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.
The WSE is enabled when
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
where T_{wc} is the temperature of condenser water leaving the cooling tower, ΔT_{wse,off} = 0.6 K is the offset temperature.
The control strategy is as follows:
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.
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 temperature 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:
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:
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}.
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.
Name | Description |
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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 |