Thermal energy storage (TES) is an advanced technology that could address the energy supply-demand balance in building air conditioning systems. TES is also important in view of the increasing utilisation of solar energy. Although sensible heat storage systems and latent heat storage systems have been heavily researched and are widely used domestically and industrially, thermochemical heat storage (THS) systems are currently undergoing a surge in research and development. Recent studies suggest that THS has significant advantages when compared with the other heat storage methods including higher storage density, lower volume requirements, low heat loss (approaching zero), and lower charging temperature. On the other hand the complexity and higher investment costs remain the main obstacles to rolling out these systems commercially. A comprehensive review has been presented here into all aspects of the current, state of the art THS research. It includes comparisons with other storage methods, description of absorption/adsorption cycles, the materials used in these systems and open/closed storage methods, as well as chemical heat pumps. Recent advances in the field are also investigated and discussed.

The latest advancements on thermochemical heat storage systems

Thermal energy storage and conversion aims to improve the high inefficiency of the industrial processes and renewable energy systems (supply versus demand). Chemical sorption processes and chemical reactions based on solid–gas systems are a promising way to store and convert thermal energy for heating or cooling applications and, thereby to increase the efficiency of the processes and to reduce the greenhouse effect. Although more efforts are required to bring this technology to the market, some important breakthrough have been made regarding to system efficiency. Over the last two decades, the experimental research in this field has increased largely to validate these advances under practical conditions. Therefore, this paper gives a state-of-art review of performances obtained under practical conditions by the different prototypes built over the last two decades. In addition, the main advantages and disadvantages of solid–gas chemical sorption processes and chemical reactions are summarized.

Thermochemical energy storage and conversion: A-state-of-the-art review of the experimental research under practical conditions

An overview of developments in adsorption refrigeration systems towards a sustainable way of cooling

The nucleation of crystals in liquids is one of nature's most ubiquitous phenomena, playing an important role in areas such as climate change and the production of drugs. As the early stages of nucleation involve exceedingly small time and length scales, atomistic computer simulations can provide unique insight into the microscopic aspects of crystallization. In this review, we take stock of the numerous molecular dynamics simulations that in the last few decades have unraveled crucial aspects of crystal nucleation in liquids. We put into context the theoretical framework of classical nucleation theory and the state of the art computational methods, by reviewing simulations of e.g. ice nucleation or crystallization of molecules in solutions. We shall see that molecular dynamics simulations have provided key insight into diverse nucleation scenarios, ranging from colloidal particles to natural gas hydrates, and that in doing so the general applicability of classical nucleation theory has been repeatedly called into question. We have attempted to identify the most pressing open questions in the field. We believe that by improving (i.) existing interatomic potentials; and (ii.) currently available enhanced sampling methods, the community can move towards accurate investigations of realistic systems of practical interest, thus bringing simulations a step closer to experiments.

Crystal Nucleation in Liquids: Open Questions and Future Challenges in Molecular Dynamics Simulations

Review on sorption materials and technologies for heat pumps and thermal energy storage

Effect of nucleation time on freezing morphology and type of a water droplet impacting onto cold substrate

Permeability and thermal conductivity of compact chemical and physical adsorbents with expanded natural graphite as host matrix

A new target-oriented methodology of decreasing the regeneration temperature of solid–gas thermochemical sorption refrigeration system driven by low-grade thermal energy

Understanding the behaviours of ice nucleation in non-isothermal conditions is of great importance for the preparation and retention of supercooled water. Here ice nucleation in supercooled water under temperature gradients is analyzed thermodynamically based on classical nucleation theory (CNT). Given that the free energy barrier for nucleation is dependent on temperature, different from a uniform temperature usually used in CNT, an assumption of linear temperature distribution in the ice nucleus was made and taken into consideration in analysis. The critical radius of the ice nucleus for nucleation and the corresponding nucleation model in the presence of a temperature gradient were obtained. It is observed that the critical radius is determined not only by the degree of supercooling, the only dependence in CNT, but also by the temperature gradient and even the Young’s contact angle. Effects of temperature gradient on the change in free energy, critical radius, nucleation barrier and nucleation rate with different contact angles and degrees of supercooling are illustrated successively. The results show that a temperature gradient will increase the nucleation barrier and decrease the nucleation rate, particularly in the cases of large contact angle and low degree of supercooling. In addition, there is a critical temperature gradient for a given degree of supercooling and contact angle, at the higher of which the nucleation can be suppressed completely.

Suppression of ice nucleation in supercooled water under temperature gradients

Supercooling is one of the major drawbacks that hinder the wide application of phase change thermal storage technology, where supercooling needs to be suppressed, or nucleation triggered at desired supercooling or time. The disturbance of mechanical impact can effectively promote nucleation. However, up to now, its nucleation mechanism has not been well explained, and the corresponding nucleation triggering technology has not been reasonably developed. In this paper, the effect of mechanical impact in supercooled water on nucleation is investigated experimentally by impact rod falling freely to hit the bottom of container. The nucleation temperature under impact at different heights of free fall are measured. The results show that 80% supercooling reduction can be achieved by a low impact energy of 0.007 J. In addition, we modify the existing model based on the experimental data to develop a revised model, which can estimate the impact stimulus required to trigger nucleation at desired supercooling. Moreover, the nucleation mechanism under impact based on classical nucleation theory is analyzed. By comparing the changes in pre-exponential factor and nucleation barrier with and without impact, the enhancement in nucleation rate under impact can be explained as a reduction on the free energy barrier for nucleation. Our experimental and theoretical analysis results preliminarily reveal the impact nucleation mechanism, which are expected to promote more in-depth research on the related topics. These results could also better serve to predict whether nucleation will occur in the supercooled water under mechanical impact, and to actively trigger nucleation or suppress supercooling as needed through mechanical impact. 

Liping Wang

Papers

Effect of nucleation time on freezing morphology and type of a water droplet impacting onto cold substrate

Permeability and thermal conductivity of compact chemical and physical adsorbents with expanded natural graphite as host matrix

A new target-oriented methodology of decreasing the regeneration temperature of solid–gas thermochemical sorption refrigeration system driven by low-grade thermal energy

Suppression of ice nucleation in supercooled water under temperature gradients

Nucleation in supercooled water triggered by mechanical impact: Experimental and theoretical analyses