Capacity margin penalty to CUR/kW #308
Description
Capacity margin penalty is at the moment CUR/MWh, which is actually quite confusing. It means that you can get 1 MWh of capacity margin for each hour of the year at the cost of the penalty. It means that it is very cheap penalty in comparison to other penalties also expresssed as CUR/MWh - especially when not modelling full year timeline. It is mixing operations with investment time scales. Capacity margin penalthy should therefore be equated with investments - CUR/kW because that's what one is essentially getting. The capacity margin penalty variable does not have time dimension (it has period dimension though).
It could be questioned whether the capacity margin penalty makes sense at all. As long as there is an investment opportunity, it should be cheaper and used by the model anyway. So, we could instead check that there is something the model can invest in - although this is complicated when the investment is not just about MW but has other constraints (VRE, electricity storage). The issue with the capacity margin penalty is that setting it to a high value can increase the coefficient range of the objective function. So, at least the user should be advised to use a relatively small value (but higher than true investments).