Between On and Off: The Impact of Thermal Generator Start-up and Shutdown Representation in Long-Term Electricity Capacity Expansion Modeling

Abstract

This study evaluates how the representation of thermal generator start-up and shutdown behavior in capacity expansion modeling influences future power system investment decisions. Greater renewable energy penetration has increased operational variability in power systems because renewable output is not fully controllable, requiring generators to respond to more dynamic conditions. Although the transitional behavior of thermal generators is often simplified in capacity expansion models, the variability introduced by renewable resources calls for an examination of whether this simplified representation remains adequate for accurate grid planning. To address this question, a deterministic mixed-integer linear programming model is developed that simultaneously solves capacity expansion, unit commitment, and economic dispatch problems to minimize the total cost of a small utility-scale power system over a 20-year planning horizon. A baseline model formulation with simplified start-up and shutdown representation is first constructed, followed by a detailed transition formulation that incorporates path-indexed, multi-hour start-up and shutdown trajectories for thermal generating technologies. Comparing the two formulations isolates the effect of increased transition resolution on investment decisions, dispatch outcomes, cycling behavior, carbon emissions, and total system cost. The results show that the detailed transition formulation remains broadly consistent enough to be a suitable substitute for the baseline model, but it produces meaningful differences in operational and planning outcomes. In particular, the baseline formulation appears to overstate thermal flexibility, underestimate operational friction, and distort the resulting investment portfolio when compared with the detailed formulation. Overall, the findings demonstrate that start-up and shutdown behavior is an important modeling choice in capacity expansion analysis, and that greater transition detail can improve the credibility of long-term power system planning results.