Chapter 5 - Thermal energy storage for enhanced building energy flexibility

Flexible engineering of advanced phase change materials

Thermal energy storage plays an important role in energy conservation and reducing CO2 emissions. Thermal energy storage involves sensible heat storage, latent heat storage and thermochemical heat storage. Compared with sensible heat storage, latent heat storage and thermochemical heat storage benefits of their high energy storage densities, which helps to reduce the initial cost of the construction of heat storage systems. However, the thermal conductivities of the phase change and thermochemical reaction materials are usually lower than 1 W m - 1 K - 1, which impedes the development and further applications of the corresponding energy storage systems. Porous materials, e.g. metal foams and expanded graphite, combining with other materials to form composites is an effective method for heat transfer enhancement. In this paper, the feasibility of using metal foams to enhance the heat transfer characteristics of heat storage materials in thermal energy storage systems was assessed. Heat transfer in solid/liquid phase change and thermochemical reaction of porous materials (metal foams and expanded graphite) was investigated. Organic commercial paraffin wax and inorganic calcium chloride hydrate were employed as the low-temperature materials, whereas sodium nitrate was used as the high- temperature materials in the experiment. Heat transfer characteristics of these PCMs embedded with open-cell metal foams and expanded graphite were studied. Composites of paraffin and expanded graphite with a graphite mass ratio of 3%, 6%, and 9% were prepared. The heat transfer performances of these composites were tested and compared with the results using metal foams. It is shown that heat transfer can be enhanced by adding these porous materials. Metal foams have better heat transfer performance due to their continuous inter-connected structures than expanded graphite. However, porous materials can suppress the effects of natural convection in liquid zone, particularly for PCMs with low viscosities, thereby leading to different heat transfer performances at different regimes (solid, solid/liquid, and liquid regions). This implies that porous materials do not always enhance heat transfer in every regime; thereby an optimal metal foam structure or expanded graphite fraction can be developed using PCMs for the overal thermal energy storage performance. For thermochemical heat storage systems, the feasibility of using metal foams to enhance the heat transfer capability of heat storage materials was assessed. Reversible reaction MgH2↔Mg+H2 was used as thermochemical heat storage reaction. The effective thermal conductivities of metal foams with various porosities (0.88–0.98) were estimated with Boomsma & Poulikakos model. A two dimensional mathematical model for the Mg/MgH2 system was estabilished to study the transient heat and mass transfer process. Heat release characteristics of chemical reaction in fixed beds with/without metal foams were compared to illustrate the effects of metal foams. Various factors influencing the reaction time for fixed reaction beds with metal foams were analyzed. The results show that metal foams shorten the reaction time and increase the output power by decreasing the average temperatures of the fixed beds. After adding metal foams with a porosity of 0.92, a 40% reduction of the reaction time and 60% promotion of the exothermic power can be achieved. The parametric study shows that there exists an optimal porosity of metal foams for the highest output power under a certain reaction condition. The cooling fluid temperature and hydrogen pressure are confirmed to have a more significant impact on the reaction rate when metal foams are embeded in fixed beds. In general, as heat transfer is coupled to phase change and chemical reaction processes in latent heat storage and thermochemical heat storage, the effects of porous materials on these heat storage systems are complex. The porous materials need to be carefully selected in order to optimizing the performance of heat storage systems.

A characteristic-oriented strategy for ranking and near-optimal selection of phase change materials for thermal energy storage in building applications

Liquid Thermo-Responsive Smart Window Derived from Hydrogel

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.

Thermal energy storage systems have gained increasing attention in recent years as effective solutions for improving energy efficiency and promoting sustainability. In this study, an eco-friendly phase change material (PCM) composite was developed by upcycling spent coffee grounds (SCG) into a value-added thermal energy storage material. SCG were employed as a natural supporting matrix, while a eutectic mixture of lauric acid (LA) and stearic acid (SA) with a mass ratio of 70:30 served as latent heat storage component. The composite was preparedby using a vacuum impregnation method. To determine the optimal PCM loading, the LA-SA content was varied between 10 and 70 wt%, and a maximum stable loading of 30 wt% was identified based on leakage performance. The leakage behavior, morphological structure, chemical composition, and thermal properties of composites were systematically investigated by leakage tests, FTIR, XRD, DTA/TG, and DSC analyses. 5%CuO@e-PCMC codedcomposite showed aneffective heat storage efficiency (E), relative thermal storage efficiency (μ), and enthalpy efficiency (λ) as 9%, 80%, and 0.1, respectively. Thermal analysis tests confirmed that the composite structure remained stable without significant degradation over repeated phase change cycles. The results demonstrated that the developed SCG-based PCM composite was a cost-effective and anenvironmentally friendly candidate for low-temperature thermal energy storage applications, particularly in 33-36°C range, making it well suited for textile and thermal comfort applications. The ability of SCG to stably accommodate up to 30 wt% of the LA-SA eutectic mixture highlighted their potential as a sustainable and natural alternative supporting material for thermal energy storage and thermal management systems.

Thermal energy storage (TES) systems using phase change materials (PCM) have been lately studied and are presented as one of the key solutions for the implementation of renewable energies. These systems take advantage of the latent heat of phase change of PCM during their melting/ solidification processes to store or release heat depending on the needs and availability. Low thermal conductivity and latent heat are the main disadvantages of organic PCM, while corrosion, subcooling and thermal stability are the prime problems that inorganic PCM present. Nanotechnology can be used to overcome these drawbacks. Nano-enhanced PCM are obtained by the dispersion of nanoparticles in the base material and thermal properties such as thermal conductivity, viscosity and specific heat capacity, within others, can be enhanced. This paper presents a review of the patents regarding the obtaining of nano-enhanced materials for thermal energy storage (TES) in order to realize the development nanotechnologies have gained in the TES field. Patents regarding the synthesis methods to obtain nano-enhanced phase materials (NEPCM) and TES systems using NEPCM have been found and are presented in the paper. The few existing number of patents found is a clear indicator of the recent and thus low development nanotechnology has in the TES field so far. Nevertheless, the results obtained with the reviewed inventions already show the big potential that nanotechnology has in TES and denote a more than probable expansion of its use in the next years.

Experimental Study on Thermal Energy Storage Performance of Water Tank with Phase Change Materials in Solar Heating System

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,

Novel strategies and supporting materials applied to shape-stabilize organic phase change materials for thermal energy storage–A review

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

Thermal Energy Storage Innovations for Tropical Buildings Through Materials Integration and Smart Cooling Strategies

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 (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.

Preparation of capric acid/halloysite nanotube composite as form-stable phase change material for thermal energy storage