Salt hydrates are a promising class of thermochemical energy storage materials (TCMs) for heat storage applications. However, their practical deployment is hindered by severe particle agglomeration, especially at large scale, which limits cyclability and leads to performance degradation in conventional packed-bed reactors. Here, we propose and experimentally validate a novel lattice-structured reactor bed designed to overcome these limitations. The engineered lattice geometry promotes uniform mass transport and mitigates bulk agglomeration, resulting in significantly improved long-term stability and energy performance. Compared to a traditional packed bed, the lattice-structured configuration maintains over 98% of its initial thermal output after 10 hydration–dehydration cycles, whereas the packed bed retains less than 50% under the same operating conditions. The lattice system delivers a thermal energy output of up to 0.13 MJ after 10 cycles, 76.8% higher than that of the packed bed. Remarkably, after 15 cycles, the lattice bed exhibited an 8% increase in energy release relative to the initial cycle, indicating self-optimizing behavior over time. Micro X-ray computed tomography (micro-CT) analysis revealed no evidence of macroscopic agglomeration within the lattice-structured bed. However, minor cracking was observed between unit cells at the base of the reactor. Subsequent computational fluid dynamics (CFD) analysis revealed that these cracks, along with increased porosity around lattice walls, redistribute vapor flow, activate previously stagnant regions, and promote more homogeneous water uptake throughout the bed. These findings demonstrate the potential of lattice-structured reactor designs to enable durable, efficient, and scalable thermochemical energy storage solutions, supporting the broader integration of renewable energy systems.

Heating and cooling account for half of global energy demand, making efficient thermal energy storage (TES) central to decarbonization. Phase change materials (PCMs) can store large amounts of heat, but their low thermal conductivity slows charging and discharging, limiting system performance. To overcome this, metallic fins and foams have been traditionally used to accelerate heat transfer, yet these add complexity, cost, and durability challenges. Here, we explore an alternative pathway: embedding recycled ceramic particles into PCMs to enhance heat transfer while avoiding corrosion and stability issues. Using a lab-scale TES unit, we test composites with different ceramic particle sizes and show that finer particles accelerate charging, enable faster discharge, and suppress overheating compared with coarser particles. This simple modification improves PCM cycling efficiency and reliability.

The effectiveness of salt hydrate particles as thermochemical materials is significantly hindered by agglomeration and volume variation. To address these challenges, this study adopts a particle-level approach, focusing on the dynamic processes of swelling and shrinkage within potassium carbonate particles during cycling. Through insitu measurements within a micro-climate chamber, the size variations of the material under different operating conditions are closely monitored. The analysis of the obtained images allowed for the calculation of the equivalent diameter of salt grains. Additionally, Micro-CT scans were employed to assess changes in particle porosity. The experiments are conducted under varied conditions, such as maintaining a constant temperature while adjusting the relative humidity and maintaining a constant relative humidity while increasing temperature. The results revealed both an increased equivalent particle diameter and porosity. This indicates increased hydration and thermal expansion, respectively. The findings emphasize the sensitivity of the material to environmental conditions. They highlight the material's ability to undergo significant structural changes in response to variations in temperature and humidity. When a large force, mimicking a real packed bed system, is exerted on the grains, agglomeration is observed, contrary to the absence of force under the same operating conditions. This finding is significant for explaining agglomeration in a packed bed system. The results are an initial step towards understanding agglomeration at the particle level. They illuminate the interplay between environmental conditions, structural changes, and cohesive forces within these materials.

In this article a dual heat storage system comprising thermochemical heat storage (TCS) and hot water storage for managing the mismatch between heat generation and demand in district heating systems (DHs) is evaluated. TCS is known as technology suitable for long-term heat storage due to its high energy density and negligible heat losses over a longer period. However, the integration of TCS in DHs is significantly influenced by the operating conditions of DHs. Here we evaluate the feasibility of integrating TCS into DHs in the Enschede region of the Netherlands. DHs models are established to simulate heat generation, demand, and storage, and a control strategy is designed to manage storage coordination. The obtained results show that the dual storage system outperforms the single hot water storage system in reducing peak load generation. Depending on the TCS's operational condition, annual energy generation from peak load in dual storage systems could drop by 30-60 % compared to the single hot water storage system. It is achieved mainly by managing the energy capacity remaining in the storage system. The technical feasibility and benefits of implementing a TCS system in DHs with a dual storage system are shown to be more energy efficient.

