Multiscale analysis of low temperature thermochemical energy storage with composite materials

Fisher, Robin Nicolas ORCID: 0009-0003-8625-7455 (2023). Multiscale analysis of low temperature thermochemical energy storage with composite materials. University of Birmingham. Ph.D.

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Abstract

Residential space heating accounts for ~60% of private domestic energy use, and for approximately 10%-15% of global greenhouse gas emissions. Increasing the penetration of renewable energies for power provision, and in particular solar power, is essential for future energy systems. Discrepancy between the high space heating demand during winter and high solar energy availability during summer leads to the need for efficient long-term thermal energy storage solutions. Storing heat by means of reversible endothermic chemical reactions is a promising strategy, since energy is maintained without losses and at higher energy storage densities than other typical methods, such as sensible or latent thermal energy storage. Energy can be stored thermochemically through reversible water adsorption into inorganic salts, and discharged later at the low to medium temperatures required for domestic space heating.

Thermochemical energy storage’s current level of maturity is between fundamental research and practical applications. Composite thermochemical materials with two or more components are being developed by the scientific community. These components include at least one active material which provides thermochemical energy storage capacity, and at least one supporting matrix. Dispersion of the active material into a matrix modifies the thermochemical equilibrium, improves heat and mass transfer, and in some instances contributes to the energy storage capacity. While the behaviour of pure active materials is reasonably well understood, their behaviour when dispersed into a matrix requires further investigation. The mechanisms behind the heat discharge phase are particularly unclear and are further complicated by the number of available host matrixes. Furthermore, technical hurdles, such as material instability, high storage volume and low discharge power, have emerged from the development of larger scale thermochemical energy storage systems, leading to poor economic performance. These issues tend to occur when operating thermochemical storage systems with pure active materials, whereas use of composites is usually confined to sample-scale experiments. At this stage, it is unclear what the performance of large-scale systems would be if operated with composite thermochemical materials, and whether the technical hurdles at reactor and system scales may be addressed this way. In this thesis, thermochemical energy storage is analysed in a cross-scale manner to help bridge the gap between material, reactor, and system scales. The overarching aim is to improve the understanding of the behaviour over time of thermochemical composite materials during heat charge/discharge and evaluate how this behaviour affects performance of thermochemical energy storage systems.

After a general introduction and a review of the TCS literature relevant for this thesis, results and contributions are presented. First of all, an analysis is carried out of the heat discharge kinetics of thermochemical materials in both pure form and dispersed into a matrix. The purpose of this analysis is to identify the rate-limiting processes of the reaction and propose suitable kinetic models to improve precision in thermochemical energy storage simulations. It was found that while the hydration of pure K2CO3 is severely kinetically hindered and difficult to model using conventional solid-state kinetic models, integration into a vermiculite host matrix provided improvements and was accurately predicted by nucleation models at 25°C and phase-boundary control model at 40°C. MgCl2 hydration shifted from an intraparticle diffusion to an interparticle diffusion kinetic control when dispersed into vermiculite, with a 5 to 10-fold reaction rate increase. Thus, the effect of salt impregnation, specifically in vermiculite, was precisely quantified in terms of rate-controlling mechanism and reaction rate. These results can feasibly be extrapolated to other salt and matrix combinations which demonstrate similar thermal energy storage capabilities. Secondly, the performance of a thermochemical energy storage reactor, integrated into a dwelling equipped with solar thermal, is carried out through numerical simulation. It became apparent that open TCS in European domestic context requires additional humidification to reach high enough temperature lifts. The choice of kinetic models (identified in the previous chapter) and reaction rates was found to affect performance during simulations by up to 5%. When modelling for different thermochemical storage materials, MgCl2-based composites showed a promising balance between power output, energy storage density and overall system cost. Finally, the techno-economic viability of a thermochemical energy storage system coupled to power-to-heat technologies is assessed. Reviewing electrified reactor technologies showed that microwave and radiofrequency heating could be more efficient (~75% to 80% versus over 80%) than conventional heating, however depth of penetration of electromagnetic waves could be a significant technical hurdle. This part of the thesis also aims to evaluate performance at the system scale, with the additional objective of quantifying the potential of combining solar photovoltaics with thermochemical energy storage. It was found that with TCS capital costs below 20 €/kWh which is conventionally the technological objective by the TCS community, domestic thermal energy storage with levelized costs between 400 and 500 €/MWh are achievable.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):