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As renewable energies are incorporated in larger shares in the electricity grid and district heating and cooling networks, the integration of storage solutions becomes more important. Thermal Energy Storage is an effective way to store heat and utilize the synergies between different energy carriers. Stratified storage tanks are a promising technology because of their low cost, simplicity and reliability. However, the modeling of the thermocline region in a stratified tank remains a challenge. There is a need to develop a fast and accurate 1D model for simulations and optimizations of TES. In this paper, a new discretization scheme is applied to the vertical axis of the storage tank. Orthogonal Collocation accurately represents the temperature profiles inside the storage tank with less points than the traditional multinode model, therefore running faster. Oscillations appear in the temperature profiles computed with orthogonal collocation if the thermocline represented is too steep and a low number of discretization points is used. But if a realistic thermocline is used as initial condition, the model performs well. Thus, it is appropriate to represent the real behavior of a storage tank, where uniform temperature conditions are avoided. Orthogonal Collocation on Finite Elements runs even faster and represents a good perspective for optimization studies. The model developed in this paper is validated with real data from a solar thermal plant with storage. A continuous and smooth model is also developed for natural convection inside the storage tank. The limitations of the model are discussed and perspectives on the modeling of natural convection for optimization models are given. • Orthogonal collocation is suggested for the 1D storage tank model discretization. • The proposed model runs faster than the multinode model for the same accuracy. • Orthogonal collocation on finite elements is suggested for optimization studies. • The storage tank model is validated with real plant data. • A continuous and smooth model for natural convection is developed. [ABSTRACT FROM AUTHOR]

Highlights: [•] A novel dynamic model for thermocline thermal energy storage systems is presented. [•] Adaptive grid technique used. [•] Accuracy is improved while using an order of magnitude fewer states and equations. [•] Effects of numerical diffusion are minimized. [•] Model more accurate under temperature (and density) inversion. [ABSTRACT FROM AUTHOR]

Strong convergence problems may arise during the dynamic simulation of the phase transitions of adsorption systems subjected to larger and/or random fluctuations of the boundary conditions, which do not follow the conventional cycle sequence (e.g., adsorptive thermal storage systems). This sequence could even be reversed at some instants and its stages not completed. Such convergence problems preclude solving those phase transitions, and thus the successful dynamic simulation of the adsorption systems. A set of numerical procedures for solving those problems is proposed, consisting of a small group of new conditions to be added to the conventional numerical algorithms. Its successful use is demonstrated in the dynamic simulation of a thermal energy storage system provided with an adsorption module, modeled in two distinct but coupled platforms (TRNSYS and MATLAB), for which the convergence difficulties are identified and described. Results indicate successful convergence by applying the proposed methodology, in contrast with the original algorithm, attesting for its capability to overcome the convergence problems that occur during the system's dynamic simulation. This set of numerical recipes can be adapted for other systems' algorithms, or to serve as a basis for its successful dynamic simulation, thus showing a promising and enlarged range of use. [ABSTRACT FROM AUTHOR]