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Radiative cooling (RC) shows good potential for building energy saving by throwing waste heat to the cosmos in a passive and sustainable manner. However, most available radiative coolers suffer from low cooling flux. The situation becomes even deteriorated in the daytime when radiative coolers are exposed to direct sunlight. To tackle this challenge, an idea of employing both a spectrally selective cover and a spectrally selective emitter is proposed in this study as an alternative approach. A comparative study is conducted among four RC modules with different spectral characteristics for the demonstration of how the spectral profiles of the cover and the emitter affects the RC performance. The results under given conditions show that the RC module with a spectrally selective cover and a spectrally selective emitter (SC/SE) reaches a net RC power of 62.4 W/m2 when the solar radiation is 800 W/m2, which is about 1.8 times that of the typical RC module with a spectrally non-selective cover and a spectrally selective emitter (n-SC/SE). When the ambient temperature is 30°C, the SC/SE based RC module realizes a daytime sub-ambient temperature reduction of 20.0°C, standing for a further temperature decrement of 9.2°C compared to the n-SC/SE based RC module.

Carbon neutrality is an ambitious goal that has been promulgated to be achieved on or before 2060. However, most of the current energy policies focus more on carbon emission reduction, efficiency and high penetration of renewable energy. Thus, this paper presented a review strategy towards carbon neutrality by presenting the concept of a multi-energy system (MES) in terms of its technologies, configuration, modelling and feasibility as zero-emission equipment. The paper addressed some prominent challenges associated with zero-carbon multi-energy systems (ZCMES). Various proven solutions in the extant studies that have been affirmed to alleviate some of these challenges were presented. In the end, we identified and summarised the current research gaps, and the future directions to ensure the feasibility of ZCMES as a primary strategy towards the actualization of carbon neutrality. Hence, this review work serves as a reference for revising the current energy policies to incorporate a carbon neutrality framework.

Improving the thermal insulation of glazing units is a common strategy of reducing building energy use for spacing cooling. This paper newly examined the application of aerogel glazing technology in Hong Kong by the means of laboratory testing and simulation. Nine prototypes of granular aerogel glazing were selected to examine their optical properties, and the measured optical properties of aerogel were used to calculate the total window thermal performance indices. A typical 40-story commercial office building was chosen for energy simulation to compare the thermal performance of aerogel glazing with different glazing technologies in Hong Kong. The results showed that aerogel glazing could achieve the promising reduction of window heat gain up to 57% and cooling energy up to 8.5% compared with double glazing. The heat insulation performance of aerogel glazing is even better than the double glazing with low-E coating in Hong Kong. Therefore, aerogel glazing can be a good alternative of glazing to comply with the existing local Overall Thermal Transfer Value (OTTV) requirement and to reduce the building energy use for space cooling in Hong Kong and other regions.

Due to the fluctuation and intermittency of distributed PV generation, battery energy storage is required with higher renewable installation towards carbon neutrality. Thus, the photovoltaic battery (PVB) system receives increasing attention. This study provides a critical review on PVB system design optimization, including system component sizing and strategy improvement studies, from mathematical modeling, evaluation system establishment to feasibility and optimization studies. Several PVB simulation software packages are compared and evaluated, and acknowledged system models are presented. The evaluation indicators are summarized from various aspects with cases of various evaluation systems combining different indicators or using the Pareto front for multi-criteria system designing. The PVB system feasibility study is analyzed from system configuration variation, critical technical and economic parameter analyses, rule-based operation strategies to future expectations like large-scale energy storage profitability, grid parity, and energy community trading platform. The targets, methods, tariff and time resolution influences, and PVB system capacity optimization design recommendations are critically discussed. The research directions for system operation development and future expectations are analyzed from system feasibility, flexibility to resilience. The co-planning of PVB system capacity and operation design optimization makes the problem complicated, leading to relatively short time resolution but more flexibility to system operation strategy. This study could provide guidance and references to distributed PVB system future design and optimization studies.

Abstract Geothermal energy with abundance and large quantity can partially cover building heating/cooling loads and promote the carbon-neutrality transitions. Shallow geothermal ventilation (SGV) system, with a little initial investment cost, is one of promising technologies to partly replace the conventional air-conditioning system for air pre-cooling/pre-heating. This paper reviews applications of SGV system for improving thermal performance over latest two decades, which mainly includes the reclassification of SGV system, coupling with other advanced energy-saving technologies, application potentials for building cooling/heating under various weather conditions. Heat transfer mechanism and mathematical modelling techniques have been reviewed, together with in-depth analysis on current research trends, existing limitations, and recommendations of SGV system. Phase change materials, with considerable latent energy density, can stabilize the thermal performance with high reliability. The review identifies that optimization designs and advanced approaches need to be investigated to address the existing urgent issues of SGV system (e.g., large land occupation, difficulty in centralized collection of condensate water timely for horizontal buried pipe, bacteria growth, polluted supply air, and high construction cost for vertical buried pipe). A plenty of studies show that the SGV system could greatly expand the application scope and improve system energy efficiency by combining with other energy-saving technologies. This paper will provide some guidelines for the scientific researchers and engineers to keep track on recent advancements and research trends of SGV system for the building thermal performance enhancement and pave path for future research works.

Internet of Things (IoT) technologies are increasingly implemented in buildings as the cost-effective smart sensing infrastructure of building automation systems (BASs). They are also dispersed computing resources for novel distributed optimal control approaches. However, wireless communication networks are critical to fulfill these tasks with the performance influenced by inherent uncertainties in networks, e.g., unpredictable occurrence of link failures. Centralized and hierarchical distributed approaches are vulnerable against link failure, while the robustness of fully distributed approaches depends on the algorithms adopted. This study therefore proposes a fully distributed robust optimal control approach for air-conditioning systems considering uncertainties of communication link in IoT-enabled BASs. The distributed algorithm is adopted that agents know their out-neighbors only. Agents directly coordinate with the connected neighbors for global optimization. Tests are conducted to test and validate the proposed approach by comparing with existing approaches, i.e., the centralized, the hierarchical distributed and the fully distributed approaches. Results show that different approaches are vulnerable against to uncertainties of communication link to different extents. The proposed approach always guarantees the optimal control performance under normal conditions and conditions with link failures, verifying its high robustness. It also has low computation complexity and high optimization efficiency, thus applicable on IoT-enabled BASs.