Buildings.DHC.Plants.Combined.Controls.BaseClasses

Package with base classes

Information

This package contains base classes that are used to construct the main plant controller. Those include an interface class for the controller as well as blocks that implement the sequence of operation for each component of the plant.

Extends from Modelica.Icons.BasesPackage (Icon for packages containing base classes).

Package Content

Name	Description
CoolingTowerLoop	Cooling tower loop control
DirectHeatRecovery	Block controlling HRC in direct heat recovery mode
IntegerArrayHold	Block that holds the value of an integer array for a given time
ModeCondenserLoop	Block that determines the condenser loop mode
ModeHeatRecoveryChiller	Block that computes the cascading cooling and direct HR switchover signals
PartialController	Interface class for plant controller
StageIndex	Block that computes the stage index out of staging signals
StagingPlant	Block that computes plant stage and command signals for chillers and HRC
StagingPump	Pump staging
TankCycle	Block that determines the tank cycle flag
ValveCondenserEvaporator	Controller for chiller and HRC condenser and evaporator valves
Validation	Package with validation models

Buildings.DHC.Plants.Combined.Controls.BaseClasses.CoolingTowerLoop

Cooling tower loop control

Information

This block implements the control logic for the CT pumps and CT fans.

CT supply temperature setpoint

When Heat Rejection mode is enabled, the setpoint is equal to min(25 °C, TChiWatSupSet + dTLif - dTConWat - dTHexCoo_nominal), where TChiWatSupSet is the CHW supply temperature setpoint, dTLif is the target chiller lift (see below), dTConWat is the chiller condenser Delta-T averaged over a 5-minute moving window, and dTHexCoo_nominal is the design heat exchanger approach. The target chiller lift is reset from the minimum chiller lift to the design chiller lift when the plant required cooling capacity varies from 10 % to 100 % of the design value.

In any other mode the setpoint is equal to the minimum setpoint value of the active tank cycle minus the design heat exchanger approach.

CT pumps

The lead pump is enabled whenever the TES tank bypass valve commanded position is lower than 1 for 1 min and the CW mass flow rate through the secondary side of the cooling heat exchanger is higher than 2.5 % of design condition for 1 min. The lead pump is disabled whenever the TES tank bypass valve commanded position is equal to 1 for 1 min or the CW mass flow rate through the secondary side of the cooling heat exchanger is lower than 2.5 % of design condition for 5 min.

The lag pump is enabled whenever the pump speed command (common to all pumps) is higher than 80 % for 5 min. The lag pump is disabled whenever the pump speed command is lower than 80 % for 5 min or the lead pump is disabled.

The pump speed command is the lower of that output by two loops, each loop being enabled whenever any pump is proven on. The first loop maintains the heat exchanger leaving CW temperature on plant side at setpoint. The setpoint is equal to the CT supply temperature setpoint plus the design HX approach. The second loop maintains the Delta-T across the primary side (CT loop) of the HX at a setpoint equal to the Delta-T across the secondary side (plant side) of the HX minus 1 K. (This loop keeps the primary and secondary HX flow rates close to each other and prevents pump speed runaway when target CT supply temperature setpoint cannot be met.) The output of each loop is mapped to the minimum pump speed at 80 % to 100 % at 100 %. The minimum pump speed is provided as a parameter for each pump stage since different speeds are required for each stage to maintain minimum tower flow.

CT fans

When any of the CT pumps is commanded On, a control loop maintains the tower water supply temperature at setpoint by resetting the tower fan speed from 0 % to a maximum value varying between 70 % and 100 % when the plant required cooling capacity varies from 0 % to 50 % of the design value. Otherwise, the loop is disabled and its output set to 0 %.

Note that the fan cycling On and Off is implicitly modeled in the cooling tower component which uses a low limit of the control signal to switch to a free convection regime at zero fan power.

