  The transition towards decarbonized energy systems requires the expansion of
renewable and flexibility technologies in power sectors. In a model comparison,
we examine the optimal expansion of such technologies with six capacity
expansion power system models. The technologies under investigation include
base- and peak-load power plants, electricity storage, and transmission. We
define four highly simplified and harmonized use cases that focus on the
expansion of only one or two specific technologies to isolate their effects on
model results. We find that deviating assumptions on limited availability
factors of technologies cause technology-specific deviations between optimal
capacity expansion in models in almost all use cases. Fixed
energy-to-power-ratios of storage can entirely change model optimal expansion
outcomes, especially shifting the ratio between short- and long-duration
storage. Fixed initial and end storage levels can impact the seasonal use of
long-duration storage. Models with a pre-ordered dispatch structure
significantly deviate from linear optimization models, as limited foresight and
flexibility can lead to higher capacity investments. A simplified net transfer
capacity approach underestimates the need for grid infrastructure compared to a
more detailed direct current load flow approach. We further find deviations in
model results of optimal storage and transmission capacity expansion between
regions and link them to variable renewable energy generation and demand
characteristics. We expect that the general effects identified in our stylized
setting also hold in more detailed model applications, although they may be
less visible there.
