Improvement of Phase Change Materials in Thermal Energy Storage Systems: A Comprehensive Review

Comparative analysis of single and paired metal hydrides based thermal energy storage system

Thermal energy storage (TES) system integrated with concentrated solar power provides the benefits of extending power production, eliminating intermittency issues, and reducing system LOCE. Infinia Corporation is under the contract with DOE in developing TES systems. The goal for one of the DOE sponsored TES projects is to design and build a TES system and integrate it with a 3 KWe free-piston Stirling power generator. The Phase Change Material (PCM) employed for the designed TES system is a eutectic blend of NaF and NaCl which has a melt temperature of 680° C and energy storage capacity of 12 KWh. This PCM was selected due to its low cost and desired melting temperature. This melt temperature ensures the Stirling being operated at designed operating hot end temperature. The latent heat of this eutectic PCM offers 5 to 10 times the energy density of a typical molten salt. The technical challenges associated with low cost molten salt TES systems are the low thermal conductivity of the salt and large thermal expansion. To address these challenges, an array of sodium filled Heat Pipes (HP) is embedded in the PCM to enhance the heat transfer from solar receiver to PCM and from PCM to Stirling engine. The oversized dish provides sufficient thermal energy to operate a 3KWe Stirling engine at full power and to charge up the TES. The HP arrays are optimally distributed so that the solar energy is transferred directly from receiver to Stirling engine heat receiver. During the charge phase, the Stirling engine absorbs and converts the transferred solar energy to electricity and the excess thermal energy is re-directed and stored to PCM. The stored energy is transferred via distributed HP from PCM to Stirling engine heat receiver during discharge phase. The HP based PCM thermal energy storage system was designed, built, and performance tested in laboratory. The TES/engine assembly was tested in two different orientations representing the extremes of system operation when mounted on sun-tracking dish, horizontal and vertical. Horizontal represents the zero elevation at sun rise and the vertical represents the extreme of solar noon. The testing allows the examination of orientation effect on the heat pipe performance and the maximum charge and discharge rates. The total energy stored and extracted was also examined. The areas for further system refinements were identified and discussed.

Recently, Concentrated Solar Power (CSP) is attracting more research attentions because it can store the excessive heat from the solar field and extend the power generation at night, CSP can also levelized the mismatch between energy demand and supply. To make CSP technology competitive, thermal energy storage (TES) system filled with energy storage media is a critical component in all CSP plant. TES system can be operated by using sensible materials, phase change materials (PCMs) or a combination of both. Because the phase change materials can store more heat due to the latent during the melting/freezing process, it becomes promising to use PCM in latent heat thermal energy storage (LHTES) system for large scale CSP application. Unfortunately, LHSS has relatively low energy storage efficiency compared to SHSS alone because of the fact that LHSS has more parameters to be controlled and optimized. To realize a complete utilization of PCM and a high energy storage/extraction efficiency and a high exergetic efficiency, one approach is to adopt a cascade configuration of multiple PCMs modules in TES tank, which can also be called as a cascade latent heat thermal energy storage (CLHTES) system. The melting temperatures of the PCMs placed in the TES tank should be cascaded from low to high temperature, where the latent heat of PCM can completely be used to absorb the heat from the solar field for energy storage purpose. Due to the complexity of a CLHTES system, it is necessary to provide a comprehensive study from the heat transfer perspective. This paper presents a preliminary parametric study of CLHTES system using a previously developed enthalpy-based 1D transient model for energy storage/extraction in CLHTES system. The effects of material properties (such as latent heat, specific heat at solid and liquid phase) and CSP plant operation conditions (such as charging/discharging time period) are to be explored. The results from the preliminary parametric study is expected to be beneficial to the community of solar thermal engineering.

This paper presents an overview of thermal energy storage (TES) materials and systems for storage applications. A TES system is composed of a storage medium (TES material), a heat exchanger and a storage tank. TES systems employ storage technology by heating/cooling a medium so that the stored energy can be used later in various applications. In recent years, TES systems have attained significant interest in the scientific community, finding multiple applications in air heating/cooling, water heating, buildings, and more. TES systems depend on capacity, power, efficiency, storage period, and cost. TES systems are divided into three main categories, depending on how the energy is stored: sensible systems (with hot water), systems using phase change materials (PCMs), and systems based on chemical reactions. Among these three types, PCM-based systems are outstanding in terms of both performance and cost-effectiveness. These advanced materials contribute to the conservation of heat and solar energy, as well as improving their efficient use. This paper addresses different aspects of PCMs utilization. The classification of PCMs is based on the thermophysical properties of composite PCMs, their methods of production, the main challenges associated with them, and the solutions to these challenges. The progress in creating more efficient TES systems and finding the appropriate PCMs is also reviewed.