Parameters

Connectors

Modelica definition

Buildings.DHC.Plants.Combined.Controls.BaseClasses.DirectHeatRecovery

Block controlling HRC in direct heat recovery mode

Information

In direct heat recovery mode, the HRC is internally controlled in heating mode and tracks a HW supply temperature setpoint. The CHW supply temperature setpoint is maintained by means of supervisory controls that act on the evaporator flow rate and condenser entering water temperature as described below.

A direct acting control loop runs for each HRC operating in direct heat recovery mode. Each loop is enabled with a bias of 50 % whenever the HRC is commanded On and in direct heat recovery mode. The loop is disabled with output set to 50 % otherwise. The loop output is mapped as follows. From 0 % to 33 % the evaporator flow setpoint of cooling-only chillers is reset from 1.2 times its minimum value to 1.2 times its design value. From 33 % to 67 % the evaporator flow setpoint of the HRC is reset from 1.2 times its minimum value to 1.2 times its design value. From 67 % to 100 % the HRC condenser entering temperature setpoint is reset from THeaWatRet + 0.5 °C to THeaWatRet - 15 °C.

Parameters

Connectors

Modelica definition

Buildings.DHC.Plants.Combined.Controls.BaseClasses.IntegerArrayHold

Block that holds the value of an integer array for a given time

Information

This blocks updates the value of the output array to match the value of the input array only if the time since the last update exceeds holdDuration. Otherwise, the value of the output array is kept equal to its value at the time of the last update. At initial time, the value of the output array is set to the value of the input array, and this is considered as the first update time.

Parameters

Connectors

Modelica definition

Buildings.DHC.Plants.Combined.Controls.BaseClasses.ModeCondenserLoop

Block that determines the condenser loop mode

Information

Tank charge fraction and rate of change

The tank charge fraction fraChaTan (-) is computed as the 5-minute moving average of the following expression: (∑_i (TTan_i - min(TTanSet)) / (max(TTanSet) - min(TTanSet)) / nTTan - fraUslTan) / (1 - fraUslTan), where TTan_i is the measurement from the i-th temperature sensor along the vertical axis of the tank, TTanSet are the tank temperature setpoints (two values for each tank cycle), nTTan is the number of temperature sensors along the vertical axis of the tank, fraUslTan is the useless fraction of the tank which is computed as follows: fraUslTan = ((max(TTanSet[2]) - min(TTanSet)) / (max(TTanSet) - min(TTanSet)) * hThe + hHee) / hTan, where hThe is the height of the thermocline (1 m by default) which is considered useless only during the second tank cycle, hHee is the upper and lower heel heights above and below the diffusers (1 m by default), hTan is the tank height (used as an approximation for the normal operating level of the tank at minimum temperature).

The rate of change of the tank charge fraction ratFraChaTan (h-1) is computed over several time periods (10, 30, 120, 240 and 360 min) as: ratFraChaTan(Δt) = (fraChaTan(t) - fraChaTan(t - Δt)) / Δt * 3600, where Δt is the time period in seconds.

Operating modes

Three operating modes are defined within Buildings.DHC.Plants.Combined.Controls.ModeCondenserLoop.

Charge Assist

The mode is enabled whenever any of the following conditions is true for 5 min. Any of the rates of change exceeds the values specified with the parameter ratFraChaTanLim. The tank charge fraction is lower than 97 % and 1 - fraChaTan > 0.08 * abs(nHouToWarUp - 2), where fraChaTan is the tank charge fraction and nHouToWarUp is the number of hours between the present time and the start time of morning warmup (4 AM by default). The 2-hour offset forces Charge Assist mode two hours before morning warmup if the tank is not fully charged.

The mode is disabled whenever none of the Enable conditions is true for 15 min, or the flow rate out of the lower port of the tank is positive for 5 min and the temperature at the bottom of the tank is higher than the maximum tank temperature setpoint minus 2 K for 5 min.

Heat Rejection

The mode is enabled whenever all of the following conditions are true. The Charge Assist mode is disabled for 5 min. The flow rate out of the lower port of the tank is positive for 5 min. The tank charge fraction is higher than 97 % for 5 min or the temperature at the bottom of the tank is higher than the maximum tank temperature setpoint minus 2 K for 5 min.

