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Air-to-Water Heat Pumps

In the air-water heat pump catalog as well as in the water-water heat pump catalog catalog there are two ways in which you can define the reference data for the heat pump. The first is to insert the reference points directly into the appropriate fields of the catalog (e.g. “Heating power at A20W35“, “Electrical power at A20W35“). Alternatively the reference data can be read from a file by inserting the name of the reference file in the column „File name“. The file must be saved in the Polysun folder “profiles” (in Windows, the default path is C:UsersPublicPolysunprofiles). The file is saved in “csv” format and is structured as follows (in Windows 7 the default path is ”C:UsersPublicPolysunprofiles“). The file is in “csv” format. File formats vary depending on the type of heat pump and are specified in the respective sub-chapters.

The file data substitute the data in the columns „Heating power at XX“ and „Electrical power at XX“ and will no longer need to be defined. The other columns will have to be filled in, particularly „DeltaT at A7/W35“.

Within the file you may indicate reference points for any evaporator and condenser temperature.

Standard Heat Pumps

The air-to-water heat pump is depicted in Polysun as a component equipped with two hydraulic connections. The model is based on the input values Heating power and Power consumption; such values were measured according to test standards EN 255 or EN 14511 on predefined sampling points (e.g. A2/W35). The electrical power absorbed by the heat-pump is calculated by linear interpolation of the sampling points. The thermal heating power is defined by linear interpolation of the corresponding quality grade of the sampling points. 

Frosting and de-frosting behaviour are likewise taken into account through the norm and may be thus simulated in Polysun. This model only applies to intermittent heat pumps; variable-speed heat pumps are described in the chapter Modulating air-to-water heat pump.

The results “Energy from/to system“ and “End Energy“ include the power of the heat pump and that of the internal heating element, in case the auxiliary heating was switched on. To calculate the COP (Coefficient of Performance), the power of the internal heating element is, however, to be deducted from the results. The result “COP“, therefore, only applies to the compressor and not to the heating pump as a whole. 

The absorbed power of compressor, internal circulation pump, fan as well as the supplied heat output are taken into account in the COP calculation.

Within a hydraulic scheme you will be able to switch from a boiler to a heat-pump by accordingly setting the selection of the heat generator.

Figure: selection of air-to-water heat pump

The heat-pump-specific operating times may be defined by means of the minimum operation times and minimum idle period that may found in the heating controller.

Q is implemented from the measured values; this requires \(T_{v}\). This is approximated through \(T_{U} = \frac{Q^{‘}}{k} \cdot V – T_{i}\) (1) where  \(k = \frac{Q_{ref}}{V_{ref}} \cdot \frac{1}{deltaT_{ref}}\) (2) and Q’ from the previous time-step or on switching on the heat-pump \(Q^{‘} = Q_{ref}\). This allows Polysun to calculate with Q’, V, Ti the outflow temperature.

All calculations are carried out with \(deltaT_{ref}\).

As a result: the calculation enables a correct calculation also for arbitrarily selected v.

Exceptions: for (1) and (2): the internal heat-exchanger is capable of transferring the overall power. The formulism will become inaccurate for V << Vref.

Catalog Entries with Reference Points from a File

The file format for the definition of reference points in a profile file is as follows:

#Heat pump name;;;;

#Evaporator[°C];Condenser[°C];Heating power[W];Electrical power [W]



Modulating Heat Pumps

The modulating air-water heat pump is integrated in Polysun as an extension to the air-water heat pump. Its working principle very closely follows the air-water heat pump model. Save as otherwise defined, the information provided for the air-water heat pump model also applies to the modulating air-water heat pump. Below is an overview of the different types of modulation available in Polysun (the type of modulation is available as a field in the air-water heat pump catalog):

Table: Types of modulation for the air/water heat pumps

NoneNon-modulating heat pump, see chapter Model for the heat exchange with the ground.
Based on outdoor temperatureA heat pump that adjusts its power level based on outdoor temperature so that the thermal power output remains constant over time. This kind of heat pump can be simulated with the model for   basic air-water heat pumps (see chapter Model for the heat exchange with the ground). In the process, reference points are entered whose modulation has already been taken into account, i.e. the thermal power is identical for all reference points, whilst the electrical power consumption varies.
Based on demandThe modulation takes place steplessly based on the desired thermal power output. For more details see the following sub-section.
Two-stageThe modulation takes place in two steps based on the desired thermal power output.

