Allow for fixing investments

src/oemof/solph/_models.py

@@ -368,7 +368,9 @@ class Model(BaseModel):
         InvestNonConvexFlowBlock,
     ]
 
-    def __init__(self, energysystem, discount_rate=None, **kwargs):
+    def __init__(
+        self, energysystem, discount_rate=None, **kwargs
+    ):
         if discount_rate is not None:
             self.discount_rate = discount_rate
         elif (
@@ -383,6 +385,7 @@ def __init__(self, energysystem, discount_rate=None, **kwargs):
             self._set_discount_rate_with_warning()
         else:
             pass
+        self._fix_investments = False
         super().__init__(energysystem, **kwargs)
 
     def _set_discount_rate_with_warning(self):
@@ -521,3 +524,16 @@ def _add_parent_block_variables(self):
                 if (o, i) in self.UNIDIRECTIONAL_FLOWS:
                     for p, t in self.TIMEINDEX:
                         self.flow[o, i, p, t].setlb(0)
+
+    def fix_investments(self):
+        """Fix investment results of an already solved model"""
+        if self.solver_results is None:
+            msg = (
+                "Cannot fix investments as model has not yet been solved!\n"
+                "You have to first solve your model and then call method "
+                "`fix_investments()` on your model instance."
+            )
+            raise ValueError(msg)
+        self._fix_investments = True
+        if hasattr(self, "InvestmentFlowBlock"):
+            self.InvestmentFlowBlock.fix_investments_results()

src/oemof/solph/flows/_investment_flow_block.py

@@ -87,6 +87,7 @@
             attribute investment and the associated source (s) and target (t)
             of flow e.g. groups=[(s1, t1, f1), (s2, t2, f2),..]
         """
+        m = self.parent_block()
         if group is None:
             return None
 
@@ -1175,3 +1176,15 @@
         self.maximum_rule_build = BuildAction(rule=_max_invest_rule)
 
         return self.maximum_rule
+
+    def fix_investments_results(self):
+        """Fix investments if `_fix_investments` is set to True for model"""
+        m = self.parent_block()
+        for (i, o) in self.INVESTFLOWS:
+            for p in m.PERIODS:
+                self.invest[i, o, p].fix()
+                self.total[i, o, p].fix()
+                if m.es.periods is not None:
+                    self.old[i, o, p].fix()
+                    self.old_end[i, o, p].fix()
+                    self.old_exo[i, o, p].fix()

tests/test_scripts/test_solph/test_multi_period_model/test_multi_period_investment_fixed.py

@@ -0,0 +1,22 @@
+from oemof.solph import processing
+from test_multi_period_investment_model import (
+    set_up_multi_period_investment_model,
+)
+
+
+def test_multi_period_investment_fixed(solver="cbc"):
+    """test fixing investments in a repeated solve"""
+    om = set_up_multi_period_investment_model(approach="DLR")
+    om.receive_duals()
+    om.solve(solver=solver)
+
+    assert om.solver_results is not None
+    om.fix_investments()
+    om.solve(solver=solver)
+    for (i, o) in om.InvestmentFlowBlock.INVESTFLOWS:
+        for p in om.PERIODS:
+            assert om.InvestmentFlowBlock.invest[i, o, p].fixed
+            assert om.InvestmentFlowBlock.total[i, o, p].fixed
+            assert om.InvestmentFlowBlock.old[i, o, p].fixed
+            assert om.InvestmentFlowBlock.old_end[i, o, p].fixed
+            assert om.InvestmentFlowBlock.old_exo[i, o, p].fixed