Ferrohydrodynamic or magnetic pumping enables the design of a magnetocaloric refrigerator with no moving parts. Existing magnetic pumps utilize travelling wave magnetic fields with frequencies in the range of 100 to 1000 Hz. Such high frequencies when utilized in the proposed refrigerator could cause heating which is detrimental to its performance. Hence, a magnetic pump that works with low magnetic field frequencies (¡ 1 Hz) is designed and its performance is experimentally characterized and compared against an one-dimensional model. The design of the magnetic pump consists of a rising and falling pipe, circumscribed by an electromagnetic coil. On application of a magnetic field, due to the inward acting force on either end of the pipes, the ferrofluid progresses in the rising pipe and reaches the falling pipe. On removal of the magnetic field, the portion of the fluid in the falling pipe falls down due to gravity, thereby achieving a net pumping action. Thus on continuously cycling the magnetic field, an intermittent motion of the ferrofluid is obtained. The maximum cross-sectional area and time-averaged mass flow rate of the proposed design is 1.8 g s-1 cm-2 at 0.74 Hz and 35.7 mT. This mass flow rate is comparable to pump designs that work on travelling wave magnetic fields, whose operational frequency is three orders of magnitude higher.

Abstract One-seventh of the worldwide electricity is used for refrigeration related activities. When compared to the predominantly used vapor compression refrigeration system, magnetocaloric refrigeration provides an energy-efficient and eco-friendly alternative. It utilizes a magnetocaloric material which undergoes temperature changes when exposed to magnetic field changes. Nearly 60% of the total system cost is due to the permanent magnets used. For a given cooling power, the amount of permanent magnet needed is determined by the residence time required by the heat transfer fluid in each cycle. The start-of-the-art systems utilizes water-alcohol mixture as the heat transfer fluid, and a mechanical pump for its circulation. By utilizing non-toxic and non-hazardous gallium-indium-tin based liquid metals as heat transfer fluid, which has three order of magnitude higher thermal diffusivity, the residence time and the permanent magnetic material is reduced nearly by a factor of 10. Even accounting for the cost of the liquid metal, it results in considerable savings in the system cost. We present experimental results on the entropy change of using liquid metal with (Mn,Fe)2(P,Si)-based magnetocaloric material, and numerical results on the system level analysis. Further, to improve the reliability of the system, we propose to use magnetic pumping for heat transfer fluid circulation, which does not have any moving parts. The state-of-the art magnetic pump works at frequencies in the range of 100 to 1000 Hz. However, they cause detrimental heating effects due to eddy current losses. To overcome this, we have designed a pump that operates at 1 Hz and still achieves comparable flow rates. We present numerical results on the pump design by studying the dynamics of ferrofluid rise in a vertical pipe with multiple electromagnetic coils.

This paper studies the effect of successive (de)hydration cycles on the structure of potassium carbonate K2CO3 & sdot;1.5H2O grains for low-temperature heat storage applications. Such structural changes are caused by exposure of the salt to water vapor or removal of water from it, accompanied by successive swelling and shrinkage. Understanding the material's internal structure is key to predicting its behaviour and optimizing its design. However, due to the simultaneous and persistent occurrence of structural changes and transport mechanisms throughout the process, gaining a complete understanding of the phenomenon can be challenging. Unlike conventional experimental approaches and two-dimensional imaging techniques used for porosity assessment, our study showcases the qualitative and quantitative alterations in the porosity and microstructure of potassium carbonate. This analysis is achieved by using Micro-X-ray computed tomography (Micro-CT). The study focuses on the impact of cycling on grain microstructure, investigating pore volume distribution, radial variation of pore sizes, and density of individual grains. It was noted that the porosity increased from 6.4 % to 19.7 % after seven cycles. Initially, we observed a greater number of pores in the core of the uncycled salt grain. However, after cycling, we noticed a more even distribution, with a higher number of pores in the outer region of the grain, which caused a radial change in porosity. Lastly, this research provides the intrinsic and apparent densities of both non-cycled and cycled potassium carbonate specimens. Micro-CT is a good tool for a better understanding of changes in thermochemical material at a structural level. Calculation of porosity provided a pathway to calculate apparent and intrinsic density. The demonstrated method can be used for a wide range of salt hydrates, enhancing the scope and applicability of this study in the field of low-temperature heat storage applications. Additionally, it gives the measuring parameter needed to calculate energy density and change in volume during the reaction.

Thermochemical storage using salt hydrates presents a promising energy storage method. Ensuring the long-term effectiveness of the system is critical, demanding both chemical and mechanical stability of material for repetitive cycling. Challenges arise from agglomeration and volume variations during discharging and charging, impacting the cyclability of thermochemical materials (TCM). For practical usage, the material is often used in a packed bed containing millimetre-sized grains. A micro-level analysis of changes in a packed bed system, along with a deeper understanding involving quantifying bed characteristics, is crucial. In this study, micro X-ray computed tomography (XCT) is used to compare changes in the packed bed before and after cycling the material. Findings indicate a significant decrease in pore size distribution in the bed after 10 cycles and a decrease in porosity from 41.34 to 19.91% accompanied by an increase in grain size, reducing void space. A comparison of effective thermal conductivity between the uncycled and cycled reactor indicates an increase after cycling. Additionally, the effective thermal conductivity is lower in the axial direction compared to the radial. XCT data from uncycled and cycled experiments are further used to observe percolation paths inside the bed. Furthermore, at a system scale fluid flow profile comparison is presented for uncycled and cycled packed beds. It has been observed that the permeability decreased and the pressure drop increased from 0.31 to 4.88 Pa after cycling.

