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• Dynamic thermal performance of a new flat-plate solar collector (VFPC) is studied. • Three dynamic thermal performance prediction models are established and compared. • The short-term thermal performance predicted by the DHTM is the most accurate. • The long-term thermal performance predicted by the three models are similar. The flat-plate solar collector with a V-corrugated absorber (VFPC) has been shown to be able to improve the steady-state optical and thermal efficiency by up to 15.8% and 10.7%, respectively. To predict the dynamic thermal performance of VFPC, the Quasi-Dynamic Test Model (QDTM), Transfer Function Model (TFM) and Dynamic Heat Transfer Model (DHTM) are compared in this study. The coefficients of the dynamic test models (including QDTM and TFM) are obtained by the regression of experimental data, and the DHTM is presented based on the analysis of detailed heat transfer processes inside collector. Tested data under four kinds of test conditions is selected to identify parameters in the dynamic test models, and tested data under one more test condition with varied solar irradiances, fluid inlet temperatures and mass flow rates is selected to validate the prediction accuracy of the three models. For the short-term prediction results by QDTM/TFM/DHTM, the maximum absolute value of relative error of fluid outlet temperature T fo and rate of useful heat gain Q u are 6.8%/5.5%/3.0% and 64.9%/43.8%/29.6% respectively. While for the long-term prediction results with the operation period of about 1 h, the averaged absolute value of relative error of useful heat gain Q u.o are 1.4%/1.0%/1.2% and the averaged absolute error of averaged thermal efficiency η t.o are 1.1%/0.8%/0.9%. Results indicate that the DHTM has the most potential to predict the short-term thermal characteristic of VFPC accurately, followed by the TFM and QDTM, while the three kinds of models have the similar accuracy level to predict the long-term thermal characteristic of VFPC. [ABSTRACT FROM AUTHOR]

Effective use of selective absorbing coating in surface based receivers and nanofluids in volumetric based receivers for better thermal performance have been confirmed in previous studies. However, in practical application, these receivers suffered from the problem of performance degradation after long-term high-temperature operation, and they were high-cost. This study proposed a novel kind of parabolic trough receiver by locating the absorber tube inside heat transfer fluid (HTF) in a twin glass tube, and developed its mathematical heat transfer model. Based on this model, the novel receiver was verified to be desirable when the inlet temperature of HTF was lower than 150 °C (with an efficiency sacrifice within 4%), and particularly in the range of 100–120 °C (with an efficiency sacrifice within 1.5%). Strategies including replacing the selective absorbing coating with non-selective absorbing coating, lowering the velocity of outer HTF and lowering the emissivity of the inner glass tube were investigated and verified to be effective in specific conditions. [ABSTRACT FROM AUTHOR]

The thermal performance of a novel solar air collector with metal corrugated packing in the buildings of cold regions is studied in this paper. Mathematical models are developed to investigate the thermal performance of the collector and the results are verified by experiments. The hydraulic analysis is conducted experimentally to study the pressure drops of the air flows in the corrugated packing. Effects of the structural and operating parameters, such as the collector width, height, specific surface area and solar radiation intensity, ambient air temperature, air inlet temperature and velocity are studied to optimize the thermal performance of the collector. Comparisons are conducted among the metal corrugated packing solar air collector, the unglazed transpired solar collector, the glazed transpired solar collector and the packed bed solar collector with iron chips. The results indicate that the metal corrugated packing solar air collector is more appropriate to be used in the rural buildings of cold regions for its advantages of large heat transfer area, high heat transfer coefficient and good economic performance. [ABSTRACT FROM AUTHOR]

The thermal performance of seawater-source heat pump with radiant floor heating system during the whole heating season in cold regions of China is studied. High-density polyethylene helical coil heat exchanger and a new type radiant floor heating system with aluminium layer is adopted to improve the thermal performance of the system. The heat transfer characteristics of the heat exchanger in flowing seawater and the thermal resistance of each component is experimentally studied, which will be much helpful in the optimizing of heat exchangers used for the seawater-source heat pump system. The temperature distribution of the radiant floor heating system with aluminium layer is tested and the system thermal efficiency is studied during the whole heating days. The results indicate that the heating system has a large popularizing and utilizing value. [ABSTRACT FROM PUBLISHER]

In order to improve the thermal efficiency, reduce the heat losses and achieve high freezing resistance of the solar device for space heating in cold regions, a new type of serpentine compound parabolic concentrator solar collector is presented in this paper, which is a combination of a compound parabolic concentrator solar collector and a flat plate solar collector. A detailed mathematical model for the new collector based on the analysis of heat transfer is developed and then solved by the software tool Matlab. The numerical results are compared with the experimental data and the maximum deviation is 8.07%, which shows a good agreement with each other. The experimental results show that the thermal efficiency of the collector can be as high as 60.5%. The model is used to predict the thermal performance of the new collector. The effects of structure and operating parameters on the thermal performance are mathematically discussed. The numerical and experimental results show that the new collector is more suitable to provide low temperature hot water for space heating in cold regions and the mathematical model will be much helpful in the designing and optimizing of the solar collectors. [ABSTRACT FROM AUTHOR]

Solar heating applied to rural buildings is without a doubt an interesting alternative for reducing energy consumption in cold regions. The thermal performance of transpired solar collector is enhanced by the impingement of jet. To make a better use of the jet impingement, a GTC (glazed transpired solar collector) with perforating corrugated plate is developed. A mathematical model based on the energy balance equations is built to predict the thermal performance of the collector. The simulated results are validated by experiments and they show good agreement with each other. The effects of various key parameters, such as inlet and ambient temperature, total volume flow of air, radiation intensity, height and width of the collector and porosity of the absorber plate, on the thermal performance of the GTC are studied. The thermal performance and economic characteristics of the collector are compared with other transpired solar collectors. The results indicate that the GTC with perforating corrugated plate is applicative enough for its advantages in economy and thermal performance in rural areas of cold regions. [ABSTRACT FROM AUTHOR]

The heat transfer performance of a high-density polyethylene helical coil heat exchanger (HCHE) which is adopted by a seawater-source heat pump system (SWHP) is experimentally and numerically studied. The effects of seawater flow rate on heat transfer performance of the HCHE are investigated and the correlation between Nusselt and Reynolds number is fitted by the experimental results of cooling condition. The fitted results are compared with other similar studies of helical heat exchangers. Mathematical models in cooling condition, non-icy condition and icy condition are developed with the calculated external convective heat transfer coefficient. The simulation results are verified by the experimental data of icy condition and they show good agreement with each other. The effects of different key parameters on the heat transfer performance of the HCHE are investigated, such as inlet temperature, velocity of intermediate medium, pipe length and diameter, temperature of seawater and icing outside the pipes. The results indicates that the consideration of seawater flow rate in the study of subsea heat exchangers is much closer to the reality and the development of the mathematical models is very helpful in the designing and optimizing of the subsea heat exchangers used for the SWHP system. [ABSTRACT FROM AUTHOR]

The research concerning ocean energy exploitation in North China is undoubtedly of significant importance to the development of the national ocean energy. The use of seawater heat pump (SWHP) systems is a better choice than air-source heat pump (ASHP) systems. The latter systems have a lower efficiency because of the formation of frost on the surface of its airside heat exchanger when the ambient air temperature is low. This paper reports research investigating the system efficiency of SWHP systems in areas of severe cold during winter. Experiments are conducted to compare the thermal performance of the beach well infiltration intake systems (BWIS) and the helical coil heat exchanger (HCHE) for SWHP systems in the Bohai Sea, Tianjin, North China. Heat transfer rate, COP (coefficient of performance) values, exergy efficiencies and exergy destruction rates of the SWHP systems with BWIS and HCHE are presented and compared with the ASHP systems. The study results indicate that the BWIS for SWHP systems has the highest thermal performance during the whole heating season in areas of severe cold during winter. That performance is observed because icing on the outer surfaces of the tubes of the HCHE and frost formation on the surface of the heat exchanger of the ASHP have great effects on the heat transfer performance. The Heating seasonal performance factor (HSPF) is used to express the performance of heat pump systems over the entire heating season. Economic analyses of the BWIS and HCHE are performed according to local economics to compare the application of the HCHE and BWIS, and the results indicate that the HCHE has a better economic value. [ABSTRACT FROM AUTHOR]

