modelHeatRecoveryHeatPump

Information

Model of an Energy Transfer Station with heat recovery heat pump, buffer tanks and optional domestic hot water preparation and optional water-side economizer.

The figure below shows the schematic diagram. The heat recovery heat pump preferentially operates in heat recovery mode, but if heating (or cooling) demand persists while the cold (or hot) water-side buffer tank is fully charged, the evaporator (or condenser) temperature is reset, the corresponding tank is decoupled to avoid flushing the tank, and heat is exchanged with the district energy system.

The operation is as follows:

Domestic Hot Water Tank and Buffer Tanks

The chilled water and heating hot water tanks are by default sized for five minutes and used as buffer tanks. The DHW tank is by default sized for 24 hours of storage. Each tank generates a signal to request charging. If the temperature of its supply side (top for the hot tank, bottom for the cold tank) deviates from the set point with hysteresis, charging is enabled until the temperature of its return side (bottom for the hot tank, top for the cold tank) achieves the set point with hysteresis.

When the space heating or cooling tank does not request charging, the diversion valve VAL_DIV_CON or VAL_DIV_EVA to the respective tank is closed, and the isolation valve VAL_ISO_CON or VAL_ISO_EVA is opened to allow energy exchange with the district heat exchanger. The diversion valves are necessary because, for example, when the ETS operates cooling only mode, rejecting heat to the ambient loop, the condenser outputs hot water at the minimum leaving temperature is 15°C. Without the diversion valve, the cool water from the condenser would flush out the energy stored in the space heating tank. This causes energy waste. It also causes short cycling because the tank will request charging repeatedly as its temperature falls below the heating set point, bring the system into a limit cycle.

Two-tank Coordination on the Condenser Side

The integration of the DHW tank is optional, as not all buildings prepare DHW using the ETS. When integrated, the space heating and DHW tank share the same condenser loop. The table below explains how the two loops are coordinated through valve control.

Heat Recovery Heat Pump

The heat recovery heat pump can produce heating, cooling, or both simultaneously. The condenser pump PUM_CON and the evaporator pump PUM_EVA are enabled when any of the respective tanks requests charging. The heat pump is turned on 30 seconds after PUM_CON and PUM_EVA are running.

When on, the primary pumps are operated at constant speed, and the condenser (resp. evaporator) mixing valve VAL_CON (resp. VAL_EVA) are modulated with a P controller to track the set point for the water that leaves the heat pump, with a small offset to open first the valve and then ramp up the compressor speed.

The compressor speed is controlled based on the same temperature measurement as the mixing valves. Based on a moving average of the compressor speed signal for heating and cooling, the heat pump control is switched into heating or cooling dominated operation, and the respective compressor speed setpoint is sent to the heat pump.

If only heating (or only cooling) is requested from the tank, then the evaporator (or condenser) set point temperature is reset to minimize the temperature lift across the heat pump.

District Heat Exchanger

The district heat exchanger hydraulically decouples the buildings system and the district system. Its primary and secondary circuits are enabled to operate if either any of the tanks request charging, and if an isolation valve VAL_ISO_CON or VAL_ISO_EVA is open. When enabled, the pumps PUM1_DHX and PUM2_DHX operate at a constant speed.

Temperature Set Points

The set points for the supply temperatures are input to this model. For the heating supply water, use THeaWatSupSet, for the cooling supply water, use TChiWatSupSet.

If a domestic hot water supply is present, as declared through the parameter have_hotWat, then use THotWatSupSet for the set point temperature to the end user (such as shower), and use TColWat for the temperature of the cold water supply and QReqHotWat_flow for the heat flow rate associated with the hot water supply, i.e., QReqHotWat_flow = mHotWat_flow cwat (THotWatSupSet-TColWat), where mHotWat_flow is the hot water mass flow rate, and cwat is the specific heat capacity of water.

Domestic Hot Water Preparation

The DHW preparation is optional. If present, a fresh water station is used. The fresh water station allows to store heat in the heating rather than the domestic hot water, therefore avoiding the potential problem of Legionella bacteria getting from the hot water tank to the DHW circuit. This allows to operate the storage at a lower temperature, thereby increasing the heat pump COP.

The integration of the domestic hot water is further described in Buildings.DHC.ETS.Combined.Subsystems.DHWConsumption, which uses the fresh water station that is described and shown with a schematic diagram at Buildings.DHC.ETS.Combined.Subsystems.StorageTankWithExternalHeatExchanger.

Water-side economizer

The water-side economizer is optional. Use of the water-side economizer can improve resilience during heat waves when power consumption of the ETS need to be curtailed by switching of the chiller, for example during a grid outage when the site operates on emergency power.

To use the water-side economizer, if the temperature conditions are favorable, the valves (or pumps) are activated in order to cool the chilled water supply to the building. This model and its operation is described in Buildings.DHC.ETS.Combined.Subsystems.WatersideEconomizer. and in Gautier et al. (2022).

References

Antoine Gautier, Michael Wetter and Matthias Sulzer.
Resilient cooling through geothermal district energy system.
Applied Energy, 325, November, 2022.

Parameters

Connectors

Components

Revisions

• November 14, 2025, by Michael Wetter:
  First implementation.