Thermal energy storage technologies mainly include three types: sensible heat storage, latent heat storage, and thermochemical storage. These technologies are constantly improving in terms of materials, structures, and system design. The review by Quasi-Efahah and Okopako systematically summarizes the latest progress of various energy storage mechanisms, particularly emphasizing the cutting-edge developments in nano-enhanced phase change materials, hybrid energy storage systems, and intelligent integration strategies. The article also points out that the energy storage density of thermochemical storage is 300-600 kWh/m³, significantly higher than that of latent heat storage (100-150 kWh/m³) and sensible heat storage (25-80 kWh/m³); adding 1.0 wt% carbon nanotubes can increase the thermal conductivity of paraffin by 210%, machine learning optimization scheduling can reduce operating costs by 12%-18%; the "sensible heat + latent heat" hybrid configuration increases the storage density by more than 35%; digital twin applications can reduce the failure rate of energy storage units by 22%. This review provides a clear performance benchmark and evolution path for the next-generation TES technologies.Addressing the seasonal mismatch between renewable energy supply and building heat demand is one of the core challenges of current low-carbon heating systems. Schmidt et al. reported the first-of-its-kind pilot application of a thermal chemical energy storage system based on the calcium oxide/water reaction in a real building environment. This system provides stable thermal energy at 60°C, with a theoretical energy storage density of 450 kWh/m³. It can be directly connected to the existing building heating infrastructure and successfully replaces fossil fuel boilers. The technical maturity has been enhanced to TRL 5 level and a continuous operation zero-emission seasonal heating demonstration has been achieved.In areas lacking stable power supply, simple and reliable thermal energy storage systems are crucial for promoting clean cooking. Nydal et al. proposed a passive temperature control method based on natural oil circulation for a cooking thermal storage system. This system automatically breaks through the liquid seal barrier and starts the circulation when the oil temperature reaches 180°C, without the need for external control. At a heating power of 2.5 kW, the cooking platform temperature stabilizes at 200-220°C. A single charge can store 2.6 kWh of thermal energy, meeting the needs of two meals for a household. After completing 150 charge-discharge cycles, the temperature control deviation is less than ±5°C. It provides a highly robust and clean cooking solution for resource-constrained areas.Combining heat pumps with thermal energy storage systems can significantly enhance the system's energy efficiency and operational flexibility. Agalave and Kulkarni conducted experimental research on an integrated system that combines a heat pump using the environmentally friendly refrigerant R290 with a phase change material storage unit. The waste heat from condensation is recovered through shell-and-tube heat exchangers. The average storage power of the system is 3,481 W, the outlet temperature of the hot fluid is 63.3°C, and the total storage capacity is 28.6 MJ.The theoretical COP of the heat pump is 4.0, while the measured average COP is 2.6. This is the first time that the feasibility of the coordinated operation of low-GWP refrigerants and phase change energy storage has been verified at the system level.The research included in this special issue showcases the vitality and diversity of thermal energy storage technology at multiple levels, ranging from basic materials and system innovation to practical applications. Future research should continue to focus on addressing several key challenges: including further reducing the cost of storage materials and systems, enhancing cycle stability and service life, developing standardized and modular designs to facilitate large-scale deployment, and deepening the integration research of intelligent control strategies and multi-energy complementary systems. We look forward to promoting the significant role of thermal energy storage technology in building a sustainable, resilient, and inclusive global energy system through continuous technological innovation and interdisciplinary collaboration.The successful publication of this special issue would not have been possible without the outstanding contributions of all the authors, reviewers and the editorial team. We sincerely hope that these research results will stimulate greater attention from the academic community and the industrial sector towards thermal energy storage technology, and jointly accelerate its progress from the laboratory to large-scale application.