The mode is disabled whenever there is reverse flow through the cooling heat exchanger for 1 min.

Tank Charge/Discharge

The mode is enabled whenever neither Charge Assist nor Heat Rejection mode is enabled.

Parameters

Connectors

Modelica definition

Buildings.DHC.Plants.Combined.Controls.BaseClasses.ModeHeatRecoveryChiller

Block that computes the cascading cooling and direct HR switchover signals

Information

This block computes the command signals to the HRCs to initiate the switchover into either cascading cooling mode (with the evaporator indexed to the CHW loop and the condenser indexed to the CW loop) or direct heat recovery mode (with the evaporator indexed to the CHW loop and the condenser indexed to the HW loop). Switching a HRC to cascading cooling mode is done starting from the unit nearest to the CW interconnection, that is the unit with the highest index. Switching a HRC to direct heat recovery mode is done starting from the unit nearest to the CW interconnection and that is not operating in cascading cooling, that is the unit with the highest index below the lowest index of HRCs operating in cascading cooling mode.

Parameters

Connectors

Modelica definition

Buildings.DHC.Plants.Combined.Controls.BaseClasses.PartialController

Interface class for plant controller

Information

This block serves as an interface class for the plant controller.

Parameters

Connectors

Modelica definition

Buildings.DHC.Plants.Combined.Controls.BaseClasses.StageIndex

Block that computes the stage index out of staging signals

Information

This block is used to compute the stage index of the plant or of multiple lead/lag units such as pump groups.

At initial time, stage #0 is active. The transition to stage #1 is triggered when stage #0 has been active for the minimum runtime and when either the Enable signal u1 has a rising edge or when the stage up signal u1Up is true.

From stage #i, the transition to stage #i+1 (resp. i-1) is triggered when stage #i has been active for the minimum runtime and when the stage up signal u1Up (resp. stage down signal u1Dow) is true. From stage #i, the transition to stage #0 is triggered when stage #i has been active for the minimum runtime and the Enable signal u1 is false.

Parameters

Connectors

Modelica definition

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

Block that computes plant stage and command signals for chillers and HRC

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.

Parameters

Connectors

Modelica definition

Buildings.DHC.Plants.Combined.Controls.BaseClasses.StagingPump

Pump staging

Information

This block implements the logic for staging the lag pump of a multiple-pump group.

The first staging criterion is optional and is based on ratFlo, the ratio of current flow rate to design flow rate. The lag pump is enabled whenever any of the following is true.

• The pump speed command is higher than yUp for 5 min.
• Optionally if have_flowCriterion is set to true, the ratio ratFlo is higher than n / nPum - 0.03 for 10 min, where n is the number of operating pumps and nPum is the number of pumps operating at design conditions.

The lag pump is disabled whenever any of the following is true.

• The lead pump is disabled.
• The pump speed command is lower than yDow for 5 min.
• Optionally if have_flowCriterion is set to true, the ratio ratFlo is higher than (n - 1) / nPum - 0.03 for 10 min.

Parameters

Connectors

Modelica definition

Buildings.DHC.Plants.Combined.Controls.BaseClasses.TankCycle

Block that determines the tank cycle flag

Information

The tank operating conditions should match the operating limits and selection conditions of the chillers and HRCs. The highest temperature (at the top of the tank) should be limited by the maximum evaporator entering temperature per manufacturer’s recommendations. The lowest temperature (at the bottom of the tank) should be limited by the design CHW supply temperature. This gives the tank maximum ΔT and the actual storage capacity. However, this value is likely above the maximum ΔT that chillers and HRCs can achieve across the condenser or evaporator barrel (typically 11 K). Therefore, in order to fully leverage the TES capacity, two temperature cycles are needed.

The first tank cycle (higher temperature setpoint) is activated whenever all of the following conditions are true.

• All tank temperature sensors measure a value higher than the mean setpoint value of the second tank cycle for 5 min.
• The flow rate out of the lower port of the tank is positive (tank is charging) for 5 min.

