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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.

At present, thermal conductivity is usually taken as a constant value in the calculation of building energy consumption and load. However, in the actual use of building materials, they are exposed to the environment with continuously changing temperature and relative humidity. The thermal conductivity of materials will inevitably change with temperature and humidity, leading to deviations in the estimation of energy consumption in the building. Therefore, in this study, variations in the thermal conductivity of eight common building insulation materials (glass wool, rock wool, silica aerogel blanket, expanded polystyrene, extruded polystyrene, phenolic foam, foam ceramic and foam glass) with temperature (in the range of 20–60 °C) and relative humidity (in the range of 0–100%) were studied by experimental methods. The results show that the thermal conductivity of these common building insulation materials increased approximately linearly with increasing temperature with maximum growth rates from 3.9 to 22.7% in the examined temperature range. Due to the structural characteristics of materials, the increasing thermal conductivity of different materials varies depending on the relative humidity. The maximum growth rates of thermal conductivity with humidity ranged from 8.2 to 186.7%. In addition, the principles of selection of building insulation materials in different humidity regions were given. The research results of this paper aim to provide basic data for the accurate value of thermal conductivity of building insulation materials and for the calculation of energy consumption.

Internal thermal insulation composite system (ITICS) can be an important measure for the energy-saving retrofitting of buildings. However, ITICS may cause harmful effects on the hygrothermal performance of building envelopes. This work investigated the influence of the materials' hygric properties on the hygrothermal performance of a typical ITICS in different climate conditions in China. Two base wall materials, the traditional concrete and a new type aerated concrete, were tested and compared for their hygric properties firstly. The influence of the hygroscopicity of exterior plasters, the permeability of insulation materials and the climate conditions were then analyzed with WUFI simulations. The hygrothermal performance was evaluated with consideration of the total water content (TWC) of the walls and the moisture flux strength, the relative humidity (RH) and the mould growth risk at the interface between the base wall and the insulation layer (B-I interface). The numerical analysis implies that the TWC of internal insulated walls depends mainly on the hygroscopicity of exterior plaster and the wind-driven rain intensity. The upper limits for the water absorption coefficient of exterior plasters used in Beijing, Shanghai and Fuzhou are 1e-9, 1e-10, 1e-10 m2/s respectively. When such limits are guaranteed, a vapour tight system created by using insulation materials with a large vapour resistance factor or adding a vapour barrier can improve the hygrothermal performance of ITICS, especially for concrete walls in cold climate.

The flow and thermal breakthrough phenomenon in a forced external circulation standing column well (FECSCW) directly affects heat transfer efficiency and load-carrying capacity. A numerical model for FECSCW is developed to analyze the migration of the temperature and velocity front under the flow and thermal breakthrough. The results indicated that thermal breakthrough began after simulation running 2.5 min and was completely formed after 12 min. The inlet water, which directly entered the production well without heat exchange with the aquifer, accounted for 12.8%. When the porosity of the backfill material decreased from 0.35 to 0, the coefficient of performance (COP) of the heat pump unit increased by 1.6% on average, and the thermal breakthrough strength decreased by an average of 45.3% within 25 min. Where seepage velocity near the well wall was greater than 1 × 10−3 m•s−1, faster velocity front migration was observed, while the migration advantage of the temperature front was more prominent outside of this region. Through quantitative analysis of flow and thermal breakthrough, temperature and velocity front migration, and COP change of heat pump unit, theoretical suggestions were provided for the thermal transfer mechanism near the thermal well wall. The extended research in this study can be applied to the design and optimization of forced external circulation standing column well system.

