Encapsulation of hydratable salt for advanced thermochemical energy storage

Du, Zheng ORCID: 0000-0002-3683-458X (2023). Encapsulation of hydratable salt for advanced thermochemical energy storage. University of Birmingham. Ph.D.

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Abstract

The rapid growth in renewable energy brings significant challenges in managing the mismatch between intermittent supply and demand. Thermal energy storage (TES) provides a potentially highly effective solution to address such challenges. As one of three TES methods, thermochemical energy storage (TCS) has attracted increasing attention due to its high energy density, good scalability, and suitability for long-term energy storage. This thesis concerns TCS materials and devices, two crucially important aspects for bringing the TCS technology to real-world applications. The focus of the work is the use of hydratable salt as the TCS material for low-to-medium-temperature storage of solar heat. Such TCS store the thermal energy in chemical bonds of inorganic salts in anhydrous state, and release heat from the gas-solid reaction with water absorbed in those salts. It is one of the most promising types of TCS for its favourable thermochemical properties and low cost. However, pure salt has insufficient stability due to deliquesces, coalesce, and caking, resulting to a significant degradation in reactivity after cycles. It is thus essential to be enhanced by forming composites. The salt-based composites are being developed at an early stage, focusing mainly on the study of “salt-in-matrix” structures, which the salt fill the continue pores in a porous matrix. However, the existing structure has barriers to increasing energy density above 1000kJ/kg and improving stability. In addition, most of those composites are designed for the packed-bed device, which lacks the flexibility of charging/discharging, and has an unfavourable flow resistance in the cases of large storage capacity and high airflow rate. So far, it is still challenging to find an effective solution for TCS with high density, highly flexible, and high power. This forms the major motivation of this research.

This PhD study aims to address the gaps in hydratable salt-based composite by developing an encapsulation of salt. The key novelties lie in the incorporation of solar-absorbing materials in encapsulated salt manufactured via optimised water-in-oil (W/O) Pickering emulsions. The use of Pickering emulsions as the template to manufacture the functional materials is an emerging technology, benefiting from the tailorable, facile, robust, and potentially scale-up. This work brings a fundamental understanding of Pickering emulsions, especially using commercial raw materials. A general method to manufacture encapsulation of hydratable salt is successfully developed. To the best of my knowledge, this work is the first report of the encapsulated salt with size adjustable from the nanoscale to microscale and energy storage density above 1700kJ/kg. Notably, this work first demonstrates the use of encapsulated salt in fluidized bed device. Furthermore, the integration of solar harvesting ability to encapsulate salt is also achieved for the first time in this work. All the innovations in this work develop a breakthrough solution towards an advanced TCS with high energy density, large storage capacity, high flexibility, and the capability of storing renewable energy.

Both fundamental and experimental work was performed in this work, with the experimental including lab-scale demonstrations and some of the highlights are summarized in the following:

• The formation mechanism of Pickering emulsions with hydrophobic fumed silica nanoparticles is studied, which explains how the nanoparticle is adsorbed at the W/O interface to stabilize the emulsion and what is the preferable conditions for W/O emulsions.
• A comprehensive study is carried out on the W/O Pickering emulsions as a template for tailoring the structure of the capsules and establishing
a relationship between emulsion and encapsulation sizes.
• The size effect on the thermal performance of the encapsulated salt is studied, and a performance index is proposed to evaluate the efficiency
of the encapsulation.
• Upon optimization, nanoscale encapsulated salt is found to have superior reactive kinetics, high-energy density, and good stability.
• The fluidised bed device demonstrates the fluidisation of encapsulated salt, showing higher thermal performance and high flexibility compared to the conventional packed beds.
• The introduction of solar-absorbing materials to the encapsulated salt is shown to significantly improve the solar absorption of the materials.
• The solar charging device with the solar absorption-enhanced encapsulated salt demonstrates the directly coupled TCS and renewable
energy, realising the solar direct charging, long-term storage with zero loss, and on-demand discharging to supply instant warm air.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):