Polycrystalline silicon thin films and solar cells prepared by rapid thermal CVD

Polycrystalline silicon (poly-Si) films ( 10 μm) were grown from dichlorosilane by a rapid thermal chemical vapor deposition (RTCVD) technique, with a growth rate up to 100 Å/s at the substrate temperature (T_s) of 1030°C. The average grain size and carrier mobility of the films were found to be dependent on the substrate temperature and material. By using the poly-Si films, the first model pn⁺ junction solar cell without anti-reflecting (AR) coating has been prepared on an unpolished heavily phosphorus-doped Si wafer, with an energy conversion efficiency of 4.54% (AM 1.5, 100 mW/cm², 1 cm²).     

Experimental performance of a thermoelectric ceiling cooling panel

An experiment has been performed to investigate the cooling performance of a thermoelectric ceiling cooling panel (TE‐CCP). The TE‐CCP was composed of 36 TE modules. The cold side of the TE modules was fixed to an aluminum ceiling panel to cool a test chamber of 4.5 m³ volume, while a copper heat exchanger with circulating cooling water at the hot side of the TE modules was used for heat release. Tests were conducted using various system parameters. It was found that the cooling performance of the system depended on the electrical supply, cooling water temperature and flow rate through the heat exchanger. A suitable condition occurred at 1.5 A of current flow with a corresponding cooling capacity of 289.4 W which gives the coefficient of performance (COP) of 0.75 with an average indoor temperature of 27°C. Using thermal comfort test data in literature for small air movements under radiant cooling ceilings, results from the experiments show that thermal comfort could be obtained with the TE‐CCP system. Copyright © 2007 John Wiley & Sons, Ltd.     

Six Sigma-based approach to optimise the diffusion process of crystalline silicon solar cell manufacturing

Silicon-based photovoltaics (PV) plays the dominant role in the history of PV due to the continuous process and technology improvement in silicon solar cells and its manufacturing flow. In general, silicon solar cell process uses either p-type- or n-type-doped silicon as the starting material. Currently, most of the PV industries use p-type, boron-doped silicon wafer as the starting material. In this work too, the boron-doped wafers were considered as the starting material to create pn junction and phosphorus was used as n-type doping material. Industries use either phosphorous oxy chloride (POCl₃) or ortho phosphoric acid (H₃PO₄) as the precursor for doping phosphorous. While the industries use POCl₃ as the precursor, the throughput is lesser than that of the industries’ use of H₃PO₄ due to the manufacturing limitations of the POCl₃-based equipments. Hence, in order to achieve the operational excellence in POCl₃-based equipments, business strategies such as the Six Sigma methodology have to be adapted. This paper describes the application of Six Sigma Define–Measure–Analyze–Improve–Control methodology for throughput improvement of the phosphorus doping process. The optimised recipe has been implemented in the production and it is running successfully. As a result of this project, an effective gain of 0.9 MW was reported per annum.     

Numerical investigation of thermal runaway propagation in a Li-ion battery module using the heat pipe cooling system

It is a promising cooling strategy to use the heat pipe for the Li-ion battery module, which can maintain the temperature of the battery module properly and prevent high temperature, triggering the thermal runaway among adjacent batteries. In this study, the thermal runaway model is simulated through the internal short circuit, which couples with Volume of Fluid (VOF) model of the heat pipe cooling and solves in ANSYS FLUENT to realize the heat and mass transfer between batteries and heat pipes. A user-defined function (UDF) code including mass source and energy source is used to calculate the heat and mass transfer in VOF model during the thermal runaway process. Numerical simulations are adopted to probe thermal runaway processes of a single battery under different operation conditions and the thermal runaway propagation from a battery to adjacent batteries. It is concluded that the heat pipe cooling system cannot prevent the thermal runaway of a single battery, but it can prevent the thermal runaway propagation from a battery to adjacent batteries.     

