Change the way the NPV appraisal tool works

[ahawkes]

Is your feature request related to a problem? Please describe

At the moment, the NPV appraisal tool uses a profitability index (for candidate assets) and total annualised surplus (TAS) for existing assets. Existing assets are always favoured over candidate assets. This means that existing assets never get decommissioned unless demand reduces. We need an approach in the NPV tool that treat all assets equally.

Describe the solution you'd like

Replace the two metrics currently used with a new one - the specific net annualised surplus (i.e. the net annual surplus per unit of demand served). This is defined as (assuming act is the output of the CoI):

SNAS = (sum(act_tAC_t)-AFCcap)/sum(act_t)

Describe alternatives you've considered

I also considered an approach where instead of dividing by sum(act), you divide by the derated capacity of the asset. i.e. derated capacity of a wind farm is its capacity multiplied by the time-weighted availabilities (I think this exactly equals 0.4 in the simple model). But I'm not sure this is a good approach, as it may favour processes that have low availability as peakers.