.Buildings.DHC.Plants.Combined.Controls.BaseClasses.StagingPlant

Information

This block implements the staging logic for the chillers and HRCs. The units are staged in part based on an efficiency condition using the operative part load ratio. The units are also staged based on failsafe conditions using the CHW and HW supply temperature and differential pressure.

For the sake of simplicity, equipment rotation (i.e. the possibility at a given stage that either one unit or another unit can be operating) is not taken into account.

Plant stages

At cooling (resp. heating) stage #i, a number i of units are operating in cooling (resp. heating) mode. The cooling (resp. heating) stage #0 (no unit operating in that given mode) is active whenever the plant is disabled based on the cooling and heating Enable condition (see below) or when the plant is enabled and has been staged down due to the efficiency or failsafe conditions. The plant stage is given by the couple (cooling stage, heating stage). The minimum runtime of each plant stage is set to 15 min.

Direct heat recovery mode

All HRCs are allowed to operate in direct heat recovery mode, that is when their condenser is indexed to the HW loop and their evaporator is indexed to the CHW loop. Switching a HRC to operate in direct heat recovery mode is done on a load requirement basis. This means that HRCs are first switched over to cascading heating or cascading cooling mode. Only when all HRCs are operating and when a new stage up event is initiated, an additional HRC is then switched to operate in direct heat recovery mode. As described in Buildings.DHC.Plants.Combined.Controls.BaseClasses.ModeHeatRecoveryChiller the HRC with the highest index that is not operating operating in cascading cooling mode is the next to be switched into direct heat recovery mode.

For example, considering a plant with two chillers and three HRCs, the table below shows the plant stages derived from this logic. In this table, the chiller with index #i is denoted CHI#i, the HRC with index #i is denoted HRC#i, the units enumerated before (resp. after) the semicolon sign (:) operate in cooling (resp. heating) mode, the units marked with a star (*) operate in direct heat recovery mode.

Cooling and heating Enable condition

The cooling and heating Enable signals u1Coo and u1Hea shall be computed outside of the plant model, at least based on a time schedule and ideally in conjunction with a signal representative of the demand such as the requests yielded by the consumer control valves. Based on those signals, the cooling (resp. heating) stage #1 is activated whenever the cooling (resp. heating) Enable signal switches to true and when cooling (resp. heating) has been disabled for at least 15 min.

Operative part load ratio

The efficiency condition is based on the operative part load ratio which is computed as the ratio of the required capacity relative to design capacity of a given stage, which is the sum of the design capacity of each unit active in a given stage. The required capacity is calculated based on the primary mass flow rate and the temperature difference between supply and primary return, and averaged over a 5-minute moving window.

Cooling and heating staging

A stage up event is initiated if any of the following conditions is true.

• Efficiency condition: the operative part load ratio of the current stage exceeds the value of the parameter PLRStaTra for 15 min.
• Failsafe conditions: the CHW (resp. HW) supply temperature is 1 K higher (resp. lower) than setpoint for 15 min, or the CHW (resp. HW) differential pressure is 1.5E4 Pa lower than setpoint for 15 min.

A stage down event is initiated if both of the following conditions are true.

• Efficiency condition: the operative part load ratio of the next lower stage falls below the value of the parameter PLRStaTra for 15 min.
• Failsafe conditions: the failsafe stage up conditions are not true.

Revisions

• February 24, 2023, by Antoine Gautier:
  First implementation.