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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A numerical model of a vertical earth-to-air heat exchanger (VEAHE) system was developed for the first time. Compared to conventional EAHE system models, the developed model considered the vertical distribution of soil temperature and thermal conductivity. The model validation against experimental data showed a good agreement. A parametric study was then conducted to investigate the effects of tube parameters, insulation parameters, mass flow rates and soil types. Results indicate that a system with a smaller tube diameter provides more thermal capacity with a fixed air mass flow rate, while a larger tube diameter can enhance the system's COP. An increase in tube depths causes the outlet air temperatures to decrease/increase in summer/winter, with a smaller daily oscillation. Compared to PVC and PE, stainless steel can be recommended as the most appropriate tube material, taking into account both heat exchange and soil pressure. Polyurethane, rubber or rock wool as insulation materials can be used as thermal barriers to shield the undesirable heat gains from the shallow soil with the almost same insulation effects. A thickness of 30 mm and a length of from 4 to 5 m can be recommended as the most appropriate insulation parameters for this proposed system.@en

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

In this study, a new vertical earth-to-air heat exchanger (VEAHE) system coupled with tubular phase change material (PCM) components was proposed. The proposed system has smaller occupied areas and higher geothermal energy use efficiency than traditional EAHE systems. Moreover, the tubular PCM component enables the system's air temperature at the outlet to be more stable, thereby enhancing the system's thermal performance. In particular, the tubular components' simple geometry, easy fabrication, convenient assembling-disassembling and low-cost facilitate their usage in practical applications. To explore the system's thermal performance, an experimental set-up was established and its numerical model was developed. Then the model was verified through a comparison between the system's simulated and monitored air temperatures at the outlet. The verification produced acceptable results with the maximum absolute relative error of 1.33%. Based on this model, the influences of the tubular PCM component, tube depth, PCM conductivity and container length on the proposed system's thermal performance were investigated. Results indicated that the tubular PCM components can effectively reduce the VEAHE system's air temperature peak and fluctuation at the outlet, and increase its average cooling capacity. For the proposed system, as the tube depths increase from 12 to 24 m, the air temperature peak and fluctuation at the outlet decrease from 25.74 °C and 3.59 °C to 21.01 °C and 0.62 °C, respectively. Different thermal conductivity of PCM has almost same influences on the system's air temperature at the outlet as the container diameter is 50 mm. Such influences, however, become apparent and require to be considered as the container diameter increases to 150 mm. The air temperature fluctuation at the outlet decreases as the container lengths increase, and 12 m can be considered as an appropriate length for the proposed system. Additionally, the system's static payback period was calculated as 18.03 years.

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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