Due to the current trends aiming to reduce carbon dioxide emissions by increasing the use of renewable energy sources, changes are required in the operation of coal-fired steam units. The unstable nature of renewable energy sources, depending on weather conditions, means that the amount of energy produced varies and is not always in line with peak demand. To ensure the security and stability of energy supplies in the energy system, renewable sources should cooperate with units independent of environmental conditions. With conventional steam systems, the main issue of such energy storage applied to steam turbine units is presented in this article, which, in the event of a need for a sudden reduction of the system load, prevents overloading of the boiler and turbines, improving the safety of the system. This article presents a thermodynamic model of this energy storage. A zero-dimensional (0D) model was implemented, including the operating parameters of the unit. This model directly relates to the thermodynamic parameters defined at specific points of the thermodynamic cycle. Based on the 0D model, it was shown that the process of loading the energy storage with steam leads to a load reduction of up to 4%. Conversely, when discharging the stored energy, the net power of the steam block may increase by 0.4%. For more detailed analysis, a three-dimensional (3D) nonequilibrium with including cross effects approach was applied. This approach is based on flow models, with phase transitions that determine temperature fields, densities, and phase transition in relevant space, and is used for more accurate analysis. Here, we investigate the relationship between the 0D and 3D approaches in the context of steam storage. The combination of these two approaches is the fundamental novelty of this article.