Include interest during construction (IDC) in cost model

[amanmajid]

Dear PyPSA-Eur Developers,

Firstly, thank you all for this excellent resource.

I'd like to suggest an enhancement to the cost model that is currently being applied in PyPSA-EUR. Currently, the model starts with overnight capital costs (OCC) and then annualises these to net present values by applying the annuity factor. However, I believe there is an important missing step here: before the OCCs are annualised, they should be converted to capital costs by factoring in the interest incurred during construction (IDC). It is important to include this step to account for the costs incurred in the construction phase. NREL do this by multiplying the OCCs by a construction finance factor, which is a function of the typical construction time of the specific technology, capital distribution during the construction phase and the IDC (see here).

@abhishek0208 has previously applied this in OSeMOSYS and I have been working with him to implement this enhancement in a fork of pypsa-eur. If you believe this enhancement would be a valuable addition to pypsa-eur, we would be happy to raise a PR (with some math) for your review.

Best,
Aman