Allow for connections including slice specifications (#774)

* move tests for split indices

* revert obsolete changes to plant schema

* fix bugs in length 3 connections for src_indices

* remove use of eval

* add ValueError for non-zero destination slice starts

* allow for src_indices tiling into destination

* update wind solar electrolyzer example to demonstrate different finance and connection approaches

* add tests for each finance/connection approach added to wind solar electrolyzer example

* add documentation for variable slicing in technology connections

* update changelog

* fix typo

* Update docs/user_guide/connecting_technologies.md

improve comment

Co-authored-by: elenya-grant <116225007+elenya-grant@users.noreply.github.com>

---------

Co-authored-by: elenya-grant <116225007+elenya-grant@users.noreply.github.com>

Author: jaredthomas68

CHANGELOG.md

@@ -57,6 +57,7 @@
   feedstock data with the resource data methodology
   [PR 801](https://github.com/NatLabRockies/H2Integrate/pull/801).
 - Added capability to specify demand technology for system-level control, and renamed the framework-derived system-level control classification dict from `slc_config` to `slc_topology` to distinguish it from the user-authored `control_parameters` block. [PR 784](https://github.com/NatLabRockies/H2Integrate/pull/784)
+- Add support for slice notation in technology connections to allow users to connect between variables of different shapes. [PR 774](https://github.com/NatLabRockies/H2Integrate/pull/774)
 
