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Underground Thermal Energy Storage (UTES) store unstable and non-continuous energy underground, releasing stable heat energy on demand. This effectively improve energy utilization and optimize energy allocation. As UTES technology advances, accommodating greater depth, higher temperature and multi-energy complementarity, new research challenges emerge. This paper comprehensively provides a systematic summary of the current research status of UTES. It categorized different types of UTES systems, analyzes the applicability of key technologies of UTES, and evaluate their economic and environmental benefits. Moreover, this paper identifies existing issues with UTES, such as injection blockage, wellbore scaling and corrosion, seepage and heat transfer in cracks, etc. It suggests deepening the research on blockage formation mechanism and plugging prevention technology, improving the study of anticorrosive materials and water treatment technology, and enhancing the investigation of reservoir fracture network characterization technology and seepage heat transfer. These recommendations serve as valuable references for promoting the high-quality development of UTES.

This paper introduces, describes, and compares the energy storage technologies of Compressed Air Energy Storage (CAES) and Liquid Air Energy Storage (LAES). Given the significant transformation the power industry has witnessed in the past decade, a noticeable lack of novel energy storage technologies spanning various power levels has emerged. To bridge this gap, CAES and LAES emerge as promising alternatives for diverse applications. The paper offers a succinct overview and synthesis of these two energy storage methods, outlining their core operational principles, practical implementations, crucial parameters, and potential system configurations. The article also highlights approaches to enhance the efficiency of these technologies and underscores the roles of thermal energy storage within their processes. Furthermore, it delves into the discussion of the significance of hybrid systems and polygeneration in the contexts of CAES and LAES technologies. Moreover, we briefly explore the potential integration of these technologies into other power systems.

The heat storage performance of latent heat storage systems is not good due to the poor thermal conductivity of phase change materials. In this paper, a new type of pointer-shaped fins combining rectangular and triangular fins has been employed to numerically simulate the melting process in the heat storage tank, and the fin geometry parameter effects on heat storage performance have been studied. The results indicate that compared with the bare tube and the rectangular finned tank, the melting time of the phase change material in the pointer-shaped finned tank is reduced by 64.2% and 15.1%, respectively. The closer the tip of the triangular fin is to the hot wall, the better the heat transfer efficiency. The optimal height of the triangular fin is about 8 mm. Increasing the number of fins from 4 to 6 and from 6 to 8 reduces the melting time by 16.0% and 16.7% respectively. However, increasing the number of fins from 8 to 10 only reduces the melting time by 8.4%. When the fin dimensionless length is increased from 0.3 to 0.5 and from 0.5 to 0.7, the melting time is shortened by 17.5% and 13.0%. But the melting time is only reduced by 2.9% when the dimensionless fin length is increased from 0.7 to 0.9. For optimising the design of the thermal storage system, the results can provide a reference value.

When considering the deployment of renewable energy sources in systems, the challenge of their utilization comes from their time instability when a mismatch between production and demand occurs. With the integration of thermal storages into systems that utilize renewable energy sources, such mismatch can be evened out. The use of phase-change materials (PCMs) as thermal storage has a theoretical advantage over the sensible one because of their high latent heat that is released or accumulated during the phase-change process. Therefore, the present paper is a review of latent thermal storages in hydronic systems for heating, cooling and domestic hot water in buildings. The work aims to offer an overview on applications of latent thermal storages coupled with heat pumps and solar collectors. The review shows that phase-change materials improve the release of heat from thermal storage and can supply heat or cold at a desired temperature level for longer time periods. The PCM review ends with the results from one of the Horizon2020 research projects, where indirect electrical storage in the form of thermal storage is considered. The review is a technological outline of the current state-of-the-art technology that could serve as a knowledge base for the practical implementation of latent thermal storages. The paper ends with an overview of energy storage maturity and the objectives from different roadmaps of European bodies.

Thermal Energy Storage Materials (TESMs) may be the missing link to the “carbon neutral future” of our dreams. TESMs already cater to many renewable heating, cooling and thermal management applications. However, many challenges remain in finding optimal TESMs for specific requirements. Here, we combine literature, a bibliometric analysis and our experiences to elaborate on the true potential of TESMs. This starts with the evolution, fundamentals, and categorization of TESMs: phase change materials (PCMs), thermochemical heat storage materials (TCMs) and sensible thermal energy storage materials (STESMs). PCMs are the most researched, followed by STESMs and TCMs. China, the European Union (EU), the USA, India and the UK lead TESM publications globally, with Spain, France, Germany, Italy and Sweden leading in the EU. Dissemination and communication gaps on TESMs appear to hinder their deployment. Salt hydrates, alkanes, fatty acids, polyols, and esters lead amongst PCMs. Salt hydrates, hydroxides, hydrides, carbonates, ammines and composites dominate TCMs. Besides water, ceramics, rocks and molten salts lead as STESMs for large-scale applications. We discuss TESMs’ trends, gaps and barriers for commercialization, plus missing links from laboratory-to-applications. In conclusion, we present research paths and tasks to make these remarkable materials fly on the market by unveiling their potential to realize a carbon neutral future.

