Modeling of heat transfer for energy efficiency prediction of solar receivers

© 2019 Elsevier. This manuscript is made available under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International licence (CC BY-NC-ND 4.0). Please see https://creativecommons.org/licenses/by/4.0/ for further details.In this article, a new heat transfer model for solar receivers with metal foam is developed for design optimization. The proposed model facilitates analysis of heat transfer processes in terms of forced convection, natural convection, heat conduction and radiation, accurately predicting the energy efficiency and percentage contribution of each form of heat loss. The results show good agreement between the predicted results and the experimental data. Specifically, sensitivity analysis is performed to predict the energy efficiency of solar receivers under different operating conditions. To explore the influence of inlet temperature, a series of simulations under high inlet temperature are carried out, resulting in poorer energy performance and heavier radiant heat loss. Non-radiant heat loss, however, accounts for less than 1.1% of the total energy loss in all cases. The results reveal that reduction of radiant loss is conducive to energy efficiency improvement.Peer reviewe

Consecutive Insulator-Metal-Insulator Phase Transitions of Vanadium Dioxide by Hydrogen Doping

We report modulation of a reversible phase transition in VO2 films by hydrogen doping. A metallic phase and a new insulating phase are successively observed at room temperature as the doping concentration increases. It is suggested that the polarized charges from doped hydrogens play an important role. These charges gradually occupy V3d-O2p hybridized orbitals and consequently modulate the filling of the VO2 crystal conduction band-edge states, which eventually evolve into new valence band-edge states. This demonstrates the exceptional sensitivity of VO2 electronic properties to electron concentration and orbital occupancy, providing key information for the phase transition mechanism.Comment: 16 pages, 4 figure

Experimental Study of the Energy and Exergy performance for a Pressurized Volumetric Solar Receiver

This document is the Accepted Manuscript of the following article: Jianqin Zhu, Kai Wang, Guoqing Li, Hongwei Wu, Zhaowu Jiang, Feng Lin, and Yongliang Li, 'Experimental study of the energy and exergy performance for a pressurized volumetric solar receiver', Applied Thermal Engineering, Vol. 104, July 2016, pp. 212-221. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/. The Version of Record is available online at doi: https://doi.org/10.1016/j.applthermaleng.2016.05.075. © 2016 Elsevier Ltd. All rights reserved.This article presents an experimental investigation of the heat transfer characteristics as well as energy and exergy performance for a pressurized volumetric solar receiver under variable mass flow rate conditions. During a two-hour period of continuous operation in the morning, the solar irradiance is relatively stable and maintained at approximately 600 W/m2, which is beneficial for analyzing the energy and exergy performance of the solar receiver. Experimental results show that the mass flow rate fluctuation has insignificant effect on the solar receiver outlet temperature, whereas the mass flow rate plays an important role in the solar receiver power, energy efficiency and exergy efficiency. The efficiency of the solar receiver is normally above 55% with the highest efficiency of 87%, and under steady state operating conditions the efficiency is maintained at approximately 60%. A very low value of the heat loss factor (0.014 kW/K) could be achieved during the current steady state operating conditions. The highest exergy efficiency is approximately 36%. In addition, as the temperature difference increases, the impact of the exergy factor increases. The highest exergy factor is 0.41 during the entire test.Peer reviewe

Insight into Electronic and Structural Reorganizations for Defect-Induced VO₂ Metal–Insulator Transition

An oxygen vacancy defect in monoclinic VO₂ has been shown to modulate the metal–insulator transition (MIT) at room temperature. However, as the electronic and structural reorganizations occur simultaneously, the origin of MIT is still unclear. Here we performed first-principles calculations to examine electronic variations separately from structural reorganizations during MIT. It was found that the oxygen defect induces electronic reorganization by creating polarized 3d orbitial electrons, while structure reorganization makes the conduction band edge states available for occupation. The conduction band states thus hold polarized charges that delocalize over space, bestowing metallic property on the originally insulated VO₂. A linear relationship for the number of polarized electrons and the defect concentration is revealed, which would lead to cost-effective control of VO₂ MIT behavior by defect engineering

A simple but actionable metric for assessing inequity in resident greenspace exposure

An increasing number of studies aim to improve and perfect the evaluation system for assessing greenspace exposure, yet it may also become more difficult to apply the evaluation index to landscape planning. Here we propose a simple but actionable index system – Greenspace Exposure Inequity index (GEII), for assessing the inequity of residents' greenspace exposure. GEII includes quantity-based availability, distance-based accessibility, and inequity-based Gini index for assessing the difference in greenspace exposure pattern. Then we selected Shanghai as a case to test the feasibility of GEII, analyzing the spatiotemporal evolution of greenspace exposure patterns, and further demonstrating the operability of the index. (1) Availability inequity for 2012, 2015, 2018, and 2021 was 0.603, 0.512, 0.514, and 0.489. The Gini index was between 0.4 and 0.6, and presented a downward trend. (2) Accessibility inequity for 2012, 2015, 2018, and 2021 was 0.372, 0.368, 0.364, and 0.344. It can be clearly seen that it has changed less over ten years, but overall equality has been rising. (3) Using GEII to calculate the inequity of Shanghai, the Gini index for 2012, 2015, 2018, and 2021 was 0.392, 0.378, 0.373, and 0.357. The inequity of greenspace exposure assessed by GEII is gradually decreasing similarly, which illustrates the positive impact of urban greening policies. The GEII has three highlights: serviceability, human-oriented, and expandability. GEII abandons the complex computational evaluation procedures of numerous indicators and bridges the gap between theoretical research on inequity and practical planning, so GEII is of great value for alleviating the uneven exposure of residents' greenspace and scientifically optimizing landscape planning

Circular RNAs in cell cycle regulation: Mechanisms to clinical significance

10.1111/cpr.13143Cell Proliferation5412e1314

Suppressing Electron–Phonon Coupling through Laser-Induced Phase Transition

Using first-principle calculations, we introduced a strategy of laser-induced phase transition that suppress electron–phonon couplings in crystal lattice. We explained unusual irreversible phase transitions in previous experiments on MoTe₂ and NaYF₄ crystals. Laser irradiations produced local heats in 2H-MoTe₂ and Hex NaYF₄, driving atom reorganizations toward new lattices. The reorganization with effective electron–phonon couplings continues with spontaneously generated heats, whereas a 1T′-MoTe₂ and a metastable cubic NaYF₄ phases were kept because of suppressed vibrational relaxations. Long time laser treatments create phases with weak electron–phonon couplings. Such irreversible transitions guarantee complete conversions, opening a new door to selective material modifications