Prediction of thermal runaway and thermal management requirements in cylindrical Li‐ion cells in realistic scenarios

Li‐ion cells suffer from significant safety and performance problems due to overheating and thermal runaway. Effective thermal management can lead to increased energy conversion efficiency and energy storage density. Critical needs towards these goals include the capability to predict thermal behavior in extreme conditions and determine thermal management requirements to prevent thermal runaway. This paper presents an experimentally validated theoretical model to predict the temperature distribution in a cell in response to nonlinear heat generation rate that is known to occur during thermal runaway. This problem is solved by linearization of the nonlinear term over successive time intervals. Experimental measurements carried out on a thermal test cell in conditions similar to thermal runaway show good agreement with the theoretical model. Experimental measurements and model predictions indicate strong dependence of the fate of the cell on its reaction kinetics, thermal properties, and ambient conditions. Specifically, a sudden change in thermal runaway behavior is predicted once the ambient temperature crosses a certain threshold, consistent with past experimental observations. The impact of increasing cell thermal conductivity on improved thermal runaway performance is quantified. Results presented here provide a fundamental understanding of thermal runaway, and may lead to improved performance and safety of Li‐ion–based energy conversion and storage systems.     

Experimental results of a sensible heat storage system for residential and light commercial application

This paper presents the performance results for a sensible heat storage system. The system under study operates as an air source heat pump which stores the compressor heat of rejection as domestic hot water or hot water in a storage tank that can be used as a heat source for providing building heating. Although measurements were made to quantify space cooling, space heating, and domestic water heating, this paper emphasizes the space heating performance of the unit. The heat storage system was tested for different indoor and outdoor conditions to determine parameters such as heating charge rate, compressor power, and coefficient of performance (COP). The thermal storage tank was able to store a full charge of heat. The rate of increase of storage tank temperature increased with outdoor temperature. The heating rate during a charge test, best shown by the normalized rate plots, increased with evaporating temperature due to the increasing mass flow rate and refrigerant density. At higher indoor temperature during the discharge tests, the rate of decrease of storage tank temperature was slower. Also, the discharge heating rate decreased with time since the thermal storage tank temperature decreased as less thermal energy became available for use. Copyright © 1999 John Wiley & Sons, Ltd.     

A comprehensive thermal analysis for the fast discharging process of a Li-ion battery module with liquid cooling

Appropriate temperature range and distribution is necessary for Li-ion battery module, especially in real application of electric vehicles and other energy storage devices. In this study, a comprehensive design of liquid cooling–based thermal management system for a Li-ion battery module's fast discharging process is investigated, and thermal analysis and numerical computation are conducted. The effects of different flow directions, different shapes of the liquid channels, different widths of channels, different thicknesses of cold plate, and the comparison between uniform and nonuniform channels' distribution are analyzed. Simulation results indicate that the liquid cooling system provides acceptable cooling performance in preventing heat runaway of the battery module under 5C discharging current rate. A five-channel cooling plate can reduce the maximum temperature with appropriate design. Additionally, specific flow direction mini-channels, different shapes of the liquid-channels, and nonuniform channels are designed to compare the maximum temperature and uniformity of temperature distribution in the module. Maximum temperature can be improved through the increase of channel width and thickness of the cooling plate. The original design is proved to be the best design considering the maximum temperature, maximum temperature deviation, and final temperature standard deviation of the fast discharging process.     

Experimental and Theoretical Evaluation of On-Chip Micro Heat Pipe

The performance of a specially fabricated micro heat pipe etched on a silicon wafer is analyzed to evaluate its cooling potential. Triangular microgrooves are etched on a silicon wafer via a lithographic process with two reservoirs at the two ends. Anodic bonding and electrochemical spark erosion techniques are used to seal the microgrooves with a thin Pyrex glass and to create holes for vacuum connections and coolant insertion, respectively. The cooling potential of the micro heat pipe is evaluated by accurately measuring the temperature distributions along the channel length at different values of the applied heat load. A mathematical model is proposed and numerically solved to evaluate the axial variation of the radius of curvature of the coolant film to examine the cooling potential of the on-chip micro heat pipe system.     