Using thermal storage materials with excellent thermal properties in the energy utilization system enables efficient use of renewable energy sources. Organic phase change materials (PCMs) have the advantages of high heat storage density, no corrosion, and low cost, but low thermal conductivity and insufficient heat transfer capacity have always been the bottlenecks in their application. In this paper, melamine foam@ reduction graphene oxide (MF@rGO) and carbon foam@ reduction graphene oxide (CF@rGO) composite foams with double carbon networks were prepared by self-assembly method and further employed in 1-octadecinal (OD) PCMs. The microstructure, chemical composition, phase change behavior, thermal conductivity, and photothermal conversion performance of MF@rGO/OD and CF@rGO/OD were studied in detail using SEM, FTIR, Raman DSC, and LFA. The melting and solidification enthalpies of CF@rGO/OD composite PCMs were 208.3 J/g and 191.4 J/g, respectively, its thermal conductivity increased to 1.54 W/m·K, which is 6.42 times that of pure OD. The porous structure and high thermal conductivity of the double carbon network substantially enhance the efficiency of energy storage and release in composite PCMs. CF@rGO/OD composite PCMs have excellent heat storage performance and heat transfer capacity, and a wide range of application prospects in the fields of low-temperature solar heat storage, precision instrument temperature control, and intelligent buildings.

Based on the square hole and triangular hole type honeycomb ceramic thermal storage material, this paper experimentally studies the influence of hole type, length, gas flow rate and Reynolds number on the resistance loss characteristics, and further explores the conversion characteristics of VOCs in different bed temperatures. The results show that the square hole type honeycomb ceramic has a larger equivalent diameter and less resistance loss than the triangular hole type honeycomb ceramic under the same porosity. At the same flow rate, the gas has sufficient heat transfer in the triangular hole honeycomb ceramic disturbance, and the VOCs oxidation conversion efficiency is high. When the honeycomb ceramic length is greater than 50 cm and the Reynolds number is greater than 550, the pressure loss shows a good linear increase trend with the change of the material length. When the honeycomb ceramic thermal storage material length exceeds 70 cm, the inlet effect can be ignored. When the bed temperature of the two honeycomb ceramic thermal storage material with different pore types reaches above 750 °C, the outlet VOCs concentration is below 15 mg/m 3 , and the conversion rate reaches above 97%.

Steam natural gas reforming is the preferred technique presently used to produce hydrogen. Proposed in 1932, the technique is very well established but still subjected to perfections. Herein, first, the improvements being sought in catalysts and processes are reviewed, and then the advantage of replacing the energy supply from burning fuels with concentrated solar energy is discussed. It is especially this advance that may drastically reduce the economic and environmental cost of hydrogen production. Steam reforming can be easily integrated into concentrated solar with thermal storage for continuous hydrogen production. Steam methane reforming (SMR) is the favored technique to produce hydrogen, well established but subjected to perfections in catalysts and processes. Herein, the decomposition of CH4 in the presence of Ni on TiO2 support is shown. Especially replacing energy supply from burning fuels with concentrated solar energy is avenue to drastically reduce economic and environmental cost of hydrogen production by SMR.

A solar water heater (SWH) system is the most frequently used devices for utilization of solar energy. Thermal energy storage devices are required to improve the performance of the solar water heating system. This study investigates the influence of storage thickness on the performance of a collector that incorporates phase change material (PCM) into an absorber plate collector at the bottom for thermal storage. The approach utilized in this work is an experimental method, which involves conducting trials to determine the influence of storage thickness on the efficiency of solar water heater collectors utilizing paraffin wax as thermal storage medium. The experiments were conducted using variations in the thickness of the PCM, with a flowrate parameter of 10 liters/hour and a PCM thickness of 4 mm, 7 mm, and 10 mm, taking into account the inlet and outlet temperatures. The results of the study show a comparison of efficiency between standard collector plates with variations in the thickness of the PCM. The efficiency of the standard flat collector plate is 79.84%, while for plate efficiency with variations in PCM thickness of the 4 mm is 83.93%, for the collector plate efficiency of 7 mm is 81.29%, and for the collector plate efficiency of 10 mm is 82.30%.

The objective of this study is to demonstrate and validate the Dynamic Energy Transport and Integration Laboratory (DETAIL) preliminary scaling analysis using Modelica language system-code Dymola. The DETAIL preliminary scaling analysis includes a multisystem integral scaling package between thermal-storage and hydrogen-electrolysis systems. To construct the system of scaled equations, dynamical system scaling (DSS) was applied to all governing laws and closure relations associated with the selected integral system. The existing Dymola thermal-energy distribution system (TEDS) facility and high-temperature steam electrolysis (HTSE) facility models in the Idaho National Laboratory HYBRID repository were used to simulate a test case and a corresponding scaled case for integrated system HYBRID demonstration and validation. The DSS projected data based on the test-case simulations and determined scaling ratios were generated and compared with scaled case simulations. The preliminary scaling analysis performance was evaluated, and scaling distortions were investigated based on data magnitude, sequence, and similarity. The results indicated a necessity to change the normalization method for thermal storage generating optimal operating conditions of 261 kW power and mass flow rate of 6.42 kg/s and the possibility of reselecting governing laws for hydrogen electrolysis to improve scaling predictive properties. To enhance system-scaling similarity for TEDS and HTSE, the requirement for scaling validation via physical-facility demonstration was identified.