Buildings.Examples.ChillerPlant

Chiller plant with water side economizer for data center

Information

System Configuration

This example demonstrates the implementation of a chiller plant with water-side economizer (WSE) to cool a data center. The system schematics is as shown below.

The system is a primary-only chiller plant with integrated WSE. The objective was to improve the energy efficiency of the chilled water plant by optimizing the control setpoints. The room of the data center was modeled using a mixed air volume with a heat source. Heat conduction and air infiltration through the building envelope were neglected since the heat exchange between the room and the ambient environment was small compared to the heat released by the computers.

The control objective was to maintain the temperature of the supply air to the room, while reducing energy consumption of the chilled water plant. The control was based on the control sequence proposed by Stein (2009). To simplify the implementation, we only applied the controls for the differential pressure of the chilled water loop, the setpoint temperature of the chilled water leaving the chiller, and the chiller and WSE on/off control.

Enabling/Disabling the WSE

The WSE is enabled when

1. The WSE has been disabled for at least 20 minutes, and
2. T_ws > 0.9 T_wet + ΔT_t + ΔT_w

where T_ws is the temperature of chilled water leaving the cooling coil, T_wet is the wet bulb temperature, ΔT_t is the temperature difference between the water leaving the cooling tower and the air entering the cooling tower, ΔT_w is the temperature difference between the chilled water leaving the WSE and the condenser water entering the WSE.

The WSE is disabled when

1. The WSE has been enabled for at least 20 minutes, and
2. T_ws < T_wc + ΔT_wse,off

where T_wc is the temperature of condenser water leaving the cooling tower, ΔT_wse,off = 0.6 K is the offset temperature.

Enabling/Disabling the Chiller

The control strategy is as follows:

• The chiller is enabled when T_chw,ent > T_chi,set + ΔT_chi,ban
• The chiller is disabled when T_chw,ent ≤ T_chi,set

where T_chw,ent is the tempearture of chilled water entering the chiller, T_chi,set is the setpoint temperature of the chilled water leaving the chiller, and ΔT_chi,ban is the dead-band to prevent short cycling.

Setpoint Reset

The setpoint reset strategy is to first increase the different pressure, Δp, of the chilled water loop to increase the mass flow rate. If Δp reaches the maximum value and further cooling is still needed, the chiller remperature setpoint, T_chi,set, is reduced. If there is too much cooling, the T_chi,set and Δp will be changed in the reverse direction.

There are two implementations for the setpoint reset.

The model Buildings.Examples.ChillerPlant.DataCenterDiscreteTimeControl implements a discrete time trim and respond logic as follows:

• A cooling request is triggered if the input signal, y, is larger than 0. y is the difference between the actual and set temperature of the suppuly air to the data center room.
• The request is sampled every 2 minutes. If there is a cooling request, the control signal u is increased by 0.03, where 0 ≤ u ≤ 1. If there is no cooling request, u is decreased by 0.03.

The model Buildings.Examples.ChillerPlant.DataCenterContinuousTimeControl uses a PI-controller to approximate the above trim and respond logic. This significantly reduces computing time.

For both models, the control signal u is converted to setpoints for Δp and T_chi,set as follows:

• If 0 ≤ u ≤ x then Δp = Δp_min + u  (Δp_max-Δp_min)/x and T = T_max
• If x < u ≤ 1 then Δp = Δp_max and T = T_max - (u-x) (T_max-T_min)/(1-x)

where Δp_min and Δp_max are minimum and maximum values for Δp, and T_min and T_max are the minimum and maximum values for T_chi,set.

Reference

Stein, J. (2009). Waterside Economizing in Data Centers: Design and Control Considerations. ASHRAE Transactions, 115(2), 192-200.
Taylor, S.T. (2007). Increasing Efficiency with VAV System Static Pressure Setpoint Reset. ASHRAE Journal, June, 24-32.

Extends from Modelica.Icons.ExamplesPackage (Icon for packages containing runnable examples).

Package Content

Name	Description
DataCenterContinuousTimeControl	Model of data center that approximates the trim and respond logic
DataCenterDiscreteTimeControl	Model of data center with trim and respond control
DataCenterRenewables	Model of a data center connected to renewable energy generation
BaseClasses	Package with base classes for Buildings.Examples.ChillerPlant

Buildings.Examples.ChillerPlant.DataCenterContinuousTimeControl

Model of data center that approximates the trim and respond logic

Information

This model is the chilled water plant with continuous time control. The trim and respond logic is approximated by a PI controller which significantly reduces computing time. The model is described at Buildings.Examples.ChillerPlant.

See Buildings.Examples.ChillerPlant.DataCenterContinuousTimeControl for an implementation with the discrete time trim and respond logic.

Extends from Buildings.Examples.ChillerPlant.BaseClasses.DataCenter (Primary only chiller plant system with water-side economizer), Modelica.Icons.Example (Icon for runnable examples).

Parameters

Connectors

Modelica definition

Buildings.Examples.ChillerPlant.DataCenterDiscreteTimeControl

Model of data center with trim and respond control

Information

This model is the chilled water plant with trim and respond control, which is a discrete time control logic.

The trim and respond logic is approximated by a PI controller which significantly reduces computing time. The model is described at Buildings.Examples.ChillerPlant.

See Buildings.Examples.ChillerPlant.DataCenterContinuousTimeControl for an implementation that approximates the trim and respond logic by a continuous time controller.

Extends from Buildings.Examples.ChillerPlant.BaseClasses.DataCenter (Primary only chiller plant system with water-side economizer), Modelica.Icons.Example (Icon for runnable examples).

Parameters

Connectors

Modelica definition

Buildings.Examples.ChillerPlant.DataCenterRenewables

Model of a data center connected to renewable energy generation

Information

This model illustrates a data center with DC and AC load. The electrical supply is from a grid, from wind turbines and from PV. The battery is charged during the night and discharged during the day in such a way that it is fully charged and discharged. This control logic is implemented using a finite state machine inside the model con.

Extends from Modelica.Icons.Example (Icon for runnable examples).

Connectors

Modelica definition