Different modes of operation can be set for on-demand and two-stage controlled heat pumps via the controller. All modes of operation are subordinated to the heat pump’s “Status“ controller value. If the latter is set to “0“, the heat pump will be switched off in any case, regardless of the mode of operation. If the status is set to “1“, the heat pump will at least run, in any case, at the lowest power level. In this way, the heat pump will not switch off if, for example, while running in the ”Defined power level“ operating mode a “0” is forwarded to it through ”Desired power level”. The heat pump must be switched off by through its status.

Should the mode of operation need to be controlled, the “Heat pump: status“ should be set as an output in the programmable controller. For each operating status, a numerical value corresponding to a given mode of operation should be assigned to this output. In the following chart, these numerical values are shown in the “Controller value“ column.

Based on the respective mode of operation, additional information should be provided to the heat pump. Such information is to be entered via an additional output in the controller. Subsequently, for each operating status the controller will need to have been allocated by these additional outputs a value which, however, will only be taken into account if the mode of operation is appropriately set. The following chart lists the possible modes of operation.

Table: Modes of the heat pump operation

DescriptionController valueAdditional controller outputsFunction description
Fixed maximum power0The heat pump runs at the maximum power level
Heat-driven operation mode1„Controlled power heat generator“The heat pump delivers, as far as possible, the desired thermal power set via the “Controlled power heat generator“ controller value. (default mode)
Elctrically-driven operation mode2“Controlled power heat generator“The heat pump draws, as far as possible, the available electrical power set via the “Controlled power heat generator“ controller value.
Defined power level3“Controlled Power level“The heat pump is run at the power level closest to the “Desired Power Level“.

In the event that the additional controller output is not configured in the mode of operation 1,2 or 3, the heat pump will be run at the maximum power level.

In the event that the mode of operation is not set through the controller, the heat pump will be run in the “Heat-driven“ operation mode as a standard.

In the event that the heat pump selected from the catalog does not provide any selectable power controller (field ”Modulating“), the controller values shown in the above chart will not be taken into account. In this case, “Controller mode“, “Controlled power heat generator“ and “Controlled Power level“ may not be selected in the controller configuration menu.

In the typical case of power control being heat-driven, two configuration options are made available to the user. Two options are provided for setting the controller parameter. Similarly to the boiler model, the heat generator controller may be set to a constant inlet temperature for tank loading purposes. In so doing, the “control mode“ is implicitly set to “Heat-driven”. The controller in/outputs are set to the same settings as for a modulating boiler.

More complex control settings are performed through a programmable controller, whereby, with the aid of the HEATING CURVE function, i.e. by means of the heating curve, the current inlet temperature required for the heating loop and, as a result, with the aid of the MATCHFLOW function, also the required thermal power output are calculated. The desired thermal power output is then transferred to “Controlled power heat generator” via a controller output. When dealing with similarly controlled systems, special attention should be paid that heat pump size and the parameters of the HEATINGCURVE function (for a more accurate description see the Programmable controller chapter) are appropriately matched to the building to be heated and its location. In the worst-case scenario, should the heating curve not be appropriately configured, the heat pump will modulate to too low a power level and will be unable to match the energy demand as a result. If, on the contrary, the curve is set so that the heat pump constantly delivers more energy than required, instead of running non-stop at a lower power level and with a higher efficiency, this reverts back to the on-off operation. In this case, the potential of the power controller are not fully exploited.

In the event that the heat pump is also used for DHW preparation purposes, it should be ideally run at reduced power for this purpose. The “Defined power level“ operating mode may be used to this end.

Stepless modulating Air/Water-Heat pump

The model for stepless modulating air/water heat pumps was developed within the framework of a research project, “Implementierung der leistungsgeregelten Luft/Wasser-Wärmepumpen aus dem BFE-Projekt „Effiziente Air/Water-Heat pumps durch kontinuierliche Leistungsregelung in der Simulationssoftware Polysun“ [Implementation of power-controlled air/water heat pumps from the BFE-Project “Efficient air/water heat pumps through continuous power control” in the simulation software, Polysun”].

