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Infeasibility with Higher Carbon Tax
Dear VEDA Support Forum Members,

I am a new VEDA user and this is my first post, so please let me know how I can better ask this question. I am running a model that has both an all-sector carbon tax and a Zero Emission Vehicle mandate. I based the carbon tax on the Demo instructions and the ZEV mandate on the examples given in the forum thread called "Modeling investments and mandates." In my model, the mandate applies to new capacity for heavy-duty vehicles, requiring 30% of new capacity to be electric in 2030 and 100% to be electric in 2050, rising linearly. I started with a carbon tax of $25/ton in 2025 rising to $100/ton in 2050. I ran combinations of the tax and the mandate and all cases completed in about 25 minutes each. Then, I tried a higher tax of $100/ton in 2025 rising to $340/ton by 2050. I was able to run a scenario with this higher tax and no mandate, but I started to get issues when I tried the higher tax with the mandate. The model would run for 13 hours and then give up, reporting "intermediate infeasible." I am not sure where the conflict is, but hope that someone here might have an idea of what's wrong.

I've attached my ZEV mandate file, my carbon tax file, and my heavy-duty vehicle UC_COMPRD constraint file (I've set most constraints to 0 to clear room for electric technologies). Note that the model period is 2010-2077. The mandate covers TBS (school bus), TBT (transit bus), TC (commercial truck), TM (medium-duty truck), THS (heavy-duty short-haul truck), and THL (heavy-duty long-haul truck) sectors. Please let me know if there is other information I can provide.

Thank you very much!

Attached Files
.xlsx   Scen_UC_TRNHDV_MOU030322_forum.xlsx (Size: 53.52 KB / Downloads: 6)
.xlsx   Scen_UC_ZEV.xlsx (Size: 15.13 KB / Downloads: 5)
.xlsx   Scen_CO2_TAX2_AZ021422.xlsx (Size: 18.48 KB / Downloads: 9)
Welcome to the Forum.

The Forum has been mostly about technical issues, for which the developers of VEDA or other users can give their help, but more general modeling questions have also been posted.

From your description, I am not able to identify any technical issue related to the VEDA or TIMES software.  It sounds more like a numerical stability issue of the model, which may be tedious to resolve. Numerical problems, which occur in the solver during the solution, are often such that they may be appearing rather randomly, so that some scenarios solve well but others may end up in endless simplex iterations after crossover (when crossover is used).

From your description, I got the impression that you are indeed running the solver using barrier with crossover, and that the solver is getting "stuck" in the simplex iterations after crossover, and terminates perhaps in iteration or time-limit interrupt, is that correct? If that is so, I have also seen such issues earlier with models having numerical problems. The model status "intermediate infeasible" in such cases does not mean that the model has been proven infeasible. And surely, just changing some cost parameters (such as imposing higher carbon taxes) basically cannot make a feasible model infeasible, unless you have also defined some constraint for the costs/taxes.

As a first thing to do, I would suggest to check that you don't have any dummy imports active in the solutions you have for the combinations of the tax and the mandate. If there are any dummy imports, you should correct the model first, so that they are eliminated. Dummy imports may often cause some numerical problems, especially if you have defined very high costs for them.

For some other discussions about troubleshooting numerical problems, see for example the following:
Thank you for the suggestions, Antti-L. I will look into it and do some more reading.

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