A liquid metal cooling system for the thermal management of high power LEDs

An active cooling solution using liquid metal as the coolant was proposed for high power light emitting diodes (LEDs). The typical thermal-physical properties of liquid metal were presented. Then a series of experiments under different operation conditions were performed to evaluate the heat dissipation performance of the liquid metal cooling system, and the results were compared with that of water. In order to better understand the cooling capability of liquid metal cooling system, a theoretical thermal resistance model was established and discussed. Both the experiments and theoretical analysis indicated that liquid metal cooling was a powerful way for heat dissipation of high power LEDs, and the fabrication of practical liquid metal cooling devices was feasible and useful.     

Thermal effects in packaging high power light emitting diode arrays

The package and system level temperature distributions of a high power (>1 W) light emitting diode (LED) array have been investigated using numerical heat flow models. For this analysis, a thermal resistor network model was combined with a 3D finite element submodel of an LED structure to predict system and die level temperatures. The impact of LED array density, LED power density, and active versus passive cooling methods on device operation were calculated. In order to help understand the role of various thermal resistances in cooling such compact arrays, the thermal resistance network was analyzed in order to estimate the contributions from materials as well as active and passive cooling schemes. Finally, an analysis of a ceramic packaging architecture is performed in order to give insight into methods to reduce the packaging resistance for high power LEDs.     

Experimental and Numerical Investigation of a Microjet-Based Cooling System for High Power LEDs

The optical extraction efficiency and reliability of light emitting diodes (LEDs) relies heavily on successful thermal management due to their inherit dependence on the low junction temperature of LED chips. In this paper, a microjet-based cooling system is proposed for the thermal management of high power LEDs. Experimental and numerical investigations on such an active cooling system were conducted. Thermocouples were packaged with LED chips to conduct an online measurement of the temperature and evaluate the cooling performance of the proposed system. The experimental results demonstrate that the microjet-based cooling system has good cooling performance. For a 2 × 2 LED chip array, when the input power is 5.6 W and the environmental temperature is 28°C, the temperature of the 2 × 2 LED chip array reaches 72°C within 2 minutes and continues to increase sharply if no active cooling technique is applied. By using the proposed cooling system to cool the LEDs, however, the maximum LED temperature measured by thermocouples will remain stable at about 36.7°C, when the flow rate of the micropump is 9.7 mL/s. With consideration of the experimental difficulty, a numerical investigation was conducted on flow and temperature distribution in the microjet device. The feasibility of the numerical model was proven by comparison with experimental results. The numerical results showed that at a flow rate of 3.2 mL/s, the heat transfer coefficient of the impinging jets in the proposed system was about 5523 W/m²·K, and the pressure drop in the microjet device was about 1368 Pa.     

中倍聚光太阳电池液浸散热特性的模拟实验

提出了一种将太阳电池液浸到冷却介质中,电池与介质间进行直接接触式换热的冷却方式.设计了小型实验装置,选用二甲基硅油为冷却介质,采用长弧氙灯模拟聚光条件,选择模拟材料制作太阳电池模拟片,进行了模拟实验研究.结果表明:采用液浸冷却方式,能够从模拟片表面带走72.2kW/m~2的热负荷,同时将模拟片的温度冷却到70℃以下;模拟片与介质间的单相对流传热系数能够达到约750W/(m~2·℃);介质温度的改变对模拟片与介质间的对流换热温差和对流传热系数影响较小.     

Environmental and economical impact of LED lighting systems and effect of thermal management

Solid‐state lighting (SSL) technologies such as light emitting diodes (LEDs) have been of interest for the last 15 years. This article focuses on inorganic LED technology and their evolving applications, energy efficiency, and economic impact as well as the effect of thermal management on LED lighting systems. The efficacy of the best commercial 1 W LED packages currently surpasses 120 lm/W, which is more efficient than typical metal‐halide and fluorescent lamps. This high efficacy will eventually allow LED lighting systems to be used in specialty and general illumination applications. However, higher lumen requirements for LED systems will inevitably lead to significant thermal challenges at both the chip and the system level that need to be addressed to enable practical applications at low costs. In this article, the basics of LED lighting will be discussed first. It will be followed by the potential economic benefits for high efficiency LED lighting systems in the general illumination market. We will then discuss the thermal challenges and possible candidate cooling technologies in LED lighting systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Research on ratio selection of a mixed liquid desiccant: Mixed LiCl– CaCl2 solution

Liquid desiccant cooling system is a new type of air-conditioning system capable of saving energy. The dehumidification process dominates the performance of this system, while the thermal properties of the liquid desiccant play a key role in improving dehumidification effect. However, there is little work about how to choose a proper liquid desiccant that has a better performance. To settle this problem, a novel method is proposed to search an ideal liquid desiccant by applying the nonrandom two-liquid equation (NRTL equation). This idea is further applied to mixed LiCl and CaCl₂ solution to work out the right mixture ratio with a better dehumidification effect under certain working conditions. Moreover, the related experiments were carried out. The results show that: compared to single LiCl solution, the dehumidification effect could be raised by more than 20% with mixed LiCl and CaCl₂ solution.     

