15 September 2020, Volume 60 Issue 9

    

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SPECIAL SECTION: ENERGY GEOSTRUCTURE AND ENGINEERING
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  Tsinghua thermodynamic soil model for simulating energy engineering projects

  CHENG Xiaohui, ZHAO Naifeng, WANG Hao, ZHANG Zhichao

  Journal of Tsinghua University(Science and Technology). 2020, 60(9): 707-714. https://doi.org/10.16511/j.cnki.qhdxxb.2020.25.024

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  Energy piles utilize pile foundations to exchange heat with the surrounding rock and soil to facilitate efficient use of shallow geothermal energy supplies. An energy pile design is given that improves the thermo-mechanical coupling of the soil with the energy pile. Specifically, this paper summaries the influences of temperature and stress on the volume change of saturated clays with evaluations of the thermo-mechanical constitutive models in the literature. A Tsinghua thermodynamic soil (TTS) model is then used in finite element simulations of the thermo-mechanical behavior of elevated temperature oedometer tests of Kaolin clay with an energy pile. The predictions show that heating reduces the bearing capacity of the energy pile foundation, the interfacial normal stress and the shear stress. The results also show that the interfacial shear stress increases with increasing temperature and foundation depth, while the interfacial normal stress changes little. Cooling reduces the energy pile-soil interface interactions, increases foundation settling and reduces the bearing capacity.
  
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  Heat exchange behavior of the phase change energy pile under cooling condition

  CUI Hongzhi, ZOU Jinping, BAO Xiaohua, QI Xuedong, QI He

  Journal of Tsinghua University(Science and Technology). 2020, 60(9): 715-725. https://doi.org/10.16511/j.cnki.qhdxxb.2020.26.016

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  Phase change materials which absorb large amounts of heat can be used as backfill material around heat transfer piles to improve the heat transfer efficiency and reduce the underground space required by the heat transfer piles. This paper describes a scale model test of a 0.2 m diameter and 1.5 m long concrete phase-change energy storage pile. The pile was buried in saturated sand in a 2.45 m×2.45 m×2 m box. The heat transfer fluid temperature was kept constant by a temperature controller. The three tests used flow rates of 0.15, 0.30 and 0.45 m³/h. Each case included three cooling-heating cycles. The tests measured the thermal response and the heat transfer rates to the phase change energy pile including the pile-soil temperature differences for various flow rates and the influence of the flow rate and the flow temperatures on the heat transfer capacity of the phase change concrete pile. The results are compared with the heat transfer capacity of an ordinary concrete pile. Cooling test results show that the heat transfer to the phase change energy pile in the saturated sand is mainly in the radial direction with the sand temperature influenced over an area about twice the pile diameter as the heat transfer approached steady state. The temperature difference between the system inlet and outlet decreased as the heat transfer capacity of the phase change pile increased with increasing flow rate.
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  Effective stress principle in saturated soil with the effect of temperature

  DENG Yuebao, MAO Weiyun, KONG Gangqiang, CHENG Guanchu

  Journal of Tsinghua University(Science and Technology). 2020, 60(9): 726-732. https://doi.org/10.16511/j.cnki.qhdxxb.2020.22.013

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  Thermo-mechanical coupling is a key research topic in geotechnical engineering today for solving thermal geotechnical engineering problems, including underground energy engineering projects, radioactive waste disposal, oil transport and thermal pipelines, and thermal soft ground stabilization methods. The classical principle of the effective stress in soil mechanics describes the pressure distribution between the soil skeleton and the pore fluid for saturated soil. However, the pressure distribution and its changes between the soil skeleton and the pore water with thermal-mechanical coupling have rarely been reported. This study combines the macroscopic thermal response of the soil consolidation with the classical effective stress principle to analyze the influence of heating on the soil skeleton and the pore water to develop an effective stress and pore pressure prediction formula that includes the influence of temperature. The effective stress principle considering the temperature effect is used to analyze the effective stress and pore pressure distributions for various displacement and drainage boundary conditions. The results show that the thermo-mechanical coupling affects the total stress, the excess pore water pressure and the effective stress in the soil. The temperature stress and the thermal excess pore pressure caused by heating change with time and affect the soil consolidation and compression. The thermal stresses affect the soil stress state and indirectly affect the soil compressibility and strength. The research results can be used to develop thermal consolidation theory and to analyze problems related to thermal geotechnical engineering.
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  Field tests of the thermal-mechanical characteristics of energy piles during thermal interactions

