Fix load dispatch for variable-flow chillers with a low leaving temperature limit

src/EnergyPlus/ChillerElectricEIR.cc

@@ -1145,6 +1145,8 @@ void ElectricEIRChillerSpecs::oneTimeInit(EnergyPlusData &state)
     if (this->FlowMode == DataPlant::FlowMode::LeavingSetpointModulated) {
         // reset flow priority
         DataPlant::CompData::getPlantComponent(state, this->CWPlantLoc).FlowPriority = DataPlant::LoopFlowStatus::NeedyIfLoopOn;
+        // this chiller sets its own evaporator flow rate to hold the leaving setpoint
+        DataPlant::CompData::getPlantComponent(state, this->CWPlantLoc).ModulatedFlow = true;
         // check if setpoint on outlet node
         if ((state.dataLoopNodes->Node(this->EvapOutletNodeNum).TempSetPoint == Node::SensedNodeFlagValue) &&
             (state.dataLoopNodes->Node(this->EvapOutletNodeNum).TempSetPointHi == Node::SensedNodeFlagValue)) {

src/EnergyPlus/ChillerIndirectAbsorption.cc

@@ -833,6 +833,8 @@ void IndirectAbsorberSpecs::oneTimeInit(EnergyPlusData &state)
     if (this->FlowMode == DataPlant::FlowMode::LeavingSetpointModulated) {
         // reset flow priority
         this->CWPlantLoc.comp->FlowPriority = DataPlant::LoopFlowStatus::NeedyIfLoopOn;
+        // this chiller sets its own evaporator flow rate to hold the leaving setpoint
+        this->CWPlantLoc.comp->ModulatedFlow = true;
 
         if ((state.dataLoopNodes->Node(this->EvapOutletNodeNum).TempSetPoint == Node::SensedNodeFlagValue) &&
             (state.dataLoopNodes->Node(this->EvapOutletNodeNum).TempSetPointHi == Node::SensedNodeFlagValue)) {

src/EnergyPlus/ChillerReformulatedEIR.cc

@@ -936,6 +936,8 @@ void ReformulatedEIRChillerSpecs::oneTimeInit(EnergyPlusData &state)
     if (this->FlowMode == DataPlant::FlowMode::LeavingSetpointModulated) {
         // reset flow priority
         DataPlant::CompData::getPlantComponent(state, this->CWPlantLoc).FlowPriority = DataPlant::LoopFlowStatus::NeedyIfLoopOn;
+        // this chiller sets its own evaporator flow rate to hold the leaving setpoint
+        DataPlant::CompData::getPlantComponent(state, this->CWPlantLoc).ModulatedFlow = true;
         // check if setpoint on outlet node
         if ((state.dataLoopNodes->Node(this->EvapOutletNodeNum).TempSetPoint == Node::SensedNodeFlagValue) &&
             (state.dataLoopNodes->Node(this->EvapOutletNodeNum).TempSetPointHi == Node::SensedNodeFlagValue)) {

src/EnergyPlus/Plant/Component.hh

@@ -309,6 +309,7 @@ namespace DataPlant {
         DataPlant::HowMet HowLoadServed;                  // nature of component in terms of how it can meet load
         Real64 MinOutletTemp;                             // Component exit lower limit temperature
         Real64 MaxOutletTemp;                             // Component exit upper limit temperature
+        bool ModulatedFlow;                               // true if this component modulates its own flow to hold a leaving setpoint
         bool FreeCoolCntrlShutDown;                       // true if component was shut down because of free cooling
         Real64 FreeCoolCntrlMinCntrlTemp;                 // current control temp value for free cooling controls
         DataPlant::FreeCoolControlMode FreeCoolCntrlMode; // type of sensor used for free cooling controls
@@ -325,7 +326,7 @@ namespace DataPlant {
               FlowPriority(LoopFlowStatus::Invalid), ON(false), Available(false), NodeNumIn(0), NodeNumOut(0), MyLoad(0.0), MaxLoad(0.0),
               MinLoad(0.0), OptLoad(0.0), SizFac(0.0), CurOpSchemeType(DataPlant::OpScheme::Invalid), NumOpSchemes(0), CurCompLevelOpNum(0),
               EquipDemand(0.0), EMSLoadOverrideOn(false), EMSLoadOverrideValue(0.0), HowLoadServed(DataPlant::HowMet::Invalid), MinOutletTemp(0.0),
-              MaxOutletTemp(0.0), FreeCoolCntrlShutDown(false), FreeCoolCntrlMinCntrlTemp(0.0),
+              MaxOutletTemp(0.0), ModulatedFlow(false), FreeCoolCntrlShutDown(false), FreeCoolCntrlMinCntrlTemp(0.0),
               FreeCoolCntrlMode(DataPlant::FreeCoolControlMode::Invalid), FreeCoolCntrlNodeNum(0), IndexInLoopSidePumps(0), TempDesCondIn(0.0),
               TempDesEvapOut(0.0), compPtr(nullptr)
         {

