packageDataCenter

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 temperature 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 differential 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.DataCenter.DiscreteTimeControl 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 supply 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.DataCenter.ContinuousTimeControl 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.

Contents

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