• An unsteady state model of seawater heat exchanger with wave theory is established. • Thermal performance of the heat exchanger under icing condition is investigated. • Heating performance of SWHP system at various seawater temperature is analyzed. • A macro spatial analysis of SWHP in numerous coastal cities is performed. Seawater-source heat pump (SWHP) system exhibits merits of building energy conservation and reducing carbon emissions, while enhancing the heat transfer of seawater heat exchangers is the key to improve the system performance. Here, a model of SWHP system with staggered tube-bundle heat exchanger is established and its performance in winter is numerically studied and verified by experiments. The wave theory is introduced into the mathematical model to describe the flow field of actual ocean and improve the accuracy. The heat transfer performance of staggered tube-bundle exchanger is investigated under icing conditions, and the results indicate that the ice layer thermal resistance outside the tube accounts for more than 28% of the total thermal resistance. The impacts of seawater temperature on the SWHP system performance are discussed and the average increment of heating capacity and COP is 0.12 kW and 0.046 respectively with the increase of seawater temperature by 1 °C. Furthermore, a spatial analysis of SWHP system in 29 coastal cities of China are analyzed and compared with air-source heat pump (ASHP) system. The results show that SWHP system has greater advantages than ASHP system, especially in the southern cities. This research comprehensively analyzes the technology, economy and geography of SWHP system from a more macro perspective, and it can promote the application of ocean thermal energy. [ABSTRACT FROM AUTHOR]

[Display omitted] • Zone-level ANN model is established to predict load and energy consumption of HVAC. • Building energy flexibility is activated in a time-of-use electricity price scenario. • Rule-based control is adopted to optimize HVAC operation and conserve operation cost. • Operation cost is mitigated by 39.22% and 44.41% in the heating and cooling season. Precise and quick thermal load prediction for buildings is imperative in realizing the flexibility of building energy systems. Operation optimization based on the prediction results can effectively mitigate the energy consumption and operation cost. Here, an artificial neural network (ANN) model is built to predict the load demand and energy consumption of an office building. Thermal zones division is considered to improve the prediction accuracy. Measured data are collected to establish and validate the ANN model. The total number of samples for training, validation and testing are 13674, 3978 and 3977, respectively. A rule-based control optimization model is introduced and combined with ANN model to optimize the operation of heating, ventilation and air-conditioning system. Building energy flexibility is activated by optimization model in a time-of-use electricity price scenario. The results indicate that ANN model has high precision and the coefficient of variation of root mean squared errors corresponding to the load demand prediction and energy consumption prediction are 10.76% and 15.59%, respectively. Furthermore, the optimization results can reduce the operation cost by 39.22% and 44.41% in the heating and cooling season, respectively. [ABSTRACT FROM AUTHOR]

Clean heating transformation and photovoltaic (PV) promotion gradually have become the keys to realize dual carbon strategy goals. Based on this, the combined application of building-integrated PV (BIPV) systems and air-source heat pump (ASHP) systems has received widespread attention. However, the current combined system still presents problems such as the mismatching in time between PV output and heating load. Therefore, in this paper, a novel BIPV and ASHP coupling system is proposed, and the operation parameters were optimized using low-carbon community as an example. A conventional PV and heat pump (HP) combined system, ASHP heating, and coal-fired heating were employed as the baseline and compared with the novel system. The results show that the optimized system can improve the PV power local accommodation rate by 43% compared with a conventional PV and ASHP coupling system. In terms of economic and environmental benefits, the operating cost of the optimized system is 50% of the cost of the ASHP system. Compared with coal-fired heating system, the annual air pollutant and carbon emissions can be reduced by 92% and 57%, respectively. The system also can improve the stability of power grid operation. The demand for power capacity can be improved by 62% and the annual load balance of the power grid can be improved by 29.5%. [ABSTRACT FROM AUTHOR]

Due to the serious problems with energy efficiency, carbon emissions, and thermal comfort of rural residences in northern China, an optimization of active and passive heating technologies for rural residences is necessary. In this paper, an optimization for rural residences in northern China is conducted with four objectives: the whole life cycle carbon emission; the annual energy consumption through heating, ventilation, and air conditioning systems; the annual cost; and thermal comfort. In addition, the optimization model with active–passive heating technology synergy is resolved by NSGA-II genetic algorithm. The active and passive design variables, including the type of air source heat pump, orientation, the type and thickness of envelope insulation, the layer of window glass, the window-to-wall area ratio, as well as sunspace parameters are preferred to obtain the optimal solution. The results indicate that the optimal solution obtained by the ideal point method gives the most outstanding performance. Compared with the prototype, the optimized carbon emissions in severe cold and cold regions decreased by 56.1% and 54.6%, respectively. The annual energy consumption decreased by 59.7% and 62.2%. Finally, the roof insulation thickness is the most sensitive design variable in Pareto-optimal solution sets. This paper offers significant guidance in the application of the optimization method of active–passive technology synergy to the energy-saving design of buildings. [ABSTRACT FROM AUTHOR]

In China, the energy policy of switching charcoal heating to electric heating is pushing hard. Radiant floor heating system (RFHS) with air-source heat pump system is considered to be a suitable scheme for energy retrofit. A new type of lightweight RFHS with the aluminum layer is put forward in this article, which has the benefit of lowering the supply water temperature by enhancing the heat transfer performance and homogenizing the floor surface temperature distribution. The theoretical model of the new lightweight RFHS is built and solved by the Matlab program. Experiments are conducted to study the thermal performance of the RFHS and verify the theoretical model. The effects of supply water temperature and water flow velocity on the thermal performances of the new RFHS are studied. Then, the effects of aluminum layer on the heat transfer performance of the new system are researched. Finally, comparative research of the new lightweight RFHS and traditional heavy RFHS is carried out. The results show that the average floor surface temperature, heat transfer rate, and annual exploitation cost of the new RFHS are all better than the traditional one. [ABSTRACT FROM AUTHOR]

Highlights • Numerical model of an HDH system driven by heat pump is built and validated. • Influences of critical parameters on system performance are numerically studied. • Estimation of GOR and unit fresh water cost shows competitive system performance. • Numerical correlation for system productivity is regressed for further predictions. Abstract In this paper, a numerical investigation of a humidification-dehumidification (HDH) desalination system driven by heat pump is presented. The presented system consists of heat pump unit, plate heat exchanger, humidifier, first stage dehumidifier and second stage dehumidifier. A mathematical model based on the heat and mass balance equations of each component was built and verified by the experimental results. The effects of several important parameters of air and seawater into the system on system performance were numerically studied. The parametric analysis indicates that the air temperature has insignificant influence on the system yield, the increment of air humidity ratio or seawater flow rate is obviously beneficial for the productivity improvement and the productivity has different trends with increasing seawater temperature. It is also revealed that the productivity firstly increases and then decreases with the increment of air flow rate. Furthermore, gained output ratio (GOR) of the proposed system and the cost of fresh water are estimated. The average values of GOR and cost are 2.532 and 0.0412 USA dollar (USD) per kilogram respectively, which are competitive in comparison with other reported HDH systems. Additionally, a numerical correlation for the system productivity was regressed with an acceptable deviation. [ABSTRACT FROM AUTHOR]

