blockTableData2DLoadDep

Information

This block implements two core features for some chiller and heat pump models within Buildings.Fluid.HeatPumps.ModularReversible and Buildings.Fluid.Chillers.ModularReversible.

• Ideal controls: The heating or cooling load is calculated based on the block inputs. The block returns the required part load ratio PLR to meet the load, within system capacity.¹
• Capacity and power calculation: The capacity and power are interpolated from user-provided data along the load side fluid temperature, the ambient side fluid temperature and the part load ratio yMea provided as input.²

¹ The part load ratio is defined as the ratio of the actual heating (or cooling) heat flow rate to the maximum capacity of the heat pump (or chiller) at the given load-side and ambient-side fluid temperatures. It is dimensionless and bounded by 0 and max(PLRSup), where the upper bound is typically equal to 1 (unless there are some capacity margins at design conditions that need to be accounted for). In this block, the part load ratio is used as a proxy variable for the actual capacity modulation observable. For systems with VFDs, this is the compressor speed. For systems with on/off compressors, this is the capacity of the enabled compressors divided by the total capacity. When meeting the load by cycling on and off a single compressor, this is the time fraction the compressor is enabled. In all cases, the algorithm assumes continuous operation and only approximates the performance on a time average. Finally, note that while the part load ratio is used for generalization purposes, either the part load ratio or the actual capacity modulation observable (e.g., the normalized compressor speed) may be used to map the performance data. The only requirement is that this variable be normalized, as the algorithm assumes it equals 1 at design (selection) conditions.

² The reason why the part load ratio is both calculated (PLR) and exposed as an input (yMea) is to allow for modeling internal safeties that can limit operation. If no safeties are modeled, a direct feedback of PLR to yMea can be used.

Capacity and power calculation

When the machine is enabled (input signal on is true) the capacity and power are calculated by partitioning the PLR values into three domains, as illustrated in Figure 1.

1. Normal capacity modulation
   This domain corresponds to the capacity range where the machine adapts to the load without false loading or cycling on and off the last operating compressor. Depending on the technology, this is achieved for example by modulating the compressor speed, throttling the inlet guide vanes or staging a varying number of compressors. In this domain, both the machine PLR and the compressor PLR vary. The capacity and power are linearly interpolated based on the performance data provided in an external file, which syntax is specified in the following section. The interpolation is carried out along three variables: the load-side fluid temperature, the ambient-side fluid temperature and the part load ratio. Note that no extrapolation is performed. The capacity and power are limited by the minimum or maximum values provided in the performance data file.
2. Compressor false loading
   This domain corresponds to the capacity range where the machine adapts to the load by false loading the compressor. For a chiller, this is achieved by bypassing hot gas directly to the evaporator. In this domain, the machine PLR varies while the compressor PLR stays roughly the same. The input power is considered equal to the interpolated value at TLoa, TAmb, min(PLRSup). This domain may not exist if the parameter PLRCyc_min is equal to min(PLRSup), which is the default setting.
3. Last operating compressor cycling
   This domain corresponds to the capacity range where the last operating compressor cycles on and off. In this domain, the capacity is linearly interpolated between 0 and the value at TLoa, TAmb, min(PLRSup), while the power is linearly interpolated between P_min and the value at TLoa, TAmb, min(PLRSup), where P_min corresponds to the remaining power when the machine is enabled and all compressors are disabled.

Figure 1. Input power as a function of the part load ratio.

Performance data file

The performance data are read from an external ASCII file that must meet the requirements specified in the documentation of Modelica.Blocks.Tables.CombiTable2Ds.

In addition, this file must contain at least two 2D-tables that provide the maximum heating (resp. minimum cooling) heat flow rate and the input power of the heat pump (resp. chiller) at 100 % PLR. Each row of these tables corresponds to a value of the load-side fluid temperature, each column corresponds to a value of the ambient-side fluid temperature. This could be either the leaving temperature if use_T*OutForTab is true, or the entering temperature if use_T*OutForTab is false. The load and ambient temperatures must cover the whole operating domain, knowing that the model only performs interpolation and no extrapolation of the capacity and power along these variables.

The table providing the capacity values must be named q@X.XX where X.XX is the PLR value formatted with exactly 2 decimal places ("%.2f"). Similarly, the table providing the power values must be named p@X.XX.