Flashing characteristics in a pipe downstream from a depressurizing tank and temperature fluctuation characteristics at a mixing tee junction with cold water injection

The flashing characteristics in a pipe downstream from a depressurizing tank were experimentally and analytically investigated on the basis of the transient test and two‐phase flow analysis. The following conclusions were obtained. (1) When the pressure margin of the pump inlet side and the distance to obtain an isothermal condition were sufficient, flashing phenomena did not occur in spite of the decreasing pressure. (2) When the ratio of the cold water injection flow rate to the hot water flow rate M_c/M_h increased, the peak distance of the water temperature fluctuation moved from L/D = 1 to 0, and the maximum water temperature fluctuation ratio was about 40% of the temperature difference between hot and cold water near the mixing tee junction. Because no problem occurred regarding the pipe material thermal fatigue, reliability of the mixing tee junction was assured. (3) Due to suppression of flashing phenomena of the mixing pipe system, the decision diagram on the flashing occurrence was obtained from the test and the analytical results, taking into consideration three factors: the depressurizing ratio in the tank; the cold water injection flow rate due to remaining subcooling; and the delay time of thermal mixing. The simplified analytical equation was used to decrease the cold water injection flow rate by the optimized pipe length between the mixing tee junction and the drain pump. The cold water injection flow rate was minimized when the pipe length was about 15 to 20 times the pipe inner diameter. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 411–429, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10096     

Thermal analysis of flip chip plastic ball grid array assembly

Steady‐state thermal performance of the 164‐Lead flip chip plastic ball grid array (FC‐PBGA) under low to moderate convective air cooling conditions is simulated with CFX software. A package with three different substrates is investigated. Package performance is presented in the form of a linear relationship between the normalized junction to ambient thermal parameter (θ_JA) versus the normalized board to ambient thermal parameter (θ_JB). The chip corner and the contact surface have a larger temperature difference, produce a mismatch phenomenon, and result in the destruction of the solder ball. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(6): 371–382, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20026     

The thermal conductivity of solar-cell wafers molded from a powdered stock

A process for impulse heat treatment of a silicon surface is considered; the thermal conductivity of a silicon powder and a polycrystalline wafer molded from it is defined and compared with monocrystalline silicon; it is found that their significant difference is due to the wafer’s porosity.     

Reversible increase of photocurrents in excimer laser-crystallized silicon solar cells

Excimer laser-crystallized silicon solar cells fabricated show a steady increment of the current densities with exposure to simulated sunlight, over a 30 min period. The current density of the amorphous silicon cell under identical conditions remains steady, with no significant change. The process was observed to be reversible upon cooling, and the performance increase is attributed to the energy barrier introduced by the enhanced bandgap of a nanocrystalline silicon middle layer, created as a result of the crystallization. It is suggested that the thermal energy due to prolonged illumination allows carriers to cross the barrier increasing output currents.     

INTRINSIC-CARRIER THERMAL RUNAWAY IN SILICON MICROCANTILEVERS

A variety of micromachined sensors and actuators use coupled electrical and thermal transport in doped silicon bridges and cantilevers. One example is thermomechanical data storage cantilevers, in which Joule heating and atomic-scale forces yield indentations in an organic substrate. The thermal isolation of these structures augments the temperature rise during Joule heating, which can generate more intrinsic carriers and lead to thermal runaway in the presence of a constant bias voltage. This article develops a simple model for the thermal runaway effect in doped silicon cantilevers. The model relates the electrical conductivity in the cantilever to the temperature-dependent carrier concentrations in silicon and is consistent with the available experimental data.     