  WANG Yanran, KONG Gangqiang, SHEN Yang, SUN Zhiwen, WANG Xinyue, XIAO Hanyu

  Journal of Tsinghua University(Science and Technology). 2020, 60(9): 733-739. https://doi.org/10.16511/j.cnki.qhdxxb.2020.21.006

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  The heat transfer efficiency, induced thermal stresses and energy pile deformation are key problems in energy pile designs. However, there has been little research on the heat transfer efficiencies and thermal-mechanical characteristics of energy piles caused by thermal effects. Field tests using a row of energy piles (1×3) with 3.0 kW heating were conducted to study the thermal-mechanical characteristics of the piles. The temperatures, strains and stresses of the middle and end piles were measured to study the differences with comparisons to a single energy pile with 3.0 kW heating. The results showed the heat transfer efficiencies, thermally induced displacements, thermally induced lateral friction resistances and the neutral point positions of the energy pile. The results show that the heat transfer efficiency of the 1×3 energy row piles is about 93% for these test conditions. The neutral point of the thermally induced lateral friction appears at about 0.4 times the pile length. The displacement of the top of the middle pile is 0.5‰ times the pile diameter (0.28 mm) while that of the edge pile is 0.3‰ times the pile diameter (0.18 mm).
ELECTRICAL ENGINEERING
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  Steady-state error and global stability analysis of energy balance control

  SHI Bingqing, ZHAO Zhengming, YUAN Liqiang, FENG Gaohui

  Journal of Tsinghua University(Science and Technology). 2020, 60(9): 740-750. https://doi.org/10.16511/j.cnki.qhdxxb.2020.21.008

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  Energy balance control uses electromagnetic energy conversion for system control. Energy balance control can provide coordinated, unified control of multiple goals that conditional controllers cannot provide by replacing the voltage and current with the energy into the energy storage elements. A system model is used to determine the control variable from the energy or power control target to improve the control system. However, the energy balance control method does not have either a steady state error analysis or a method to reduce the steady state error and there is no global stability analysis. This paper presents a steady-state error and global stability analysis for energy balance control. The three existing energy balance control methods have equivalent forms of the outer loop sliding mode surface and the inner loop controller. These are used to quantitatively analyze the steady-state error. A global Lyapunov stability analysis is provided along with a method for setting the energy balance control parameters.
  
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  Analysis of the high-frequency oscillation characteristics of a multi-port converter with an AC link

  WEI Shusheng, ZHAO Zhengming, WEN Wusong, LI Kai, YUAN Liqiang, CAI Weiqian

  Journal of Tsinghua University(Science and Technology). 2020, 60(9): 751-762. https://doi.org/10.16511/j.cnki.qhdxxb.2020.21.009

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  High-frequency links based on modular multi-active bridges (MMAB) are useful for high power multi-port converters since they are modular, efficient and isolate the various ports. High-frequency AC buses have intrinsic oscillations that do not occur in DC buses which increase the converter losses and make the system less safe. Also, oscillations in one module affect the operations of other modules connected to the common AC bus. This study modeled two kinds of converters with each having two modules to study the effects of the high-frequency oscillations in the AC-link in an MMAB. The frequency domain analysis method was then used to investigate the relationship between the oscillations and the resonant tank parameters, including the stray parameters of the high-frequency transformer (HFT) and the stray capacitance of the switching devices. The results showed the key factors leading to the oscillations for both the uncontrolled mode and the controlled mode. Tests were conducted for the two modes and for switching between the two modes. The analytical results are verified by experimental data which would lay a foundation to reduce the oscillations in MMAB in the future.
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  Energy balancing control for locomotive converter based on a discrete state event driven method

  JU Jiahe, ZHAO Zhengming, SHI Bochen, ZHU Yicheng, YU Zhujun, LUO Yunfei, ZHANG Zhixue, HU Sideng, HE Xiangning

  Journal of Tsinghua University(Science and Technology). 2020, 60(9): 763-772. https://doi.org/10.16511/j.cnki.qhdxxb.2020.25.021