Recently, the requirement for cooling capacity decreased when the driving energy changed from liquid fuel to lithium batteries. Therefore, the structure and location of the forecabin could be adjusted based on the aerodynamic performance. The current study conducted a significant number of simulations in order to find out the effects of the internal flow through forecabin in an Ahmed body. The following conclusions have been identified:1, The flow through the forecabin would always increase the resistance of the entire body, and the drag coefficient increases, on average, by approximately 85%. 2, When the aspect ratio is higher or the position of the inlet opening is lower, the total drag coefficient is lower due to a weaker vortex strength, a simpler vortex structure and a relatively simple flow. 3, The existence of the forecabin will largely increase the oscillation frequency of the flow field by approximately 15 times compared to the original Ahmed model. Finally, the high drag coefficient moment always appears to be due to the formation of more complex or intense vortex motion. These conclusions can offer useful results and references for the structural design of the front cabin for new energy vehicles.

Numerous studies have demonstrated that commercial activities have significantly reduced during COVID-19, while there are few studies disclosing the consequent impacts on the energy consumption of commercial buildings. This study explores the changes in energy consumption of different types of commercial buildings in Singapore under the impact of the pandemic, using commercial building energy performance data from 2017 to 2020 (n=540). The sampled buildings include 93 hotel buildings, 303 office buildings, 106 retail buildings, and 38 mixed developments. The analysis mainly used linear regression and paired sample t-test. The results showed that relative to 2019, the mean energy use intensity (EUI) of sampled commercial buildings decreased by 56.77 kWh/m² in the pandemic year (2020), a plunge of 19.9%. The extent to which the EUI of each type of commercial building is affected by the pandemic is found as: mixed development>retail>office>hotel. The study also identified the factors that significantly influenced the EUI of commercial buildings before and during the pandemic. The results of the study complement existing knowledge about the factors influencing energy consumption in commercial buildings by considering the impact of the pandemic and furthermore contribute to the improvement of energy management in commercial buildings by providing directions for building energy efficiency approaches.

Molten salt has been widely used in latent heat thermal energy storage (LHTES) system, which can be incorporated into hybrid photovoltaic/thermal solar system to accommodate the built environment. Solar salt (60 wt.% NaNO3 and 40 wt.% KNO3) was employed as the phase change materials (PCMs) in this study, and both aluminum oxide (Al2O3) nanopowder and metal foam were used to improve the properties of pure solar salt. The synthesis of the salt/metal foam composites seeded with Al2O3 nanopowder were performed with the two-step and impregnation methods, and the composite PCMs were characterized morphologically and thermally. Then pure solar salt, the salt/2 wt.% Al2O3 nanopowder and salt/copper foam composite seeded with 2 wt.% Al2O3 nanopowder were encapsulated in a pilot test rig, respectively, where a heater of 380.0 W was located in the center of the LHTES unit. The charging and discharging processes of the LHTES unit were conducted extensively, whereas the heating temperatures were controlled at 240 °C, 260 °C and 280 °C respectively. Temperature evolutions at radial, angular and axial positions were recorded, and the time-durations and volumetric mean powers during the charging and discharging processes were obtained and calculated subsequently. The results show that physical bonding between Al2O3 nanopowder and nitrate molecule has been formed from the morphological pictures together with XRD and FTIR curves. Slight changes are found between the melting/freezing phase change temperatures of the salt/metal foam composites seeded with Al2O3 nanopowder and those of pure solar salt, and the specific heats of the salt/Al2O3 nanopowder composite slightly increase with the addition of Al2O3 nanopowder. The time-duration of the charging process for the salt/copper foam composite seeded with Al2O3 nanopowder at the heating temperature of 240 °C can be reduced by about 74.0%, compared to that of pure solar salt, indicating that the heat transfer characteristics of the LHTES unit encapsulated with the salt/copper foam composite seeded with Al2O3 nanopowder can be enhanced significantly. Consequently, the mean volumetric powers of the charging process were distinctly enhanced, e.g., the volumetric mean power of heat storage can reach 110.76 kW/m3, compared to 31.94 kW/m3 of pure solar salt. However, the additive has little effect on the volumetric mean power of heat retrieval because of the domination of natural air cooling.