Dynamic modelling for thermal micro-actuators using thermal networks

Thermal actuators are extensively used in microelectromechanical systems (MEMS). Heat transfer through and around these microstructures are very complex. Knowing and controlling them in order to improve the performance of the micro-actuator, is currently a great challenge. This paper deals with this topic and proposes a dynamic thermal modelling of thermal micro-actuators. Thermal problems may be modelled using electrical analogy. However, current equivalent electrical models (thermal networks) are generally obtained considering only heat transfers through the thickness of structures having considerable height and length in relation to width (walls). These models cannot be directly applied to micro-actuators. In fact, micro-actuator configurations are based on 3D beam structures, and heat transfers occur through and around length. New dynamic and static thermal networks are then proposed in this paper. The validities of both types of thermal networks have been studied. They are successfully validated by comparison with finite elements simulation and analytical calculations.     

隔热材料导热系数的数值模拟预测

对氧化锆空心球隔热材料进行合理的简化,用数值模拟方法计算氧化锆空心球隔热材料导热系数,并分析空心球半径、温度、发射率等对导热系数的影响。数值模拟结果表明,减小空心球半径,降低空心球表面发射率,抽真空等都有助于降低隔热材料导热系数。数值模拟与实验测量结果良好吻合,用数值模拟计算隔热材料导热系数是一个行之有效的方法。     

Influence of thermal stresses on thermal diffusion of hydrogen

Thermal diffusion of hydrogen atoms in zirconium taking into account thermal stresses is investigated. As mathematical model the steady-state temperature in the hollow cylinder is considered. The first invariant of the tensor of thermal stresses in the hollow cylinder has a logarithmic dependence on the radial coordinate. Such dependence permits an exact analytical solution of diffusion kinetics problem in view of thermal stresses.     

Simple thermal decomposition reactions for storage of solar thermal energy

Simple thermal decomposition reactions have been investigated for the purpose of solar thermal energy storage. Ten criteria regarding the thermodynamics and kinetics of the reaction and the physical properties of the components of the reaction have been established. One particular reaction, the decomposition of ammonium hydrogen sulfate, has been evaluated in a preliminary manner and appears to satisfy all of the established criteria. The efficiency of storage is high and the decomposition occurs in the vicinity of 500°C. Other compounds such as ammonium halides, alkali and alkaline earth metal hydroxides, carbonates, sulfates and oxides have also been examined.     

Modeling the thermal absorption factor of photovoltaic/thermal combi-panels

In a photovoltaic/thermal combi-panel solar cells generate electricity while residual heat is extracted to be used for tap water heating or room heating. In such a panel the entire solar spectrum can be used in principle. Unfortunately long wavelength solar irradiance is poorly absorbed by the semiconductor material in standard solar cells. A computer model was developed to determine the thermal absorption factor of crystalline silicon solar cells. It was found that for a standard untextured solar cell with a silver back contact a relatively large amount of long wavelength irradiance is lost by reflection resulting in an absorption factor of only 74%. The model was then used to investigate ways to increase this absorption factor. One way is absorbing long wavelength irradiance in a second absorber behind a semi-transparent solar cell. According to the model this will increase the total absorption factor to 87%. The second way is to absorb irradiance in the back contact of the solar cell by using rough interfaces in combination with a non-standard metal as back contact. Theoretically the absorption factor can then be increased to 85%.     

Combined thermal storage

In this study a combination of the two basic storages, the water storage and the bed storage, is examined. Formulas and limitations of the combined storage process, for a specific geometrical shape, are presented and the exploitation of the thermal losses of the water storage is attempted.     

Segmented thermal storage

Standard methods of charging solar thermal energy into rock bed storage for space heating tend to reduce stratification because of decreasing collector temperatures in the afternoon. This creates inaccessible regions of higher temperature rocks and may reduce the amount of energy within the bed. Stratification of the segmented bed has the advantages of supplying the highest temperature air to the dwelling and the lowest temperature air to the collector. A method of preserving the stratification by segmenting the storage bed was numerically studied. Discharging was not studied.Segmenting a standard rock bed and routing the flow to segments cooler than the inlet air during charging was shown to preserve stratification throughout the bed. The simulated segmented bed contained 1% less maximum energy than an otherwise identical standard bed for an approximated solar day. This was a consequence of the segment control temperature locations.     