Magnetic refrigeration (MR) is a cutting-edge technology that promises high energy efficiency and eco-friendliness, making it an exciting alternative to traditional refrigeration systems. However, the main challenge to its widespread adoption is cost competitiveness. In this context, the use of liquid metals as heat transfer liquids in the MR has been proposed as a game-changing solution. Unfortunately, the toxicity and flammability of these liquid metals have raised serious concerns, limiting their practical use. In this study, we investigate the compatibility of a nontoxic and nonflammable GaInSn-based liquid metal with a magnetocaloric material, La(Fe,Mn,Si)13Hz, over a 1.5 year period. Our findings reveal nearly a 14% reduction in specific cooling energy and peak-specific isothermal magnetic entropy change for the considered magnetocaloric material. Our study provides valuable insights into the long-term stability of magnetocaloric materials and their compatibility with liquid metals, facilitating the development of more cost-effective and sustainable MR systems.

Thermochemical energy storage (TCES) is becoming increasingly important in the energy transition, as it can effectively bridge the gap between renewable energy supply and demand. In this study, the reaction kinetics of K2CO3 were characterized and validated. Based on this kinetic model, a numerical model of a packed bed of particles was developed using a coupled CFD-DEM approach. The results of the model were validated against experimental data of a particle bed, showing good agreement. The reaction rate of the system was found to be limited by the diffusion of water vapor into the material, which led to unsatisfactory performance on the bed scale due to significant temperature drop-offs. Although reducing particle size was found to be an effective way to improve system performance, practical concerns such as agglomeration and bed permeability limited its effectiveness. As an alternative, a multi-reactor system with adaptive flow rates was proposed, which improved system performance without the limitations of reducing particle size. The proposed modular system is capable of delivering 10 kW power at the temperature of 45 degrees for a duration of 19.5 h.

Underground hydrogen storage will be an essential part of the future hydrogen infrastructure to provide flexibility and security of supply. Storage in porous reservoirs should complement storage in salt caverns to be able to meet the projected high levels of required storage capacities. To assess its techno-economic feasibility, a case study of hydrogen storage in a depleted gas field in the Netherlands is developed. Subsurface modelling is performed and various surface facility design concepts are investigated to calculate the levelized cost of hydrogen storage (LCOHS). Our base case with hydrogen as cushion gas results in an LCOHS of 0.79 EUR/kg (range of 0.58–1.04 EUR/kg). Increasing the number of full-cycle equivalents from 1 to 6 lowers the storage cost to 0.25 EUR/kg. The investment cost of the cushion gas represents 76% of the total cost. With nitrogen as cushion gas, LCOHS is reduced to 0.49 EUR/kg (range of 0.42–0.56 EUR/kg).

A typical solar domestic water heating system suffers from low energy efficiency due to multiple heat transfer process among components, i.e., the solar thermal collector and the thermal energy storage. In this work, a compact design of storage-integrated solar thermal collector and its compatible phase change material (PCM) storage material were explored. The salt hydrate based composite PCM, the sodium acetate trihydrate (SAT), was used to create a shape stabilized PCM (ssPCMs) for a feasible PCM by its low leakage and high thermal conductivity. The developed ssPCMs has been tested experimentally and a numerical model has been developed to obtain the effective thermal property of the developed ssPCMs by comparing the predicted results with the measurements. Furthermore, the model has been used to study the (dis)charging behavior of PCM in a storage-integrated solar thermal collector which is composed of an evacuated tube and a double spiral coils heat exchanger. A geometrical optimization has been performed to achieve 2.6 times longer of discharging period with a minimum outlet temperature of 55 °C. Moreover, the circulating water is found useful in increasing the PCM charging rate with 9% by transferring the heat from the surface to the center of tube.

A problematic issue with the solar water heaters is the storage tank requirement, which takes considerable space and makes the piping and installation more difficult. This study is the first report on experimentally applying a shape-stabilized PCM to a tankless direct-absorption evacuated tube solar collector to address this challenge and directly store solar energy. The proposed salt hydrate PCM was synthesized at various concentrations of related components and after detecting the optimum compound, it was tested under several cycles to ensure its sustainable heat storage capability. Furthermore, after charging the solar system in the stagnation mode (without water flow), it was discharged at 10, 27, and 40 L per hour (LPH) flow rates. It was revealed that the thermal efficiency in the stagnation mode was improved from 66 % to 82 % using this collector-storage system. In addition, it was concluded that changing the flow rate from 10 to 27 LPH does not considerably reduce the heat gain of collector; however, using the flow rate of 40 LPH plunges the discharge efficiency. Ultimately, cost and carbon footprint analyses of the proposed system were conducted and a payback period of 6 years and annual reduction of 5.4 tons of CO2 emissions were reported.

