Clarification of MACRO equation COST_ENERGY

[Jihoon]

There have been questions raised for the Taylor formulation of Equation COST_ENERGY in MACRO.

• Internal discussion: https://iiasa-ece.slack.com/archives/CD0GBHHA4/p1637231296047600
• External user: https://github.com/iiasa/message_ix/discussions/615

The question is about how the second Taylor coefficient term is derived. We need to clarify how internally and elaborate the conclusion in the documentation.

https://github.com/iiasa/message_ix/blob/c497a5fd1993d2d63e8c2587b83764471de17d76/message_ix/model/MACRO/macro_core.gms#L273-L290 or as rendered in the docs:

[Jihoon]

The theoretical paper behind this: https://www.sciencedirect.com/science/article/pii/S0360544299000638