A Thermal Energy Storage (TES) system is meant for holding thermal energy in the form of hot or cold materials for later utilization. A TES system is an important technological system in providing energy savings as well as efficient and optimum energy use. The main types of a TES system are sensible heat and latent heat. A latent heat storage is a very efficient method for storing or releasing thermal energy due to its high energy storage density at constant temperatures, and a latent heat storage material can store 5-14 times more heat per unit volume than a sensible heat storage material can. Phase Change Materials (PCMs) are called latent heat storage materials. PCMs can save thermal energy, and use energy efficiently because PCMs can absorb thermal energy in the solid state, and the thermal energy can be released in the liquid state. Therefore, PCMs as new materials for saving energy can be applied into building applications. PCMs have been widely researched, but the current issues are lack of accurate and detailed information about thermophysical properties of PCMs to apply to buildings and inaccurate materials properties measured by existing methodology. The objective of this study is to develop a methodology and procedure to accurately determine the thermophysical properties of PCMs based on salt hydrates. TES systems of PCMs are measured and analyzed by various methods, such as DSC method and heat flow method. In addition, this study demonstrates to design a building roof with PCMs to save energy using Finite Element Analysis (FEA). The developed methodology is designed based on ASTM C1784-14, Standard Test Method for Using a Heat Flow Meter Apparatus for Measuring Thermal Storage Properties of Phase Change Materials and Products, for measuring the thermal energy storage properties of PCMs. The thermophysical properties and thermal stabilities are evaluated by using a Differential Scanning Calorimetry (DSC), which is made with DSC Q 200 equipment from TA Instruments and DSC STA 8000 equipment from Perkin Elmer Company. The thermal conductivities are assessed by heat flow meter, which is FOX 314 equipment from TA Instruments, and the enthalpy changes of the PCMs are determined by DSC method and heat flow method. Numerical FEA to evaluate potential energy savings is conducted using ABAQUS software. Four types of Phase Change Materials (PCMs), which have phase changes at 21ºC, 23ºC, 26ºC, and 30ºC, respectively, are used for measuring the thermophysical properties. The onset/peak temperature,

Solar-powered compact thermal energy storage system with rapid response time and rib-enhanced plate via techniques of CFD, ANN, and GA

Compared to Solar Photovoltaics (PV), Concentrated Solar Power (CSP) can store the excess solar thermal energy, extend the power generation at night and cloudy days, and levelize the mismatch between energy demand and supply. To make CSP competitive, Thermal Energy Storage (TES) system filled with phase change material (PCM) is a promising indirect energy storage technique, compared to the TES system using concrete or river rocks. It is of great interests to solar thermal community to apply the latent heat thermal energy storage (LHTES) system for large scale CSP application, because PCMs can store more thermal energy due to the latent heat during the melting/freezing process. Therefore, a comprehensive parametric analysis of LHTES system is necessary in order to improve its systematic performance, since LHTES system has a relatively low energy storage efficiency compared to TES systems using sensible materials. In this study, an 11-dimensionless-parameter space of LHTES system was developed, by considering only the technical constraints (materials properties and operation parameters), instead of economic constraints. Then the parametric analysis was performed based on a 1D enthalpy-based transient model, and the energy storage efficiency was used as the objective function to minimize the number of variables in the parameter space. It was found that Stanton number (St), PCM radius (r), and void fraction (ε) are the three most important ones. The sensitivity study was conducted then based on the three dimensionless-parameter space which will significantly influence the system performance. The results of this study make LHTES system competitive with TES system using sensible materials in terms of energy storage efficiency.

This paper presents an energy performance and feasibility study of Thermal Energy Storage (TES) System compared with direct supply system of chilled water to an engineering complex in UiTM. In principle, TES system is a load management technology which shift load from peak to off-peak utility periods thus reduces maximum demand of electrical needs during the day and takes advantage of the low off peak tariff offered by the utility company (TNB). Data of a month's load profile was analysed and it was found that the TES system consumed more energy by 27% compared to the direct supply system of chilled water but its maximum demand reduced by 23%. The estimated cost (RM) per day was less by only 4%, under no energy loss, due to the introduction of thermal energy storage system compared with the direct system. Under the current electricity tariff, TES system in Malaysia is not feasible economically. However, if the maximum demand electrical tariff is revised the TES system can be attractive.

This study focuses on the development and testing under lab-controlled conditions of a hybrid sensible–latent thermal energy storage (TES) system for domestic hot water (DHW) provision in residential buildings. The TES system’s design is based, for the first time in the literature, on a commercial tank-in-tank architecture integrating a macro-encapsulated commercial phase change material (PCM) inside the external tank to guarantee the safe provision of DHW and increase overall energy storage density at a reasonable cost. The PCM is a salt hydrate with a nominal melting temperature of 58 °C. The overall tank-in-tank TES volume is about 540 dm3. Almost one tenth of this volume is occupied by the PCM macro-capsules. The developed TES system was comparatively tested against the same configuration operated as a sensible TES. The obtained results showed the ability of the PCM to increase the thermal inertia inside the external tank, thus guaranteeing the quite stable provision of heat to the integral DHW tank during the stand-by periods. This effect was confirmed by the PCM’s ability to achieve an energy storage capacity up to 16% higher than the reference sensible TES system.