src/EnergyPlus/PlantChillers.cc

@@ -2082,6 +2082,8 @@ namespace PlantChillers {
             if (this->FlowMode == DataPlant::FlowMode::LeavingSetpointModulated) {
                 // reset flow priority
                 DataPlant::CompData::getPlantComponent(state, this->CWPlantLoc).FlowPriority = DataPlant::LoopFlowStatus::NeedyIfLoopOn;
+                // this chiller sets its own evaporator flow rate to hold the leaving setpoint
+                DataPlant::CompData::getPlantComponent(state, this->CWPlantLoc).ModulatedFlow = true;
 
                 // check if setpoint on outlet node
                 if ((state.dataLoopNodes->Node(this->EvapOutletNodeNum).TempSetPoint == Node::SensedNodeFlagValue) &&
@@ -4145,6 +4147,8 @@ namespace PlantChillers {
             if (this->FlowMode == DataPlant::FlowMode::LeavingSetpointModulated) {
                 // reset flow priority
                 DataPlant::CompData::getPlantComponent(state, this->CWPlantLoc).FlowPriority = DataPlant::LoopFlowStatus::NeedyIfLoopOn;
+                // this chiller sets its own evaporator flow rate to hold the leaving setpoint
+                DataPlant::CompData::getPlantComponent(state, this->CWPlantLoc).ModulatedFlow = true;
                 // check if setpoint on outlet node
                 if ((state.dataLoopNodes->Node(this->EvapOutletNodeNum).TempSetPoint == Node::SensedNodeFlagValue) &&
                     (state.dataLoopNodes->Node(this->EvapOutletNodeNum).TempSetPointHi == Node::SensedNodeFlagValue)) {
@@ -6106,6 +6110,8 @@ namespace PlantChillers {
             if (this->FlowMode == DataPlant::FlowMode::LeavingSetpointModulated) {
                 // reset flow priority
                 DataPlant::CompData::getPlantComponent(state, this->CWPlantLoc).FlowPriority = DataPlant::LoopFlowStatus::NeedyIfLoopOn;
+                // this chiller sets its own evaporator flow rate to hold the leaving setpoint
+                DataPlant::CompData::getPlantComponent(state, this->CWPlantLoc).ModulatedFlow = true;
 
                 // check if setpoint on outlet node
                 if ((state.dataLoopNodes->Node(this->EvapOutletNodeNum).TempSetPoint == Node::SensedNodeFlagValue) &&

src/EnergyPlus/PlantCondLoopOperation.cc

@@ -3604,6 +3604,12 @@ void AdjustChangeInLoadByHowServed(EnergyPlusData &state,
 
         //- Retrieve data from the plant loop data structure
         CurMassFlowRate = state.dataLoopNodes->Node(this_component.NodeNumIn).MassFlowRate;
+        // A component in a modulated flow mode controls its own flow rate to hold the leaving setpoint, so it can raise
+        // its flow up to whatever the branch can deliver. Judging its capacity by the current flow, which may still be
+        // sitting at the branch pump minimum, understates it and spills load onto the next machine in the list.
+        if (this_component.ModulatedFlow) {
+            CurMassFlowRate = max(CurMassFlowRate, state.dataLoopNodes->Node(this_component.NodeNumIn).MassFlowRateMaxAvail);
+        }
         ToutLowLimit = this_component.MinOutletTemp;
         Tinlet = state.dataLoopNodes->Node(this_component.NodeNumIn).Temp;
         CurSpecHeat = plantLoc.loop->glycol->getSpecificHeat(state, Tinlet, RoutineName);
@@ -3688,11 +3694,13 @@ void AdjustChangeInLoadByHowServed(EnergyPlusData &state,
             this_component.Available = true;
             this_component.FreeCoolCntrlShutDown = false;
             CurMassFlowRate = state.dataLoopNodes->Node(this_component.NodeNumIn).MassFlowRate;
+            if (this_component.ModulatedFlow) {
+                CurMassFlowRate = max(CurMassFlowRate, state.dataLoopNodes->Node(this_component.NodeNumIn).MassFlowRateMaxAvail);
+            }
             ToutLowLimit = this_component.MinOutletTemp;
             Tinlet = state.dataLoopNodes->Node(this_component.NodeNumIn).Temp;
             CurSpecHeat = plantLoc.loop->glycol->getSpecificHeat(state, Tinlet, RoutineName);
             QdotTmp = CurMassFlowRate * CurSpecHeat * (Tinlet - ToutLowLimit);
-
             //        !- Don't correct if Q is zero, as this could indicate a component which this hasn't been implemented or not yet turned
             //        on
             if (CurMassFlowRate > 0.0) {