Heat output and uniformity of surface temperature distribution are primarily concerned by designers and users of radiant floor heating. The testing approach, testing conditions and calculation of the heat output at each stage of program are different among the test standards. In this paper, a mathematical model to calculate surface temperature and heat output of radiant floor heating system was proposed. Measured data from experiments was used to validate the proposed model. Based on this simulation model, four current test standards in China, Europe, America and Japan were simulated. The simulation results show that the heat output of the same sample is different under the four test standards. Meanwhile, simulations were carried out at various non-heating surface temperatures. The results indicate that the non-heating surface temperature has a significant impact on the heat output of the radiant floor. The differences in the simulations of heating capacity vary from 2% to 7%. Hence, a parameter, the temperature difference between average water temperature of test sample and area weighted average temperature of non-heating surfaces, is introduced to revise the heat output characteristic equation based on the Chinese test standard. [ABSTRACT FROM AUTHOR]

In order to address global warming, most countries in the world have established carbon neutral targets and are continuously taking action to achieve carbon neutrality. The building sector accounts for 36% of end-use energy consumption and 37% of energy-related CO2 emissions globally, so carbon mitigation in building sector is considered to be the most critical step in completing the "last mile" of global carbon neutrality. Low-carbon buildings and communities are the foundation for achieving low-carbon cities and the key transition to reach the goal of carbon neutrality. Therefore, this review aimed to: (a) provide a comprehensive review of countries' policies on low-carbon buildings and communities and a theoretical basis for improving the corresponding laws and regulations; (b) investigate low-carbon technologies related to building and community construction and operation, as well as identify the current technology gaps; (c) provide a comprehensive overview of low-carbon buildings and communities assessment systems to analyze and evaluate the economic, technical, environmental and social benefits of current building and community energy systems; and (d) identify enablers and barriers in low-carbon buildings and communities to provide direction for future research. The results of this paper can provide comprehensive insights in to further achieving low-carbon buildings and communities. [ABSTRACT FROM AUTHOR]

A large amount of heat and piston wind is generated during the train braking. The piston wind will be heated by the braking energy simultaneously, which has a significant influence on the air temperature and the flow field of subway stations and tunnels. The study conducted a train-induced experiment in Tai yuanjie (TYJ) station, and the air temperature and velocity data were recorded. Meanwhile, a train-induced numerical simulation was carried out for full-scaled model based on dynamic mesh and the theory of braking energy. The experimental data was compared with the simulation results to analyze the unsteady air temperature field and unsteady air flow field of the actual station. The results indicate that the braking energy of the train cannot be ignored in the simulation of the subway station in winter. However, the utilization of the braking energy is inefficient. To improve the utilization of the braking energy, this paper put forward one passive approach and three active approaches. The simulation results indicated that the active approaches could improve the heat utilization of braking energy significantly. Ventilation system could transport part of the piston wind to the station by fans so that it can prevent cold air entering the station. [ABSTRACT FROM AUTHOR]

The district heating area in China is continuously increasing, which brings an increase in district heating load. In order to solve the shortage of heating power and realize the carbon neutral target, this study proposes two retrofit schemes for district heating system by integrating air source heat pump and water source heat pump, respectively. Mathematical models are established to study the performance of the integrated systems and a bilevel optimization model is proposed to optimize them. The results show that the air source heat pump combined district heating system has better performance compared to the conventional system, which reduces 50% energy consumption, 10.8% carbon emissions and achieves better economy. The dynamic coupling property of the district heating network and power grid are also considered, and the results indicate that the introduction of air source heat pump can effectively improve the efficiency and stability of power grid and reduce the seasonal fluctuation. The potential of large-scale application of air source heat pump combined system in Beijing is evaluated. The results reveal that retrofit scheme of integrating air source heat pumps into district heating system can cover 2930 MW heating load and bring 362 million Chinese Yuan profit by reducing 219,000 tons of carbon emissions and 539,000 tons of standard coal consumption in 2025. [ABSTRACT FROM AUTHOR]

• A novel Temperature-Humidity-Image (T-H-I) method for defrosting is proposed. • T-H-I control logic is analyzed with fractal theory and image processing technology. • Comparative experiments are conducted between T-T method and T-H-I method. • T-H-I method is proved to be more accurate and effective than T-T method. Temperature-time (T-T) defrosting control strategy is widely used in traditional air-source heat pump (ASHP) units. Its fatal problem that urgently need to be addressed is mal-defrosting, which happens frequently in heating season and causes extra energy loss. In order to accurately control the defrosting time and avoid the mal-defrosting problem, a novel Temperature-Humidity-Image (T-H-I) defrosting control strategy is proposed to improve the defrosting process. With the aid of image processing technology, T-H-I defrosting control strategy applies fractal analysis can accurately and effectively perform defrosting operation. Through the experimental investigation on frost formation characteristics, T-H-I defrosting control strategy operates defrosting when the frost thickness reaches 0.726 mm and the fractal dimension reaches 2.839, while it terminates when the fractal dimension decreases to 2.324. Comparative experiments of ASHP unit with traditional temperature-time (T-T) method and the proposed T-H-I method are conducted. Compared with the traditional T-T method, 'delayed defrosting' and 'unnecessary defrosting' are avoided effectively with the T-H-I defrosting control strategy, and the energy consumption of defrosting can be reduced by 10.6% and 22.3%, respectively. In view of its high accuracy in defrosting and effectiveness in energy saving, the proposed T-H-I method has a promising prospect for future application. [ABSTRACT FROM AUTHOR]

Frosting on the coil of the air source heat pump (ASHP) significantly affects its performance. An accurate defrosting control method is essential for the ASHP unit. Here, a novel visual defrosting control method based on machine vision is proposed. The frosting degree of the unit is accurately obtained by frosting image analysis with digital image processing to achieve on-demand defrosting. Fractal dimension, frosting degree coefficient, and single-circuit frosting evenness value are introduced to evaluate the frosting degree of coil images. The image acquisition point is determined with confusion matrix. Based on this, the logical framework of the proposed method is constructed, in which the defrosting starting and termination conditions are set as the moment when the frosting degree coefficient is equal to 0.9 and 0, respectively. A multi-variable model of fractal dimension is fitted, which improves the generalization of the proposed method. Finally, comparative experiments are conducted between the proposed method and the traditional temperature-time (T-T) method in typical cases. The results indicate that the COP and total heating capacity increase by 11.8 %, 10.6 % and 7.4 %, 6.6 %, respectively, and the defrosting frequency is reduced by 75 %, which demonstrates dominant advantages of the proposed method in accuracy, energy-saving, and indoor thermal comfort management. • A novel visual defrosting control method based on machine vision is proposed. • Microscopic image acquisition point is selected by multi-case statistical analysis. • The defrosting control conditions are determined by frosting degree coefficient. • Comparative experiments are conducted between T-T method and the novel method. [ABSTRACT FROM AUTHOR]

Combining phase change materials (PCMs) with building envelopes has emerged as an effective means to improve the thermal performance of envelopes and achieve the energy conservation and emission reduction in buildings. However, the PCM envelopes with constant phase change temperature and thermal conductivity typically work in a single season and have limited thermal regulation functions. This study proposed a double-layer heterogeneous phase change (DHPC) wall, composing two kinds of PCMs with different thermal conductivities and phase change temperatures. The thermal storage capacity, energy-saving rate and indoor thermal comfort of DHPC wall were analyzed under passive and active regulation strategies to evaluated its thermal performance in different seasons. In summer, the peak temperature of DHPC wall/box decreased by 6.6 °C/1.5 °C compared with the gypsum wall/box, which was further lowered by 1.7 °C/3.3 °C after introducing the cool water. In winter, the average temperature drop rate of gypsum/DHPC wall was 3.7/1.6 °C/h, and the thermal comfort interval duration of DHPC box increased to 92.8% ∼ 87.7% after introducing the hot water. Compared with gypsum wall/box, the attenuation factor and heat load level of DHPC wall/box decreased by 6.7% ∼ 27.3% and 4.0% ∼ 67.6% respectively, the heat storage coefficient increased by −4.2% ∼ 197.1%, and the energy-saving rate reached 9.0% ∼ 22.7%. The results showed that DHPC wall could efficiently meet the thermal requirements in winter and summer, and thus had broad application prospects. • A phase change wall with dual melting points and thermal conductivities was tested. • The seasonal thermal performance was evaluated under passive/active strategies. • The thermal insulation, storage and comfort were better under active strategies. • The thermal performance of proposed DHPC wall was superior to gypsum wall. [ABSTRACT FROM AUTHOR]

