Zhang, Yong
(2025)
Investigation of low-temperature thermochemical energy storage for water heating in residential application.
PhD thesis, University of Nottingham.
Abstract
This study aims to develop a low-temperature thermochemical energy storage (TCES) water heating system for domestic use, designed to integrate with low-temperature sustainable energy sources like solar energy. The goal is to improve the utilization of renewable energy in residential applications, reduce reliance on fossil fuels, and lower greenhouse gas emissions.
This thesis provides a comprehensive review of the latest advancements in the field of low-temperature TCES systems, and finds research gaps at the system, material and reactor levels. At the system level, open TCES systems demonstrate advantages in output performance and cost-effectiveness compared to closed systems. However, open TCES systems are difficult to integrate directly into most existing domestic central heating systems due to their reliance on different heat transfer mediums. To address this, the thesis proposes two open TCES water heating systems: one incorporating an internal bare microchannel flat-tube heat exchanger (IBHEX-TCES system) and the other incorporating a detached finned microchannel flat-tube heat exchanger (DFHEX-TCES system). The performance of these two TCES water heating systems was subsequently studied through numerical simulations. The performance comparison under identical conditions shows that the IBHEX-TCES system has a slight advantage in water temperature increase, with the difference generally being less than 1 °C. However, the DFHEX-TCES system consistently outperforms in overall thermal efficiency, leading by 10 to 15 percentage points.
Subsequently, a multifunctional TCES system was developed to experimentally test the two proposed open TCES water heating systems. The test results show that while both systems achieved similar peak water temperature lifts, the DFHEX-TCES exhibited superior performance in terms of duration and operational efficiency. Further tests on the multilayer DFHEX-TCES revealed a slightly lower peak temperature lift. However, the duration during which the water temperature was lifted above 5 °C was at least twice as long, underscoring the effectiveness of the multilayer configuration.
Additionally, this thesis explores various impregnation methods for synthesizing vermiculite-CaCl2 composites (VCMs), focusing on analysing the effect of these methods on the performance of VCMs. The optimized synthesis methods achieved salt contents exceeding those obtained with traditional single-step impregnation methods (reporting a maximum of 68 wt%). Among these methods, VCM-vac (synthesised by vacuum impregnation) exhibited the highest volume energy storage density, reaching 2.05 GJ·m-3, which is approximately 1.5 times higher than the VCM sample synthesised by single-step. VCM-vac also demonstrated excellent cycling stability.
Finally, the structure of packed bed reactor was optimized based on experimental and simulation studies. The proposed two-layer and four-layer reactors extended the discharging duration by factors of 1.7 and 3 as compared to the conventional single-layer setup. Concurrently, these configurations achieved a doubling and a quadrupling of peak output power. Such performance improvements are difficult to achieve by simply scaling up traditional packed bed reactors.
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