Net Billing: Credit expiry and same month credit application (Inflow/Outflow)

[berg-michael]

Iowa has an "Inflow/Outflow" regulatory scheme for distributed generation. I'm not familiar with other states but it appears Michigan has adopted a similar scheme (though I have not checked to see whether this PR conforms exactly to Michigan's standards).

A full description of this scheme can be found in the MidAmerican Tariffs under Rate IO.

The important parts are excerpted below, emphasis mine:

At the end of the monthly billing period:
• All inflow kWh will be summed and the Customer will be charged, at the
applicable tariffed rate for which the Customer is eligible, including
applicable adjustment clauses and riders, for the electricity delivered by the
Company.
• All outflow kWh will be summed and the Customer will receive an outflow
purchase credit, expressed in dollars, at the outflow purchase rate for
energy exported to the Company.
o The Customer may use the outflow purchase credits to offset any
applicable volumetric charges, including applicable adjustment
clauses and riders, billed on a kWh basis.
o Any unused outflow purchase credits may be carried forward,
expressed in dollars, and applied to subsequent billing periods to
offset any applicable volumetric charges, including applicable
adjustment clauses and riders, billed on a kWh basis, in those billing
periods until all outflow purchase credits are used or until the end of
the annual period or termination of service.
o At no time shall the Company be required to convert the outflow
purchase credits to cash.

It's possible current functionality of SAM allows for this kind of modelling, but if so, I missed it.

This PR includes two minor features that can be enabled only if Net billing with carryover to next month is selected under Metering and Billing in SAM.

1. The banked credits can expire at the end of the true-up period. Instead of a lump sum payout they are voided.
2. Any earned credit in a month can be applied to the energy charges of current month's bill (along with previously earned credit). After previously earned credits and any credit earned in the current month are applied to the energy charges, any excess rolls over to the next month (or is lost the current month is the end of the true up period.

These changes (IMO) make SAM much more useful for financial modelling under this regulatory scheme.

[brtietz]

@berg-michael Fantastic insight and addition! I agree that this is not a case that is handled in the current set of options in SAM. A few notes:

1. This PR adds variables, and is therefore more appropriate to merge into NREL:develop instead of patch. We can work towards incorporating this into our fall release, when input variable changes are viable.
2. I'm wondering if we need new variables here, or if we can just change the name of "ur_nm_credit_rollover" to handle both compensation types.

Can you send me an email at brian.mirletz (at) nrel.gov so we can discuss (2) on a call?