Here is an example of chiller data ("-----" is not part of the file content):

-----------------------------------------------------
#1
double q@1.00(5,5)                    # Cooling heat flow rate at 100 % PLR
0 292.0 297.4 302.8 308.2             # CW temperatures as column headers
280.4 -493241 -555900 -495611 -312372 # Each row provides the capacity at a given CHW temperature
282.2 -470560 -578165 -562822 -424529
284.1 -418413 -573462 -605561 -514711
285.9 -342290 -542284 -619329 -573426
double p@1.00(5, 5)                   # Input power at 100 % PLR
0 292.0 297.4 302.8 308.2             # CW temperatures as column headers
280.4 60430 80413 80830 55530         # Each row provides the input power at a given CHW temperature
282.2 54399 80278 89151 73950
284.1 45251 76017 92822 87633
285.9 34546 68567 91833 95401
-----------------------------------------------------

In addition, for machines that have capacity modulation other than cycling on and off a single compressor, the whole range of normal capacity modulation must be covered by providing similar 2D-tables at different PLR values. The lowest PLR value will be considered as the minimum PLR value before false loading the compressor. If the machine has no hot gas bypass (PLRCyc_min = min(PLRSup)) this will correspond to the minimum PLR value before cycling the last operating compressor.

All the PLR values used in the performance data file must be specified in the array parameter PLRSup[:].

Compressor cycling

Compressor cycling is not explicitly modeled. Instead, the model assumes continuous operation from 0 to max(PLRSup). The only effect of cycling taken into account is the impact of the remaining power P_min when the machine is enabled and the last operating compressor is cycled off. Studies on chillers and heat pumps show that this is the main driver of efficiency loss due to cycling (Rivière, 2004). When a compressor is staged on, energy losses occur due to the overcoming of the refrigerant pressure equalization and the heat exchanger temperature conditioning. However, a large part of these losses is recovered when staging off the compressor, unless the machine is disconnected from the load when compressors are disabled. This disconnection does not happen when staging multiple compressors, and the research shows no significant performance degradation when a chiller cycles between different stages without completely shutting down. And even when disabling the last operating compressor, most plant controls require continuous operation of the primary pumps when the chillers or heat pumps are enabled. The European Standard for performance rating of chillers and heat pumps at part load conditions (CEN, 2022) states that the performance degradation due to the pressure equalization effect when the unit restarts can be considered as negligible for hydronic systems. The only effect that will impact the coefficient of performance when cycling is the remaining power input when the compressor is switching off. If this remaining power is not measured, the Standard prescribes a default value of 10 % of the effective power input measured during continuous operation at part load.

Heat recovery chillers

Heat recovery chillers can be modeled with this block. In this case, the same chiller performance data file is used for both cooling and heating operation. The model assumes that all dissipated heat from the compressor is recovered by the refrigerant. This assumption enables computing the heating capacity as the sum of the cooling capacity and the input power.

When configured to represent a heat recovery chiller, this block uses an additional input connector coo which must be true when cooling mode is enabled, and false when heating mode is enabled. The load side input variables must externally be connected to the evaporator side variables in cooling mode, and to the condenser side variables in heating mode. The output connector Q_flow is always the cooling heat flow rate, whatever the operating mode. The heating heat flow rate in heating mode can be computed externally as P-Q_flow.

Ideal controls

The block implements ideal controls by solving for the part load ratio required to meet the load (more precisely the minimum between the load and the actual capacity for the current load and ambient temperatures). This is done by interpolating the PLR values along the heat flow rate values for a given load.

The load is calculated based on the load side variables and the temperature setpoint provided as inputs. The setpoint either represents a leaving (supply) temperature setpoint if use_TLoaLvgForCtl is true (default setting) or the entering (return) temperature if use_TLoaLvgForCtl is false.

The required PLR value is returned as an output while the actual heat flow rate and power are calculated using the PLR value yMea provided as input, which allows limiting the required PLR to account for equipment internal safeties.

References

• CEN, 2022. European Standard EN 14825:2022 E. Air conditioners, liquid chilling packages and heat pumps, with electrically driven compressors, for space heating and cooling, commercial and process cooling - Testing and rating at part load conditions and calculation of seasonal performance.
• Rivière, P. (2004). Performances saisonnières des groupes de production d’eau glaçée [Seasonal performance of liquid chillers]. École Nationale Supérieure des Mines de Paris. [In French]. https://pastel.hal.science/pastel-00001483

Parameters

Connectors

Contents

Revisions

• March 21, 2025, by Antoine Gautier:
  First implementation.