Frontiers in Energy

ISSN 2095-1701 (Print)
ISSN 2095-1698 (Online)
CN 11-6017/TK
Postal Subscription Code 80-972
Formerly Known as Frontiers of Energy and Power Engineering in China
2018 Impact Factor: 1.701
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Thermal performance of a single-layer packed metal pebble-bed exposed to high energy fluxes
Shengchun ZHANG, Zhifeng WANG, Hui BIAN, Pingrui HUANG
Front. Energy
Abstract   HTML   PDF (3022KB)

It is difficult to accurately measure the temperature of the falling particle receiver since thermocouples may directly be exposed to the solar flux. This study analyzes the thermal performance of a packed bed receiver using large metal spheres to minimize the measurement error of particle temperature with the sphere temperature reaching more than 700°C in experiments in a solar furnace and a solar simulator. The numerical models of a single sphere and multiple spheres are verified by the experiments. The multiple spheres model includes calculations of the external incidence, view factors, and heat transfer. The effects of parameters on the temperature variations of the spheres, the transient thermal efficiency, and the temperature uniformity are investigated, such as the ambient temperature, particle thermal conductivity, energy flux, sphere diameter, and sphere emissivity. When the convection is not considered, the results show that the sphere emissivity has a significant influence on the transient thermal efficiency and that the temperature uniformity is strongly affected by the energy flux, sphere diameter, and sphere emissivity. As the emissivity increases from 0.5 to 0.9, the transient thermal efficiency and the average temperature variance increase from 53.5% to 75.7% and from 14.3% to 27.1% at 3.9 min, respectively. The average temperature variance decreases from 29.7% to 9.3% at 2.2 min with the sphere diameter increasing from 28.57 mm to 50 mm. As the dimensionless energy flux increases from 0.8 to 1.2, the average temperature variance increases from 13.4% to 26.6% at 3.4 min.

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MPC-based interval number optimization for electric water heater scheduling in uncertain environments
Jidong WANG, Chenghao LI, Peng LI, Yanbo CHE, Yue ZHOU, Yinqi LI
Front. Energy
Abstract   HTML   PDF (1115KB)

In this paper, interval number optimization and model predictive control are proposed to handle the uncertain-but-bounded parameters in electric water heater load scheduling. First of all, interval numbers are used to describe uncertain parameters including hot water demand, ambient temperature, and real-time price of electricity. Moreover, the traditional thermal dynamic model of electric water heater is transformed into an interval number model, based on which, the day-ahead load scheduling problem with uncertain parameters is formulated, and solved by interval number optimization. Different tolerance degrees for constraint violation and temperature preferences are also discussed for giving consumers more choices. Furthermore, the model predictive control which incorporates both forecasts and newly updated information is utilized to make and execute electric water heater load schedules on a rolling basis throughout the day. Simulation results demonstrate that interval number optimization either in day-ahead optimization or model predictive control format is robust to the uncertain hot water demand, ambient temperature, and real-time price of electricity, enabling customers to flexibly adjust electric water heater control strategy.

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A novel cryogenic insulation system of hollow glass microspheres and self-evaporation vapor-cooled shield for liquid hydrogen storage
Jianpeng ZHENG, Liubiao CHEN, Ping WANG, Jingjie ZHANG, Junjie WANG, Yuan ZHOU
Front. Energy
Abstract   HTML   PDF (1279KB)

Liquid hydrogen (LH2) attracts widespread attention because of its highest energy storage density. However, evaporation loss is a serious problem in LH2 storage due to the low boiling point (20 K). Efficient insulation technology is an important issue in the study of LH2 storage. Hollow glass microspheres (HGMs) is a potential promising thermal insulation material because of its low apparent thermal conductivity, fast installation (Compared with multi-layer insulation, it can be injected in a short time.), and easy maintenance. A novel cryogenic insulation system consisting of HGMs and a self-evaporating vapor-cooled shield (VCS) is proposed for storage of LH2. A thermodynamic model has been established to analyze the coupled heat transfer characteristics of HGMs and VCS in the composite insulation system. The results show that the combination of HGMs and VCS can effectively reduce heat flux into the LH2 tank. With the increase of VCS number from 1 to 3, the minimum heat flux through HGMs decreases by 57.36%, 65.29%, and 68.21%, respectively. Another significant advantage of HGMs is that their thermal insulation properties are not sensitive to ambient vacuum change. When ambient vacuum rises from 103 Pa to 1 Pa, the heat flux into the LH2 tank increases by approximately 20%. When the vacuum rises from 103 Pa to 100 Pa, the combination of VCS and HGMs reduces the heat flux into the tank by 58.08%–69.84% compared with pure HGMs.

