.Buildings.Templates.Plants.HeatPumps.AirToWater .Buildings.Templates.Plants.HeatPumps.AirToWater
This template represents an air-to-water heat pump plant
with closed-loop controls. While the heat pump plant configuration can be changed
through parameters, the image below shows a typical configuration with
two reversible air-to-water heat pumps, a primary-secondary distribution system
and a sidestream heat recovery chiller.
For a detailed schematic of the actual plant configuration, refer to the diagram
view of the plant component. In Dymola, for example, you can access this by right-clicking
the component pla in the model
Buildings.Templates.Plants.HeatPumps.Validation.AirToWater
and selecting "Show Component" from the context menu.
Currently, only identical heat pumps are supported. Although the template can accommodate any number of identical heat pumps, the graphical feedback for system configuration via the diagram layer is only accurate for up to 6 devices.
The supported plant configurations are enumerated in the table below.
The first option displayed in bold characters corresponds to the default
configuration.
| Configuration parameter | Options | Notes |
|---|---|---|
| Function |
Heating and cooling Heating-only |
The plant always provides heating hot water. Setting the parameter have_chiWat to true (default setting) allows
modeling a plant that provides both heating hot water and chilled water.
|
| Heat recovery |
Without sidestream heat recovery chiller With sidestream heat recovery chiller |
This option is only available for heating and cooling plants. When selected, the template includes a chiller and its associated dedicated primary CHW and CW pumps. The chiller is considered connected in a sidestream configuration to both the CHW return and the HW return. |
| Type of distribution |
Constant primary-variable secondary centralized Variable primary-only |
It is assumed that the HW and the CHW loops have the
same type of distribution, as specified by this parameter. Most AWHPs on the market use a reverse cycle for defrosting. This requires maximum primary flow during defrost cycles. Consequently, variable primary plants commonly adopt a high minimum flow setpoint, typically close to the design flow rate, effectively operating akin to constant primary plants but with variable speed pumps controlling the loop differential pressure. While the flow rate directed towards the loads varies, the bypass valve control loop ensures a constant primary flow for a given number of staged units. "Centralized secondary pumps" refers to configurations with a single group of secondary pumps that is typically integrated into the plant. Distributed secondary pumps with multiple secondary loops served by dedicated secondary pumps are currently not supported. |
| Type of primary pump arrangement |
Dedicated Headered |
It is assumed that the HW and the CHW loops have the same type of primary pump arrangement, as specified by this parameter. |
| Separate dedicated primary CHW pumps |
False True |
This option is only available for heating and cooling plants with dedicated primary pumps. If this option is not selected (default setting), each AWHP uses a common dedicated primary pump for HW and CHW – this pump is then denoted as the primary HW pump. Otherwise, each AWHP relies on a separate dedicated HW pump and a separate dedicated CHW pump. |
| Type of primary HW pumps |
Variable speed Constant speed |
For constant primary-variable secondary distributions, the variable speed primary pumps are commanded at fixed speeds, determined during the Testing, Adjusting and Balancing phase to provide design AWHP flow in heating and cooling modes. The same intent is achieved with constant speed primary pumps through the use of balancing valves. |
| Type of primary CHW pumps |
Variable speed Constant speed |
See the note above on primary HW pumps. |
| HW buffer tank |
HW buffer tank in the primary supply HW buffer tank in the primary return No HW buffer tank |
By default, the HW buffer tank is considered integrated into the primary supply to mitigate the impact of defrost cycles on the temperature of the HW supplied to the loads. This assumes that the buffer tank is well-mixed. The default sizing of the tank corresponds to 4 min of the design primary flow rate. This is based on manufacturer recommendations, which account for the fact that defrost cycles can take 3 to 5 min to complete. |
| CHW buffer tank |
CHW buffer tank in the primary return CHW buffer tank in the primary supply No CHW buffer tank |
By default, the CHW buffer tank is considered integrated into the primary return to mitigate the impact of rapid load variations on the plant controls. This assumes that the buffer tank is well-mixed. The default sizing of the tank corresponds to 2 min of the design primary flow rate, based on manufacturer recommendations. |
| Controller |
Closed-loop controls with supply temperature and differential pressure reset |
Most parts of the sequence of operation are similar to that
described in ASHRAE, 2021 for chiller plants. See the documentation of Buildings.Templates.Plants.Controls.HeatPumps.AirToWater for more details. An open loop controller is also available for validation purposes. |
The control sequence implemented in this template requires the external input points specified in the documentation of the controller Buildings.Templates.Plants.HeatPumps.Components.Controls.AirToWater.
The pressure drops of the heat pump CHW and HW heat exchangers are calculated
within the isolation valve component valIso based on lumped flow
coefficients for the sake of computational efficiency.