Avoid creating DispatchGenByFuel for single fuel projects

Author: staadecker

switch_model/generators/core/dispatch.py

@@ -233,10 +233,47 @@ def init(m, gen, period):
     mod.DispatchGen = Var(
         mod.GEN_TPS,
         within=NonNegativeReals)
-    mod.DispatchGenByFuel = Var(mod.GEN_TP_FUELS, within=NonNegativeReals)
+
+    ##########################################
+    # Define DispatchGenByFuel
+    #
+    # Previously DispatchGenByFuel was simply a Variable for all the projects and a constraint ensured
+    # that the sum of DispatchGenByFuel across all fuels was equal the total dispatch for that project.
+    # However this approach creates extra variables in our model for projects that have only one fuel.
+    # Although these extra variables likely get removed during Gurobi pre-solve, we've nonetheless
+    # simplified the model here to reduce time in presolve and ensure the model is always
+    # simplified regardless of the solving method.
+    #
+    # To do this we redefine DispatchGenByFuel to be an
+    # expression that is equal to DispatchGenByFuelVar when we have multiple fuels but
+    # equal to DispatchGen when we have only one fuel.
+
+    # Define a set that is used to define DispatchGenByFuelVar
+    mod.GEN_TP_FUELS_FOR_MULTIFUELS = Set(
+        dimen=3,
+        initialize=mod.GEN_TP_FUELS,
+        filter=lambda m, g, t, f: g in m.MULTIFUEL_GENS,
+        doc="Same as GEN_TP_FUELS but only includes multi-fuel projects"
+    )
+    # DispatchGenByFuelVar is a variable that exists only for multi-fuel projects.
+    mod.DispatchGenByFuelVar = Var(mod.GEN_TP_FUELS_FOR_MULTIFUELS, within=NonNegativeReals)
+    # DispatchGenByFuel_Constraint ensures that the sum of all the fuels is DispatchGen
     mod.DispatchGenByFuel_Constraint = Constraint(
         mod.FUEL_BASED_GEN_TPS,
-        rule=lambda m, g, t: sum(m.DispatchGenByFuel[g, t, f] for f in m.FUELS_FOR_GEN[g]) == m.DispatchGen[g, t])
+        rule=lambda m, g, t:
+        (Constraint.Skip if g not in m.MULTIFUEL_GENS
+         else sum(m.DispatchGenByFuelVar[g, t, f] for f in m.FUELS_FOR_MULTIFUEL_GEN[g]) == m.DispatchGen[g, t])
+    )
+
+    # Define DispatchGenByFuel to equal the matching variable if we have many fuels but to equal
+    # the total dispatch if we have only one fuel.
+    mod.DispatchGenByFuel = Expression(
+        mod.GEN_TP_FUELS,
+        rule=lambda m, g, t, f: m.DispatchGenByFuelVar[g, t, f] if g in m.MULTIFUEL_GENS else m.DispatchGen[g, t]
+    )
+
+    # End Defining DispatchGenByFuel
+    ##########################################
 
     # Only used to improve the performance of calculating ZoneTotalCentralDispatch and ZoneTotalDistributedDispatch
     mod.GENS_FOR_ZONE_TPS = Set(