To address the intermittent challenges of new energy and waste heat recovery as well as counteract the issues of corrosion and overcooling in phase-change materials, this study develops and investigates a medium-temperature phase-change capsule (PCC) through experiments and numerical simulations. The thermal cycle stability testing of the developed PCC, subjected to 150 cycles to evaluate its performance, demonstrated that the capsule’s surface remained intact with no signs of leakage. The enthalpy-porosity method combined with the volume-of-fluid method was used to establish a numerical model to simulate the phase-change process in capsules with two structures: one with a central cavity and the other with a top cavity. Results indicated that when using 304 stainless steel as the wall material for both structures, the PCC with the centrally located cavity melted 28.3% faster than that with the cavity at the top. When using different materials as wall coverings, the melting rate of the PCC made of polytetrafluoroethylene (PTFE) was 22.1% slower than that of the capsule made of 304 stainless steel. Conversely, the modified PTFE PCC melted 15.2% faster than the stainless steel-based PCC. Furthermore, when using PCCs having different diameters, the time differences for complete melting between the PTFE and stainless steel-based PCCs were 118 s and 66 s for the capsules having diameters of 24 mm and 16 mm, respectively, indicating that the time difference decreased with decreasing capsule diameter.

Solid particle heat storage technology offers a potential solution to the challenges posed by the significant growth of renewable energy sources, particularly in terms of grid security and stability. Consequently, it has the capability to optimize the energy utilization efficiency of the power system. In order to investigate the transport regulation characteristics of solid particles in the thermal storage and release system of a circulating fluidized bed (CFB), a test rig with a capacity of 0.1 MW (th) was established. This rig was utilized to systematically study the transport regulation characteristics of solid particles under the double U-type valve feed structure and U-type valve discharge structure. The experimental findings indicate that the system’s design enables efficient and rapid storage and release of solid particles in the CFB. The air distribution mode, specifically the double U-type valve feed structure and the U-type valve discharge structure, significantly influence the feed and discharge characteristics of the ash storage bin. It was observed that the impact of loose air on these characteristics is more substantial than that of the return air, irrespective of the feed structure or the return structure. When adjusting the feed and discharge rate, it is recommended to adopt a scheme that involves coarse adjustment through loose air and fine adjustment through return air.

Compressed air energy storage (CAES) is an important technology in the development of renewable energy. The main advantages of CAES are its high energy capacity and environmental friendliness. One of the main challenges is its low energy density, meaning a natural cavern is required for air storage. High-pressure air compression can effectively solve the problem. A liquid piston gas compressor facilitates high-pressure compression, and efficient convective heat transfer can significantly reduce the compression energy consumption during air compression. In this paper, a near isothermal compression method is proposed to increase the surface area and heat exchange by using multiple tube bundles in parallel in the compression chamber in order to obtain high-pressure air using liquid-driven compression. Air compression with a compression ratio of 6.25:1 is achieved by reducing the tube diameter and increasing the parallel tube number while keeping the compression chamber cross-sectional area constant in order to obtain a high-pressure air of 5 MPa. The performances of this system are analyzed when different numbers of tubes are applied. A system compression efficiency of 93.0% and an expansion efficiency of 92.9% can be achieved when 1000 tubes are applied at a 1 minute period. A new approach is provided in this study to achieve high efficiency and high pressure compressed air energy storage.

The widespread diffusion of renewable energy sources calls for the development of high-capacity energy storage systems as the A-CAES (Adiabatic Compressed Air Energy Storage) systems. In this framework, low temperature (100°C–200°C) A-CAES (LT-ACAES) systems can assume a key role, avoiding some critical issues connected to the operation of high temperature ones.
In this paper, two different LT-ACAES configurations are proposed. The two configurations are characterized by the same turbomachines and compressed air storage section, while differ in the TES section and its integration with the turbomachinery. In particular, the first configuration includes two separated cycles: the working fluid (air) cycle and the heat transfer fluid (HTF) cycle. Several heat exchangers connect the two cycles allowing to recover thermal energy from the compressors and to heat the compressed air at the turbine inlet. Two different HTFs were considered: air (case A) and thermal oil (case B). The second configuration is composed of only one cycle, where the operating fluid and the HTF are the same (air) and the TES section is composed of three different packed-bed thermal storage tanks (case C). The tanks directly recover the heat from the compressors and heat the air at each turbine inlet, avoiding the use of heat exchangers.
The LT-ACAES systems were modelled and simulated using the ASPEN-Plus and the MATLAB-Simulink environments. The main aim of this study was the detailed analysis of the reciprocal influence between the turbomachinery and the TES system; furthermore, the performance evaluation of each plant was carried out assuming both on-design and off-design operating conditions. Finally, the different configurations were compared through the main performance parameters, such as the round-trip efficiency.
A total power output of around 10 MW was set, leading to a TES tank volume ranging between 500 and 700 m3. The second configuration with three TES systems appears to be the most promising in terms of round-trip efficiency since the energy produced during the discharging phase is greater. In particular, the round-trip efficiency of the LT-ACAES ranges between 0.566 (case A) to 0.674 (case C). Although the second configuration assures the highest performance, the effect of operating at very high pressures inside the tanks should be carefully evaluated in terms of overall costs.

