CoolingController
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Use
The CoolingController controls a cold generator such as a compression or adsorption cooling machine. It ensures that the specified temperature is reached. This setpoint can be the desired room temperature or a desired storage tank temperature.
The CoolingController covers the following applications:
Basic: The CoolingController provides the generator with a switch-on and switch-off signal as well as the flow rate of the circulation and source pumps, i.e. the generator is either on at full power or completely off. The flow rate of the circulation pumps is controlled using a flow setpoint temperature.
Modulation: The output of the generator is additionally modulated, i.e. the generator is not only switched on and off, but can also only call up part of its maximum output during operation.
Cascade: This assumes that the generator has several generator stages (e.g. in the case of a compression cooling machine compressors) or that there is a cascade of generators. These are used one after the other depending on the power requirement.
HeatSource: If the generator is an absorption or adsorption cooling machine, the CoolingController must also take into account and control the source side of the cooling machine , i.e. the CoolingController not only regulates the volume flow on the cold sink side of the cooling machine, but also the volume flow on the heat source side.
Parameters and Connectors
Basic
The controller requires the setpoint temperature specification (▶TRef) and the actual value (▶TAct) of the temperature to which is to be regulated. If the actual value is above the setpoint by the difference of deltaTCoolUp, the generator is switched on (▶CoolingOn). If the actual value falls below the setpoint by the difference deltaTCoolLow, the generator is switched off. In this case the cooling demand is satisfied. In addition, the flow temperature (▶TFlow) and the return temperature (▶TReturn) are required at the generator in order to switch off the generator (emergency shutdown) if limit values (limit parameters TFlowMin and TReturnMin) are exceeded.
In addition, a specification for the desired flow temperature (▶TFlowRef) is required to determine the volume flow from the generator to the cold sink (volume flow of the circulation pumps - ▶qvRef) . This is regulated in the limits qvMax and qvMin using the difference between the actual flow temperature and the set flow temperature as well as the temperature limits deltaTFlowRefMax and deltaTFlowRefMin. In addition, a Boolean signal is provided via the output ▶CPon to control the pumps.
As compression cooling machines always have to dissipate heat, the source side of these is also controlled . When the cooling machine is switched on by the controller, a switch-on signal for the source pump is also output (▶SPon). In this case, the pump setpoint volume flow ▶qvSourceRef for the source pump corresponds to the constant value of the parameter qvSource (uncontrolled).
Delays (tDelayCM, tDelayCPComp, tDelaySP, tDelaySPComp) are set between switching the circulation and source pumps and the generator on and off in order to prevent excessively low temperatures in the generator and the resulting switch-off. In addition, minimum switch-on and switch-off times (tCMminOn, tCMminOff) of the generator are defined to prevent excessive clocking of the generator.
The ▶Enable input can be used for the targeted switch-off of the generator by a higher-level control system, seasonal releases or similar.
Modulation
The Modulation parameter allows the generator not only to be switched on and off, but also to only call up part of its maximum output during operation. The technologically defined minimum modulation is mapped by the parameter RelModMin. The power is adjusted within the limits (deltaTModUp, deltaTModDown) using the actual and setpoint temperature and provided to the generator via the available connector (▶RelativeModulation) for connection. It is recommended that the temperature limits for switching on and off (deltaTFlowRefMax and deltaTFlowRefMin) are greater than or equal to the limits of the modulation. Otherwise the generator will be switched on or off before the modulation limits are reached.
Cascade
The nStages parameter can be used to control several (compressor) stages of the cold generator or a cascade of generators. The ▶CompNumbers connector is used for this purpose. Here too, the limits for the next stage (deltaTPrevStage, deltaTNextStage) should be smaller than the limits for switching the generator on and off in order to enable meaningful control. If modulation is also active, the limits for the next stage should be greater than the limits for modulation so that a stage or a generator from the cascade is fully utilised first. The following hierarchy is therefore recommended:
Reference temperature (deltaTActRef) > Stages (deltaTStage) > Modulation (deltaTMod)
The tCompStateChange parameter can be used to specify the time that must elapse before the next change of the stage takes place. This should be adapted to the inertia of the actual temperature.
HeatSource
When the CtrlHeatSource parameter is activated, the ▶TActHeat and ▶TRefHeat inputs are enabled. These show the current temperature of the heat source and the temperature level required to operate the chiller. If the temperature of the heat source is within the deltaTHeatLow and deltaTHeatUp limits around the setpoint temperature, the cooling machine can be operated. During operation, a constant volume flow ▶qvHeatRef (defined by parameter qvHeat) from the heat source to the cooling machine is assumed.
Model Background
Volume Flow Control
The volume flow of the circulation pump is defined by a linear approach as shown in the following figure. The decisive factor is the temperature difference between ▶TFlow and ▶TFlowRef. At the value deltaTFlowRefMax (in the image 3 K) the maximum volume flow qvMax is assumed to reduce the temperature stroke inside the generator. At the temperature difference deltaTFlowRefMin (in the image -10 K) the minimum volume flow qvMin is used to higher the temperature stroke. Linear interpolation takes place between the extreme values in order to achieve the desired set flow temperature ▶TFlowRef.
Calculation of Modulation
The modulation of the generator is determined using the following formula. In the case of a multi-stage generator, all stages have the same modulation.
Switch on/off
The switching on and off of the generator, the stages and the auxiliary heating is controlled via a hysteresis with the specified upper and lower limits of the temperature difference between setpoint and actual temperature.
Pre- and Post-run Time of Pumps
The logic of the time delay can be seen in the following figure. The cooling machine (▶CoolingOn) is only switched on after the circulation pump is switched on and the time tDelayCM (3 seconds in the figure) has elapsed. The logic for tDelaySP is analogue. This results in the ▶UNITon signal being switched on at 4 s after the circulation pump is switched on at time step 1 s. If the parameters are set differently, both pumps start at the same time, but the compressor is only switched on after the longer of the two delays has elapsed. The circulation pump is then switched off 4 s (tDelayCPComp) after the unit is switched off. The logic for tDelaySPComp is analogue.
