Qualitative explanations of OBLONG are given here: https://iea-etsap.org/wp-content/uploads...riants.pdf
> Is my reading of the trapezoidal-vs-rectangular distinction correct, and is there a documentation reference (rather than just the source) that describes it?
Yes. The trapezoidal investment accounting method causes economic inconsistencies in the model, because the activities (and available capacities) are indeed assumed rectangular (under OBJ=AUTO/STD/MOD), and therefore the trapezoidal incremental investments would not result in the investment costing being "in sync" with the activities or available capacities in each period. For example, with the trapezoidal method (which is applied when NCAP_ILED is not used) you would be often assumed to have parts of the new investments occurring even before the base year, increasing gradually up to the milestone year, even though the investment costs should in that case represent the lump-sum investment costs at the commissioning year (including interest during constriction). Similarly, the corresponding salvage value accounting fails to be consistent without using OBLONG, again due to the trapezoidal method. These inconsistencies are avoided when using NCAP_ILED, but then you will be missing the smooth cost trajectory (based on assuming incremental investments). OBLONG thus takes care of both: it results in consistent investment cost accounting and still uses a smooth cost trajectory over the assumed investment spread.
> Is exact reconciliation between reported lump-sum investment cost and INVCOST only guaranteed under OBLONG=NO, or is there a different switch/reporting option that lets me keep the recommended OBLONG=YES formulation while still getting a lump-sum figure that ties out cleanly to INVCOST?
I am not sure which kind of issues you see about "exact reconciliation between reported lump-sum investment cost and INVCOST" (and neither been able to verify such). So yes, a small test model reproducing and demonstrating the issue(s) you are seeing would indeed be very helpful, together with some notes pinpointing them ("there's a small but systematic discrepancy"), so that I can better respond to the question.
> Aside from the "few inconsistencies" that OBLONG=YES corrects, is there a downside to sticking with OBLONG=NO for this purpose, does it materially change the optimal solution itself (not just the reporting)?
As explained in the document, the downside is that the model solutions may be economically somewhat inconsistent, with those distortions occurring according to the lifetimes of technologies and the related periods definition over that lifetime, and can thus be different for each technology and may easily distort the true competitiveness of technologies. The document referred to above gives some illustrations about those distortions. The effect of OBLONG is in the objective function formulation, and not just reporting.
> Is my reading of the trapezoidal-vs-rectangular distinction correct, and is there a documentation reference (rather than just the source) that describes it?
Yes. The trapezoidal investment accounting method causes economic inconsistencies in the model, because the activities (and available capacities) are indeed assumed rectangular (under OBJ=AUTO/STD/MOD), and therefore the trapezoidal incremental investments would not result in the investment costing being "in sync" with the activities or available capacities in each period. For example, with the trapezoidal method (which is applied when NCAP_ILED is not used) you would be often assumed to have parts of the new investments occurring even before the base year, increasing gradually up to the milestone year, even though the investment costs should in that case represent the lump-sum investment costs at the commissioning year (including interest during constriction). Similarly, the corresponding salvage value accounting fails to be consistent without using OBLONG, again due to the trapezoidal method. These inconsistencies are avoided when using NCAP_ILED, but then you will be missing the smooth cost trajectory (based on assuming incremental investments). OBLONG thus takes care of both: it results in consistent investment cost accounting and still uses a smooth cost trajectory over the assumed investment spread.
> Is exact reconciliation between reported lump-sum investment cost and INVCOST only guaranteed under OBLONG=NO, or is there a different switch/reporting option that lets me keep the recommended OBLONG=YES formulation while still getting a lump-sum figure that ties out cleanly to INVCOST?
I am not sure which kind of issues you see about "exact reconciliation between reported lump-sum investment cost and INVCOST" (and neither been able to verify such). So yes, a small test model reproducing and demonstrating the issue(s) you are seeing would indeed be very helpful, together with some notes pinpointing them ("there's a small but systematic discrepancy"), so that I can better respond to the question.
> Aside from the "few inconsistencies" that OBLONG=YES corrects, is there a downside to sticking with OBLONG=NO for this purpose, does it materially change the optimal solution itself (not just the reporting)?
As explained in the document, the downside is that the model solutions may be economically somewhat inconsistent, with those distortions occurring according to the lifetimes of technologies and the related periods definition over that lifetime, and can thus be different for each technology and may easily distort the true competitiveness of technologies. The document referred to above gives some illustrations about those distortions. The effect of OBLONG is in the objective function formulation, and not just reporting.