 ## 0.8 [April 15, 2026]
 

docs/user_guide/connecting_technologies.md

@@ -61,6 +61,31 @@ There are two connection formats:
 The `source_parameter` and `destination_parameter` should be input into the array as another array. If it's input as a tuple the model will raise an error.
 ```
 
+##### Slice notation for mismatched shapes
+
+3-element connections with different shared parameter names allow the user to append a NumPy-style slice in square brackets to the source and/or destination parameter name.
+This is used to connect variables whose shapes differ, for example feeding a scalar finance output into a per-timestep input.
+The slice is parsed into OpenMDAO `src_indices`, and the bracketed text is stripped from the parameter name before the connection is made.
+
+```yaml
+technology_interconnections: [
+  # select a subset of the source to connect to the destination
+  ["tech_a", "tech_b", ["source_param[0:100]", "dest_param"]],
+  # tile a single source element across an 8760-length destination
+  ["finance_subgroup_electricity", "grid_buy", ["LCOE[0]", "electricity_buy_price[0:8760]"]],
+]
+```
+
+Behavior depends on which side carries a slice:
+
+- **Source only** (`"source_param[0:100]"`): the slice selects source indices directly (supports start/stop/step, e.g. `[0:100:2]`, and `[:]` for the full range).
+- **Destination only** or **both sides equal**: no `src_indices` are applied (the slice is treated as documentation of the target shape).
+- **Both sides** (`"LCOE[0]"` → `"electricity_buy_price[0:8760]"`): the source indices are *tiled* to fill the destination length. A single index is repeated (e.g. `[0]` → 8760 copies) and a multi-index source such as `[0,1]` is cycled to fill the destination.
+
+```{note}
+When the destination has a slice, it must (1) start at `0` (a non-zero start raises a `ValueError`) and (2) include the destination length, e.g. `[0:8760]`. The length is required because the input shape is not known until `prob.setup()` has run.
+```
+
 ### Internal connection logic
 
 H2Integrate processes these connections in the `connect_technologies()` method of `h2integrate_model.py`. Here's what happens internally:

examples/15_wind_solar_electrolyzer/plant_config.yaml

@@ -24,10 +24,32 @@ sites:
 # with the reverse definition.
 # this will naturally grow as we mature the interconnected tech
 technology_interconnections:
+  # Physical plant: wind + solar are combined and used to run a single electrolyzer.
+  # This same physical plant is financed three different ways below (see finance_subgroups).
   - [wind, combiner, electricity, cable]
-                                                # source_tech, dest_tech, transport_item, transport_type = connection
   - [solar, combiner, electricity, cable]
   - [combiner, electrolyzer, electricity, cable]
+
+  # --- Approach B wiring: upstream LCOE used as a feedstock price --------------
+  # Compute the wind + solar levelized cost of electricity (LCOE) in the
+  # `electricity` finance subgroup, then charge the electrolyzer for the
+  # electricity it consumes at that LCOE through the `electricity_feedstock` tech.
+  #
+  # 1. Feed the computed LCOE in as the feedstock purchase price (scalar slice):
+  - [finance_subgroup_electricity, electricity_feedstock, [LCOE, price]]
+  # 2. Charge the feedstock for exactly the electricity the electrolyzer consumes:
+  - [electrolyzer, electricity_feedstock, electricity_consumed]
+  # 3. Provide the feedstock supply profile (used for its capacity factor):
+  - [electricity_feedstock_source, electricity_feedstock, electricity_out]
+
+  # --- Approach C wiring: upstream LCOE used as a grid purchase price ----------
+  # Same idea as Approach B, but the electricity is "purchased" through a
+  # `grid_buy` interconnection priced at the LCOE instead of a generic feedstock.
+  #
+  # 1. Feed the computed LCOE in as the grid buy price (scalar tiled to 8760 h):
+  - [finance_subgroup_electricity, grid_buy, ['LCOE[0]', 'electricity_buy_price[0:8760]']]
+  # 2. Buy from the grid exactly the electricity the electrolyzer consumes:
+  - [electrolyzer, grid_buy, [electricity_consumed, electricity_set_point]]
 resource_to_tech_connections:
   # connect the wind resource to the wind technology
   - [wind_site.wind_resource, wind, wind_resource_data]
@@ -61,14 +83,32 @@ finance_parameters:
         depr_period: 5  # 5  years - for clean energy facilities as specified by the IRS MACRS schedule https://www.irs.gov/publications/p946#en_US_2020_publink1000107507
         refurb: [0.]
   finance_subgroups:
+    # Upstream electricity finance group: levelized cost of the wind + solar
+    # electricity. Its LCOE output feeds Approaches B and C below.
     electricity:
       commodity: electricity
       commodity_stream: combiner  # use the total electricity output from the combiner for the finance calc
       technologies: [wind, solar]
+    # Approach A - "same finance model": wind + solar + electrolyzer are all
+    # financed together, so the renewable capital is embedded directly in the LCOH.
     hydrogen:
       commodity: hydrogen
       commodity_stream: electrolyzer
       technologies: [wind, solar, electrolyzer]
+    # Approach B - "upstream LCOE feedstock": only the electrolyzer capital plus the
+    # purchased-electricity cost (electricity_feedstock priced at the LCOE above)
+    # are financed here. This should give an LCOH close to Approach A.
+    hydrogen_elec_feedstock:
+      commodity: hydrogen
+      commodity_stream: electrolyzer
+      technologies: [electricity_feedstock, electrolyzer]