[Display omitted] Particulate matter (PM) is an important air pollutant in subway systems. Exposure to high PM concentrations poses a threat to human health and raises concerns. Piston effect can provide the outdoor fresh air to subway stations through station exits, but it also brings PM from tunnels to platforms when the train enters the station. In this paper, a transient numerical model is developed according to an actual subway station and validated by field tests. The model is used to investigate PM 10 concentration at platform induced by train movement. The cumulative PM 10 concentration at platform in one day is calculated according to the train schedule. Platform screen door (PSD) with adjustable vents is studied to control the PM 10 level at platform and energy saving in 27 cities of five climatic zones in China are discussed. The results show that adjustable vent height should be less than 0.70 m to ensure that PM 10 concentrations at platform can be maintained at an acceptable level and the requirement of fresh air can be met. Moreover, energy consumption can be reduced by more than 70 % in different cities compared with the conventional mechanical ventilation system. The optimized adjustable vent can provide an important reference for improving indoor air quality (IAQ) and saving energy consumption of ventilation systems in underground facilities. [ABSTRACT FROM AUTHOR]

• The effects of AAEMs on the APR of bio-based surrogates over Ni/α-MoO 3 were studied. • AAEMs increased hydrogen yield and promoted TOC removal. • AAEMs prevented Ni leaching and promoted oxygen vacancy formation. • The carbon deposition on the catalyst with Mg was the lowest. Aqueous phase reforming (APR) of biomass derived products is a promising hydrogen generation approach, and many internal alkali and alkaline earth metals (AAEMs) have been proven capable of catalyzing these processes. In this study, APR of various functional groups in biomass derived products over Ni/α-MoO 3 catalyst toward hydrogen production and pollutants control was investigated. The effect of different AAEMs was comprehensively studied under a temperature of 225 °C and a residence time of 30 min. Findings demonstrated that Na, K, Ca, and Mg all showed hydrogen generation and total organic carbon removal capability, whose performances were competitive to Ni/α-MoO 3 catalyst. Moreover, although the presence of single K suppressed the removal of total nitrogen (TN), the combination of K and the Ni/α-MoO 3 catalyst exhibited the optimal removal efficiency of TN (69.92 %). A series of characterization results revealed that the AAEMs effectively prevented Ni leaching in the hydrothermal environment of the Ni/α-MoO 3 catalysts and promoted the formation of oxygen vacancies, inhibiting carbon deposition and deactivation of the catalyst. [ABSTRACT FROM AUTHOR]

• A mathematic model is built to evaluate the air pollution of heating consumption. • The coal substitution policy can reduce the PM2.5 concentrations by about 20 μg/m3. • Effect of renewable energy and its development to air quality is considered. • PM2.5 concentration in 2030 will drop by about 27% in Northern China compared to 2016. The Air Pollution Prevention and Control Action Plan proposed in 2013 has contributed to the fossil fuel reduction in central heating, but the consumption of scattered coal in rural areas should not be overlooked as it accounts for nearly 50% pollutant emissions during the heating season. Air Pollution Prevention and Control Action Plan for Beijing-Tianjin-Hebei and Surrounding Areas was issued to solve the problem of scattered coal consumption and improve the heating energy structure in 2017. In this paper, an air quality-heating model is established to evaluate the air quality improvement potential and far-reaching influence of scattered coal consumption reduction in rural areas. Distinct from other air quality evaluation models, the proposed model is to evaluate the regional air quality related to the heating factors. The results indicate that the coal substitution policy can reduce PM2.5 concentrations in the heating season by about 20 μg/m3 in Beijing-Tianjin-Hebei and surrounding areas. Furthermore, predictions about the variation of PM2.5 concentration in the heating season are conducted, which indicate that the PM2.5 concentration of Beijing, Tianjin, Hebei and surrounding areas in 2030 will drop by 48%, 35%, 29% and 23% respectively compared to that in 2016. The reduction could be further enhanced to be 55%, 40%, 32% and 27% with the consideration of clean power development. This study is conducive to evaluate the clean heating technologies role in improving the environment and air quality. [ABSTRACT FROM AUTHOR]

[Display omitted] • The dynamic variations of smoke back-layering in motion train fire are studied. • The maximum back-layering length varies logarithmically with the wind pressure. • The effect of HRR, speed and deceleration on back-layering length are analyzed. • An empirical model is proposed to predict the maximum back-layering length. The exploitation and utilization of urban underground space effectively contribute to the sustainable development of society. Tunnel fire safety is necessary to ensure normal operation. Motion train fire is more serious and complex than stationary fire in tunnels due to the influence of unsteady piston wind. This study investigates the smoke back-layering characteristics of a burning subway train during emergency braking. A three-dimensional unsteady numerical model adopting the dynamic mesh technique is built to investigate the smoke diffusion characteristics of motion train fire. Experimental tests are conducted based on a 60 m-long scaled-down experimental setup to verify the accuracy of the numerical model. On this basis, the dynamic characteristics of smoke back-layering length as the longitudinal wind pressure is less than the critical value are studied, considering the influence of heat release rate (HRR), train speed, and train deceleration. The results indicate that the maximum back-layering length follows the logarithmic law with the variation of longitudinal wind pressure. An empirical model is proposed to predict the maximum back-layering length at various longitudinal wind pressure, and it is helpful to the optimization design of ventilation systems for tunnel motion fire. [ABSTRACT FROM AUTHOR]

Increasing low-grade and renewable energy sources in the HVAC systems has become necessary to reduce heating energy consumption. Hence, seeking a new air conditioning system that can be combined with renewable energy is significant to maintain a healthy and comfortable indoor environment with a low cost. This paper proposes a novel air source heat pumps assisted solar hot water with interactive cascade ventilation system (shortened as SWAP-ICV). Taking the traditional air source heat pumps with mixing ventilation system (AP-MV) as a reference, the virus control performance and energy efficiency of SWAP-ICV is investigated through CFD and TRNSYS simulations. The results demonstrate that interactive cascade ventilation under full-fresh air operation mode and normal-epidemic linkage mode can make the annual infection risk reduce by 41.16% and 9.33% compared with mixing ventilation under full-fresh air operation mode. Meanwhile, the proposed system can reduce energy consumption by 59.92% and 69.8% during the normal period and epidemic period by introducing solar energy. The average system COP during the heating season indicates that SWAP-ICV can improve the system efficiency by 1.9–2.58 times than AP-MV system. The conclusions obtained from this research can provide new ideas for the design and operation of air conditioning systems in the post-epidemic era. • A novel interactive cascade ventilation (SWAP-ICV) system was established. • Normal-epidemic linkage operation mode of SWAP-ICV was proposed. • The SWAP-ICV system can make the annual infection risk reduce by 41.16% and 9.33%. • The new operation mode can reduce energy consumption by 69.8%. • The SWAP-ICV system can improve COP by 1.9–2.58 times during the heating season. [ABSTRACT FROM AUTHOR]