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Theoretical and technological exploration of deep in situ fluidized coal mining
Heping XIE, Yang JU, Shihua REN, Feng GAO, Jianzhong LIU, Yan ZHU
Front. Energy
Abstract   HTML   PDF (2144KB)

Mining industries worldwide have inevitably resorted to exploiting resources from the deep underground. However, traditional mining methods can cause various problems, e.g., considerable mining difficulty, environmental degradations, and frequent disastrous accidents. To exploit deep resources in the future, the concept of mining must be reconsidered and innovative new theories, methods, and technologies must be applied. To effectively acquire coal resources deeper than 2000 m, new theoretical and technological concepts about deep in situ fluidized mining are required. The limits of mining depth need to be broken to acquire deep-coal resources in an environmentally friendly, safe, and efficient manner. This is characterized by ‘There are no coal on the ground and no men in the coal mine’. First, this paper systematically explains deep in situ fluidized coal mining. Then, it presents a new theoretical concept, including the theories of mining-induced rock mechanics, three-field visualization, multi-physics coupling for in situ transformation, and in situ mining, transformation and transport. It also presents key technological concepts, including those of intelligent, unmanned, and fluidized mining. Finally, this paper presents a strategic roadmap for deep in situ fluidized coal mining. In summary, this paper develops new theoretical and technological systems for accomplishing groundbreaking innovations in mining technologies of coal resources in the deep underground.

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Exergy losses in premixed flames of dimethyl ether and hydrogen blends
Tongbin ZHAO, Jiabo ZHANG, Dehao JU, Zhen HUANG, Dong HAN
Front. Energy
Abstract   HTML   PDF (1413KB)

A second-law thermodynamic analysis was conducted for stoichiometric premixed dimethyl ether (DME)/hydrogen (H2)/air flames at atmospheric pressure. The exergy losses from the irreversibility sources, i.e., chemical reaction, heat conduction and species diffusion, and those from partial combustion products were analyzed in the flames with changed fuel blends. It is observed that, regardless of the fuel blends, chemical reaction contributes most to the exergy losses, followed by incomplete combustion, and heat conduction, while mass diffusion has the least contribution to exergy loss. The results also indicate that increased H2 substitution decreases the exergy losses from reactions, conduction, and diffusion, primarily because of the flame thickness reduction at elevated H2 substitution. The decreases in exergy losses by chemical reactions and heat conduction are higher, but the exergy loss reduction by diffusion is slight. However, the exergy losses from incomplete combustion increase with H2 substitution, because the fractions of the unburned fuels and combustion intermediates, e.g., H2 and OH radical, increase. The overall exergy losses in the DME/H2 flames decrease by about 5% with increased H2 substitution from 0% to 100%.

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Numerical and experimental research of the characteristics of concentration solar cells
Zilong WANG, Hua ZHANG, Binlin DOU, Weidong WU, Guanhua ZHANG
Front. Energy
Abstract   HTML   PDF (1449KB)

The development of automatic tracking solar concentrator photovoltaic systems is currently attracting growing interest. High concentration photovoltaic systems (HCPVs) combining triple-junction InGaP/lnGaAs/Ge solar cells with a concentrator provide high conversion efficiencies. The mathematical model for triple-junction solar cells, having a higher efficiency and superior temperature characteristics, was established based on the one-diode equivalent circuit cell model. A paraboloidal concentrator with a secondary optic system and a concentration ratio in the range of 100X–150X along with a sun tracking system was developed in this study. The GaInP/GalnAs/Ge triple-junction solar cell, produced by AZUR SPACE Solar Power, was also used in this study. The solar cells produced by Shanghai Solar Youth Energy (SY) and Shenzhen Yinshengsheng Technology Co. Ltd. (YXS) were used as comparison samples in a further comparative study at different concentration ratios (200X–1000X). A detailed analysis on the factors that influence the electrical output characteristics of the InGaP/lnGaAs/Ge solar cell was conducted with a dish-style concentrating photovoltaic system. The results show that the short-circuit current (Isc) and the open-circuit voltage (Voc) of multi-junction solar cells increases with the increasing concentration ratio, while the cell efficiency (ηc) of the solar cells increases first and then decreases with increasing concentration ratio. With increasing solar cell temperature, Isc increases, while Voc and ηc decrease. A comparison of the experimental and simulation results indicate that the maximum root mean square error is less than 10%, which provides a certain theoretical basis for the study of the characteristics of triple-junction solar cell that can be applied in the analysis and discussion regarding the influence of the relevant parameters on the performance of high concentration photovoltaic systems.