The second tank cycle (lower temperature setpoint) is activated whenever both of the following conditions are true.

• All tank temperature sensors measure a value lower than the mean setpoint value of the first tank cycle for 5 min.
• The flow rate out of the lower port of the tank is negative (tank is discharging) for 5 min.

At initial time the tank cycle flag is set based on the tank temperature condition only from the above two clauses without a time delay. If neither condition is true, the tank cycle flag is set based on the flow condition only in the above two clauses without a time delay.

The minimum runtime of each tank cycle is fixed at 30 min.

Parameters

Connectors

Modelica definition

Buildings.DHC.Plants.Combined.Controls.BaseClasses.ValveCondenserEvaporator

Controller for chiller and HRC condenser and evaporator valves

Information

This block implements the control logic for the chiller isolation valves, the HRC isolation and switchover valves, the CHW and HW minimum flow bypass valves, the HRC evaporator CW mixing valve, and the CW chiller bypass valve. It also computes the lead pump Enable signal for the CHW, HW, CWC and CWE pump groups.

Chiller evaporator isolation valve

When a chiller is enabled, the valve position is controlled as follows.

• If no HRC is concurrently operating and connected to the CHW loop, the valve is commanded to a fully open position,
• If any HRC is concurrently operating in cascading cooling mode, but no HRC is in direct heat recovery mode, the valve is commanded to a fixed position ensuring flow balancing proportionally to design flow.
• If any HRC is concurrently operating in direct heat recovery mode, the valve is modulated with a control loop tracking an evaporator flow setpoint which is reset as described hereunder. The loop output is mapped to a valve position of 10 % (resp. 100 %) at 0 % (resp. 100 %) output signal. The loop is biased to launch from 100 %.

Otherwise, the valve is commanded to a closed position.

Chiller evaporator flow setpoint

The setpoint is computed based on the logic implemented in Buildings.DHC.Plants.Combined.Controls.BaseClasses.DirectHeatRecovery.

Chiller condenser isolation valve

When a chiller is enabled, the condenser isolation valve is modulated with a control loop tracking a condenser flow setpoint which is reset as described hereunder. The loop output is mapped to a valve position of 10 % (resp. 100 %) at 0 % (resp. 100 %) output signal. The loop is biased to launch from 100 %.

Otherwise, the valve is commanded to a closed position.

Chiller condenser flow setpoint

The condenser flow setpoint varies based on the condenser loop mode and on the tank cycle index.

• When the condenser loop mode is Charge Assist, a control loop maintains the condenser loop return temperature at a target setpoint equal to the highest temperature setpoint of the active tank cycle. The loop output is mapped to a flow setpoint of 10 % (resp. 100 %) of design flow at 0 % (resp. 100 %) output signal. The loop is biased to launch from 20 %.
• When the condenser loop mode is Tank Charge/Discharge, a control loop maintains the chiller condenser leaving temperature at target setpoint equal to the highest temperature setpoint of the active tank cycle. The loop output is mapped to a flow setpoint of 5 % (resp. 100 %) of design flow at 0 % (resp. 100 %) output signal. The loop is biased to launch from 50 %.
• When the condenser loop mode is Heat Rejection, the condenser flow setpoint is set at design value.

HRC evaporator isolation valve

When a HRC is enabled, the valve position is controlled as follows.

• If the HRC is operating in cascading cooling mode, the valve is commanded to a fixed position ensuring flow balancing proportionally to design flow.
• If the HRC is operating either in cascading heating mode or in direct heat recovery mode, the valve is modulated with a control loop tracking an evaporator flow setpoint which is reset as described hereunder. The loop output is mapped to a valve position of 10 % (resp. 100 %) at 0 % (resp. 100 %) output signal. The loop is biased to launch from 100 %.

Otherwise, the valve is commanded to a closed position.

HRC evaporator flow setpoint

In direct heat recovery mode, the setpoint is reset based on the logic implemented in Buildings.DHC.Plants.Combined.Controls.BaseClasses.DirectHeatRecovery.