• The effects of diffuse solar radiation on indoor thermal comfort are studied. • CPMV is found to deviate from TSV at high diffuse solar radiation intensity. • CPMV* is proposed to assess thermal comfort under direct and diffuse solar radiation. • CPMV* is verified to be consistent with TSV through field experiments. • Thermal acceptability in solar conditions is analyzed and compared with Fanger's PPD. With the wide application of curtain walls in modern buildings, the effect of solar radiation on indoor thermal comfort becomes significant. To assess the thermal comfort under solar conditions, the corrected predicted mean vote (CPMV) was proposed due to the neglect of solar radiation in the predicted mean vote (PMV) developed by Fanger. However, CPMV only considers direct solar radiation and ignores diffuse solar radiation. Therefore, the effects of diffuse solar radiation on thermal comfort are studied in this research, and field experiments are conducted in transparent envelope buildings. 916 thermal responses are collected in summer. The results reveal that CPMV deviates from the thermal sensation vote (TSV) at high diffuse solar radiation. Then the CPMV* is proposed to consider both direct and diffuse solar radiation, and it is validated to be accurate in quantitatively predicting the thermal sensation under global solar radiation. The thermal acceptability in solar conditions is obtained through questionnaire. The results of thermal unacceptability are different from the predicted percentage dissatisfied (PPD) developed by Fanger. The minimum percentage of dissatisfaction is 12% instead of 5% based on Fanger's PPD, and occurs at the CPMV* of −0.34 other than 0. [ABSTRACT FROM AUTHOR]

Hybrid humidification-dehumidification (HDH) and heat pump desalination system is a newly developed type of desalination plant. The hybrid system is entirely driven by electricity, and therefore, it is essential to investigate its optimal operation for certain optimization targets. In the present work, the multi-objective particle swarm optimization (MOPSO) algorithm is introduced to discover the optimal operation of a hybrid HDH and heat pump desalination system. Based on a mathematical model of the hybrid system, multi-objective optimization is performed to obtain the optimal air flow rate and seawater flow rate in terms of the maximum system production rate, minimum unit cost of fresh water and maximum gained output ratio (GOR). It can be concluded that the inlet air temperature has insignificant impact on the optimization objectives. As the inlet air humidity ratio increases within the concerned range, the increment of optimized system production rate as well as GOR are 3.79% and 4.15% respectively, and the decrement of fresh water cost is 8.50%. The variation tendencies of optimization objectives with increasing inlet seawater temperature are similar to those with increasing inlet air humidity ratio. It is also revealed that a near-linear relationship exists between the optimized cost and electricity price. • A mathematical model of a hybrid HDH and heat pump desalination system is built. • Multi-objective optimization is conducted for the hybrid system with MOPSO algorithm. • The effect of inlet air temperature on optimization results is insignificant. • Increasing inlet air humidity ratio or seawater temperature favors optimization objectives. • The optimized cost of fresh water has a near-linear relationship with the electricity price. [ABSTRACT FROM AUTHOR]

• A new type of refrigerant-heated radiator coupled with ASHP system is proposed. • The heat flux of this new radiator varies from 89.2 W/m2 and 211.4 W/m2. • The COP of the system with new radiator could be as high as 3.5 during the tests. • The system with RHR is superior in indoor thermal with the reasonable PMV. Air-source heat pump (ASHP) is considered to be one of the most energy conservation heating system in the areas without district heating. To improve the performance of the ASHP heating system, a new type of refrigerant-heated radiator (RHR) is proposed. As a new direct-condensation terminal, it avoids the secondary heat exchange and is conducive to reduce the condensing temperature and heat loss. Experiments are conducted to investigate the temperature and operating characteristics of the heating system with the new radiator, as well as the thermal performance and system efficiency during the heating period. The results indicate that the radiator has favorable thermal performance with the heat flux varies from 89.2 W/m2 to 211.4 W/m2 during the testing period. Heat storage material is employed in the radiator to maintain the stability of indoor thermal environment during the defrosting process, and it can provide the heat of 750.9–1699.6 kJ. The convective heat transfer coefficient of the RHR was regressed and the coefficient of performance (COP) of the new system could be as high as 3.5 during the tests. Meanwhile, the ASHP heating system with RHR is superior in the indoor thermal comfort with the reasonable indoor PMV and lower vertical temperature gradient. It is demonstrated that the refrigerant-heated radiator matches well with the heat pump heating system, and can effectively improve the thermal performance and indoor thermal environment, as well as it has significant effects on the application of ASHP in the severe cold areas. [ABSTRACT FROM AUTHOR]

Integrated energy system (IES) is characterized by high self-consumption ratio of on-site generated renewable energy, high efficiency of conventional energy utilization and possesses a significant flexibility in its operation. This overall, constitute to the foundation of low-carbon communities. Considering economic and environmental benefits, this paper proposes a two-layer co-optimization model with the upper layer optimizing the IES configuration and the lower layer optimizing IES operation. A community in Beijing is introduced as a case study to analyze the benefits of IES and compared with the conventional energy system. Multiple scenarios are researched, including (a) IES for low-carbon communities with PV roof area limitation, (b) IES for each of the four building types with PV roof area limitation, (c) IES for low-carbon communities without PV roof area limitation. The results indicate that the optimized IES for community can reduce the cost by 25% and CO 2 emissions by 32% per year by considering the limitation of roof area for the PV system. Without this constraint, the costs and emissions could be reduced by 20% and 62%, respectively. In addition, IES is more appropriate for office and commercial buildings since their load characteristics allow for load shifting and community-level IES costs and emissions are 8% and 10% lower than building-level due to the complementary of community load. This study provides suggestions for the IES planning and application in low-carbon community. [Display omitted] • An IES with multiple renewable energy and energy storage subsystems is proposed. • A two-layer co-optimization model of IES design and operation is developed. • IES optimization results for building-level and community-level are compared. • Impact of roof area limitation for PV on IES optimization results is investigated. [ABSTRACT FROM AUTHOR]

Highlights • The dynamic mesh numerical method is validated by the model experiment. • Energy conservation by effective utilization of piston wind is significant. • The energy saving rates are about 13%∼32% in different climatic regions. • The minimum train speed is calculated based on the CO 2 concentration. • From the perspective of thermal environment, the optimal train speed is 30 m/s. Abstract With the flourishing development of the subway construction, it becomes increasingly urgent to improve the subway tunnel environment and reduce the energy consumption of the tunnel ventilation system. The tunnel environment is significantly affected by the piston wind, which is influenced by the train speed. In this paper, a three-dimensional computational model of a subway tunnel is developed and validated through experiments. The model is used to study the carbon dioxide concentration and thermal environment of the subway tunnel. The optimal train speed is proposed with the aim to minimize the volume of mechanical supply air and to optimize the carbon dioxide concentration and thermal environment of the tunnel. In parallel with the considerations of tunnel environment, the subways in 25 cities of China are analyzed to study the energy conservation of the tunnel ventilation system by making full use of piston wind. The results indicate that the optimal train speed is 30 m/s based on the carbon dioxide concentration and thermal environment. The effective utilization of the piston wind can reduce 13%∼32% of the energy consumption for tunnel ventilation. The calculation method of the optimal train speed developed in this paper is also applicable to ordinary railway tunnels and high-speed railway tunnels. [ABSTRACT FROM AUTHOR]

Highlights • A new V-corrugated absorber with multi-channels is presented in this study. • The optical/thermal efficiency of the new collector are increased by 15.8%/10.7%. • The pump power consumption of the new collector is lowered by about 73%. • Effects of key parameters on the collector thermal performance are investigated. Abstract In this study, a new V-corrugated absorber with multi-channels is presented and applied in the liquid flat-plate solar collector. The absorber is extruded with aluminium and specific triangular channels are directly formed to improve the solar absorptance, reduce the heat conduction thermal resistance and decrease the pressure drop. Optical and heat transfer models for the new collector and a conventional sheet-and-tube collector are developed and experiments are carried out to validate the accuracy. Performance of the new and conventional collector are compared under different operational conditions and the results indicate that: (i) The daily average optical/thermal efficiency of the new and conventional collector are about 84.9%/69.4% and 69.1%/58.6%, respectively. (ii) The average exergy efficiency of the new and conventional collector are about 3.8% and 3.3% respectively under the mass flow rate of 10–90 g/s. (iii) The pressure drop and pump power consumption of the conventional collector are about 4.1 times higher than those of the new collector. The designing of V-corrugated absorber can improve the collector optical and thermal efficiency, and reduce the pressure drop and pump power consumption significantly, but exerts little effect on the exergy efficiency. Notwithstanding so, the performance improvement potential of the V-corrugated absorber is huge and the designing and manufacturing of the collector can be further developed. [ABSTRACT FROM AUTHOR]