+    # Approach C - "grid buy": like Approach B, but the electricity is purchased
+    # through a grid_buy interconnection priced at the LCOE above. This should also
+    # give an LCOH close to Approaches A and B.
+    hydrogen_elec_grid_buy:
+      commodity: hydrogen
+      commodity_stream: electrolyzer
+      technologies: [grid_buy, electrolyzer]
   cost_adjustment_parameters:
     cost_year_adjustment_inflation: 0.025  # used to adjust modeled costs to target_dollar_year
     target_dollar_year: 2022

examples/15_wind_solar_electrolyzer/run_wind_solar_electrolyzer.py

@@ -8,3 +8,25 @@
 model.run()
 
 model.post_process()
+
+# Compare three ways of financing the hydrogen produced by the same physical plant:
+#   Approach A ("same finance model"): wind + solar + electrolyzer financed together.
+#   Approach B ("upstream LCOE feedstock"): the electrolyzer buys electricity as a
+#       generic feedstock priced at the wind + solar LCOE.
+#   Approach C ("grid buy"): the electrolyzer buys electricity through a grid
+#       interconnection priced at the wind + solar LCOE.
+# Approaches B and C should both give an identical result for LCOH, which should be
+# close to the LOCH in Approach A.
+lcoe = model.prob.get_val("finance_subgroup_electricity.LCOE", units="USD/kW/h")[0]
+lcoh_integrated = model.prob.get_val("finance_subgroup_hydrogen.LCOH", units="USD/kg")[0]
+lcoh_feedstock = model.prob.get_val(
+    "finance_subgroup_hydrogen_elec_feedstock.LCOH", units="USD/kg"
+)[0]
+lcoh_grid_buy = model.prob.get_val("finance_subgroup_hydrogen_elec_grid_buy.LCOH", units="USD/kg")[
+    0
+]
+
+print(f"Upstream electricity LCOE:                      {lcoe:.4f} USD/kWh")
+print(f"LCOH - A: wind + solar + electrolyzer together: {lcoh_integrated:.4f} USD/kg")
+print(f"LCOH - B: electrolyzer + LCOE feedstock:        {lcoh_feedstock:.4f} USD/kg")
+print(f"LCOH - C: electrolyzer + grid buy at LCOE:      {lcoh_grid_buy:.4f} USD/kg")

examples/15_wind_solar_electrolyzer/tech_config.yaml

@@ -63,6 +63,42 @@ technologies:
       performance_parameters:
         commodity: electricity
         commodity_rate_units: kW
+  electricity_feedstock:
+    performance_model:
+      model: FeedstockPerformanceModel
+    cost_model:
+      model: FeedstockCostModel
+    model_inputs:
+      shared_parameters:
+        commodity: electricity
+        commodity_rate_units: kW
+      performance_parameters:
+        rated_capacity: 500000000.0  # kW
+      cost_parameters:
+        cost_year: 2022
+        price: 0.0  # USD/kWh - to be set through connections
+        annual_cost: 0.
+        start_up_cost: 0.
+  # Grid interconnection used purely to purchase electricity for the electrolyzer
+  # (Approach C). NOTE: the tech name MUST start with "grid_buy" for the financial
+  # logic to treat its variable cost as a purchase (expense) rather than a coproduct.
+  grid_buy:
+    performance_model:
+      model: GridPerformanceModel
+    cost_model:
+      model: GridCostModel
+    model_inputs:
+      shared_parameters:
+        interconnection_size: 5.e8  # kW - large so it never limits the electrolyzer's purchases
+      cost_parameters:
+        cost_year: 2022
+        # Purchase price is set through a connection to the wind + solar LCOE.
+        electricity_buy_price: 0.0  # USD/kWh - overwritten by the finance_subgroup_electricity.LCOE
+        buy_price_mode: per_timestep
+        # No interconnection capital/O&M charged here so only the energy cost matters.
+        interconnection_capex_per_kw: 0.0  # $/kW capital cost
+        interconnection_opex_per_kw: 0.0  # $/kW/year O&M cost
+        fixed_interconnection_cost: 0.0  # $ one-time fixed cost
   electrolyzer:
     performance_model:
       model: ECOElectrolyzerPerformanceModel

examples/test/test_all_examples.py

@@ -1322,6 +1322,28 @@ def test_wind_solar_electrolyzer_example(subtests, temp_copy_of_example):
             == 5.3063358423
         )
 
+    with subtests.test("Check LCOH from LCOE feedstock"):
+        assert (
+            pytest.approx(
+                model.prob.get_val("finance_subgroup_hydrogen_elec_feedstock.LCOH", units="USD/kg")[
+                    0
+                ],
+                rel=1e-5,
+            )
+            == 5.50083
+        )
+
+    with subtests.test("Check LCOH from grid buy"):
+        assert (
+            pytest.approx(
+                model.prob.get_val("finance_subgroup_hydrogen_elec_grid_buy.LCOH", units="USD/kg")[
+                    0
+                ],
+                rel=1e-5,
+            )
+            == 5.50083
+        )
+
     wind_generation = model.prob.get_val("wind.electricity_out", units="kW")
     solar_generation = model.prob.get_val("solar.electricity_out", units="kW")
     total_generation = model.prob.get_val("combiner.electricity_out", units="kW")

h2integrate/core/h2integrate_model.py

@@ -1670,8 +1670,16 @@ def connect_technologies(self):
                     )
 
             elif len(connection) == 3:
-                # connect directly from source to dest
                 source_tech, dest_tech, connected_parameter = connection
+                src_indices = None
+
+                # initialize src_indices to allow connections between different shaped variables
+                if isinstance(connected_parameter, list):
+                    connected_parameter, src_indices = (
+                        self._split_indices_from_connected_parameter_definition(connected_parameter)
+                    )
+
+                # connect directly from source to dest
