Natural ventilation is a green building strategy that improves building energy efficiency, indoor thermal environment, and air quality. However, in practice, it is not always clear when and how to ...utilize the natural ventilation and coordinate its operation with the HVAC system. This paper introduces a reinforcement learning control strategy, specifically through model-free Q-learning, that makes optimal control decisions for HVAC and window systems to minimize both energy consumption and thermal discomfort. This control system evaluates the outdoor and indoor environments (temperature, humidity, solar radiation, and wind speed) at each time step, and responds with the best control decision that targets both immediate and long-term goals. The reinforcement learning control is evaluated through numerical simulation on a building thermal model and compared with a rule-based heuristic control strategy. Case studies in hot-and-humid Miami and warm-and-mild Los Angeles demonstrated the superior performance of reinforcement learning control, which led to 13% and 23% lower HVAC system energy consumption, 62% and 80% lower discomfort degree hours, and 63% and 77% fewer high humidity hours compared to heuristic control. Unlike heuristic control that requires specific knowledge of individual buildings and the creation of exhaustive decision-making scenarios to improve performance, reinforcement learning control guarantees optimality through self-advancement on given goals and cost functions and is able to adapt to stochastic occupancy and occupant behaviors, which is difficult to accommodate by heuristic control.
In high-density megacities, air pollution has a higher impact on public health than cities of lower population density. Apart from higher pollution emissions due to human activities in densely ...populated street canyons, stagnated air flow due to closely packed tall buildings means lower dispersion potential. The coupled result leads to frequent reports of high air pollution indexes at street-side stations in Hong Kong. High-density urban morphologies need to be carefully designed to lessen the ill effects of high density urban living. This study addresses the knowledge-gap between planning and design principles and air pollution dispersion potentials in high density cities. The air ventilation assessment for projects in high-density Hong Kong is advanced to include air pollutant dispersion issues. The methods in this study are CFD simulation and parametric study. The SST κ–ω model is adopted after balancing the accuracy and computational cost in the comparative study. Urban-scale parametric studies are conducted to clarify the effects of urban permeability and building geometries on air pollution dispersion, for both the outdoor pedestrian environment and the indoor environment in the roadside buildings. Given the finite land resources in high-density cities and the numerous planning and design restrictions for development projects, the effectiveness of mitigation strategies is evaluated to optimize the benefits. A real urban case study is finally conducted to demonstrate that the suggested design principles from the parametric study are feasible in the practical high density urban design.
•The study clarifies the air pollutant dispersion mechanism in high-density cities.•The study addresses the urban traffic pollutant problems and provides the solutions.•The study relies on a parametric study that tests the performance of design options.•The study validates LES and RANS models using wind tunnel data.•The study provides understandings to map the pollutant dispersion in the urban areas.
Three-dimensional (3D) radiative transfer modeling of the transport and interaction of radiation through earth surfaces is challenging due to the complexity of the landscapes as well as the intensive ...computational cost of 3D radiative transfer simulations. To reduce computation time, current models work with schematic landscapes or with small-scale realistic scenes. The computer graphics community provides the most accurate and efficient models (known as renderers) but they were not designed specifically for performing scientific radiative transfer simulations. In this study, we propose LESS, a new 3D radiative transfer modeling framework. LESS employs a weighted forward photon tracing method to simulate multispectral bidirectional reflectance factor (BRF) or flux-related data (e.g., downwelling radiation) and a backward path tracing method to generate sensor images (e.g., fisheye images) or large-scale (e.g. 1 km2) spectral images. The backward path tracing also has been extended to simulate thermal infrared radiation by using an on-the-fly computation of the sunlit and shaded scene components. This framework is achieved through the development of a user-friendly graphic user interface (GUI) and a set of tools to help construct the landscape and set parameters. The accuracy of LESS is evaluated with other models as well as field measurements in terms of directional BRFs and pixel-wise simulated image comparisons, which shows very good agreement. LESS has the potential in simulating datasets of realistically reconstructed landscapes. Such simulated datasets can be used as benchmarks for various applications in remote sensing, forestry investigation and photogrammetry.