Cooling load density characteristics of an endoreversible variable‐temperature heat reservoir air refrigerator

The performance optimization of an endoreversible air refrigerator with variable‐temperature heat reservoirs is carried out by taking the cooling load density, i.e. the ratio of cooling load density to the maximum specific volume in the cycle, as the optimization objective in this paper. The analytical relations of cooling load, cooling load density and coefficient of performance are derived with the heat resistance losses in the hot‐ and cold‐side heat exchangers. The maximum cooling load density optimization is performed by searching the optimum pressure ratio of the compressor, the optimum distribution of heat conductance of the hot‐ and cold‐side heat exchangers for the fixed total heat exchanger inventory, and the heat capacity rate matching between the working fluid and the heat reservoirs. The influences of some design parameters, including the heat capacitance rate of the working fluid, the inlet temperature ratio of heat reservoirs and the total heat exchanger inventory on the maximum cooling load density, the optimum heat conductance distribution, the optimum pressure ratio and the heat capacity rate matching between the working fluid and the heat reservoirs are provided by numerical examples. The refrigeration plant design with optimization leads to a smaller size including the compressor, expander and the hot‐ and cold‐side heat exchangers. Copyright © 2002 John Wiley & Sons, Ltd.     

Simplification of analytical models and incorporation with CFD for the performance predication of closed‐wet cooling towers

Simplified analytical models are developed for evaluating the thermal performance of closed‐wet cooling towers (CWCTs) for use with chilled ceilings in cooling of buildings. Two methods of simplification are used with regard to the temperature of spray water inside the tower. The results obtained from these models for a prototype cooling tower are very close to experimental measurements. The thermal performance of the cooling tower is evaluated under nominal conditions. The results show that the maximum difference in the calculated cooling water heat or air sensible heat between the two simplified methods and a general computational model is less than 3%. The analytical model distribution of the sensible heat along the tower is then incorporated with computational fluid dynamics (CFD) to assess the thermal performance of the tower. It is found that CFD results agree well with the analytical results when the air flow is simulated with air supply from the bottom of the tower, which represents a uniform air flow. CFD shows the importance of the uniform distribution of air and spray water to achieve optimum design. Copyright © 2002 John Wiley & Sons, Ltd.     

The performance of natural draft dry cooling towers under crosswind: CFD study

The thermal performance of a natural draft dry cooling tower (NDDCT) under a crosswind has been investigated using a general‐purpose CFD code. A three‐dimensional study using the standard k–ε turbulence model to simulate airflow in and around an NDDCT has been conducted. A parametric study has been carried out to examine the effect of crosswind velocity profile and air dry‐bulb temperature on the thermal performance of an NDDCT. Two approaches have been considered in this study to quantify the crosswind effect. Firstly, simulations have been conducted at the nominal conditions and crosswind effect has been represented by thermal effectiveness parameter. Secondly, the ejected heat from the NDDCT has been maintained at a constant value (285 MW) and the crosswind effect has been represented by the change in the cooling tower approach parameter. After quantifying the effect of the crosswind on the thermal performance, windbreak walls have been introduced as a means of reducing this effect. The results in this paper show the importance of considering the crosswind velocity profile. Moreover, the introduction of windbreak walls has indicated an improvement in reducing the thermal performance losses due to the crosswind. Copyright © 2004 John Wiley & Sons, Ltd.     

Experimental examination of large capacity liFePO4 battery pack at high temperature and rapid discharge using novel liquid cooling strategy

