CompressionCooling
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Use
The model CompressionCooling is, in principle, a heat pump model in which the source side power is controlled. Heat pumps are devices that transfer thermal energy from a source to a sink that is at a higher temperature than the source. To provide usable thermal energy they use electrical energy which is converted into mechanical work for example by compressors. In this case, the cooling circuit is the heat source and the heat output of the heat pump has to be cooled (e.g. by cooling towers).
The Compression Cooling covers the following applications:
Basic: A switch-on and switch-off signal is provided to the CompressionCooling, so it is either on at full power or completely off. The maximum cooling power and the COP are defined by the input curves and depending on the flow temperatures at the heat and cold side.
Modulation: The cold output of the CompressionCooling is additionally modulated, so it 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 CompressionCooling has several generator stages (compressors) or that there is a cascade of CompressionCoolings. These are used one after the other depending on the input signal ▶CompNumb.
Parameters and Connectors
Basic
The CompressionCooling is connected to the heat sink side with the connectors ▶FlowCooling and ▶ReturnCooling. The cooling circuit is linked by the ▶FlowCold and ▶ReturnCold connector. The on/off signal is provided via ▶CMon and the calculated power demand to ▶LVGrid. The type of grid connector can also be specified depending on which GridType (i.e. 'Low-Voltage', 'Medium-Voltage' and 'High-Voltage') should be used to operate the heat pumps internal compressor. In addition, internal heat losses in the heat pump can be mapped via TAmbient and QlossRate.
The cold power output is simplified calculated using measurement data of heat pump system. Based on this the cold power output and coefficient of performance (COP) of the system are characterized dependent on cooling system flow and heat sink temperature. The associated curves can be adapted using the table dialog of parameter CoolingPowerCurve and COPCurve. To prevent these tables from being extrapolated too far, the minimum and maximum temperatures (TColdMin, TColdMax, TCoolingMin and TCoolingMax) must be set. In order to correctly map the electricity demand of the heat pump, the power factor of the compressor (CosPhi) must also be set accordingly.
It is also important to parameterise the properties of the medium at the cooling circuit (cpMedCold, rhoMedCold) correctly. Inertia can be modelled with the help of the source volume (VCold). The properties (cpMedCooling, rhoMedCooling) and volume (VCooling) of the heat transfer medium on the heat sink side must also be parameterised correctly.
The internal dynamics of the model regarding heat production (tCMStart) can also be adapted. If specific data for this is not available, the pre-defined parameters should be used.
Modulation
To only call up part of the CompressionCooling's maximum cold output during operation, the parameter ConstantPowerOutput can be set to false. Afterwards, the connector ▶Modulation is available. This can be used to signal the percentage of maximum power to be called up. No more than 100 % is possible. The technological limit of the minimum modulation (RelModMin) is also taken into account.
Cascade
Important to note is that when parameterizing cascaded CompressionCooling systems the parameter nStages is used to describe the number of cascaded steps. If this number is greater than 1, the connector ▶CompNumber is released. This can be used to control the active number of stages.
In general, the total cooling power must be specified in the CoolingPowerCurve. As an example, if the data sheet of a cooling machine unit is available and two stages are required, the outputs in the data sheet must be multiplied by 2 and inscribed in the CoolingPowerCurve table, accordingly.
Model Background
Due to heat pump system complexity when compared to other heating systems (e.g. condensing boilers or CHPs) simplification was necessary to reduce simulation time. Heat power output highly depends on inner system states and medium characteristics. To reduce model complexity, inner system processes were neglected completely. The heat power output is instead, calculated using measurement data of heat pump system. This data is commonly available from the manufacturer for heat pump systems. Based on this, the heat power output and coefficient of performance (COP) of the system are characterized dependent on heating system flow and heat source temperature. As a result of this approach, the source pump does not adequately interact with the corresponding heat pump system. The pump however has a non-neglectable power demand and is thus externally modeled as an additional electrical energy consumer.