tst/EnergyPlus/unit/PlantCondLoopOperation.unit.cc

@@ -55,6 +55,8 @@
 #include <EnergyPlus/BranchInputManager.hh>
 #include <EnergyPlus/Data/EnergyPlusData.hh>
 #include <EnergyPlus/DataEnvironment.hh>
+#include <EnergyPlus/DataLoopNode.hh>
+#include <EnergyPlus/FluidProperties.hh>
 #include <EnergyPlus/Plant/DataPlant.hh>
 #include <EnergyPlus/Plant/PlantManager.hh>
 #include <EnergyPlus/PlantCondLoopOperation.hh>
@@ -1445,3 +1447,43 @@ TEST_F(EnergyPlusFixture, OperationSchemePriority)
     EXPECT_EQ(coolingTower(1).CurCompLevelOpNum, 1);
     EXPECT_EQ(coolingTower(1).CurOpSchemeType, DataPlant::OpScheme::Uncontrolled);
 }
+
+TEST_F(EnergyPlusFixture, AdjustChangeInLoadByHowServed_ModulatedFlowChiller)
+{
+    // A chiller in LeavingSetpointModulated mode sets its own flow rate, so its capacity must not be truncated using the current (branch pump
+    // minimum) flow rate.
+
+    state->dataPlnt->PlantLoop.allocate(1);
+    auto &thisLoop = state->dataPlnt->PlantLoop(1);
+    thisLoop.glycol = Fluid::GetWater(*state);
+    thisLoop.LoopSide(DataPlant::LoopSideLocation::Supply).Branch.allocate(1);
+    thisLoop.LoopSide(DataPlant::LoopSideLocation::Supply).Branch(1).Comp.allocate(1);
+
+    state->dataLoopNodes->Node.allocate(1);
+    state->dataLoopNodes->Node(1).Temp = 8.0;                  // chiller inlet, low load return temp
+    state->dataLoopNodes->Node(1).MassFlowRate = 1.0;          // currently at the pump minimum
+    state->dataLoopNodes->Node(1).MassFlowRateMaxAvail = 10.0; // what the chiller could pull
+
+    auto &thisComp = thisLoop.LoopSide(DataPlant::LoopSideLocation::Supply).Branch(1).Comp(1);
+    thisComp.HowLoadServed = DataPlant::HowMet::ByNominalCapLowOutLimit;
+    thisComp.NodeNumIn = 1;
+    thisComp.MinOutletTemp = 6.7; // same as the loop setpoint, as in the defect file
+
+    PlantLocation plantLoc(1, DataPlant::LoopSideLocation::Supply, 1, 1);
+    PlantUtilities::SetPlantLocationLinks(*state, plantLoc);
+
+    Real64 const Cp = thisLoop.glycol->getSpecificHeat(*state, 8.0, "AdjustChangeInLoadByHowServed_ModulatedFlowChiller");
+    Real64 constexpr dispatchedLoad = 20000.0;
+
+    // Constant flow chiller: capacity is limited by the flow it actually has
+    thisComp.ModulatedFlow = false;
+    Real64 changeInLoad = dispatchedLoad;
+    PlantCondLoopOperation::AdjustChangeInLoadByHowServed(*state, plantLoc, changeInLoad);
+    EXPECT_NEAR(changeInLoad, 1.0 * Cp * (8.0 - 6.7), 0.001);
+
+    // Variable flow chiller: it can raise its own flow, so the full dispatched load stands
+    thisComp.ModulatedFlow = true;
+    changeInLoad = dispatchedLoad;
+    PlantCondLoopOperation::AdjustChangeInLoadByHowServed(*state, plantLoc, changeInLoad);
+    EXPECT_NEAR(changeInLoad, dispatchedLoad, 0.001);
+}