In cascading heating mode, the setpoint is reset with a control loop that maintains the evaporator leaving temperature at target setpoint equal to the lowest temperature setpoint of the active tank cycle. The loop output is mapped as follows. From 0 % to 50 %, the HRC evaporator CW mixing valve commanded position is reset from 0 % (full bypass flow) to 100 % (no bypass flow). From 50 % to 100 %, the evaporator flow setpoint is reset from minimum to design value. The loop is biased to launch from 75 %. When disabled, the loop output is set to 75 % to ensure that the HRC evaporator CW mixing valve is fully open (no bypass flow).

HRC condenser isolation valve

When a HRC is enabled, the valve position is controlled as follows.

• If the HRC condenser is indexed to the HW loop (cascading heating or direct heat recovery mode), the valve is commanded to a fully open position.
• If the HRC condenser is indexed to the CW loop (cascading cooling mode), the valve is modulated with a control loop tracking a condenser flow setpoint which is reset based on the same logic as for the chiller condenser flow setpoint (see above). The loop output is mapped to a valve position of 10 % (resp. 100 %) at 0 % (resp. 100 %) output signal. The loop is biased to launch from 100 %.

Otherwise, the valve is commanded to a closed position.

HRC condenser and evaporator switchover valve

Each valve is commanded to a fully open or fully closed position depending on the valve index and the current operating mode of the HRC (cascading cooling, cascading heating or direct heat recovery). In addition, the condenser switchover valve indexed to the HRC which is nearest to the interconnection with the condenser loop (highest index) and which is operating in direct heat recovery mode is modulated with a control loop tracking the condenser entering temperature. The condenser entering temperature setpoint is reset based on the logic implemented in Buildings.DHC.Plants.Combined.Controls.BaseClasses.DirectHeatRecovery. This allows false loading the HRC that is controlled to meet the HW supply temperature setpoint in direct heat recovery mode, and thus meeting the CHW supply temperature setpoint simultaneously.

HRC evaporator CW mixing valve

The valve is modulated based on two control loops: the HRC evaporator leaving temperature control loop (see the section HRC evaporator flow setpoint) and another control loop that maintains the HRC evaporator entering water temperature below the highest tank temperature setpoint. This latter control loop is enabled when any HRC is operating in cascading heating mode. When the loop is enabled, the loop output is mapped to a valve position of 100 % (resp. 0 %) at 0 % (resp. 100 %) output signal. When the loop is disabled, the loop output is set to 100 % (no bypass flow). The valve control signal is the minimum (maximum bypass flow) of the resulting signals of those two control loops.

CHW and HW minimum flow bypass valve

Each chiller and HRC has its own CHW (resp. HW) minimum flow control loop. The loop is enabled whenever the unit's evaporator (resp. condenser) is indexed to the CHW (resp. HW) loop and its evaporator (resp. condenser) isolation valve is commanded open (with a threshold of 10 %). When enabled, each loop tracks a flow setpoint equal to 1.1 times the minimum CHW (resp. HW) flow rate. When disabled, each loop output is set to 0 %. The valve control signal is the maximum (maximum bypass flow) of the resulting signals of all control loops.

CW chiller bypass valve

The valve control is enabled when the plant is enabled either in cooling or heating mode, the Charge Assist mode is active and all chiller condenser isolation valves are closed (based on their commanded position).

When the valve control is enabled the valve position is modulated by the same control loop used to maintain the condenser loop return temperature at a target setpoint equal to the highest temperature setpoint of the active tank cycle (see the section "Chiller condenser flow setpoint").

Otherwise, the valve is commanded to a closed position.

CHW, HW, CWC, CWE lead pump

The lead pump of each loop is enabled whenever any chiller or HRC is indexed to the loop and the corresponding evaporator or condenser isolation valve is commanded open (with a threshold of 10 %). In addition, the CWC lead pump may also be enabled if the CW chiller bypass valve is commanded open.

Parameters

Connectors

Modelica definition