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  Traction-oriented power electronic converters that are widely used in electric locomotives need to be very reliable and to avoid over-voltage problems caused by device stresses, so such devices need very stable DC bus voltages. Since the traction converter parameters are functions of the locomotive operating conditions, the single-phase rectifier control strategy must be very reliable to ensure the DC bus voltage stability. This paper presents an energy balancing control strategy for a single-phase rectifier for a traction converter which improves the converter dynamics. The control strategy needs to be tested for many different parameters and working conditions. A discrete state event driven (DSED) simulation method was used here in a numerical simulation platform for the traction converter for efficient, accurate multiple time scale (system level dynamic processes and device level transient processes) simulations. The traction converter simulation platform was used to compare the single-phase energy balance control with traditional PI control. The energy balance control is fast and effectively suppresses the DC bus fluctuations for all kinds of dynamic processes. Combining the energy balance control with the bus structure optimization also significantly reduces the turn-off electric force and allowance for the device.
CHEMISTRY AND CHEMICAL ENGINEERING
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  Input-output analysis of the economic, resource and environmental impacts of China's chemical industry

  CHEN Dingjiang, PIAN Zihao, ZHU Bing, HU Shanying

  Journal of Tsinghua University(Science and Technology). 2020, 60(9): 773-778. https://doi.org/10.16511/j.cnki.qhdxxb.2019.22.043

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  Quantitative analyses using an input-output model were used to study the relationships between China's chemical industry and the national economy and the environmental impacts of China's chemical industry and other sectors from 2002 to 2015. The results show that the contribution of the chemical industry to the increase of China's gross output value is greater than the average of all other sectors. The chemical industry does more to expand China's national economy than all other sectors. More than 75% of the fossil fuel consumption and industrial sulfur dioxide emissions of China's chemical industry can be attributed to other sectors since many chemical industry products are raw materials for these other industries. The percentage of fossil fuel consumption and industrial sulfur dioxide emissions of China's chemical industry attributed to other sectors kept increasing from 2002 to 2015.
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  Dynamic material flow analysis of polyvinyl chloride resin and additives in China

  JIANG Xiaobin, JIANG Jian, ZHOU Wenji, CHEN Dingjiang, GUO Baohua, ZHU Bing

  Journal of Tsinghua University(Science and Technology). 2020, 60(9): 779-786. https://doi.org/10.16511/j.cnki.qhdxxb.2020.21.010

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  China has become the world's largest producer and consumer of polyvinyl chloride (PVC) plastics and is facing various environmental risks from the widespread use of PVC plastics. This paper presents a dynamic material flow model to quantitatively assess PVC resins and their additives to illustrate the complex flow paths and stock distributions of PVC materials across 8 types of products, 6 sectors and 4 methods of waste management. The results showed that from 1978 to 2017, the cumulative output of PVC resins and additives was 254 million tons. PVC plastic products are mostly hard products as China has become more urbanized, with PVC additives mostly used in soft products. The building and construction sector accounted for 65.8% of the total stock of PVC resin and 41.9% of the total stock of PVC additives, which will be a future source of waste. Although the proportion of additives with potential environmental and health risks in the total inflow of additives dropped from 62.2% to 41.5%, 15 million tons of toxic additives remain in use which will have a lasting negative impact on health and the environment through migration and diffusion during their use. Although these results have some limitations due to limited data availability, the uncertainty analysis showed that the results were still very reliable. In the future, the government should strengthen the management of waste PVC plastics and increase the proportion of environmentally friendly additives through technological progress and policy guidance.
PHYSICS AND ENGINEERING PHYSICS
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  Full-scale experimental study of a fire in an island subway station with a stepped hall

  LONG Zeng, LIU Chang, YANG Yuxuan, QIU Peiyun, CHEN Jiacheng, ZHONG Maohua

  Journal of Tsinghua University(Science and Technology). 2020, 60(9): 787-794. https://doi.org/10.16511/j.cnki.qhdxxb.2020.21.004

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  Fire-induced smoke spread in a subway station was studied using full-scale experiments for various fire scenarios in an island station with a stepped hall. The smoke diffusion was characterized by measuring some key parameters, such as the smoke temperature, smoke layer height, and smoke diffusion time, for fires in the platform hall and in the stepped hall fire for two fire loads. The results show that during the platform fire, the natural wind pressure from north to south caused smoke and heat to accumulate at the south end of the platform. The smoke layer just downwind of the 3# escalator was not stable with smoke inside the connecting channel. The fire in the stepped station hall resulted in less smoke near the northern entrance and the smoke did not spread into the connecting channel or the platform. These results provide experimental data for smoke control and exhaust designs of stations with the same structure.