Compressed Air Energy Storage (CAES) is one of the methods that can solve the problems with intermittency and unpredictability of renewable energy sources. A side effect of air compression is a fact that a large amount of heat is generated which is usually wasted. In the development of CAES systems, the main challenge, apart from finding suitable places for storing compressed air, is to store this heat of compression process so that it can be used for heating the air directed to the expander at the discharging stage. The paper presents the concept of a hybrid compressed air and thermal energy storage (HCATES) system, which may be a beneficial solution in the context of the two mentioned challenges. Our novel concept assumes placing the thermal energy storage (TES) system based on the use of solid storage material in the volume of the post-mining shaft forms a sealed air pressure reservoir. Implementation of proposed systems within heavily industrialized agglomerations is a potential pathway for the revitalization of post-mine areas. The potential of energy capacity of such systems for the Upper Silesian region could exceed the value of 10 GWh. In the paper, the main construction challenges related to this concept are shown. The issues related to the possibility of storing air under high pressure in the shaft from the view of the rock mass strength are discussed. The overall concept of the TES system installation solution in the shaft barrel is presented. The basic problems related to heat storage in the cylindrical TES system with a non-standard structure of high slenderness are also discussed. The numerical calculations were based on the results of experiments performed on a laboratory stand, the geometry of which is to reflect the construction of a TES tank in a post-mining shaft. The article presents the results of numerical analysis showing the basic aspects related to difficulties that may occur at the construction stage and during the operation of the proposed HCATES system. The paper focuses on the methodology for determining the energy and exergy efficiency of a section of a Thermal Energy Storage tank, and presents the differences in the performance of this tank depending on its geometric dimensions, which are determined by the different sizes of mine shafts.

There is a critical need to develop advanced high-temperature thermal storage systems to improve efficiencies and reduce the costs of solar thermal storage system. In this work, two typical systems composed with Cu as matrix and Sn as the phase change material (PCM) are explored, namely, the 3-deimentional (3D) structure system by embedding Sn particles into Cu matrix and the 2-deimentional (2D) structure system by embedding Sn wires into Cu matrix. Given the thermophysical properties of a nanomaterial could be importantly different from that of a bulk one, we thus firstly derive the thermophysical properties of PCM and matrix theoretically, like the thermal conductivity by kinetic method and the specific heat capacity based on Lindemann’s criterion. And then, these properties are utilized to estimate the energy storage ability in both 3D and 2D structure system, and the influence of structure on heat transfer efficiency is theoretically investigated in both 3D and 2D structure system. Results turn out that 3D structure system is a better choice than a 2D structure system, because of larger specific surface area, a larger sensitive heat capacity and a larger thermal conductivity. When the feature size of the PCM decreases to be less than a critical value which is about 500 nm for Sn, the thermal conductivity of the system decreases exponentially while the heat storage capacity increases lineally. Moreover, when the feature size of Sn geometry is less than a critical value, which is 15 nm for 3D structure system and 25 nm for 2D structure, the Cu matrix can’t play a role in improving the effective thermal conductivity of the whole system.

As a variable-condition adjustment technology, the adjustable vaned diffusers (AVDs) can expand the working flow range of the compressor in the compressed air energy storage (CAES) system and improve its aerodynamic performance. In order to investigate the regulatory mechanism of AVDs and capture the details of vane loading distribution for the diffuser design optimization, additively manufactured AVDs for testing in a centrifugal compressor closed test facility are designed and implemented. Firstly, the regulation law of AVDs was summarized by numerical analysis and experimental support, and the corresponding vane loading data was extracted for the distribution law. Then, based on the distribution characteristics, 3D diffuser models were designed suitably for the adjustable components. Then, the laser selective melting (SLM) technology and die steel material 1.2709 were selected for metal printing according to the actual operating environment. Finally, performance testing and accuracy detection were performed on the finished test pieces, almost all inlet hole’s deviations were within the 0.3 mm tolerance. The research results indicated that additive manufacturing can significantly improve the accessibility of the internal flow channels of the diffuser, and derive the load of the blade on the pressure surface and suction surface in detail, also provide adjustable functions for variable operating conditions. It can not only break through the traditional processing bottleneck of the complicated internal flow channels of AVDs but also improve the design matching degree with adjustable components; simultaneously, it ensures high performance with high precision and effectively shortens the long lead time.