Polyvalent four-pipe heat pump

Owned by Bram Van Bogaert
Last updated: Feb 28, 2024 by Ruben Gielen
7 min read
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Polyvalent four-pipe heat pump

System topology

The polyvalent four-pipe heat pump is a heat-pump based production unit that can deliver simultaneous heating and cooling, independent heating and independent cooling all in one system. Conceptually, the system has 4 pipe connections (two for heating, two for cooling) , therefore also referred to as a four-pipe heat pump or four-pipe chiller. Other terms used in the industry are polyvalent units, multi-purpose units, multi-purpose heat pumps or multi-purpose chillers. For consistency regarding terminology, the Hysopt BC has been built on the recommendations of Eurovent which can be consulted here: Eurovent 18/1: Seasonal efficiency index STER for polyvalent units.

Conceptually, the polyvalent four-pipe heat pump has 6 operation modes, depending on the heat flow between three types of heat exchangers.

Three type of heat exchangers:

  1. Heating coil: The heat exchanger that is connected to the heating circuit. When used in operation, this heat exchanger will act as condenser for the heat pump system.

  2. Cooling coil: The heat exchanger that is connected to the cooling circuit. When used in operation, this heat exchanger will act as evaporator for the heat pump system.

  3. Source heat exchanger: The heat exchanger that is directly exchanging heat with the heat source, generally being air or water. The source heat exchanger can act as evaporator in heat pump mode or as condenser in chiller mode.

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Three types of heat exchangers present in polyvalent four-pipe heat pumps

 

Six operation modes:

  1. Off: No heat flow

  2. Heat pump mode (HP mode): Heat flow from source (evaporator) to heating coil (condenser)

  3. Chiller mode: Cooling flow from source (condenser) to cooling coil (evaporator)

  4. Total heat recovery mode (THR mode): Heat flow from cooling coil (evaporator) to heating coil (condenser)

  5. Total heat recovery + HP mode: Cooling demand supplied by THR-mode (partially providingheating demand), remaining heating demand with heat pump mode

  6. Total heat recovery mode + chiller mode: Heating demand supplied by THR-mode (partially providing cooling demand), remaining cooling demand with chiller mode

Hysopt Base Circuit

Hysopt icon

The Hysopt icon is a conceptual representation of the heat flows between the three heat exchangers. The representation differs significantly from practical manufacturer designs, typically comprising of at least two refrigerant circuits.

The following connections can be instantly recognised:

  • Connection cooling circuit: Connect 2-pipe cooling circuit

  • Connection heating circuit: Connect 2-pipe heating circuit

  • Activation & modulation cooling circuit: Connect control input, coming from the cooling circuit (e.g. Supply temperature control cooling circuit)

  • Activation & modulation heating circuit: Connect control input, coming from the heating circuit (e.g. Supply temperature control heating circuit)

  • Source temperature: Data input representing the source temperature. For (common) air-source polyvalent heat pumps, the source temperature equals outdoor air temperature.

Parameter overview

General BC parameters

The polyvalent four-pipe heat pump contains two general parameters:

Environment temperature: Optional parameter. The temperature of the room/environment in which the heating and cooling coil is located to include heat losses to the environment in simulation. If no value is entered, the outside temperature is taken by default.

Design configuration: This is an important parameter, as it defines the method the Hysopt software will use to size the system. There are three options:

  1. Electric demand: In this mode, Hysopt will determine the size of the polyvalent four-pipe heat pump based on the minimal electric power that is needed to satisfy both heating and cooling conditions. Based on the design heating and cooling flows, the required compressor power for each situation is calculated using COP, EER and TER tables. The situation that requires the highest compressor power is used to size the complete system.

  2. Heating demand: In this mode, Hysopt will determine the size of the polyvalent four-pipe heat pump to meet the thermal installed capacity requirement for the heating condition.

  3. Cooling demand: In this mode, Hysopt will determine the size of the polyvalent four-pipe heat pump to meet the thermal installed capacity requirement for the cooling condition.

