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Abstract The promise of redox flow batteries (RFBs) utilizing soluble redox couples, such as all vanadium ions as well as iron and chromium ions, is becoming increasingly recognized for large-scale energy storage of renewables such as wind and solar, owing to their unique advantages including scalability, intrinsic safety, and long cycle life. An ongoing question associated with these two RFBs is determining whether the vanadium redox flow battery (VRFB) or iron-chromium redox flow battery (ICRFB) is more suitable and competitive for large-scale energy storage. To address this concern, a comparative study has been conducted for the two types of battery based on their charge–discharge performance, cycle performance, and capital cost. It is found that: i) the two batteries have similar energy efficiencies at high current densities; ii) the ICRFB exhibits a higher capacity decay rate than does the VRFB; and iii) the ICRFB is much less expensive in capital costs when operated at high power densities or at large capacities.

Abstract The present study is aimed to investigate and compare effects of biodiesel-ethanol (BE) and biodiesel-n-butanol (BBu) blends on combustion, performance and emissions of a direct-injection diesel engine. Experiments were conducted on BE5 (5% ethanol and 95% biodiesel, v/v), BE10, BE15, BBu5, BBu10 and BBu15, at five engine loads and at 1800 rpm. In regard to combustion characteristics, effects on maximum heat release rate, maximum in-cylinder pressure, start of combustion, combustion duration and coefficients of variations (COVs) of IMEP and maximum increase rate of in-cylinder pressure were investigated. In regard to engine performance, effects on BSFC and BTE were investigated. The blended fuels have adverse effects on engine performance especially at low load, with the BE blends having more adverse effects than the BBu blends. Moreover, on average of the five engine loads, the BBu and BE blends increase CO emission by 13.7% and 22.8% and HC emission by 5.6% and 29.2%, respectively; but reduce NOx emission by 6.5% and 28.0%, particle mass concentration by 20.7% and 20.6% and particle number concentration by 22% and 21%, respectively. Overall, the BE blends are more effective in reducing particulate and NOx emissions but the BBu blends would lead to less increase in CO and HC emissions.

Since the utilization of renewable energy is crucial to achieve carbon neutrality, the global installation of photovoltaic (PV) devices has been growing exponentially in the past decade. As outdoor devices, PV will interact with the ambient environment, leading to impacts on power generation efficiency, system structure safety, and the surrounding microclimate. To investigate the various interactions between PV and its ambient environment, simulation with Computational Fluid Dynamics (CFD) is a frequently employed approach. Given the rapid increase of studies using CFD to investigate PV-environment interactions, this study provides a comprehensive review of the research reported in journal publications on this topic within the last decade, aiming to answer two questions: (1) Which interactions can be simulated using CFD? (2) How to simulate those interactions using CFD? A total of 69 studies were surveyed and they were categorized into six research subjects based on various interactions. According to the results, most studies applied CFD for simulations regarding PV thermal characteristics, PV cooling, and dust deposition & mitigation, whereas less were for investigating airflow & ventilation, wind loading, and microclimate. Practices of CFD setups were summarized for different steps of a simulation procedure. It was found that component scale, PV module geometry, three-dimensional modelling, Reynolds-averaged Navier-Stokes type, and SST k-ω turbulence model are generally favoured in simulations. Future research may focus on developing simplified models, boundary conditions, and parameterization methods for simulations in urban-scale and involving more complex physical phenomena.

Abstract Driven by wind and buoyancy effects in the urban environment, ventilation performance and pollutant transmission are highly related to human health. In order to investigate characteristics of the single-sided natural ventilation and interunit dispersion problem, this study conducted scaled outdoor experiments in summer and winter periods in two-dimensional street canyons. Tracer gas method was adopted to predict the ventilation rate and simulate the pollutant dispersion. It was found the ventilation performance of windward and leeward rooms showed different trends with wind velocities. Archimedes number Ar was used to examine the interactions of the buoyancy and the wind forces. It revealed that the non-dimensional ventilation rates of all rooms were generally smaller than the results of buoyancy effect only. It indicates that interactions between the buoyancy and wind effects were destructive, which reduced the ventilation rates. The interunit dispersion characteristics with the wind effect were highly dependent on source locations. The results of the tracer gas concentrations of the reentered rooms were not showing simple increasing or decreasing trends. This study provides authentic and instant airflow and pollutant dispersion information in an urban environment. The dataset of this experiment can offer validations for further numerical simulations.