Abstract Cities in North China are facing serious air pollution problems including high concentrations of particles, SO 2 , and NO x due to heating emissions. The exposure to air pollutants, especially PM 2.5 , is highly correlated to various adverse health impacts. However, few studies focused on the impact of heating emissions on the environment and health in China. This study aims to address the above-mentioned issues in North China by selecting 62 typical cities with different pollution levels and heating energy consumption as study objects. The heating environment index and PM 2.5 concentration were used to show the environmental impact of heating emissions. With respect to the health aspect, eight health endpoints, including the all-cause mortality, cardiovascular mortality, respiratory mortality, lung cancer mortality, chronic bronchitis, cardiovascular diseases, asthma attacks, and acute bronchitis, were evaluated. The results show that the air quality index considering heating emissions is 118.4 [95% confidence interval (CI): 111.5 to 125.2], while that without heating emissions is only 74.3 (95% CI: 68.6 to 79.9). The heating environment index is 0.64 (95% CI: 0.60 to 0.68). The proportion of polluted cities (i.e., with an air quality index larger than 100) is 77.42% considering heating emissions, while it is only 17.74% without considering heating emissions. The average PM 2.5 concentration during the heating season in the study region is 75.3 μg/m3 (95% CI: 68.2 to 82.5) and 47.7 μg/m3 (95% CI: 42.6 to 52.7) with and without considering heating emissions, respectively. In terms of the health impact, the total number of premature deaths attributable to heating emissions in the studied cities is 51154, accounting for 0.18‰ of the total population. The analysis shows that heating emissions are the major cause of the health endpoints during the heating season in most cities in North China. This study helps to confirm the quantitative environment and health impacts caused by heating emissions to timely reduce the damage to the human health of residents in North China. Graphical abstract Image Highlights • The environment and health impacts caused by heating emissions were investigated. • A novel index to evaluate the environmental impact of heating emissions was proposed. • 62 typical cities were selected to analyze the heating emission related impacts. • Our findings provide guidelines for improving environmental quality in China. [ABSTRACT FROM AUTHOR]

Abstract In this paper, a simple one-dimensional heat transfer model for the nanofluid-based direct absorption parabolic trough collector (NDAPTC) was developed, which was quick to solve, easy to understand, and unnecessary to solve the complicated differential equations. In an actual system, the vacuum condition between the inner tube and glass cover may be changed gradually over time. Therefore, a continuous function for calculating the convective heat transfer in the annulus from near vacuum to atmospheric pressure was established. Combined with it, the derived heat transfer model was suited to evaluating the long-running thermal performance of a NDAPTC quickly. The heat transfer model for a conventional indirect absorption parabolic trough collector (CIAPTC) was also constructed, aiming at comparing the heat transfer processes of this two kinds of collectors. Both the heat transfer models were validated by the test data from the reference. Additionally, the effects of physical parameters, operational and weather conditions on the collector thermal performance were analyzed. It was shown that improvement of the glass transmittance and emittance was crucial to raise the collector efficiency, especially at a higher inlet temperature. The pressure change in the annulus affected the collector thermal performance greatly, thus it tried to avoid the vacuum damage. The collector efficiency was raised with the solar irradiance and ambient temperature increasing and wind velocity reducing, which was more sensible to these weather conditions at a higher inlet temperature and pressure in the annulus. The results in this paper were useful for guiding the collector improvements and engineering designs theoretically, and were beneficial for cleaner production in civil and industrial heat utilization further. Highlights • A one-dimensional heat transfer model for the novel NDAPTC was developed. • A continuous convective heat transfer function in the annulus was derived. • The model was simple and feasible compared to the results from other research. • Effects of glass properties, operation and weather conditions on η c were studied. • The results were useful for evaluating collector long-running thermal performance. [ABSTRACT FROM AUTHOR]

Highlights • The bottom-supply mode and side-supply mode were studied contrastively. • The simulation results were validated by the physical experiment in the laboratory. • The indoor thermal environment, PPD and energy utilization efficiency were explored. • The bottom-supply mode provided better thermal comfort and lower energy saving. Abstract The growing energy consumption and thermal comfort of an air-conditioned room have attracted increasing public attention. As such, the performance of air-conditioning systems with two air supply modes for a simplified office room was studied by numerical simulation and physical experiment. An experimental investigation was developed in an environmental chamber to validate the simulation model. Numerical investigation was carried out subsequently for an office setting by the validated CFD model. The indoor thermal environment, human thermal comfort and energy utilization efficiency of two air supply modes were analyzed and compared in this study. The results showed that under both air supply modes, thermal stratification occurred in the model room, and the air flow velocity in the occupied zone was low. In addition, the results indicated that under 21 °C supply temperature, the air-conditioning system with bottom-supply mode provided better indoor thermal comfort of the occupied zone, but the energy consumption was higher than the side-supply mode. [ABSTRACT FROM AUTHOR]

In this paper, the performance of a humidification-dehumidification (HDH) desalination system with heat pump unit is experimentally investigated. The newly developed HDH system is comprised of humidifier, heat pump unit, 1st stage dehumidifier, plate heat exchanger and 2nd stage dehumidifier. The proposed system takes advantages of heat pump unit, which transfers heat to the seawater through the plate heat exchanger and absorbs heat from air through the 2nd stage dehumidifier in the meantime. Fresh water is accumulated from both the 1st stage dehumidifier and 2nd stage dehumidifier and the productivity of the proposed system is acceptable compared with other reported HDH systems and unaffected by solar radiation. The effects of seawater temperature, air temperature and air flow rate on system productivity are investigated. Seawater temperature has a positive influence on the productivity, while the effect of air temperature is insignificant. As the air flow rate increases, the productivity increases to the maximum value at a certain air flow rate and then decreases as the air flow rate still increases. The maximum productivity of the proposed system and the estimated cost of the fresh water are 22.26 kg/h and 0.051 USD/kg respectively, which are superior compared with other HDH systems. [ABSTRACT FROM AUTHOR]

The Monte Carlo Ray Tracing (MCRT) method has been confirmed flexible and efficient in the optical simulation of Concentrating Solar Collectors (CSCs), but it usually needs higher computing cost and longer runtime or its results fluctuate in multiple runs. The parameters of the way of random number generation, the number of rays, running times, grid numbers, and random number generation times all exerted effects on the simulation results. It was found that running the MCRT model with less number of rays for several more times could mitigate the fluctuation of results and decrease the total runtime simultaneously. Taken the Line-focus CSC with a metal-glass receiver and a parabolic reflector as an example, the maximum ( e max ) and average ( e avg ) relative errors of the MCRT method with 1 × 10 8 rays running for once, 2 × 10 7 rays running for once and 3 × 10 6 rays running for five times were all lower than the threshold values ( E max  = 5% and E avg  = 0.5%), but the total runtime was about 410 s, 82 s and 63 s respectively. On these bases, an optimized MCRT model was proposed by combining the MCRT method with the iteration method, where the minimum running times ( t min ) and the maximum running times ( t max ) were introduced, and they could be changed conveniently to meet the requirements of different optical simulations. By applying the proposed model to the Line-focus CSC with a more complex cavity receiver or compound parabolic reflector, the total runtime varied in the range of 268–413 s and 26–102 min respectively, indicating that the runtime reduction was significant when the limit of relative errors were acceptable. The proposed model is beneficial to mitigate the fluctuation, improve the accuracy and reduce the runtime of the MCRT method. It can also be further used to the optical simulation of various kinds of CSCs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Data enhanced interpretable machine learning was used to predict biochar property. • Data enhancement improved model accuracies in average from 5.8% to 15.8%. • The optimal RF model showed Accuracy average of 94.89%, outperforming SVM and ANN. • Sensitivity analysis provided informative insights for the predicting mechanism. • The results could enhance the design of hydrothermal biochar production systems. Hydrothermal biochar is a promising sustainable soil remediation agent for plant growth. Demands for biochar properties differ due to the diversity of soil environment. In order to achieve accurate biochar properties prediction and overcome the interpretability bottleneck of machine learning models, this study established a series of data-enhanced machine learning models and conducted relevant sensitivity analysis. Compared with traditional support vector machine, artificial neural network, and random forest models, the accuracy after data enhancement increased in average from 5.8% to 15.8%, where the optimal random forest model showed the average of accuracy was 94.89%. According to sensitivity analysis results, the essential factors influencing the predicting results of the models were reaction temperature, reaction pressure, and specific element of biomass feedstock. As a result, data-enhanced interpretable machine learning proved promising for the characteristics prediction of hydrothermal biochar. [ABSTRACT FROM AUTHOR]

