Calculating selected years

Whether optimizing with perfect or limited foresight, you can restrict scenario calculations in NEMO to certain years only. To do so, specify a limited set of years in the calcyears argument for calculatescenario (or the calcyears key in your model's NEMO configuration file). This ensures that only those years are analyzed in the scenario's optimization problem. In particular, the problem's objective is to minimize discounted costs in those years. Note that if you don't supply a value for calcyears, all years in the scenario database are included in the optimization.

Calculating selected years offers a way quickly to generate results from large, complex models. While the results may not be identical to what you would get if you calculated all years, NEMO takes several steps to ensure they are a reasonable approximation. Specifically, when calculating selected years, NEMO:

• Calculates discounted investment costs for technologies and storage by distributing the costs over modeled (selected) and non-modeled years. Each modeled year is taken as the endpoint of an interval that starts after the prior modeled year (or at the beginning of the first year in the scenario database [the first scenario year], if there is no prior modeled year). For discounting purposes, investment costs incurred in a modeled year are assumed to be spread equally over the year's interval. NEMO's output variables for discounted technology and storage investment costs (vdiscountedcapitalinvestment and vdiscountedcapitalinvestmentstorage) reflect this adjustment.

• Calculates discounted transmission investment costs in the same way as discounted technology/storage investment costs if the continuoustransmission argument for calculatescenario is true. In this case, the output variable vdiscountedcapitalinvestmenttransmission includes the adjustment.

• Estimates discounted operation and maintenance (O&M) costs for technologies in non-modeled years. Fixed O&M costs are estimated by assuming that technology capacity evolves in a linear fashion between modeled years (or, if a modeled year's interval begins with the first scenario year, that there is a linear change from the residual capacity in that year to the capacity in the modeled year). Variable O&M costs are computed by assuming technology activity changes in a linear fashion between modeled years (or, if a modeled year's interval begins with the first scenario year, that the activity in the modeled year recurs in other years in the interval). Fixed and variable O&M costs are also assumed to remain constant between the last modeled year and the last year in the scenario database (the last scenario year). The output variable vdiscountedoperatingcost includes these adjustments.

• Similarly estimates discounted transmission O&M costs in non-modeled years. For exogenously specified lines (where yconstruction is defined), fixed O&M costs in non-modeled years depend on whether the line exists in those years. For candidate lines NEMO may build endogenously (yconstruction is not provided), fixed O&M costs in non-modeled years are contingent on continuoustransmission. If this argument is true, fixed O&M costs are calculated by assuming that the fraction built of each line evolves linearly between modeled years (the fraction built at the start of the first scenario year is 0). If continuoustransmission is false, fixed O&M costs are counted in non-modeled years if a line has previously been built. In both cases, fixed O&M costs of candidate lines are assumed to remain constant between the last modeled year and the last scenario year. Variable O&M costs in non-modeled years are estimated by assuming transmission activity changes in a linear fashion between modeled years (or, if a modeled year's interval begins with the first scenario year, that the activity in the modeled year also holds in other years in the interval). Variable transmission O&M costs are presumed to remain constant between the last modeled year and the last scenario year. All of these adjustments are included in the output variable vdiscountedoperatingcosttransmission.

• Estimates discounted technology emission penalties in non-modeled years. The penalties are computed by assuming that penalty amounts evolve linearly between modeled years (or, if a modeled year's interval begins with the first scenario year, that the penalty in the modeled year recurs in other years in the interval). For the period between the last modeled year and the last scenario year, penalties are assumed to remain constant. The output variable vdiscountedtechnologyemissionspenalty includes these adjustments.

All of these adjustments are also incorporated in NEMO's output variables that sum discounted costs: vtotaldiscountedcostbytechnology, vtotaldiscountedstoragecost, vtotaldiscountedtransmissioncostbyregion, and vtotaldiscountedcost. Note that salvage values for storage, transmission, and technologies are still based on asset value remaining in the last scenario year, so calculating selected years does not alter how they are computed. Whether or not selected years are calculated, NEMO discounts all costs to the first scenario year.

NEMO also makes a few other changes when you restrict calculations to selected years:

• Annual capacity addition limits for technologies (TotalAnnualMaxCapacityInvestment and TotalAnnualMinCapacityInvestment) and storage (TotalAnnualMaxCapacityInvestmentStorage and TotalAnnualMinCapacityInvestmentStorage) are scaled up by multiplying them by the number of years in each modeled year's interval.

• Energy in storage is not transferred between non-contiguous modeled years (i.e., if there is a gap between modeled years). This restriction applies even if none of the net zero attributes of storage (netzeroyear, netzerotg1, and netzerotg2) are enabled.

• The StorageLevelStart parameter is interpreted as the fraction of exogenous storage capacity that is charged at the start of the first modeled year.

• Activity in modeled and non-modeled years is taken into account when determining compliance with model period technology activity limits (TotalTechnologyModelPeriodActivityUpperLimit and TotalTechnologyModelPeriodActivityLowerLimit). Activity in non-modeled years is estimated by assuming that: 1) activity is constant in all years from the first scenario year to the first modeled year; 2) activity grows linearly between modeled years; and 3) activity is constant in all years from the last modeled year to the last scenario year. The output variable vtotaltechnologymodelperiodactivity includes activity in both modeled and non-modeled years.

• Emissions in modeled and non-modeled years are taken into account when determining compliance with model period emission limits (ModelPeriodEmissionLimit). Emissions in non-modeled years are estimated in the same way as activity in non-modeled years. The output variable vmodelperiodemissions includes emissions in both modeled and non-modeled years.

• The output variable vmodelperiodcostbyregion includes results for modeled years only.