Remove internal discounting

The (very similar) interest still needs to be removed.

docs/usage.rst

@@ -1325,13 +1325,6 @@ Besides the `invest` variable, new variables are introduced as well. These are:
 
 .. note::
 
-    * You can specify a `discount_rate` for the model. If you do not do so, 0.02 will be used as a default, corresponding
-      to sort of a social discount rate. If you work with costs in real terms, discounting is obsolete, so define
-      `discount_rate = 0` in that case.
-    * You can specify an `interest_rate` for every investment object. If you do not do so, it will be chosen the same
-      as the model's `discount_rate`. You could use this default to model a perfect competition administered by some sort of
-      social planner, but even in a social planner setting, you might want to deviate from the `discount_rate`
-      value and/or discriminate among technologies with different risk profiles and hence different interest requirements.
     * For storage units, the `initial_content` is not allowed combined with multi-period investments.
       The storage inflow and outflow are forced to zero until the storage unit is invested into.
     * You can specify periods of different lengths, but the frequency of your timeindex needs to be consistent. Also,

src/oemof/solph/_models.py

@@ -52,9 +52,6 @@ class Model(po.ConcreteModel):
         Solph looks for these groups in the given energy system and uses them
         to create the constraints of the optimization problem.
         Defaults to `Model.CONSTRAINT_GROUPS`
-    discount_rate : float or None
-        The rate used for discounting in a multi-period model.
-        A 2% discount rate needs to be defined as 0.02.
     objective_weighting : array like (optional)
         Weights used for temporal objective function
         expressions. If nothing is passed, `timeincrement` will be used which
@@ -84,17 +81,6 @@ class Model(po.ConcreteModel):
     rc : `pyomo.core.base.suffix.Suffix` or None
         Store the reduced costs of the model if pyomo suffix is set to IMPORT
 
-    Note
-    ----
-
-    * The discount rate is only applicable for a multi-period model.
-    * If you want to work with costs data in nominal terms,
-      you should specify a discount rate.
-    * By default, there is a discount rate of 2% in a multi-period model.
-    * If you want to provide your costs data in real terms,
-      just specify `discount_rate = 0`, i.e. effectively there will be
-      no discounting.
-
 
     **The following basic sets are created**:
 
@@ -134,7 +120,7 @@ class Model(po.ConcreteModel):
         InvestNonConvexFlowBlock,
     ]
 
-    def __init__(self, energysystem, discount_rate=None, **kwargs):
+    def __init__(self, energysystem, **kwargs):
         super().__init__()
 
         # Check root logger. Due to a problem with pyomo the building of the
@@ -190,9 +176,7 @@ def __init__(self, energysystem, discount_rate=None, **kwargs):
         self.dual = None
         self.rc = None
 
-        if discount_rate is not None:
-            self.discount_rate = discount_rate
-        elif energysystem.periods is not None:
+        if energysystem.periods is not None:
             self._set_discount_rate_with_warning()
         else:
             pass

src/oemof/solph/components/_generic_storage.py

@@ -1798,7 +1798,7 @@ def _objective_expression(self):
                     )
                     investment_costs_increment = (
                         self.invest[n, p] * annuity * present_value_factor
-                    ) * (1 + m.discount_rate) ** (-m.es.periods_years[p])
+                    )
                     remaining_value_difference = (
                         self._evaluate_remaining_value_difference(
                             m,
@@ -1839,7 +1839,7 @@ def _objective_expression(self):
                     investment_costs_increment = (
                         self.invest[n, p] * annuity * present_value_factor
                         + self.invest_status[n, p] * n.investment.offset[p]
-                    ) * (1 + m.discount_rate) ** (-m.es.periods_years[p])
+                    )
                     remaining_value_difference = (
                         self._evaluate_remaining_value_difference(
                             m,
@@ -1865,9 +1865,7 @@ def _objective_expression(self):
                             m.es.periods_years[p] + lifetime,
                         )
                         fixed_costs += sum(
-                            self.invest[n, p]
-                            * n.investment.fixed_costs[pp]
-                            * (1 + m.discount_rate) ** (-pp)
+                            self.invest[n, p] * n.investment.fixed_costs[pp]
                             for pp in range(
                                 m.es.periods_years[p],
                                 range_limit,
@@ -1882,9 +1880,7 @@ def _objective_expression(self):
                         m.es.end_year_of_optimization, lifetime - age
                     )
                     fixed_costs += sum(
-                        n.investment.existing
-                        * n.investment.fixed_costs[pp]
-                        * (1 + m.discount_rate) ** (-pp)
+                        n.investment.existing * n.investment.fixed_costs[pp]
                         for pp in range(range_limit)
                     )
 
@@ -1956,15 +1952,11 @@ def _evaluate_remaining_value_difference(
                     self.invest[n, p]
                     * (remaining_annuity - original_annuity)
                     * present_value_factor_remaining
-                ) * (1 + m.discount_rate) ** (-end_year_of_optimization)
+                )
                 if nonconvex:
                     return convex_investment_costs + self.invest_status[
                         n, p
-                    ] * (n.investment.offset[-1] - n.investment.offset[p]) * (
-                        1 + m.discount_rate
-                    ) ** (
-                        -end_year_of_optimization
-                    )
+                    ] * (n.investment.offset[-1] - n.investment.offset[p])
                 else:
                     return convex_investment_costs
             else: