Restructure the yaml file format to separate variable types from fuels

[danielhuppmann]

This PR is a first step towards implementing the proposal in #29, namely to separate the list of fuels from the variable definitions. This will make the yaml files more readable (because shorter) and reduce the potential for errors (because avoiding unnecessary repetition).

The key change: the variables for electricity generation by fuel types (including CCS subcategories) are generically given by Secondary Energy|Electricity|<Fuel> (rather than having to define the full list in energy-secondary.yaml) and the <Fuel> tag will be replaced by the list given in fuel_types.yaml when importing the Python package (so that the validation continues to work as expected). A test is added to make sure that the correct variable and description are available via the Python package.

This PR only changed this one instance (electricity generation) to elicit feedback - other variables can then be simplified in a similar manner after a first round of discussion, e.g., replacing numerous occurrences of Maximum Active Power|Electricity|... by just one variable Maximum Active Power|Electricity|<Fuel> in power-plant.yaml.

[erikfilias]

I propose two new sub-categories for Gas and Hydro, respectively. OCGT & CCGT for Gas. And, Run of River & Pumped Storage for Hydro.

[sandrinecharousset]

Added a few missing technologies

[danielhuppmann]

Thanks @sandrinecharousset and @erikfilias for the suggestions, I have merged them (with some minor editorial changes in the descriptions).

@openENTRANCE/coordination any other comments? (please do not add more suggestions of fuel types for now, this will be implemented via specific PRs later - but general comments on the proposed structure are welcome!)

[sandrinecharousset]

@danielhuppmann I have a question: how do you declare the variable in the data file when eg it is Coal without CCS?

[danielhuppmann]

Thank you for this question @sandrinecharousset because it shows that the proposed notation is not yet sufficiently intuitive. The idea is that every fuel in fuel_types.yaml that has the attribute ccs: True has two sub-categories.

So following your example, there would be three variables:

• Secondary Energy|Electricity|Coal (which is the total)
• Secondary Energy|Electricity|Coal|w/ CCS
• Secondary Energy|Electricity|Coal|w/o CCS

This is (tried to be) explained at the top of that file. Can you propose how to rename the attribute or extend the description to make this immediately obvious?

[sandrinecharousset]

Thanks @danielhuppmann
When reading again the file, I find it clear. But I propose to add just examples so that too-quick readers (such as myself) will catch it....

[danielhuppmann]

thank you @sandrinecharousset - I rewrote it to make it even clearer that we are talking about three variables, not just the two sub-categories. hope you agree!