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What is an Infeasible Model?

An infeasible model scenario occurs when a model cannot be solved because the constraints and inputs defined are mathematically or logically contradictory. In the context of Scenario Builder, this means the optimisation engine fails to find a valid solution that meets all the technical, economic, and policy constraints over the specified time horizon.

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Common Causes of Infeasibilities

Infeasibilities result from unrealistic or overly constrained model assumptions. Common causes include:
  1. Demand-supply mismatch
    • High demand growth that cannot be met by investment in generation
    • High peak demand which cannot be met by available dispatchable generation
  2. Over-constrained inputs
    • Emissions targets that are too strict: For example, setting net-zero emissions too early without allowing enough low-carbon technologies to be built or without enabling carbon removal.
    • Generation or capacity targets that exceed growth rates: If you require a high share of renewables without increasing their build rates or potentials, the model may not be able to meet demand.
    • Simultaneous targets that clash: For example, requiring high fossil generation and low emissions.
    • Growth rates that are too restrictive: If growth rates prevent the model from deploying needed capacity in time, it may be impossible to meet demand or targets.
    • Potentials are too low: If you limit how much of a technology (like solar or wind) can be installed, and then rely on that technology to meet a generation target, the model won’t have enough options.
    • Cost edits that discourage viable solutions: If clean technologies are made too expensive, or fossil fuels too cheap, the model may not be able to find a clean pathway within emissions limits.
    • No storage or dispatchable capacity available: If variable renewables dominate and no storage, hydro, or gas is allowed, the model may fail to balance the grid.
    • Fuel costs set to extreme values: Unrealistically low or high fuel prices may skew the model away from feasible combinations of supply.

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How to Resolve Infeasibilities

When you get an infeasible result, it helps to relax, adjust, and review your scenario inputs. Here’s a structured way to troubleshoot and resolve infeasibilities:
  1. Check for Conflicts
    • Relax tight emissions or capacity targets, especially in early years. Try intermediate targets and see if the model can find a solution.
    • Review overlapping targets. If you have multiple targets (e.g., capacity and emissions), try removing one and rerun.
  2. Loosen Constraints
    • Increase build rates for key technologies like wind, solar, batteries, or gas.
    • Increase renewable potentials if you’re limiting generation capacities.
  3. Review Assumptions
    • Double-check edited costs: Are renewables too expensive? Are fossil fuels unrealistically cheap? Are fuel prices too steep?
    • Balance between ambition and realism: Consider whether your inputs reflect plausible system development pathways.