This article reviews recent research on phase-change materials (PCMs) used in thermal energy storage systems with the aim of enhancing their performance. The study explores various methods to improve heat transfer in PCMs, such as microencapsulation, infill materials, fins and nanofluids. Additionally, it evaluates techniques to boost heat transfer in latent heat thermal energy storage (LHTES) systems and investigates ways to increase thermal conductivity using porous and low-density materials. PCMs store thermal energy, making them suitable for use in solar energy systems when solar energy is not available. The need for eco-friendly alternatives to conventional heating and cooling in global construction and the significant energy consumption of buildings has driven research on this topic. As such, this study additionally examines current advancements in free cooling systems with latent heat storage to identify the key factors affecting their effectiveness. The findings show that using PCMs for overnight cooling maintains the room temperature within the comfort zone and reduces the cooling loads in various climates. Using machine learning methods, this study also compares recent advancements in the use of PCMs in various solar energy systems, including solar thermal power plants, solar air purifiers, solar water heaters and solar appliances. Results derived feature key factors crucial for the optimal selection of PCMs and the challenges associated with sustainable green transitions in built environments. HIGHLIGHTS This review provides an in-depth examination of PCM thermal energy storage systems. This study investigates the use of fillers, nanofluids, and nanoparticles for enhanced heat transfer. Analytical methods for boosting the PCM thermal conductivity are briefly outlined. Geometric design and thermal conductivity enhancement affect the Latent Heat Thermal Energy Storage (LHTES). The assessment also evaluates advanced free-cooling systems using the LHTES. PCMs help to store heat energy in solar systems and bridge supply gaps. GRAPHICAL ABSTRACT

Energy sustainability is the cornerstone to the health and competitiveness of the industries in our global economy. It is more than being environmentally responsible, means the ability to utilize and optimize multiple sources of secure and affordable energy for the enterprises, and then continuously improve the utilization through systems analysis, energy diversification, conservation, and intelligent use of these resources. Distributed energy resources (DER) and dispersed generation systems are becoming more important in the future electricity generation. A description of distributed energy resource and types, characteristics, performances, is the subject of this chapter. Brief presentations of the power system interfaces, power electronics, and control of distributed generation systems are also included. The chapter presents an overview of the key issues concerning the integration of distributed and dispersed generation systems, the role of thermal energy storage (TES) systems and the main applications. A synopsis of the main challenges and issues that must be overcome in the process of DG and DER applications and integration are presented. Particular emphasis is placed on the need to move away from the fit and forget approach of connecting DG to electric power systems to a policy of integrating DG into power system planning and operation through active management of distribution networks and application of other novel concepts. Several distributed energy systems, together with energy storage capabilities, expected to have a significant impact on the energy market are presented and discussed. Microgrid is a new approach of power generation and delivery system that considers DG, DER, and loads, often controllable loads is set as a small controllable subsystem of a power distribution network. The microgrid subsystem has characteristics, such as the ability to operate in parallel or in isolation from the electrical grid, having the capabilities and functionalities to improve service and power quality, reliability, and operational optimality. Microgrids may also be described as a self-contained subset of indigenous generation, distribution system assets, protection and control capabilities, and end user loads that may be operated in either a utility connected mode or in an isolated from the utility mode. In addition to providing reliable electric power supply, microgrids are also capable of providing a wide array of ancillary services, such as voltage support, frequency regulation, harmonic cancellation, power factor correction, spinning, and nonspinning reserves. A microgrid may be intrinsically distributive in nature including several DGs-both renewable and conventional sourced energy storage elements, protection systems, end user loads, and other elements. In order to achieve a coordinated performance of a microgrid (or several microgrids) within the scope of a distribution company, it is required to perform distributed or cooperative control. This harvested energy through such applications can be released onto the grid, when needed, to eliminate the need for high-cost peak generators or can be used local for heat and hot water or other industrial process applications. Microcombined heat and power (CHP) systems powering up to about 10 kWe are considered as a future key technology for the building or facility energy supplies from the viewpoints of heating system users, manufacturers, and energy suppliers. CHP plants can be based on conventional diesel, gas or biomass engines, gas turbines, Stirling engines, or fuel cells. Energy storage systems are an important component of the renewable energy technology applications. Among the storage technologies, the TES, a technology that stocks thermal energy by heating or cooling a storage medium and use the stored energy at a later time for heating, cooling and power generation. TES systems are used particularly in buildings and in industrial processes, while the main advantages of using TES in an energy system, building or industrial process include an increase in overall efficiency and better reliability, leading to the reductions in investment and running costs, and less environmental pollution of the environment. Energy storage inclusion into distributed generation systems provides the user dispatchability of DER, while improving the overall system performances and capabilities. All of the DER and DG technologies require specific power electronics and control schemes to convert the generated power into useful power that can be directly interconnected with the grid or that can be used for specific applications. This chapter presents convenient resources to understand the current state-of-the art power electronic interfaces for DER and DG applications. In this chapter, a description of TES systems and microCHP generation systems is presented with references to heating, ventilation, and air conditioning systems. A discussion on the major components of such systems, load analysis and methods for improving the energy efficiency of existing systems are also included in this chapter. After completing this chapter, the readers are able to understand the importance and role of the thermal energy systems and storage, energy conservation and efficiency in building electrical and mechanical systems, and in industrial energy systems and equipment. A special attention is given to the understanding and learning about micro-CHP generation systems, components and configurations of such systems, their operation, functions, and capabilities.