太阳能热发电的显热蓄热技术进展

介绍了太阳能热发电显热蓄热的3种技术:单一流体蓄热,直接接触蓄热和间接接触蓄热。单一流体蓄热,主要有导热油、熔融盐和蒸汽3种选择。直接接触蓄热,使用廉价材料作为蓄热介质,节约了成本。间接接触蓄热使模块化安装成为可能,进一步克服了直接接触蓄热的缺点,为蓄热技术发展提供了便利。     

生物柴油与柴油热稳定性对比和热动力学分析

利用热重分析技术对生物柴油和0#柴油进行燃烧特性分析,比较两者的热稳定性。根据DTG-DTA曲线及实验数据,利用Achar微分法和Coats-Redfen积分法计算了活化能,并推断出生物柴油和柴油在低温段和高温段的非等温动力学方程。实验表明:生物柴油的挥发分较高,易于燃烧;但低温段表面活化能高于生物柴油,热稳定性优于柴油。     

Economic implications of thermal energy storage for concentrated solar thermal power

Solar energy is an attractive renewable energy source because the sun's energy is plentiful and carbon-free. However, solar energy is intermittent and not suitable for base load electricity generation without an energy backup system. Concentrated solar power (CSP) is unique among other renewable energy options because it can approach base load generation with molten salt thermal energy storage (TES). This paper describes the development of an engineering economic model that directly compares the performance, cost, and profit of a 110-MW parabolic trough CSP plant operating with a TES system, natural gas-fired backup system, and no backup system. Model results are presented for 0–12 h backup capacities with and without current U.S. subsidies. TES increased the annual capacity factor from around 30% with no backup to up to 55% with 12 h of storage when the solar field area was selected to provide the lowest levelized cost of energy (LCOE). Using TES instead of a natural gas-fired heat transfer fluid heater (NG) increased total plant capital costs but decreased annual operation and maintenance costs. These three effects led to an increase in the LCOE for PT plants with TES and NG backup compared with no backup. LCOE increased with increasing backup capacity for plants with TES and NG backup. For small backup capacities (1–4 h), plants with TES had slightly lower LCOE values than plants with NG backup. For larger backup capacities (5–12 h), plants with TES had slightly higher LCOE values than plants with NG backup. At these costs, current U.S. federal tax incentives were not sufficient to make PT profitable in a market with variable electricity pricing. Current U.S. incentives combined with a fixed electricity price of $200/MWh made PT plants with larger backup capacities more profitable than PT plants with no backup or with smaller backup capacities. In the absence of incentives, a carbon price of $100–$160/tonne CO_2eq would be required for these PT plants to compete with new coal-fired power plants in the U.S. If the long-term goal is to increase renewable base load electricity generation, additional incentives are needed to encourage new CSP plants to use thermal energy storage in the U.S.     

On the estimation of thermal resistance in borehole thermal conductivity test

Sizing of ground-coupled loop heat exchangers (GLHE) depends on the ground thermal conductivity and capacity, and the borehole thermal resistance. One popular method to estimate the thermal parameters is the interpretation of in situ thermal response tests. The modeled response is T_m=(T_in+T_out)/2, the average temperature of the fluid entering and leaving the ground. The T_m response corresponds to the physically unrealistic hypothesis of constant heat flux along a borehole. Using a 3D finite element model of the borehole, we show that T_m does not correspond to the fluid mean temperature within the borehole. Accordingly, with T_m, an overestimation of the borehole thermal resistance results. The resistance overestimation has a noticeable economic impact. We propose instead a new estimator we name “p-linear” average of T_in and T_out with parameter p→-1, as determined by numerical simulations. We show that the p-linear average closely fits the average fluid temperature computed with the numerical model, hence avoiding bias in estimation of borehole thermal resistance. Finally, we discuss the problem of collinearity arising in the estimation of thermal parameters.     

Experimental research on thermal characteristics of PCM thermal energy storage units

To explore thermal management integration in electric vehicles (EVs), a phase change materials (PCMs) thermal energy storage unit using flat tubes and corrugated fins is designed. The investigation focuses on the thermal characteristics of the PCM unit, such as the temperature variation, heat capacity, and heat transfer time, etc. Meanwhile, the heat storage and release process will be influenced by different inlet temperature, liquid flow rate, melting point of the PCM, and the combination order of the units. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Furthermore, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. The multiple PCM units with different melting temperatures are cascaded to help recycle low-grade heat energy with different temperature classes and exhibit well heat storage and release rates.     

Overall thermal energy and exergy analysis of hybrid photovoltaic thermal array

In this paper, overall thermal energy and exergy analysis has been carried out for different configurations of hybrid photovoltaic thermal (PVT) array. The hybrid PVT array (10.08 m × 2.16 m) is a series and parallel combinations of 36 numbers of PV modules. A one-dimensional transient model for hybrid PVT array has been developed using basic heat transfer equations. On the basis of this transient model, an attempt has been made to select an appropriate hybrid PVT array for different climatic conditions (Bangalore, Jodhpur, New Delhi, and Srinagar) of India. On the basis of high grade energy (i.e. overall exergy gain), case-III has been selected as the most appropriate configuration because overall exergy for case-III is 12.9% higher than case-II. The overall thermal energy and exergy gain for Bangalore is 4.54 × 10⁴ kW h and 2.07 × 10⁴ kW h respectively which is highest in comparison to the other cities.     