The electric demand mode cannot be used when locking the parameter ‘Design power’ in either Heat pump mode, Chiller mode or Total heat recovery mode.

Heating coil parameters

The heating coil is the connection between the polyvalent four-pipe heat pump's refrigerant circuit and the heating circuit. The following inputs are required

  • KV value: A parameter for the pressure drop at the heating coil (the condenser in heating modes). The KV value expresses the amount of flow that is needed for a pressure drop of 1 bar. This must be manually filled in by the user, as its value will affect the pressure drop propagated upstream during the optimise system components step. By default, this value is set at 100.

  • UA value: A parameter for the heat losses at the heating coil (the condenser in heating mode) to the environment. These heat losses depend on the temperature difference of the heat pump’s heating coil temperature and the environment temperature. By default, this parameter is set to 0 W/K.

  • Capacitance heating coil: A parameter for the thermal inertia of the heating coil (the condenser in heating mode). The capacitance represents the amount of energy that is needed to increase the coil’s temperature by one Kelvin. By default, this parameter is set to 10 000 J/K.

Cooling coil parameters

The cooling coil is the connection between the polyvalent four-pipe heat pump's refrigerant circuit and the cooling circuit. The following inputs are required

  • KV value: A parameter for the pressure drop at the cooling coil (the evaporator in cooling modes). The KV value expresses the amount of flow that is needed for a pressure drop of 1 bar. This must be manually filled in by the user, as its value will affect the pressure drop propagated upstream during the optimise system components step. By default, this value is set at 100.

  • UA value: A parameter for the heat losses at the cooling coil (the evaporator in cooling modes) to the environment. These heat losses depend on the temperature difference of the cooling coil temperature and the environment temperature. By default, this parameter is set to 0 W/K.

  • Capacitance cooling coil: A parameter for the thermal inertia of the cooling coil (the evaporator in cooling modes). The capacitance represents the amount of energy that is needed to increase the coil’s temperature by one Kelvin. By default, this parameter is set to 10 000 J/K.

Heat pump/Chiller/Total heat recovery mode parameters

Depending on the operation mode, Hysopt will use the information provided in the corresponding input fields. The easiest way to interpret the three blocks is to look at the the polyvalent four-pipe unit as three separated machines that co-operate as one system:

  1. A heat pump between source and heating coil (Heat pump mode)

  2. A Chiller between source and cooling coil (Chiller mode)

  3. A Water-water heat pump between heating and cooling coil (Total heat recovery mode)

To keep the component as generic as possible, those three machines can be specified separately. In practice however, those three machines are typically interconnected by having consistent COP, EER and TER-tables over the different operation modes, governing the following equations:

EER = COP -1

TER = 2 ⋅ COP - 1

The COP-table of the heat pump mode will thus often determine the EER-table and TER-table of the other modes. The user can ensure consistency over the machine by providing COP, EER and TER-tables that govern the above formulas or by using Hysopt’s default tables. When the COP, EER and TER-tables are not consistent with each other, the user is actually modelling three separated machines that will co-operate in simulation.

The parameter specifications are elaborated for the Heat pump mode:

COP and power table: The table that the Hysopt software uses to determine the heat flow based on the reference heating power , the reference temperatures and the modulation signal.

Reference source exchanger temperature: The reference temperature at the source exchanger (evaporator) for which the polyvalent four-pipe heat pump’s reference heating power is defined in heat pump mode.

Reference heating coil temperature: The reference temperature at the heating coil (condenser) for which the polyvalent four-pipe heat pump’s reference heating power is defined in heat pump mode.

Reference heating power: The thermal design power of the heat pump mode at the defined reference temperature conditions.

The parametrisation for the Chiller mode and THR mode follow the same logic, but make use of respectively an EER-table and TER-table. More information on the application of these tables can be found at Heat pump.

Design calculations

The design calculations for the polyvalent four-pipe heat pumps are similar to the ones used for the reversible heat pumps. An in-depth tutorial can be found here: Reversible heat pump | Impact of the design configuration on the calculated heat pump size.

This part of the wiki page is still in progress and will be expanded soon.