Abstract Renewable energy utilisation, latent energy storage, optimal system design, and robust system operation are critical elements for carbon-free buildings and communities. Machine learning methods are effective to assist the energy-efficient renewable systems during multi-criteria design and multi-level uncertainty-based operation periods. However, the current literature provides little knowledge on this topic. In this study, a state-of-the-art-review on phase change materials for cooling applications is presented, in terms of smart ventilations, intelligent PCMs charging/discharging, deterministic parametrical analysis, stochastic uncertainty-based performance prediction and optimisation. Furthermore, technical effectiveness of machine learning methods in single and multi-objective optimisations has been presented, through hybrid PCMs integrated renewable systems. Multivariables involved in the review include thermo-physical, geometrical and operating parameters of PCMs. Multi-criteria employed in the review include heat transfer rate, cooling energy storage density, heat storage and release efficiency, and indoor thermal comfort. The literature review presents technical challenges, such as tradeoff solutions between computational accuracy and efficiency, generic methods for effective selection amongst multi-diversified optimal solutions along the Pareto front, the general methodology for multi-level uncertainty quantification, smart controllers with accurate predictions under high-level parameters’ uncertainty and stochastic occupants’ behaviors. The future outlook and recommendations of machine learning methods in PCMs integrated cooling systems have also been presented as avenues for upcoming research.

Abstract Conventional vanadium redox flow batteries (VRFBs) using Nafion 115 suffered from issues associated with high ohmic resistance and high capital cost. In this work, we report a commercial membrane (VANADion), consisting of a porous layer and a dense Nafion layer, as a promising alternative to Nafion 115. In the dual-layer structure, the porous layer (∼210 μm) can offer a high ionic conductivity and the dense Nafion layer (∼20 μm) can depress the convective flow of electrolyte through the membrane. By comparing with the conventional Nafion 115 in a VRFB, it is found that the change from the conventional Nafion 115 to the composite one results in an increase in the energy efficiency from 71.3% to 76.2% and an increase in the electrolyte utilization from 54.1% to 68.4% at a current density of as high as 240 mA cm−2. In addition, although two batteries show the comparable cycling performance at current densities ranging from 80 mA cm−2 to 240 mA cm−2, the composite membrane is estimated to be significantly cheaper than the conventional Nafion 115 due to the fact that the porous layer is rather cost-effective and the dense Nafion layer is rather thin. The impressive combination of desirable performance and low cost makes this composite membrane highly promising in the VRFB applications.

202306 bcww ; Not applicable ; RGC ; Published ; 24 months

Ternary OSCs fabricated with two acceptors with similar absorption spectra achieved the best PCE of 14.13% with an impressive FF of 78.2%.

Abstract Wind pressures on buildings with different aspect ratios were investigated via higher-order dynamic mode decomposition (HODMD) in this study. Taken’s embedding theorem was used to augment the spatial dimensionality of the original snapshot matrix by introducing time delay coordinates. HODMD was applied to reveal the spatial-temporal evolution characteristics of fluctuating wind pressures on building surfaces. It was found that HODMD can successfully extract the modes and corresponding frequencies from the random pressure field. As the aspect ratio increases, the main mode is more dominant than other modes. Moreover, comparisons between HODMD and proper orthogonal decomposition (POD) suggests that the first-order mode shapes extracted by HODMD and POD are generally similar, and both can reflect the main characteristics of the fluctuating pressure field. However, the higher-order mode shapes extracted by HODMD and POD are different due to higher-order nonlinearities. The modes of HODMD oscillate at a fixed frequency while those of POD resemble random signals, indicating the physical meaning of HODMD is more evident. Furthermore, HODMD is proven to be superior to POD in reconstructing pressure fields with less root mean square errors, because HODMD reconstructs the pressure field directly while POD primarily aims to rebuild the energy field.