New challenge in advanced thermal energy transportation using functionally thermal fluids

The present review article presents the current status of some researches on thermal energy transportation using functionally thermal fluid, which is a mixture of heat transfer medium like water and other material with or without phase change like a paraffin wax as a latent heat storage material. This functionally thermal fluid offers attractive opportunities for thermal energy transportation and heat transfer enhancement of heat exchanger. This article describes classification and characteristics of functionally thermal fluids and their application. Referring to functionally thermal fluid for the usage of sensible heat, some visco-elastic fluids for flow drag reduction in a thermal energy transport system such as aqueous polymer solution and surfactant solution are mentioned. On the other hand, this article describes heat transfer and hydrodynamic characteristics of some phase change slurries like ice slurry, phase change microemulsion slurry, phase change microencapsule slurry, clathrate slurry and shape-stabilized paraffin and polyethylene pellets as functionally thermal fluids using latent heat between solid and liquid phases. Finally, it leads to the conclusion that some functionally thermal fluids are very useful for the advanced thermal energy transportation and heat exchanger systems.     

3-D thermal modelling applied to stress-induced anisotropy of thermal conductivity

The present work consists in the development of a three-dimensional model of heat diffusion in orthotropic media, based on numerical Fourier transforms, and taking into account the extent of the source. This model has been applied, together with a Gauss–Newton parameter estimation procedure, to identify the components of the conductivity tensor of a steel bar under uniaxial loading. Few percent variations of the conductivity components have been observed for applied stresses remaining in the elastic domain.     

太阳能热发电系列文章(1)聚光类太阳能热发电概述

能源短缺、资源枯竭、环境污染等问题已严重影响人们的生活和制约社会的发展,各国竞相开展水能、风能、地热能、生物质能、潮汐能、太阳能等清洁和可再生能源的应用研究。美国、德国、以色列、澳大利亚及日本等国家在太阳能应用技术研究方面起步较早,也是当今太     

Liquid cooling based on thermal silica plate for battery thermal management system

To achieve safe, long lifetime, and high‐performance lithium‐ion batteries, a battery thermal management system (BTMS) is indispensable. This is especially required for enabling fast charging‐discharging and in aggressive operating conditions. In this research, a new type of battery cooling system based on thermal silica plates has been designed for prismatic lithium‐ion batteries. Experimental and simulations are combined to investigate the cooling capability of the BTMS associated to different number of cooling channels, flow rates, and flow directions while at different discharge C‐rates. Results show that the maximum temperature reached within the battery decreases as the amount of thermal silica plates and liquid channels increases. The flow direction had no significant influence on the cooling capability. While the performance obviously improves with the increase in inlet flow rate, after a certain threshold, the gain reduces strongly so that it does not anymore justify the higher energy cost. Discharged at 3 C‐rate, an inlet flow rate of 0.1 m/s was sufficient to efficiently cool down the system; discharged at 5 C‐rate, the optimum inlet flow rate was 0.25 m/s. Simulations could accurately reproduce experimental results, allowing for an efficient design of the liquid‐cooled BTMS.     

Three-dimensional thermal finite element modeling of lithium-ion battery in thermal abuse application

In order to better understand the thermal abuse behavior of high capacities and large power lithium-ion batteries for electric vehicle application, a three-dimensional thermal model has been developed for analyzing the temperature distribution under abuse conditions. The model takes into account the effects of heat generation, internal conduction and convection, and external heat dissipation to predict the temperature distribution in a battery. Three-dimensional model also considers the geometrical features to simulate oven test, which are significant in larger cells for electric vehicle application. The model predictions are compared to oven test results for VLP 50/62/100S-Fe (3.2 V/55 Ah) LiFePO₄/graphite cells and shown to be in great agreement.     

Influence of thermal sensitivity of the materials on temperature and thermal stresses of the brake disc with thermal barrier coating

The time-dependent frictional heating of a disc with applied thermal barrier coating (TBC) on its working surface was investigated. To determine the temperature fields in the coating and the disc a one-dimensional friction heat problem during braking was formulated, with taking into account the dependence of thermal properties of materials from temperature. A model was adopted for materials with a simple non-linearity, i.e. materials whose thermal conductivity and specific heat are temperature dependent, and their ratio – thermal diffusivity is constant. The linearization of the corresponding boundary-value heat conduction problem was made by the Kirchhoff transformation and the linearizing multipliers method. A numerical-analytical solution to the obtained problem was found by Laplace transform method. Knowing the temperature distributions, quasi-static thermal stresses in the strip (TBC) with taking into account change in temperature mechanical properties, were determined. The distribution of temperature and thermal stresses in the strip made from ZrO₂ deposited on the UNS G51400 steel disc, was investigated.     

建筑绝热材料导热系数测试方法探讨

以GB/T 10294—2008为例,对使用防护热板法测试建筑绝热材料导热系数过程中易忽略及不明确的地方进行阐述,主要包括:试件的加工及干燥的处理方法 ;试件厚度的测量方式;环境温、湿度的控制范围;修正系数的标定及参数设置;测量不确定度等方面,并给出了指导性意见和建议。