ITC period fix [v3.8.11]

[softwareengineerprogrammer]

Description

This change applies ITC credit as revenue after construction rather than deduction of capital costs.

GEOPHIRES's existing ITC implementation is incorrect. It treats ITC credit as a negative capital expenditure in year 0 that is ultimately subtracted from total capital costs. For example Fervo_Project_Cape-3 actual total capital costs are $1532.78M, but are displayed/used in underlying calculations as $1072.95M via subtraction of the $459.83M ITC:

However, according to the convention used by SAM (which is presumably correct), ITC is treated as income the year after capital expenditure. This seems to make sense because taxes in general are filed and refunds received after the fiscal year in question, not during. Note the selected Federal ITC income box in year 1 in this screenshot from SAM:

This change raises LCOE for cases with ITC because the value of the credit is now discounted (like other future cash flows). For example Fervo_Project_Cape-3 LCOE increases by 35.7% from 2.77¢/kWh to 3.76¢/kWh. Other economic metrics are also negative affected: Project NPV (0.6%), IRR (10.3%), VIR=PI=PIR (24.7%), and MOIC (22%).

Testing & Validation

1. Regenerated example_ITC and Fervo_Project_Cape-3
2. Manual verification of SAM ITC discount convention based on after-tax NPV:
   1. Screenshot:
   2. Year 0 NPV = capex =-1,586,465,536.
   3. Year 1 total after-tax returns = 808,724,672
   4. Year 1 after-tax NPV = -845,467,968 ≅ -1,586,465,536 + 808,724,672 * {Year 1 discount factor ≅ 0.91628}

[softwareengineerprogrammer]

Note: I would ideally have preferred this to be to a .gov site but:

1. The original quasi-canonical government page seems to have been removed and now redirects to the DOE homepage: https://www.energy.gov/eere/solar/federal-solar-tax-credits-businesses
2. Existing .gov sites including https://www.energy.gov/sites/prod/files/2016/12/f34/Leveraging_Federal_Renewable_Energy_Tax_Credits_Final.pdf and https://www.epa.gov/sites/default/files/2018-08/documents/guide-purchasing-green-power-7.pdf point to https://programs.dsireusa.org/system/program/detail/658 so it appears to have some level of blessing as an official source of information
3. The DSIRE page links to relevant .gov pages as authorities:
4. ITC-specific .gov pages such as https://www.irs.gov/pub/irs-pdf/i3468.pdf are focused on specific tax form instructions which are an inappropriate/inaccessible level of detail in the context of GEOPHIRES documentation

(Also note the previous tooltip text erroneously instructed "see docs" when ITC is not discussed any existing documentation)

[softwareengineerprogrammer]

Rebased and closed in favor of #63