A novel vertical air-soil heat exchanger (VASHE) is proposed, coupling with diversified energy storage components, i.e., both annular and tubular phase change material (PCM) components. Compared to traditional air-soil heat exchanger systems, advantages of the new VASHE include the space-saving, higher energy efficiency, centralized discharge of condensate water and smaller fluctuation of outlet air temperature. An enthalpy-based model is developed to characterise the underlying heat transfer mechanism of the sophisticated sensible and latent heat transfer in PCMs. An experimental platform is thereafter constructed for the calibration of the developed enthalpy-based numerical model. Systematic parametrical analysis has been conducted on PCM types, PCM structure and PCM locations. Research results indicated that, the developed enthalpy-based model was accurate to predict the system performance with the maximum relative error at 1.98%. Systematic parametric analysis indicates that, within various PCM types, the RT 20 in the tubular PCM is the most promising with the smallest outlet temperature amplitude. Furthermore, accurate PCM location is an effective solution to the contradiction between daily cooling storage capacity and outlet temperature amplitude. This study demonstrates a novel air-soil heat exchanger with diversified energy storage components, which can provide concrete guidance and pave path for the geothermal energy utilisation.

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Shallow subterranean ventilation of earth-to-air heat exchanger (EAHE) system can improve renewable utilisation, decrease CO ₂ emission and promote carbon-neutral transition. However, the conventional EAHE system has drawbacks, e.g., large occupied land area, low energy-usage efficiency, small falling gradient for buried pipe and fluctuated outlet air temperature. This study proposes a vertical EAHE integrated with annular PCM with advantages, including less occupied floor space, higher energy efficiency, better centralised discharge of air condensate water and more stable outlet air temperature. An experimental test-rig was established for online testing and the real-time monitored data was for modelling calibration to characterise the sophisticated heat transfer in phase change process. Afterwards, multivariant analysis on thermo-physical PCM parameters was conducted on cooling capacity and outlet air temperature fluctuation. A dimensionality reduction approach from redundant experiments was adopted for multivariant optimization and sensitivity analysis. Results show that PCM fusion temperature and latent heat of PCM dominate the cooling capacity with percentage contribution of 37.61% and 28.91%, respectively. PCM thickness and melting temperature dominate the temperature fluctuation with percentage contribution of 31.27% and 26.18%, respectively. This study provides benchmark and guidelines on PCM thermo-physical parameters’ selection with an efficient dimensionality reduction approach, paving path for application of the vertical EAHE integrated with PCMs in buildings.

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The underground air tunnel system shows promising potentials for reducing energy consumption of buildings and for improving indoor thermal comfort, whereas the existing dynamic models using the computational fluid dynamic (CFD) method show computational complexity and are user-unfriendly for parametrical analysis. In this study, a dynamic numerical model was developed with the on-site experimental calibration. Compared to the traditional CFD method with high computational complexity, the mathematical model on the MATLAB/SIMULINK platform is time-saving in terms of the real-time thermal performance prediction. The experimental validation results indicated that the maximum absolute relative deviation was 3.18% between the model-driven results and the data from the on-site experiments. Parametrical analysis results indicated that, with the increase of the tube length, the outlet temperature decreases with an increase of the cooling capacity whereas the increasing/decreasing magnitude slows down. In addition, the system performance is independent on the tube materials. Furthermore, the outlet air temperature and cooling capacity are dependent on the tube diameter and air velocity, i.e., a larger tube diameter and a higher air velocity are more suitable to improve the system’s cooling capacity, and a smaller tube diameter and a lower air velocity will produce a more stable and lower outlet temperature. Further studies need to be conducted for the trade-off solutions between air velocity and tube diameter for the bi-criteria performance enhancement between outlet temperature and cooling capacity. This study proposed an experimentally validated mathematical model to accurately predict the thermal performance of the underground air tunnel system with high computational efficiency, which can provide technical guidance to multi-combined solutions through geometrical designs and operating parameters for the optimal design and robust operation.

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A vertical earth-to-air heat exchanger system (VEAHE) system was proposed in this study. Compared to conventional EAHE systems, it has smaller land use and higher geothermal energy utilization efficiency. To validate its thermal and economic feasibility, an experimental setup was established to examine the variation of its outlet air temperature, air temperature along the tube and soil temperature. Also, the energy metrics of the proposed system were calculated for its economic assessment. Results showed that the outlet air temperature of this system ranged from 22.4 °C to 24.4 °C in summer and from 16.0 °C to 18.0 °C in winter. A detailed air temperature profile along the whole tube revealed that, the air temperature difference under 5 m in deep soil was more than twice as many as that from the inlet to 5 m. For the soil temperature at different depths, its recovery rate approximately equaled to the corresponding changing rate at the same depth during operation. For the proposed system, its energy payback time was calculated as 8.2 years, and its CO2 emission mitigation potential and total carbon credits were calculated as 7170.42 kg and $203.43 respectively, given an economic lifespan of 20 years. In addition, its monetary payback period was calculated as 17.5 years. The above results demonstrate the feasibility and effectiveness of the proposed system in exploiting deep ground soil as a natural heat sink/source in summer/winter. Meanwhile, the energy metrics analysis indicates that the proposed system is economically viable.@en

Integrating phase change material (PCM) into building envelopes significantly reduces building energy consumption and improves indoor environment. Among different integration techniques, macro-encapsulation allows for an efficient, safe and convenient way of using PCM, and its applications have been widely investigated in recent years. However, this study argues that there is a lack of a systematic analysis regarding the thermal performance of macro-encapsulated PCM, particularly for building envelope applications. Also, a number of important issues have seldom been addressed such as material selection and PCM melting processes at a component level, and optimal locations at a system level. Such a research gap remains a barrier to architects and engineers succeeding at making rational decisions during building design stages, thereby achieving the optimal building performance. This paper aims to provide a comprehensive overview of macro-encapsulated PCM and its integration into building envelopes. The discussion mainly includes: definition and material selection for PCM macro-encapsulation, common macro-encapsulation forms and PCM melting processes within these forms, the optimal locations of systems in building envelopes, and thermal performance enhancement for PCM and shells. In addition, the key issues in future studies are discussed. It is hoped that this comprehensive review will contribute to a deeper understanding of the design and application of macro-encapsulated PCM in building envelopes.@en