By analyzing the status quo of the district heating energy consumption and air quality in China, it was found that with the environmental quality degradation, the previous assessment method using energy efficiency as the index cannot consider the impact on the environmental quality and human health simultaneously. Hence, to analyze the environmental quality deterioration and human health hazard due to the heating emissions, a novel assessment method was proposed and can be used to guide the improvement strategies. In this study, based on the quantitative influence of the heating emissions, urban population density and pollution duration, heating impact index (HII) as a new comprehensive evaluation index was introduced. It can be used to assess the urgency of the heating improvement in the studied regions. To estimate the air pollution status quo caused by heating emissions, 66 main heating cities in China were selected as the studied objects. The results showed that the calculated average heating emission-contributed air quality index (AQI) was 45% and polluted time percentage was 39%. The industrial emissions in 19 cities were the main reason to deteriorate the environmental quality during the heating season. While in the remaining 47 cities (71.2% of total studied cities), heating emissions dominated the deterioration of environmental quality. Subsequently, the cities with HII larger than 0.1 were divided into six levels to provide a guide for the Chinese government and relevant departments to improve the heating system and reduce human health hazard. This study can also be used as a reference for other polluted cities to investigate the environmental quality deterioration and human health hazard degrees due to the heating emissions. [ABSTRACT FROM AUTHOR]

In this paper, a solar air collector with slit-perforated corrugated plate is put forward, which is a combination of transpired solar air collector and corrugated packing solar air collector. The slit-perforated corrugated plate can enlarge the heat collection area and reinforce the jet impingement largely. The mathematical model is developed to predict thermal performance of the collector. Simulation results of temperature distribution in the collector are verified by the experimental data with the average bias of 4.8%. The local resistance coefficient of the collector is obtained though experimental results. The effects of key parameters on thermal efficiency are analyzed. It can be found that the height of absorber plate has the largest effect on thermal performance among structure parameters. And when air velocity in the collector increases to 1.14 m/s, effective efficiency can reach the peak, 67.83%. Compared with other transpired solar air collectors, the present solar air collector has better thermal performance and lower resistance. Case study shows that the application of the present solar air collector can produce 820.7 MJ in the heating season, which can save 43.1 kg standard coal and reduce 102.1 kg carbon dioxide emission. Hence, there is application potential for space heating in the cold region. [ABSTRACT FROM AUTHOR]

The heat source types in China were not limited to the coal, other types of energy were introduced into the heating energy consumption in recent years, and therefore the specific regional impact of the heating emissions could not be obtained accurately according to the current achievement. Based on this, this study aimed to quantify the impact of the heating emissions on the air quality in an efficient and accurate way. A new method was proposed for investigating the quantitative impact of the heating emissions on the air quality index in this paper and its assumptions and accuracy were verified as well. The data employed in this method is extracted from Ministry of Environmental Protection Data Centre, Qingyue open source and Shanghai Central Meteorological Observatory. The key prerequisites of the proposed method were discussed in this study. By calculating, the averaged absolute difference of the meteorological parameters, namely Clo, WD, WS and P, is 3.58%, 2.40%, 3.21% and 3.94%, respectively. The difference ratio of the industrial emissions relative to the total emissions is as low as 4.3%. The optimal principal component number is six for the proposed method, which can satisfy the accuracy requirement with minimum workload. Compared with the results in previous similar studies, the proposed method showed excellent performance in analyzing the regional impact of the heating emissions on ambient air quality in China. Moreover, a total of 64 North cities in China were analyzed based on the proposed method to promote the knowledge on the influence of heating emissions on the environmental quality and the related parameters were provided in this study. [ABSTRACT FROM AUTHOR]

With the flourishing development of the subway construction, more and more attentions are paid to the subway tunnel ventilation. The piston wind caused by the running trains can not only improve the tunnel environment, but also strengthen the power of the tunnel ventilation. The majority of the current researches are based on similar experiments or CFD simulations, yet most of which lack universality. It takes long time to finish the research cycles and it is difficult to generalize the conclusions based on these researches. This paper is aimed to put forward a universal prediction formula to estimate the ventilation effect of piston wind. In this paper, a uniformly accelerated Bernoulli theoretical model is established firstly, which is validated by the experimental data and numerical simulations. Secondly, a passing through model is built with the verified theoretical method and the influence of the key parameters on the ventilation rate are analyzed. Thirdly, the grey correlation analysis is conducted to study each influencing factors, meanwhile, a general prediction formula on the ventilation rate is proposed, which is applicable to the most subway tunnels in China. Finally, the effects of the trailing distance between multi-trains on the tunnel ventilation is discussed. [ABSTRACT FROM AUTHOR]

Predicting the thermal transients of district heating (DH) network is the key to simulation analysis and operation optimization of DH system. Numerical methods can provide accurate prediction and sufficient information of thermal transients. But the high computation burden restricts the application of numerical methods, especially when applied to operation optimization of large DH networks. This dilemma can be relieved by suitably increasing the scales of time and spatial steps, but do not obviously affect the precision of the numerical models. However, there are few researches concerning such topics. In this paper, the optimal scales of time and spatial steps of a newly proposed implicit upwind model and the characteristic line model were studied for fast and accurate calculation. Results show that both models can ensure the prediction errors of the pipeline outlet temperature within ±0.5 °C. For characteristic line model, the recommended time step and spatial step are 60 s and 170 m < Δ x < 520 m. And for implicit upwind method, the recommended time step and spatial step are 20 s and 30 m. Besides, the implicit upwind model is unconditionally stable and provides more information on temperature distribution along the pipeline, especially when small and fast propagation occurs. [ABSTRACT FROM AUTHOR]

Since the heating substation plays a key role in transferring the thermal energy from the primary network to the secondary network and controlling the heat output of district heating system to meet the thermal load, high operation performance of heating substation is essential for energy conservation, cost saving and emission reduction. The dynamic operation stability of heating substation is a very important dynamic characteristic of heating substation and largely affects the operation efficiency of district heating system. The operation instability of heating substation mainly manifest as flow rate and pressure oscillations, which will deteriorate the network hydraulic condition, break the network thermal balance, reduce the consumer comfort and increase the energy cost of the pumping system. Since heating substations will easily operate unstably under some conditions, this paper presents a theoretical method to analyze the stability and retune the feedback controller for operation stability of heating substation. Mathematical model of the plate heat exchanger was established and the feedback control theory was adopted to study the operation stability of heating substation. Based on the mathematical model and feedback control theory, a stability criterion was proposed for analyzing the operation stability of district heating substation effectively. The dynamic model of plate heat exchanger was validated with measured data. Simulation results show that controller tuned at certain operating condition can’t ensure operation stability of heating substation, when operating condition varies in large range. The stability analysis method proposed in this paper can be applied to analyzing the operation stability and tuning the controller of heating substation to enhance the operation stability. [ABSTRACT FROM AUTHOR]

