Fuel price and annual fuel price change might not be implemented correctly

[Bachibouzouk]

Simulations of @a-linke and @KatLamm indicate some wierd behavior if the annual fuel price is not set to 0 and even if it is, then diesel is massively preferred in defavor of other technologies, which is not expected.

The only use of the input variabe FUEL_PRICE_CHANGE_ANNUAL is in the module D0:

offgridders/src/D0_process_input.py

Line 170 in 54d80b2

fuel_price_change_annual=experiment[FUEL_PRICE_CHANGE_ANNUAL],

It is then used in the function present_value_of_changing_fuel_price

In the function call (that I copied under for convenience):

        present_value_changing_fuel_price = economics.present_value_of_changing_fuel_price(
            fuel_price=experiment[FUEL_PRICE],
            project_lifetime=experiment[PROJECT_LIFETIME],
            wacc=experiment[WACC],
            crf=experiment[CRF],
            fuel_price_change_annual=experiment[FUEL_PRICE_CHANGE_ANNUAL],
        ) 

it seems that all arguments stem directly from input variables from the .xlsx file.
Whatever comes out of the function present_value_of_changing_fuel_price is then reassigned to the variable PRICE_FUEL (note that it is not FUEL_PRICE)

Looking into the function, one can see that if fuel_price_change_annual is set to 0, then what comes out of the function present_value_of_changing_fuel_price should be exactly equal to the what value was provided to the argument fuel_price

The variable PRICE_FUEL is then used to compute the EXPENDITURES_FUEL_ANNUAL (results):

offgridders/src/G3a_economic_evaluation.py

Line 393 in 3c3349c

EXPENDITURES_FUEL_ANNUAL: oemof_results[CONSUMPTION_FUEL_ANNUAL_L]

As well as the variable_costs of the fuel energy source:

offgridders/src/G2a_oemof_busses_and_componets.py

Line 97 in 90d2274

variable_costs=experiment[PRICE_FUEL]

It is given by
experiment[PRICE_FUEL]/ experiment[COMBUSTION_VALUE_FUEL]
experiment[PRICE_FUEL] has unit currency/unit and experiment[COMBUSTION_VALUE_FUEL] kWh/unit so the variable_costs have unit currency/kWh as expected (one unit is for example one gallon, or one liter)

[Bachibouzouk]

Apparently this was issue might be fixed by #92 which would indicate that the problem would be due to outdated version of oemof-solph for offgridders