))

According to the EnergyPlus definition we have: Conso-op = f(Conso-rated / PLF)

Which is not the same thing and makes me think that PLF <> HIR-FPLR(PLR) ... Am I missing something?

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answered 2015-05-28 11:59:53 -0600

A little update as it might be useful for other users.

Neal's answer made sense the way he presented it. However, it didn't quite add up with the data I had. To find out what the calculation is in both cases, I generated hourly reports in both eQUEST and EnergyPlus. Here is what I found out:

Calculation of the fuel consumption in DOE-2:

  • Calculation of the PLR: PLR = (Load for the hour) / (Total Furnace Capacity)
  • Use the PLR to get the HIR f(PLR) using the performance curve attached to the system
  • Calculation of the operational HIR: HIR-op = HIR f(PLR) * HIR-rated
  • Fuel consumption: Qfuel = (Total Furnace Capacity) * HIR-op (eq. 1)

Here is an excerpt from an hourly report from eQUEST that illustrate the calculation:

In this example, the total furnace capacity is 39988 Btu/h and the rated HIR is 1.25.

Calculation of the fuel consumption in EnergyPlus:

  • Calculation of the PLR: PLR = (Load for the hour) / (Total Furnace Capacity)
  • Use the PLR to get the PLF f(PLR) using the performance curve attached to the system
  • Fuel consumption: Qfuel = (Total Furnace Capacity) * PLR * HIR-rated / PLF (eq. 2)

Here is an excerpt from an hourly report from EnergyPlus that illustrate the calculation:

In this example, the total furnace capacity is 39988 Btu/h and the rated HIR is 1.25.

Comparison:

Then, if we look at (eq. 1) and (eq. 2):

  • HIR f(PLR) * HIR-rated = PLR * HIR-rated / PLF
  • Which simplifies to HIR f(PLR) = PLR / PLF
  • Thus PLF = PLR / HIR f(PLR)

In conclusion PLF and HIR f(PLR) are not quite the same.

Using this relation I converted the curve used in eQUEST (by default) to the match the PLF used in EnergyPlus.

  • EnergyPlus PLF = f(PLR) is the default curve used in EnergyPlus
  • eQUEST PLF = f(PLR) is the default curve used in eQUEST converted to the EnergyPlus "format".

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Comments

This makes sense to me. I couldn't remember if PLR was implicit in the HIR curve in DOE-2, or accounted for separately.

Neal Kruis gravatar imageNeal Kruis ( 2015-05-28 13:00:26 -0600 )edit
1

Amazing post! I remember scratching my head about this when I first started using e+ / os, I stopped using it for a while and recently started back up and encountered the exact same issue! This post refreshed my memory. Thanks so much!

Jono gravatar imageJono ( 2020-07-22 17:39:02 -0600 )edit
2

answered 2015-05-22 10:23:06 -0600

updated 2015-05-22 10:23:28 -0600

The HIR in DOE-2 is the inverse of the heating efficiency (eff).

In DOE-2 the calculated efficiency is: 1/(HIR*HIR-FPLR(PLR))

In EnergyPlus the calculated efficiency is: eff/PLF

So if eff = 1/HIR, then PLF and HIR-FPLR(PLR) are the same.

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Asked: 2015-05-21 18:45:22 -0600

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Last updated: May 28 '15