A numerical study is performed to examine the influence of a non-uniform magnetic field on the thermo-hydraulic behaviour of a ferrofluid. The analysis is done in the context of a differentially heated semi-circular annulus where a magnetic dipole with its distinct location and dipole strength is used to obtain different configurations. The field variables are computed by solving the coupled set of flow equations, energy equations and the Maxwell's magneto-statics equations. A detailed description is provided on the flow and thermal response after observing different parameters at both global and local scale. Comparison of streamlines and isotherms with a reference case of natural convection concludes that the recirculation zones are responsible for the increased velocity and heat transfer magnitudes. Another key finding of the present work is about the possibility to locally improve the thermal performance of heat exchangers at any desired position along the circumference.

The unpredictability of heat demand–supply is one of the major challenges in our future sustainable energy system. Thus, coming up with new methods of heat storage for operating solar systems during low solar irradiance is among the advanced technical approaches in this field. The compact integrated solar thermal collector-storage systems have recently attracted the researchers’ attention due to the direct usage of stored thermal energy as well as the reduced space occupation. The current literature lacks a review paper on investigating various existing designs of such compact PCM-solar systems; thus, this paper analyzes the latest developments on experimentally investigated single-unit PCM-based solar collectors regarding both solar water and air heaters. Various types of solar collectors, including flat plate, evacuated tube, concentrating, and photovoltaic/thermal collectors have been thoroughly reviewed and discussed and the thermodynamic correlations for the system analysis have been given. The investigations revealed that a large proportion of integrated flat plate storage systems have been designed to provide space heating and it was deduced that paraffin and other hydrocarbons were basically used for the low-temperature heat production for domestic uses. Also, the concentrating solar collectors have mostly been utilized for providing the medium temperature for the industrial usage employing salt hydrates and nano-composite based phase change materials possessing higher melting points are adopted. Last but not least, it was concluded that the exergy efficiencies of solar collectors are remarkably low (below 5%), and the hybrid photovoltaic-thermal collectors can encounter this drawback by exploiting the high electrical exergy.

Ferrofluids have always been the centre of interest for a broad range of applications where the flow manipulation is achieved using different types of magnetic fields. In the present work, a Finite Volume Method based numerical study is performed to investigate the ferrofluid behaviour in the laminar flow regime using the complete set of Ferrohydrodynamics equations. Four distinct single layer finite solenoids of different Ls/Ds (length to diameter ratio) are considered to replicate a 3D (three-dimensional) non-uniform magnetic field, and a twofold validation is also reported to authenticate the generated field distributions. Moreover, a magnetic field dependent viscosity model is used to take into account the variation of ferrofluid viscosity in the presence of a non-uniform magnetic field. To accommodate the varying particle size distribution of ferrofluid, an effective relaxation time constant is used that can accurately predict the magnetization using Debye relaxation mechanism. The obtained flow characteristics are then further discussed and compared along with the case of stationary ferrofluid for all solenoid configurations. New insights about the flow structures are provided using the lambda 2 criteria of vortex identification technique and the critical point theory analysis. It is observed that each solenoid arrangement have a distinct magnetic field distribution which leads to the occurrence of unique vortex structures and fluid deformations. Also, different flow trajectories are noticed within the fluid domain owing to the peculiar distribution of Kelvin force density. Results from the present work have direct implications in all ferrofluid based momentum transport devices and can be effectively used to further improve our understanding about their performances.

Salt hydrates are promising candidates for long-term thermochemical heat storage (TCHS) in the building environment. In such storage systems, the surplus of energy will be exploited in an endothermic reaction to dehydrate the salt hydrates. Once it is demanded, the stored energy will be released through an exothermic reaction by hydrating the salt, which results in an increase in the mass and temperature of salt particles as well as changes in the species of material. In order to construct an improved storage system, it is very important to deeply understand the details of the heat and mass transfer processes in the packed beds of salt hydrates. Poor heat (in the closed systems) and mass transfer (in open systems) can be the bottleneck in this technology. The main objective of this work is to investigate how heat transfer will be affected by applied pressure, particle size, and packing arrangement through calculating/measuring the effective thermal conductivity of the packed beds of salt hydrates. This is achieved by applying and developing a CFD-DEM model and by experimental measurements in a vacuum oven. Comparisons are carried out for the numerical results at low and high ambient pressures with the experimental measurements, which show a very good agreement. The obtained results show the effect of natural convection in the packed bed when the higher vapor pressure is applied.