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An adsorption study of 99Tc using nanoscale zero-valent iron supported on D001 resin
Lingxiao FU, Jianhua ZU, Enxi GU, Huan WANG, Linfeng HE
Front. Energy
Abstract   HTML   PDF (1644KB)

Nanoscale zero-valent iron (nZVI) supported on D001 resin (D001-nZVI) was synthesized for adsorption of high solubility and mobility radionuclide 99Tc. Re(VII), a chemical substitute for 99Tc, was utilized in batch experiments to investigate the feasibility and adsorption mechanism toward Tc(VII). Factors (pH, resin dose) affecting Re(VII) adsorption were studied. The high adsorption efficiency of Re(VII) at pH= 3 and the solid-liquid ratio of 20 g/L. X-ray diffraction patterns revealed the reduction of ReO4 into ReO2 immobilized in D001-nZVI. Based on the optimum conditions of Re(VII) adsorption, the removal experiments of Tc(VII) were conducted where the adsorption efficiency of Tc(VII) can reach 94%. Column experiments showed that the Thomas model gave a good fit to the adsorption process of Re(VII) and the maximum dynamic adsorption capacity was 0.2910 mg/g.

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Energy conservation in China’s coal-fired power industry by installing advanced units and organized phasing out backward production
Weiliang WANG, Junfu LYU, Zheng LI, Hai ZHANG, Guangxi YUE, Weidou NI
Front. Energy
Abstract   HTML   PDF (705KB)

Coal-fired power is the main power source and the biggest contributor to energy conservation in the past several decades in China. It is generally believed that advanced technology should be counted on for energy conservation. However, a review of the decline in the national average net coal consumption rate (NCCR) of China’s coal-fired power industry along with its development over the past few decades indicates that the up-gradation of the national unit capacity structure (including installing advanced production and phasing out backward production) plays a more important role. A quantitative study on the effect of the unit capacity structure up-gradation on the decline in the national average NCCR suggests that phasing out backward production is the leading factor for the decline in the NCCR in the past decade, followed by the new installation, whose sum contributes to approximately 80% of the decline in the national average NCCR. The new installation has an effective affecting period of about 8 years, during which it would gradually decline from a relatively high value. Since the effect of phasing out backward production may remain at a certain degree given a continual action of phasing out backward capacity, it is suggested that the organized action of phasing out backward production should be insisted on.

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Key problems and solutions in supercritical CO2 fracturing technology
Haizhu WANG, Gensheng LI, Bin ZHU, Kamy SEPEHRNOORI, Lujie SHI, Yong ZHENG, Xiaomei SHI
Front. Energy
Abstract   HTML   PDF (1025KB)

Supercritical CO2 fracturing is considered to be a new method for efficient exploitation of unconventional reservoirs, such as shale gas, coal bed methane, and tight sand stone gas. Supercritical CO2 has many special properties including low viscosity, high diffusion coefficient, and lack of surface tension, which brings about great advantages for fracturing. However, these properties also cause several problems, such as difficulty in proppant transportation, high friction loss, and high pump displacement. In this paper, the above problems were analyzed by combining field test with laboratory study and specific solutions to these problems are given. The high frictionloss in the pipeline could be reduced by developing a new drag reducing agent and selecting large-size casing. Besides, for the problem of poor capacity in proppant carrying and sand plug, the methods of adding tackifier into supercritical CO2, increasing pump displacement and selecting ultra-low density proppants are proposed. Moreover, for the problem of fast leak-off and high requirement for pump displacement, the displacement can be increased or the pad fluid can be injected into the reservoir. After solving the above three problems, the field test of supercritical CO2 fracturing can be conducted. The research results can promote the industrialization process of supercritical CO2 fracturing.

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A decoupled method to identify affecting mechanism of crosswind on performance of a natural draft dry cooling tower
Weiliang WANG, Junfu LYU, Hai ZHANG, Qing LIU, Guangxi YUE, Weidou NI
Front. Energy
Abstract   HTML   PDF (1401KB)

The natural draft dry cooling tower (NDDCT) has been increasingly used for cooling in power generation in arid area. As crosswind affects the performance of a NDDCT in a complicated way, and the basic affecting mechanism is unclear, attempts have been made to improve the performance of a NDDCT based on limited experiences. This paper introduces a decoupled method to study the complicated crosswind effects on the inlet and outlet of a NDDCT separately by computational fluid dynamics (CFD) modeling and hot state experiments. Accordingly, the basic affecting mechanism of crosswind on the NDDCT performance is identified. Crosswind changes the inlet flow field of a NDDCT and induces mainstream vortices inside the tower, so as to degrade the ventilation. Besides, low crosswind deflects the upward plume at the outlet to further degrade the ventilation, while high crosswind induces the low pressure area at the outlet to reduce the ventilation degradation.

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