Add entry in SAM Economic Models docs

Author: softwareengineerprogrammer

docs/SAM-Economic-Models.md

@@ -22,6 +22,7 @@ The following table describes how GEOPHIRES parameters are transformed into SAM
 | `Net Electricity Generation`                                                                                                                    | AC Degradation                                         | `Annual AC degradation rate` schedule                                                                        | `Utilityrate5`                    | `degradation`                                                | Percentage difference of each year's `Net Electricity Generation` from `Maximum Total Electricity Generation` is input as SAM as the degradation rate schedule in order to match SAM's generation profile to GEOPHIRES                                                                                                    |
 | {`Total CAPEX` before inflation} × (1 + `Accrued financing during construction (%)`/100);                                                       | Installation Costs                                     | `Total Installed Cost`                                                                                       | `Singleowner`                     | `total_installed_cost`                                       | `Accrued financing during construction (%)` = (1+`Inflation Rate During Construction`) × 100 if `Inflation Rate During Construction` is provided or ((1+`Inflation Rate`) ^ `Construction Years`) × 100 if not.                                                                                                           |
 | `Total O&M Cost`, `Inflation Rate`                                                                                                              | Operating Costs                                        | `Fixed operating cost`, `Escalation rate` set to `Inflation Rate` × -1                                       | `Singleowner`                     | `om_fixed`, `om_fixed_escal`                                 | .. N/A                                                                                                                                                                                                                                                                                                                    |
+| `Royalty Rate`                                                                                                                                  | Operating Costs                                        | `Variable operating cost`                                                                                    | `Singleowner`                     | `om_production`                                              | Royalties are treated as a variable operating cost to the owner                                                                                                                                                                                                                                                           |
 | `Plant Lifetime`                                                                                                                                | Financial Parameters → Analysis Parameters             | `Analysis period`                                                                                            | `CustomGeneration`, `Singleowner` | `CustomGeneration.analysis_period`, `Singleowner.term_tenor` | .. N/A                                                                                                                                                                                                                                                                                                                    |
 | `Inflation Rate`                                                                                                                                | Financial Parameters → Analysis Parameters             | `Inflation rate`                                                                                             | `Utilityrate5`                    | `inflation_rate`                                             | .. N/A                                                                                                                                                                                                                                                                                                                    |
 | `Discount Rate`                                                                                                                                 | Financial Parameters → Analysis Parameters             | `Real discount rate`                                                                                         | `Singleowner`                     | `real_discount_rate`                                         | .. N/A                                                                                                                                                                                                                                                                                                                    |