•Forward photon tracing and backward path tracing are implemented in LESS.•LESS has been extended to simulate thermal infrared images.•LESS supports simulating large-scale spectral images based on 3D landscapes.•LESS provides an easy-to-use GUI and related tools to construct the 3D scene
Low wind speeds can inhibit passive ventilation and reduce thermal comfort in tropical cities. We use both experimental and numerical approaches to investigate pedestrian-level wind speed enhancement ...with void decks (empty spaces at the ground floors) in urban street canyons. Water channel experiments show that void decks allow wind to pass through them, thereby enhancing pedestrian-level wind speeds up to twofold. Computational fluid dynamics models validated with the experiments were used to conduct two parametric studies. The first study varied the void deck height and showed that the wind enhancement effects increase with taller void decks, as expected. The second study varied the canyon aspect ratio and showed that variations in the canyon aspect ratio had less influence on wind enhancement compared to the variations in the void deck height. This is because a void deck induces a secondary vortex, which counter-rotates with the vortex driven by the freestream. Increasing the canyon aspect ratio stretches the vortices vertically but does not alter the (normalized) flow fields. Our findings reveal the potential of void decks to channel wind into street canyons, including narrow (high height-to-width aspect ratio) canyons, which suffer from poor ventilation. The enhanced wind speeds are beneficial for pollutant dispersion and outdoor thermal comfort in tropical cities.
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•Voids at ground floor (void decks) allow winds to flow “through” buildings.•Void decks enhance pedestrian-level wind speeds in urban street canyons.•Void decks are equally effective in narrow urban street canyons.•Wind enhancement effects increase with increasing void deck height.
Fabrication of stretchable chemical sensors becomes increasingly attractive for emerging wearable applications in environmental monitoring and health care. Here, for the first time, chemically ...derived ionic conductive polyacrylamide/carrageenan double-network (DN) hydrogels are exploited to fabricate ultrastretchable and transparent NO2 and NH3 sensors with high sensitivity (78.5 ppm–1) and low theoretical limit of detection (1.2 ppb) in NO2 detection. The hydrogels can withstand various rigorous mechanical deformations, including up to 1200% strain, large-range flexion, and twist. The drastic mechanical deformations do not degrade the gas-sensing performance. A facile solvent replacement strategy is devised to partially replace water with glycerol (Gly) molecules in the solvent of hydrogel, generating the water–Gly binary hydrogel with 1.68 times boosted sensitivity to NO2 and significantly enhanced stability. The DN-Gly NO2 sensor can maintain its sensitivity for as long as 9 months. The high sensitivity is attributed to the abundant oxygenated functional groups in the well-designed polymer chains and solvent. A gas-blocking mechanism is proposed to understand the positive resistance shift of the gas sensors. This work sheds light on utilizing ionic conductive hydrogels as novel channel materials to design highly deformable and sensitive gas sensors.
•A DR framework is proposed to assist buildings in exploiting thermal energy storage.•Thermal dynamics of an experimental building room with a VSHP are modelled.•Operating conditions of distribution ...grid are considered for price-based DR of VSHPs.•Two-stage optimisation problems are formulated considering wind power forecast error.
Energy storage resources (ESRs) inherent in building structures are a viable, attractive option to improve power system operation by providing demand-side flexibility. This paper proposes a two-stage optimisation framework for price-based demand response of commercial buildings that include variable speed heat pumps (VSHPs). The proposed framework aims at assisting commercial building aggregators to devise a beneficial strategy for exploiting thermal ESRs in response to electricity prices. Specifically, in this paper, the thermal dynamics of VSHPs are modelled in detail using a set of piecewise linear equations for two different methods of room temperature control. The energy consumption and reserve provision of VSHPs, as well as plug-in electric vehicles, are then co-optimised considering the operating conditions of distribution networks (DNs) for the pre- and post-contingency states of wind power generation. Simulation case studies are performed to estimate the effects of building ESRs on the optimal operation of power systems and commercial buildings under various conditions characterised by: (1) temperature control methods, (2) ESR penetration levels, and (3) DN operational constraints.
