IRREVERSIBILITY EVALUATION FRAMEWORK FOR SEASONAL THERMAL ENERGY STORAGE SYSTEMS AND MIXING PHENOMENA IN HOT WATER STORAGE RESERVOIRS

Abstract: Heat storage technology can address the issue of mismatch between the supply and demand of carbon-free thermal energy in energy systems in terms of time, space, or intensity. It is one of the key technologies for achieving carbon neutrality in building and urban energy systems. For district heating systems in urban regions of northern Chinese, the storage of various low-grade waste heat collected throughout the year using seasonal heat storage technology can fully meet the winter heating needs of northern cities and take advantage of the existing infrastructure of district heating networks. However, seasonal water heat storage (also known as hot water reservoir storage) has not yet achieved a technological breakthrough and large-scale application. One of the main reasons is that the fundamental principles of flow and heat transfer within ultra-large-scale heat storage water bodies are not clear, and a temperature-grade loss model resulting from the mixing of hot and cold fluids has not been established. This has led to a lack of sufficient theoretical tools for Chinese engineering researchers to carry out the design of ultra-large-scale water heat storage devices and accurately predict and evaluate the performance of water heat storage devices.
We are in urgent needs of conducting fundamental research in fluid dynamics and heat transport for large-scale seasonal heat storage technology and establish a thermal performance evaluation framework for seasonal heat storage technology, including water heat storage. To address the research needs mentioned above, this research report intends to discuss the key theoretical aspects of seasonal heat storage technology in the following five chapters:
Chapter One introduces the overview of seasonal heat storage technology, lists the main technological options and their key characteristics, and emphasizes the common problem of excessive temperature-grade loss in existing seasonal heat storage projects. This chapter points out that irreversibility of heat transport is a key reason for temperature-grade loss during heat storage. It also shows the limitations of existing indicators, such as the exergy efficiency, in evaluating temperature-grade loss characteristics. Furthermore, the chapter specifies the research needs for constructing an irreversibility evaluation framework for heat storage devices.
Chapter Two introduces the irreversibility evaluation framework in this report. First, it proposes a reclassification method for heat storage technology based on heat storage principles: fluid displacement-based and heat conduction-based heat storage. Also, the chapter compares and analyzes the interconversion and differences of entropy, entransy, and maximum work potential as evaluation parameters in describing irreversible internal heat transfer issues, with a focus on the advantages of using entransy and its dissipation in analyzing the irreversibility characteristics of heat storage. Furthermore, considering that heat storage processes are non-equilibrium and heat storage media are continuum, and heat and momentum transport phenomena are coupled, an irreversibility evaluation framework is proposed for describing the heat quantity parameter transfer and dissipation phenomena using partial differential equations.
Chapter Three presents the entransy dissipation analysis framework for seasonal heat storage media in details. First, based on the entransy balance principle in the heat storage media, entransy quantity efficiency is defined, and an evaluation method is designed to differentiate ideal and actual processes' heat quantity dissipation based on the heat transfer principles of water heat storage and ground source heat storage. In the chapter, the analytical solutions are derived for entransy dissipation in two categories of heat storage processes under ideal and actual conditions. In particular, the chapter presents a general expression for entransy dissipation in turbulent stratified flow based on turbulence modelling theory including the eddy viscosity models. The entransy dissipation levels of the two categories of heat storage technologies are compared. And the inherent reasons why fluid displacement heat storage is more suitable for long-term heat storage are provided. The chapter also specifies the engineering design direction of minimizing the intensity of cold and hot fluid mixing and the corresponding entransy dissipation levels.
Chapter Four introduces the flow and heat transfer phenomena of heat storage reservoirs, outlining the basic phenomena and governing equations from the two aspects of basic flow processes and heat transfer processes in heat storage reservoirs characterized by thermal stratification.
Chapter Five analyzes the mechanism of hot and cold fluid mixing in hot water reservoirs. Here, the chapter first analyzes the basic principles of heat and cold fluid mixing corresponding to transport phenomena and, through the discussion of physical phenomena and model de