Subjects: Energy Science >> Engineering of Energy Sources System submitted time 2023-10-31
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
Peer Review Status:
Awaiting Review
Subjects: Energy Science >> Engineering of Energy Sources System Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2019-07-09
Abstract： The challenges of multi-area power system optimal dispatch exist on how to be in line with certain dispatching mode as well as on the modeling/solution. In this article, a multi-area system day-ahead scheduling, with a provincial systems-reducing method and a generation variances-minimizing objective, is proposed, based on the hierarchical dispatching organization and the long-term bilateral energy contract mode in China. Non-iteratively, the sub-optimal scheduling of ac/dc tie-lines and independent generation units can be derived from solving the model, which also considers some practical constraints like discrete constraints of dc power profiles and power flow limits of local ac interface, etc. Case study is carried out based on the realistic data from the multi-area hybrid ac/dc southern China power system, to verify the effectiveness and feasibility of the proposed model. It is demonstrated that with this method, the upper-level dispatching institution can generate an optimized and reliable transmission power plan with limited information, which decreases the peak-valley difference and standard variance of generation series, relieves the burden of peak regulation of its sub-systems, and hence improves the economic efficiency. The approach suits the dispatching mode of power systems in China well, handles various operation scenarios, and thus has been implemented in the system operator.
Peer Review Status:Awaiting Review
Subjects: Dynamic and Electric Engineering >> Engineering Thermophysics Subjects: Energy Science >> Engineering of Energy Sources System submitted time 2017-03-22 Cooperative journals: 《工程热物理学报》
Abstract：为了深入研究切变入流条件下大型风电机组尾流效应的非定常特性，以NREL 5MW大型海上风电机组为研究对象，建立了机组三维整机流场的全尺度结构化网格模型，基于滑移网格方法开展了风电机组额定工况下尾流效应的非定常数值模拟研究，并通过与NREL报告数据对比验证了模拟方法的可靠性。分析了风切变、机舱塔筒、叶片旋转等多种因素综合影响下风电机组性能参数的周期性变化规律，探讨了切变入流对尾流区风速分布的影响， 以及尾流涡系的结构与发展情况。
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