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What Tdb means in this performance curve for heat pumps?

asked 2014-10-15 04:16:28 -0500

AleMac's avatar

updated 2014-10-16 09:24:39 -0500

I am trying to model an air-to-water heat pump with Modelica. The heat pump should be able to heat water to a temperature between 20-23 °C during the whole year. A Boiler will assist the heat pump when outdoor temperature becomes too low in winter season.

Since I could not find any air-to-water heat pump model in the Modelica library that I am using, I decided to develop a simple model by myself and I am looking for performance curves. I have found this link that includes standard performance curves for heat pumps:

If you look at the section "Electric Heat Pump Heating Efficiency Adjustment Curve" you will see that the parameter EIR_FT is function of two variables: Toadb and Tdb. Toadb is the outdoor air dr-bulb temperature, but unfortunately I cannot understand what Tdb actually refers to. In the description it says Tdb: The entering coil dry-bulb temperature.

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@AleMac - would you like to rephrase your question including specifically what you are trying to achieve (describe how the system you want to model works) and which software you want to use?

Julien Marrec's avatar Julien Marrec  ( 2014-10-15 04:51:46 -0500 )edit

I have just added some more info to my question

AleMac's avatar AleMac  ( 2014-10-15 05:55:36 -0500 )edit

@AleMac. I believe we are using the convention of '-' (hyphen) rather than '_' (underscore) for spaces. Please use that in tags, so that things get routed properly. You may also want to add a 'modelica' tag so that folks subscribed to that tag get the question.

__AmirRoth__'s avatar __AmirRoth__  ( 2014-10-16 09:07:11 -0500 )edit
AleMac's avatar AleMac  ( 2014-10-16 09:47:59 -0500 )edit

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answered 2014-10-15 04:49:55 -0500

updated 2014-10-16 07:21:18 -0500

What you are looking at right now is a curve for an Air to Air Heat Pump.

EIR_FT curve

  • Todb is the outdoor air temperature, which is the temperature of the air entering the outdoor coil (condenser in cooling mode, evaporator in heating mode).
  • Tdb is the dry-bulb temperature of the air reaching the indoor coil (evaporator in cooling mode, condenser in heating mode).

For an Air-To-Water heat pump, maybe you could look at using a biquadratic curve such as this one:

image description


  • Ta = Evaporator inlet air temperature (either dry-bulb or wet bulb)
  • Tw = Condenser inlet water temperature

Are you familiar with EnergyPlus? In the Example files, there is one called "HeatPumpWaterHeater.idf" that you should take a look at in particular (and a few others that have air cooled chillers)

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Thanks for your answer Julien, that is exactly why I couldn't understand, I thought it was an air-to-water heat pump! Do you know where I could find performance curves for a air-to-water heat pump?

AleMac's avatar AleMac  ( 2014-10-15 05:59:49 -0500 )edit

It is worth noting that "db" stands for "dry-bulb". In this case, both temperatures are dry-bulb temperatures, but on many pieces of cooling equipment, the indoor coil independent variable will be the wet-bulb temperature.

Neal Kruis's avatar Neal Kruis  ( 2014-10-15 09:02:56 -0500 )edit

Julien, I am kind of familiar with EnergyPlus, but I could't find any explicit air-to-water heat pump model in the engineering reference document. I will have a look at the example file you mention. Thanks!

AleMac's avatar AleMac  ( 2014-10-16 07:33:34 -0500 )edit

@AleMac: there is a Heat Pump Water Heater (HPWH) object in EnergyPlus. It is intended for DHW production, but the same principles apply. Look for either HPWH or Coil:WaterHeating:AirToWaterHeatPump in the I/O or engineering reference manuals (see page 774/1444 of the Engineering reference manual, v8.2) As for cooling, E+ can model an electric chiller with either a water-cooled condenser or an air cooled condenser (your case), so you'll want to look at that too (see section Electric Chiller Model based on condenser entering temperature, page 630/1444)

Julien Marrec's avatar Julien Marrec  ( 2014-10-16 08:00:30 -0500 )edit

@Julien Marrec Those references seem very useful! Maybe I could try to use the set of equations described at page 630/1444 for the chiller and invert the procedure in order to get the condenser leaving temperature as set point (instead of original procedure where the set point is at the evaporator).

AleMac's avatar AleMac  ( 2014-10-16 08:47:14 -0500 )edit

answered 2014-10-15 11:04:59 -0500

@AleMac If you are about to start your own heat pump development in Modelica, you may consider using the development version at as a starting point. See the package Buildings.Fluid.HeatExchangers.HeatPumps.WaterToWater. They are not yet part of the next release candidate of the library as they require more testing and code review, but they may serve you as a starting point.

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Indeed I am using the LBNL building library to develop my model, so I will definitely have a look at the package you suggest.

AleMac's avatar AleMac  ( 2014-10-16 03:34:49 -0500 )edit

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Asked: 2014-10-15 04:16:28 -0500

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Last updated: Oct 16 '14