                 if isinstance(connected_parameter, tuple | list):
                     source_parameter, dest_parameter = connected_parameter
                     # Check if this is a multivariable stream connection
@@ -1685,7 +1693,9 @@ def connect_technologies(self):
                         )
                     else:
                         self.plant.connect(
-                            f"{source_tech}.{source_parameter}", f"{dest_tech}.{dest_parameter}"
+                            f"{source_tech}.{source_parameter}",
+                            f"{dest_tech}.{dest_parameter}",
+                            src_indices=src_indices,
                         )
                 else:
                     # Check if the connected_parameter is a multivariable stream
@@ -1701,6 +1711,7 @@ def connect_technologies(self):
                         self.plant.connect(
                             f"{source_tech}.{connected_parameter}",
                             f"{dest_tech}.{connected_parameter}",
+                            src_indices=src_indices,
                         )
 
             else:
@@ -2245,3 +2256,87 @@ def _get_commodity_for_tech(self, tech_name):
         tech_commodities = [e[1] for e in self.techs_to_commodities if e[0] == tech_name]
 
         return tech_commodities
+
+    @staticmethod
+    def _split_indices_from_connected_parameter_definition(connected_parameter):
+        """Extract and parse slice indices from connected parameter definitions for OpenMDAO
+        connections.
+
+        This function processes parameter names containing slice patterns in square brackets
+        (e.g., "power[0:8760]") and generates OpenMDAO-compatible src_indices for connections
+        between variables of different shapes.
+
+        Args:
+            connected_parameter (list[str]): A two-element list containing:
+                - [0] source parameter name, optionally with pattern like "var[slice_spec]"
+                - [1] destination parameter name, optionally with pattern like "var[slice_spec]"
+
+                Example: ["power[0:8760]", "demand[:]"]
+
+        Returns:
+            tuple: A two-element tuple containing:
+                - connected_parameter (list[str]): The parameter names with slices removed
+                  (e.g., ["power", "demand"])
+                - src_indices: OpenMDAO slicer object for indexing source outputs to match
+                  destination input shapes. Returns om.slicer[slice] for indexing.
+
+        Note:
+            If the destination has a slice pattern, it must include the length ":N"
+            (e.g., "[0:N]"), the function extracts N as the destination length and
+            multiplies the source slice by this factor to create properly scaled indices.
+            The length is required because the length is not known in the OpenMDAO model
+            until prob.setup() has been called.
+        """
+        source_parameter, dest_parameter = connected_parameter
+
+        def _extract_slice(parameter):
+            """Return the contents inside the brackets (e.g. '0:8760'), or None."""
+            match = re.search(r"\[(.*)\]", parameter)
+            return None if match is None else match.group(1)
+
+        def _to_indices(spec):
+            """Convert a bracket spec string into a slice or list of ints."""
+            if ":" in spec:
+                return slice(*(int(p) if p.strip() else None for p in spec.split(":")))
+            return [int(p) for p in spec.split(",")]
+
+        source_slice = _extract_slice(source_parameter)
+        dest_slice = _extract_slice(dest_parameter)
+
+        if source_slice == dest_slice:
+            src_indices = None
+        elif dest_slice is not None and source_slice is not None:
+            # Tile the source indices to fill the destination length to handle shape
+            # mismatches. Examples:
+            #   source="0",   dest_length=8760 -> [0] repeated 8760 times
+            #   source="0,1", dest_length=10   -> [0, 1] cycled to fill 10 slots
+            if dest_slice.split(":")[0] not in ("", "0"):
+                raise ValueError(
+                    "A non-zero start was provided for the slice for destination "
+                    f"parameter <{dest_parameter}>"
+                )
+            dest_length = int(dest_slice.split(":")[-1])
+
+            source_indices = _to_indices(source_slice)
+            if isinstance(source_indices, slice):
+                source_indices = list(
+                    range(
+                        source_indices.start or 0,
+                        source_indices.stop,
+                        source_indices.step or 1,
+                    )
+                )
+
+            # Repeat the source values enough times to cover the destination, then
+            # truncate so the result is exactly dest_length long. This cycles through
+            # the source values when the source is shorter than the destination.
+            n_repeats = -(-dest_length // len(source_indices))  # ceiling division
+            src_indices = om.slicer[(source_indices * n_repeats)[:dest_length]]
+        else:
+            # No destination slice pattern; use source slice pattern directly.
+            src_indices = None if source_slice is None else om.slicer[_to_indices(source_slice)]
+
+        # Remove the slice patterns from parameter names to get clean names.
+        connected_parameter = [source_parameter.split("[")[0], dest_parameter.split("[")[0]]
+
+        return connected_parameter, src_indices