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This article deals with a thermal energy storage system in the form of a water tank with a thermocline. The well-known thermocline phenomenon is modeled using computational fluid dynamics (CFD). However, the reservoir model proposed in this article is zero-dimensional. This is due to the fact that the aim of this article is to build a mathematical model that will be more useful in mathematical models of complex energy systems in which a hot water tank is one of many elements of the system. In such a zero-dimensional mathematical model, the hot water tank will be modeled using equations describing heat transfer, and the thermocline itself will be treated as a heat transfer surface with known dimensions and heat transfer coefficient. A novelty of this paper is that it addresses heat loss across the thermocline as defined in this manner. A CFD model of a thermal storage tank is created, validated with available experimental data, and used to obtain the heat transfer coefficient U. The resulting value is then analyzed quantitatively and qualitatively and the changes in the thickness of the thermocline are accounted for in the equation. The results from this groundbreaking work can be used to analyze heat storage in the form of thermocline water tanks at the level of system modeling, e.g., for the purpose of configuring the structure of other devices and control systems. [ABSTRACT FROM AUTHOR]

Summary: Growing installed capacity in renewable energy sources is driving demand for energy storage in the power systems. Compressed air energy storage (CAES) technology can provide a good alternative to pumped energy storage, with high reliability and good efficiency in terms of performance. The article presents three constant volume CAES systems: (i) without recuperation, (ii) with recuperation, and (iii) adiabatic. Dynamic mathematical models of these systems were built using Aspen HYSYS software. Adiabatic compressed air energy storage system with liquid thermal energy storage achieved round trip efficiency of 64.8%. [ABSTRACT FROM AUTHOR]

The article focuses on existing technologies developed to harvest and store solar irradiance as a source of primary energy in district heating systems. In the study particular attention was given to solar collector systems cooperating with seasonal heat storage solutions. Motivating this work was the search for a tool to assess achievable solar fraction ‒ the ratio of usable solar energy to total energy consumption. The outcomes of this work are: (i) a simplified dynamic model facilitating evaluation of the proposed solutions, and (ii) selection of optimal operating parameters of the proposed system. A brief review and discussion is made of the technical and legal limitations on possible investments. The ground rules of cooperation between solar based heat sources with seasonal storage systems and conventional industrial boilers and district heating network have been set out. The work touches on the physical components of the discussed systems. Commercially available solar collectors and heat exchangers are presented and their pros and cons discussed. Some in-depth analysis of seasonal heat storage solutions is provided, in particular on tank and pit thermal energy storage as well as storage solutions that use boreholes or aquifer layers. Examples are given of existing plants characterized by high solar fraction located in the EU region and outside of it. A simplified dynamic model developed in the Aspen Hysys software environment is described and the results discussed. Due to the high complexity of the primary problem, the model has been limited to a solar collector installation, seasonal heat storage system and auxiliary boiler. The results obtained from the model are discussed and future steps are presented in the last part of the article. These recommendations seek to further the development of an efficient way of analysis and commercial assessment of such systems. Image 1 • Dynamic model of solar heating plant with STES was developed. • Optimal operating parameters of the proposed system were selected. • Rules of cooperation between all components of the system have been set out. • Solar collectors and heat exchangers are presented with their pros and cons. • Results obtained from the model are discussed and future steps are presented. [ABSTRACT FROM AUTHOR]