According to the annual air quality report released by the Ministry of Environmental Protection of China, the cities that requiring heating reported an average of 168.3 days that could not reach the air quality standard, which was 59.8% higher than warm cities that did not require heating in 2014. Coal-fired energy conversion for building space heating can generate a large amount of haze pollutions during the winter. More seriously, the year-by-year data of district heating capacity increases with an average growth rate of 5.59% published in the China Statistical Yearbook, which means more and more heating associated pollutants will emit into the atmosphere and result in the worse and worse environment quality predictability in the further without taking any effective measure. In this context, considering the factors of energy, environment and human health simultaneously, this paper aimed to develop an air quality index (AQI) associated with energy conversion for building space heating model to guide the energy-saving measures on heating. Principal component analysis and grey relation projection evaluation theory approaches were used to preprocess the historical AQI data. Moreover, a simplified two-dimensionality heating capacity model based on EnergyPlus simulation results was introduced to expand the application range of the AQI-Heating (A-H) model. The A-H model included three components, which were the direct pollutant emission, regenerated pollutant emission and urban self-purifying capacity. Four pollutants related input coefficients including the direct emission factor A , the urban maximum purifying capacity a , the attenuation coefficient b and the regenerated coefficient ξ were solved by homemade code based on Matlab software according to a 10-year observed data (2003–2015). The performance of A-H model was compared with the actual values (data from 2015 to 2016) and the results of conventional linear and nonlinear regression techniques, in terms of the index of mean absolute error ( MAE ), mean absolute percentage error ( MAPE ), root-mean-squared error ( RMSE ) and index of agreement ( IA ). All performance metrics proved that the A-H model was accurate enough (e.g. IA = 0.983) to analyze the relationship between the AQI and the energy conversion for building space heating. [ABSTRACT FROM AUTHOR]

The PTC (parabolic trough solar collector) is widely used for space heating, heat-driven refrigeration, solar power, etc. The concentrated solar radiation is the only energy source for a PTC, thus its optical performance significantly affects the collector efficiency. In this study, four different optical models were constructed, validated and compared in detail. On this basis, a novel coupled method was presented by combining advantages of these models, which was suited to carry out a mass of optical simulations of collectors with different geometrical parameters rapidly and accurately. Based on these simulation results, the optimal configuration of a collector with highest efficiency can be determined. Thus, this method was useful for collector optimization and design. In the four models, MCM (Monte Carlo Method) and FVM (Finite Volume Method) were used to initialize photons distribution, as well as CPEM (Change Photon Energy Method) and MCM were adopted to describe the process of reflecting, transmitting and absorbing. For simulating reflection, transmission and absorption, CPEM was more efficient than MCM, so it was utilized in the coupled method. For photons distribution initialization, FVM saved running time and computation effort, whereas it needed suitable grid configuration. MCM only required a total number of rays for simulation, whereas it needed higher computing cost and its results fluctuated in multiple runs. In the novel coupled method, the grid configuration for FVM was optimized according to the “true values” from MCM of collectors with the maximum geometry difference. Following this, the initialization of photons distribution of collectors with different geometries were conducted with FVM. To demonstrate this coupled model in real practice, performance analysis of a kind of cavity receiver for PTCs was carried out. The judgment of collectors with maximum geometry difference were worth investigating further in the future work. [ABSTRACT FROM AUTHOR]

• Proposed a cascaded energy storage radiator with sensible and latent heat storage. • Aluminum foil enhanced the heat storage/release rate of radiator by up to 200 W. • Heat storage/release efficiency of cascaded radiator increased by 11.9%/14.6%. • Indoor vertical/horizontal temperature of cascaded radiator dropped by 2.5/0.2 °C. Clean heating technologies have been developed in past decades and numerous strategies have been adopted in improving the heat transfer performance of energy storage radiators. Nevertheless, the integrated heat storage/release effect by enhancing thermal conductivities of PCMs and the radiator has rarely been studied, and the energy utilization efficiency needed to be evaluated by considering the cascading and stratification effect. Based on these, three kinds of energy storage radiators were designed with different types and layouts of energy storage bricks, i.e. (I) magnesium–aluminum (Mg-Al) energy storage bricks without aluminum foil wrapping, (II) Mg-Al energy storage bricks wrapped by aluminum foil and (III) cascaded energy storage bricks with Mg-Al and paraffin wax-expanded graphite (PW-EG) energy storage bricks wrapped by aluminum foil. These radiators were experimentally evaluated from aspects of heat storage/release amount, heat transfer capacity and indoor thermal environment characteristics. After using aluminum foil to package energy storage bricks, the temperature rise/drop and heat storage/release rate of energy storage bricks were 21.0/56.8 °C higher and up to 95.7/119.0 W faster respectively (Radiator II vs Radiator I). While after using cascaded energy storage bricks, the heat storage/release efficiency was 11.9 %/14.6 % higher, the indoor vertical/horizontal temperature difference was 2.5/0.2 °C smaller, and the energy utilization efficiency by occupied space to total power consumption was 24.3 % higher (Radiator III vs Radiator II). Therefore, the aluminum foil packaging energy storage bricks were shown to improve the thermal conductivity, and the cascaded energy storage radiator owned higher thermal storage/release capacity and energy utilization efficiency. [ABSTRACT FROM AUTHOR]

Operational optimization is the key to energy reduction of the district heating (DH) system. Poor hydraulic performances of the DH network will largely increase the energy consumption. However, for most meshed DH networks with multiple heat sources, the optimal hydraulic conditions are usually not achieved. In this paper, the hydraulic performance optimization problem of meshed DH network with multiple heat sources was proposed. In order to solve the problem, the General Reduced Gradient (GRG) algorithm was adopted to minimize the total pump power through optimizing the pump frequencies and substation valve openings of the DH network. The hydraulic performances of the GRG algorithm based optimal control (OC) strategy were compared with the traditional constant pressure difference control (CPDC) and the constant speed control (CSC) strategies. Results shows that in comparison with the CPDC and CSC strategy, the total pump power can be reduced by 20% and 65% respectively, when applying the OC strategy. And the hydraulic intersection point of the DH network was changeable to reallocate the serving areas of heat sources and optimize the total pump power. Besides, increasing the pump efficiency without considering the hydraulic constraints of the DH network may not lead to optimal conditions. [ABSTRACT FROM AUTHOR]

• A quantitative assessment method of CACs' regulation capacity is proposed. • An adaptive method is proposed to allocate regulation requirements among CACs. • A fluctuation suppression control is adopted to guarantee the regulation accuracy. • An online control method is developed to ensure occupants' comfortable temperatures. The increasing renewable energies bring more generation fluctuations to power systems, which puts forward a higher requirement on regulation services for maintaining the system balance. Existing traditional generating units probably cannot provide sufficient regulation services to solve this challenge. With the progress of information and communication technologies, it is possible to control demand-side flexible loads to provide regulation services. Among various flexible loads, central air conditionings (CACs) have huge regulation potential, because CACs account for about 40% of the total energy consumption in buildings. However, the operation of CACs is a complicated dynamic process with multiple subsystems and independent control loops. It is difficult to accurately regulate CACs to reach the required regulation services within the comfortable indoor temperature. To address this issue, this paper focuses on the quantitative assessment and two-layer coordinated controls of CACs to provide regulation services for the power system. Firstly, the thermal-electrical model of the CAC is established to describe the dynamic operation process of subsystems' characteristics. On this basis, a quantitative assessment method of CAC's regulation services is proposed by discretizing the thermal-electrical operation characteristics. Then, to aggregate multiple CACs to provide significant regulation capacities, an adaptive allocation method is proposed to satisfy the power system's regulation requirement based on different CACs' available regulation potential. Furthermore, an online distribution control method is developed to regulate each individual CAC's cooling capacity for guaranteeing the regulation accuracy and different occupants' comfortable temperatures. The effectiveness of the proposed models and methods is illustrated by numerical studies. [ABSTRACT FROM AUTHOR]