cost assignment(s) might not update in myopic optimisation

[martacki]

Version Checks (indicate both or one)

• I have confirmed this bug exists on the lastest release of PyPSA-Eur.

• I have confirmed this bug exists on the current master branch of PyPSA-Eur.

Issue Description

If a technology (let's say nuclear, because this is where I noticed it) is not present after prepare_sector_network, but added in add_existing_baseyear, all following years never update the cost assumptions accordingly, because add_brownfield doesn't update them. Costs will always remain at the level of add_existing_baseyear.

Most techs exist both after prepare_sector_network and in/after add_existing_baseyear which is why this rarely causes an issue, but it does happen for nuclear and potentially other technologies.

Reproducible Example

for example, use a custom_costs.csv file to update nuclear investment costs for 2040, and start the myopic optimisation already in 2030.

Expected Behavior

Nuclear is not added in prepare_sector network, thus add_existing_baseyear uses the 2030 cost assumptions.

add_brownfield never updates costs, ultimately ignoring the adjusted costs for nuclear for 2040.

Installed Versions

it's not a versioning issue