  To decarbonize the economy, many governments have set targets for the use of
renewable energy sources. These are often formulated as relative shares of
electricity demand or supply. Implementing respective constraints in energy
models is a surprisingly delicate issue. They may cause a modeling artifact of
excessive electricity storage use. We introduce this phenomenon as 'unintended
storage cycling', which can be detected in case of simultaneous storage
charging and discharging. In this paper, we provide an analytical
representation of different approaches for implementing minimum renewable share
constraints in models, and show how these may lead to unintended storage
cycling. Using a parsimonious optimization model, we quantify related
distortions of optimal dispatch and investment decisions as well as market
prices, and identify important drivers of the phenomenon. Finally, we provide
recommendations on how to avoid the distorting effects of unintended storage
cycling in energy modeling.
