Reaction kinetics of the hydration of potassium carbonate including the influence of metastability

Eurotherm Seminar #116 brings together researchers and practitioners from academia and industry, who are working in the rapidly expanding field of thermal energy storage (TES).The seminar aims to cover most recent trends in such areas as novel storage materials, advanced storage concepts and configurations, TES for renewable energy systems, TES in buildings and industry, use of advanced techniques such as artificial intelligence in TES, and social, environmental, and educational aspects

Volume variation in a thermochemical material- An experimental study

The research focuses on swelling and shrinkage during cycling of a thermochemical material. Potassium carbonate has been cycled and the change in size has been monitored over subsequent cycles with the help of in-situ measurement in the micro-climate chamber. The experiments have been performed for different operating conditions and the resultant images were processed to calculate the equivalent diameter of the salt grains. Micro -CT scans were performed for both the samples to compare the two-dimensional results from in-situ experiments to a complete three-dimensional analysis

Microstructural changes in thermochemical heat storage material over cycles:Insights from micro-X-ray computed tomography

This paper studies the effect of successive (de)hydration cycles on the structure of potassium carbonate K2CO3·1.5H2O 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.</p

Characterizing Changes in a Salt Hydrate Bed Using Micro X-Ray Computed Tomography

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.</p

Successive melting and solidification of paraffin–alumina nanomaterial in a cavity as a latent heat thermal energy storage

Latent heat thermal energy storage (LHTES) plays a main role in many industrial applications, especially in high-powered electronics cooling systems and providing the thermal energy demand when the energy supply is unavailable. In this study, the LHTES cycle process, including successive melting and solidification, investigates in a two-dimensional annular space of a square cavity filled with nanomaterial of paraffin–alumina as a nanoPCM. In the melting process, all sidewalls of the cavity are insulated. Meanwhile, a constant heat rate generates homogeneously within the central heat source. At the end of melting, the heat generation gets off, while a time-reducing temperature lower than the paraffin melting point imposes on the sidewalls, and then, solidification triggers. The numerical simulation was accomplished using control volume method and the governing equations solved using the SIMPLE algorithm. The enthalpy-porosity method was employed to model the phase-change front. The value of thermal conductivity and the viscosity of the nanofluid have been experimentally measured before the numerical modeling. In this study, the effect of volume fraction of nanoparticles (0–0.03) has been investigated on the successive melting and solidification rate for a constant Rayleigh number of 5.74 × 105. The results show that adding nanoparticles to the PCM equal to the volume fractions of 0.01 and 0.02 improves melting rate, but the nanofluid with the volume fraction of 0.03 represents a poor heat transfer rate during melting even weaker than those for nanofluid with the volume fraction of 0.01. It also observed that the nanomaterial with the volume fraction of φ = 0.03 represents the highest solidification rate. However, taking the overall performance of successive melting and solidification system into account, the nanofluid with the volume fraction of 0.02 remarked the most effective heat transfer rate in comparison with the other examined cases

Simultaneous charging and discharging of multi-tube heat storage systems using copper fins and Cu nanoparticles

In this paper, heat transfer of a multi-tube heat exchanger filled with RT82, fins, and nanoparticles is investigated using numerical modeling. There are four heat transfer fluid (HTF) tubes embedded in a shell that contains PCM. The PCM is under simultaneous charging and discharging (SCD) conditions caused by pumping two hot heat transfer fluids (HHTF) and two cold heat transfer fluids (CHTF). The arrangement of these HHTFs and CHTFs, as geometrical factors, are compared with each other. Results demonstrated that based on cold and hot tubes positioning, steady-state liquid fraction alternates between 37% and 60% as setting hot tubes at the lower half claims the maximum 60%. Fins addition results lead to the varying outcome under SCD, unlike melting cases where fin proved more practical based on liquid fraction criterion. Under SCD conditions fins show an advantage in the short time (<100 min), however, in a long time adding fins will result in less available thermal energy. Cu nanoparticles have been added into the PCM to lower the response time, however, under SCD conditions its impact is negligible. Hence, while the PCM is under SCD condition, using nanoparticles is not recommended. The results show that among different geometrical arrangements the one with inner hot tubes located at the bottom of the shell registers the most favorable outcome