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

- The WSE has been disabled for at least 20 minutes, and
*T*_{ws}> 0.9 T_{wet}+ ΔT_{t}+ ΔT_{w}

where *T _{ws}* is the temperature of chilled water
leaving the cooling coil,

The WSE is disabled when

- The WSE has been enabled for at least 20 minutes, and
*T*_{ws}< T_{wc}+ ΔT_{wse,off}

where *T _{wc}* is the temperature of condenser
water leaving the cooling tower,

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,

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

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*and_{min}+ u (Δp_{max}-Δp_{min})/x*T = T*_{max} - If
*x < u ≤ 1*then*Δp = Δp*and_{max}*T = T*_{max}- (u-x) (T_{max}-T_{min})/(1-x)

where *Δp _{min}* and

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 |
---|---|

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 |

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