Cities suffer from low wind speeds due to the blockage effects of urban structures. Low wind speeds inhibit a city's ability to self-ventilate and reduce thermal comfort in tropical cities. Building ...porosity has been shown to be an effective architectural feature to channel winds into the pedestrian level. However, previous studies have focused on building porosity in single buildings or two-dimensional urban street canyons. We extended the investigations of building porosity to three-dimensional urban street canyons with computational fluid dynamics simulations. Validated numerical models were used to conduct parametric studies on void decks (empty spaces at the ground floors). By allowing winds to flow through them, void decks can enhance pedestrian-level wind speeds by more than twofold compared to the reference case without void decks. The wind enhancement effect is the strongest at the first canyon and decreases downstream. The effectiveness of a void deck is greatly improved by increasing the void deck height. The average wind speed in the first canyon increases from 13 percent to 59 percent (of the freestream wind speed) by increasing the void deck height from 2 m to 6 m. On the other hand, the building height and canyon (height-to-width) aspect ratio have a much smaller influence on void-deck effectiveness. Non-uniform building height in an array of buildings also has a minor influence on the effectiveness. Therefore, void decks are equally effective to enhance pedestrian-level wind speeds in urban areas with tall buildings and buildings with non-uniform height.
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•Void decks increase pedestrian-level winds by allowing winds to flow through them.•Winds in both urban street canyons and along streets can be enhanced.•Wind enhancement effects increase with void deck height.•Wind enhancement effects are insensitive to building height.
Reduced-scale experiments and full-scale field measurements show contradictory buoyancy effects from heated windward walls. Reduced-scale experiments exhibit significant thermal effects, but ...full-scale field measurements show a negligible thermal effect on the overall flow fields. This paper investigates this discrepancy by using at both scales Computational Fluid Dynamics simulations with Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES). Compared to experimental and field measurements, RANS models perform well at reduced scale but over-predict the thermal effects of heated windward walls at full scale. On the other hand, LES results agree well with measurements at both scales. Therefore, LES should be used for full-scale simulations of street canyon flows with heated windward walls. To date, there has been no explanation for the discrepancy of opposing thermal effects between reduced-scale experiments and full-scale field measurements although Richardson number similarity is satisfied. We provide an explanation by showing that in reduced-scale experiments with heated windward walls, the assumption of Reynolds number independence is invalid. In canyon flows with thermally induced buoyancy, unless the flow is proven independent of both Reynolds number and Grashof number, we should not generalize results from reduced-scale experiments to full-scale street canyons.
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•Reynolds number independence assumption is invalid in flows with opposing buoyancy.•LES results match experiments at both reduced scale and full scale.•RANS results match experiments at reduced scale but not at full scale.
Low-cost, one-step, and hydrothermal synthesized 3D reduced graphene oxide hydrogel (RGOH) is exploited to fabricate a high performance NO2 and NH3 sensor with an integrated microheater. The sensor ...can experimentally detect NO2 and NH3 at low concentrations of 200 ppb and 20 ppm, respectively, at room temperature. In addition to accelerating the signal recovery rate by elevating the local silicon substrate temperature, the microheater is exploited for the first time to improve the selectivity of NO2 sensing. Specifically, the sensor response from NH3 can be effectively suppressed by a locally increased temperature, while the sensitivity of detecting NO2 is not significantly affected. This leads to good discrimination between NO2 and NH3. This strategy paves a new avenue to improve the selectivity of gas sensing by using the microheater to raise substrate temperature.
An ultrastretchable thermistor that combines intrinsic stretchability, thermal sensitivity, transparency, and self-healing capability is fabricated. It is found the polyacrylamide/carrageenan double ...network (DN) hydrogel is highly sensitive to temperature and therefore can be exploited as a novel channel material for a thermistor. This thermistor can be stretched from 0 to 330% strain with the sensitivity as high as 2.6%/°C at extreme 200% strain. Noticeably, the mechanical, electrical, and thermal sensing properties of the DN hydrogel can be self-healed, analogous to the self-healing capability of human skin. The large mechanical deformations, such as flexion and twist with large angles, do not affect the thermal sensitivity. Good flexibility enables the thermistor to be attached on nonplanar curvilinear surfaces for practical temperature detection. Remarkably, the thermal sensitivity can be improved by introducing mechanical strain, making the sensitivity programmable. This thermistor with tunable sensitivity is advantageous over traditional rigid thermistors that lack flexibility in adjusting their sensitivity. In addition to superior sensitivity and stretchability compared with traditional thermistors, this DN hydrogel-based thermistor provides additional advantages of good transparency and self-healing ability, enabling it to be potentially integrated in soft robots to grasp real world information for guiding their actions.
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