A mathematical model describing the water flow and convective/conductive thermal energy transport in an ATES system is presented. The three-dimensional thermal process with combined groundwater and heat flow in the aquifer and heat conduction in surrounding layers is solved numerically. This paper presents the results of long-time thermal behavior of ATES system with two wells under continuous operation methods. The effects of various injection-withdrawal rates and durations on computed values of aquifer thermal behavior and final producing temperature were studied for a 10-year continuous injection and withdrawal. The hypothetical simulations indicate that the model of the twowell system will be a valuable tool in determining the most efficient system operation. The thermal behavior of the storage system is shown to depend on the aquifer’s volume relative to energy input and flow pattern of the water. Various operational and geometrical parameters including operation schedules, injection temperature, injection/production rates, and geometrical configuration of well and aquifer impact the predicted recovery water temperature. Small variations in injection temperatures, low flow rate, and large surface to volume ratio are recommended as an effective ATES because of small loss and little fluctuation in extracted thermal energy. However, aquifer thickness and hydraulic anisotropy have a minimal effect on the performance of ATES systems.

The intermittent nature of solar radiation requires a thermal energy storage (TES) system for reducing the mismatch between energy demand and supply. Solar water heating (SWH) systems can help save up to 90% of the utilized energy for water heating. In this study, a compound parabolic concentrator (CPC) solar collector has been coupled to three different configurations of TES system. A comprehensive analysis on the effects of PCMs arrangements in TES systems viz three PCMs (case 1) and five PCMs (case 2) on the energy efficiency, exergy efficiency, and overall loss coefficient of the solar collector and TES system has been made and compared with sensible TES system. An experimental data showed an augmented energy storage of 12% and 41% in "case 1" and "case 2" over sensible TES system as a result of reduction in heat losses with the cascaded arrangement of PCMs. The collector paired with case 2 configuration clearly exhibited a higher exergy efficiency due to supply of heat transfer fluid at relatively lower temperature while compared to other TES configurations. The outcomes of this study reveal the key role of cascaded arrangement of PCMs for enhancing energy and exergy efficiencies of solar collector.

The power block for a conventional Concentrated Solar Power (CSP) Plant without thermal storage follows standard power block design practices. A closed loop heat transfer fluid (HTF) is heated in the solar field, which consists of multiple solar collector assemblies (SCAs). Heat exchangers use the heat from the HTF to generate and superheat steam. The steam is sent to a steam turbine, which generates electricity. The cooled HTF is recirculated back to the solar field. In an effort to shift the period of power generation or to maintain full power output during non-peak periods of operation, a thermal energy storage (TES) system can be added. This entails adding a second closed loop fluid that is heated by the HTF during sufficient radiation hours, which in turn can heat the HTF that is supplied to the power block during periods of non-peak radiation. This article discusses the process control and design issues for the integrated solar field, TES system and power block for these plants. The article will address the following: 1) Operations with the Solar field on-line, TES system off-line, and STG on-line. 2) Operations with the Solar field on-line, TES system charging, and STG on-line. 3) Operations with the Solar field on-line, the TES system discharging, and STG on-line. 4) Operations with the solar field off-line, the TES system discharging, and the STG on-line. 5) Operations with the Solar field on-line, the TES system charging, and STG off-line. 6) Steam Turbine Issues. 7) Freeze protection. 8) HTF/TES Heat Exchanger. 9) Circulating Water and Surface Condenser.