To overcome the significant amounts of heat generated by large‐capacity battery modules under high‐temperature and rapid‐discharge conditions, a new liquid cooling strategy based on thermal silica plates was designed and developed. The superior thermal conductivity of the thermal silica plate combined with the excellent cooling effect of water led to a feasible and effective composite liquid cooling system during long cycle testing. The experimental results showed that the addition of thermal silica plates can greatly improve the cooling capacity that can allow the maximum temperature difference to be controlled at 6.1°C and reduce the maximum temperature of the battery module by 11.3°C, but still outside the optimum operating temperature range. The water flow significantly enhanced the cooling performance/stability, and slight temperature fluctuations were observed during cycling. The cooling performance obviously improved as the flow rate rose. When the velocity reached a critical value, further increase in water flow rate induced a slight influence on the cooling capacity due to the limitation of the materials. The maximum temperature (T_max ) could be reduced to 48.7°C, and temperature difference (?T ) could be maintained within 5°C when the water flow velocity increased to 4 mL/s, which was determined as the best value. The energy consumed by the water pump is only 1.37% of the total energy of the battery module. Overall, these findings should provide novel strategies for the design and optimization of battery thermal management system.     

A miniature silicon hot wire sensor for automatic wind speed measurements

In order to make air flow measurements easier and more accurate, a very small sensor has been constructed. The fabrication of such a sensor mainly consists in depositing a thin doped polycrystalline silicon layer on a 4″ silicon wafer by using a silicon—micromachined technique. At the end of the integration process, the wafer is sliced into 46 wind sensors. Each of them comprises a polycrystalline silicon layer which is 0.5 μm thick, with width running from 2 to 5 μm and length, from 45 to 58 μm. Supplied with a dc electrical current, each layer acts as a hot wire on contact with the fluid under study. Wind speed is then measured by detecting the resistance variations caused by the thermal transfer from the heated layer to the ambient atmosphere. A microcontroller-based data acquisition system has especially been designed so as to collect the wind speed measurements arising from this kind of hot wire transducer. The integrated silicon sensors have been experimented within a wind tunnel and calibrated for air speed ranging from 0 to 35 m/s. Initially intended for wall shear stress monitoring, these sensors can usefully be employed as anemometers for wind energy applications.     

Electrocatalytic properties improvement on carbon-nanotubes coated reaction surface for micro-DMFC

In this paper, the enhancement of electrocatalytic performance of carbon nanotubes (CNTs) grown on silicon substrate was demonstrated as compared with the conventional reaction surfaces including bare silicon wafer, carbon cloth and carbon paper, and all were with a thin Pt film (10 nm) coating. The peak current density was doubled by the Pt film/CNTs electrode comparing to Pt film/carbon cloth and about six times comparing to silicon wafer because of the large reaction surface area and excellent CO₂ microbubble removal capability by the CNTs nanostructure. The results indicated that the electrocatalytic performance of catalyst was significantly improved for the peak current density on better linear relationship to methanol concentration and square root of sweeping-rate, better exponential relationship to temperature and higher peak current density on different substrate tiling angles, which greatly attribute to the rapid microbubbles removal on nanostructured hydrophilic CNTs surface. These conclusions can be taken as references to designing micro-DMFC in the future.     

不可逆半导体固态热离子制冷器性能分析和优化

建立了考虑外部有限速率传热过程和热源间热漏的不可逆半导体固态热离子制冷器模型,基于非平衡热力学和有限时间热力学理论导出了热离子制冷器的制冷率和制冷系数的表达式;对比分析了不可逆热离子制冷器与可逆热离子制冷器的发射电流密度特性、电极温度特性以及制冷系数特性;研究了不可逆系统的制冷率与制冷系数最优性能,得到了制冷率和制冷系数的最优运行区间;通过数值计算,详细讨论了外部传热以及内部导热、热源间热漏损失、热源温度、外加电压、半导体材料势垒等设计参数对热离子装置性能的影响。在总传热面积一定的条件下,进一步优化了高、低温侧换热器的面积分配以获得最佳的制冷率和制冷系数特性。结果表明,由于存在内部和外部的不可逆性,热离子装置的发射电流密度及制冷系数都会明显降低;不可逆半导体固态热离子制冷器的制冷率与制冷系数特性呈扭叶型;合理地选外加电压、势垒等参数,可以使制冷器设计于最大制冷率或最大制冷系数的状态。