Like the other heat pump models available in Polysun, the model for stepless modulating air/water heat pumps, too, is based on an interpolation procedure. The family of curves is described by means of several measuring points that are saved in a text file. Reference points must be necessarily saved as a profile file. The format is:

#Heat pump name;;;;

#Power level[0-1];Evaporator[°C];Condenser[°C];Heating power[W];Electrical power[W]







where “Evaporator“ corresponds to the outdoor temperature and ”Condenser“ to the inlet temperature. This input format makes it possible to process data in a clear, intelligible way using a spreadsheet program such as Microsoft Excel.

For results to be fairly accurate, a total of at least 140 reference points should be defined covering the expected operating range. For the simulation of home systems, reference points should be ideally spread out over an outdoor temperature range from -20°C to +20°C (= evaporator temperature) and an inlet temperature range from 30°C to 60°C (= condenser temperature). However, there is also a chance that in a given time step an operating point is needed for the simulation that lies outside the temperature range defined in the file. In this case, the data curve will programmatically flatten out, as is shown in the following chart in the form of dashed lines.

Figure: outdoor-temperature heating-power data for different power levels. The dashed lines show the programmatic extension of data series.

The simulation requires that the electrical power absorbed by the heat pump as well as the thermal power output are calculated for each time step.

In the heat-driven operation mode, i.e. in the standard case, the required thermal power is calculated by a controller. The power is then transferred to the “Controlled power heat generator” of the Heat pump component via a controller output. Polysun provides the outdoor temperature for each time step. The last required input parameter is the inlet temperature at which the heat pump is currently operating. However, as this is only calculated when the system is simulated, the inlet temperature is extrapolated from the data collected in the latest time steps (similarly as for the non-modulating air/water heat pump model).

In a first step, using the known input parameters and the defined reference points, the power level is calculated by means of scattered data interpolation. If this turns out to be lower than the lowest power level specified in the reference points, it will be rounded off to the lowest power level. Then, in a second step, the current thermal power output and the currently absorbed electrical power are interpolated for the calculated power level and passed on to the overall system simulation.

The electrically driven operation mode runs in a way that is very similar to the heat-driven mode. For the determination of the power level, however, the electrical power consumption together with the outdoor and inlet temperature are used as interpolation points instead of the required thermal power.

If the required power level is sent directly by the controller, the interpolation step will no longer be required for the determination of the power level.

Schemes with and without DHW preparation respectively for a new building and an old renovated building are presented as an example in the scheme catalog under “Swiss Systems“. As has been mentioned, the parameters for the HEATINGCURVE function need to be adjusted in the programmable controller. In the template, these parameters are set to the configured building and to the location Rapperswil SG.

Two-Stage Air/Water Heat Pumps

The two-stage heat pump model is implemented as an extension to the standard air/water and water/water heat pump model. In this model, the two-stage heat pump is presented as a device containing two units operating in parallel. For the pump to run at full power, both units need to be activated. If the power is reduced to 50%, only one unit is required to be running.

The heating power delivered at the current operating point is calculated by means of the existing model. The power is then reduced to 50% by decreasing both the heating power and the electrical power by 50%.

In the heat-driven operation mode, a check is made whether 50% of the power is sufficient to meet the required power demand. If this is the case, the heat pump is run at 50% of its capacity.

In the electrically-driven operation mode, the power is reduced to 50% as soon as the available electrical power drops below 90% of the electrical power absorbed by the heat pump when running at 100% capacity.

In the “Fixed power level“ operation mode, the power is reduced to 50% if the required power level is equal or lower than 50%.

In the database or in the profile file, reference points are entered for the full power; the format is the same as for a standard air/water heat pump.

Exhaust Air Heat Pumps

In order to improve the air heat pump efficiency, the source part of the heat pump can be placed in the unheated area of the building where the temperature is higher than the ambient temperature, such as cellar, storage room, garage, etc. In this case, you can open the heat pump dialog window, select the required building and choose the unheated area in the heat source drop-down menu.

Figure: selection of the unheated area in the heat pump dialog window

In the building dialog window, you may also define the temperature of the unheated area.

Figure: definition of the mean temperature of the unheated area in the building dialog window