Thermal analysis and management of lithium-titanate batteries

Battery electric vehicles and hybrid electric vehicles demand batteries that can store large amounts of energy in addition to accommodating large charge and discharge currents without compromising battery life. Lithium-titanate batteries have recently become an attractive option for this application. High current thresholds allow these cells to be charged quickly as well as supply the power needed to drive such vehicles. These large currents generate substantial amounts of waste heat due to loss mechanisms arising from the cell's internal chemistry and ohmic resistance. During normal vehicle operation, an active cooling system must be implemented to maintain a safe cell temperature and improve battery performance and life. This paper outlines a method to conduct thermal analysis of lithium-titanate cells under laboratory conditions. Thermochromic liquid crystals were implemented to instantaneously measure the entire surface temperature field of the cell. The resulting temperature measurements were used to evaluate the effectiveness of an active cooling system developed and tested in our laboratory for the thermal management of lithium-titanate cells.     

Liquid metal enabled combinatorial heat transfer science: toward unconventional extreme cooling

As a class of newly emerging material, liquid metal exhibits many outstanding performances in a wide variety of thermal management areas, such as thermal interface material, heat spreader, convective cooling and phase change material (PCM) for thermal buffering etc. To help mold next generation unconventional cooling technologies and further advance the liquid metal cooling to an ever higher level in tackling more extreme, complex and critical thermal issues and energy utilizations, a novel conceptual scientific category was dedicated here which could be termed as combinatorial liquid metal heat transfer science. Through comprehensive interpretations on a group of representative liquid metal thermal management strategies, the most basic ways were outlined for developing liquid metal enabled combined cooling systems. The main scientific and technical features of the proposed hybrid cooling systems were illustrated. Particularly, five abstractive segments toward constructing the combinatorial liquid metal heat transfer systems were clarified. The most common methods on innovating liquid metal combined cooling systems based on this classification principle were discussed, and their potential utilization forms were proposed. For illustration purpose, several typical examples such as low melting point metal PCM combined cooling systems and liquid metal convection combined cooling systems, etc. were specifically introduced. Finally, future prospects to search for and make full use of the liquid metal combined high performance cooling system were discussed. It is expected that in practical application in the future, more unconventional combination forms on the liquid metal cooling can be obtained from the current fundamental principles.     

Integrated analysis of cooling water systems: Modeling and experimental validation

Cooling towers are widely used in many industrial and utility plants as a cooling medium, whose thermal performance is of vital importance. Despite the wide interest in cooling tower design, rating and its importance in energy conservation, there are few investigations concerning the integrated analysis of cooling systems. This work presents an approach for the systemic performance analysis of a cooling water system. The approach combines experimental design with mathematical modeling. An experimental investigation was carried out to characterize the mass transfer in the packing of the cooling tower as a function of the liquid and gas flow rates, whose results were within the range of the measurement accuracy. Then, an integrated model was developed that relies on the mass and heat transfer of the cooling tower, as well as on the hydraulic and thermal interactions with a heat exchanger network. The integrated model for the cooling water system was simulated and the temperature results agree with the experimental data of the real operation of the pilot plant. A case study illustrates the interaction in the system and the need for a systemic analysis of cooling water system. The proposed mathematical and experimental analysis should be useful for performance analysis of real-world cooling water systems.     

Experimental investigation on mini‐channel cooling–based thermal management for Li‐ion battery module under different cooling schemes

The performance of Li‐ion battery depends on the temperature. Active liquid cooling system can keep the battery temperature within an optimal range, but the system itself consumes energy. This paper reported the experimental work on the thermal performance of liquid cooling system for the battery module under different cooling schemes. It was hoped that energy consumption could be reduced as much as possible. Meanwhile, liquid cooling system could provide effective cooling for the battery module. Two identical real battery modules including 18 cylindrical cells (with and without cooling system) were manufactured for the validity of comparison. The 2 battery modules discharged at the discharge rates of 1C and 1.5C. Charge and discharge cycle test was also carried out. The results indicated that a simple hysteresis control cooling scheme could reduce the energy consumption effectively. The energy consumption was saved by 83.2% and 49% at the discharge rates of 1C and 1.5C, respectively. Meanwhile, the temperature of battery module was still kept within the optimal range. The maximum temperature appeared on different cells in the battery module during the process of charge and discharge. Thus, the temperature dynamic comparison mechanism was very necessary.     

Experimental Study of Heat Transfer in Gas Turbine Blades Using a Transient Inverse Technique

To enhance specific power output and thermal efficiency of gas turbine engines, industry searches for ways to increase the turbine inlet temperatures. Therefore, temperatures of turbine blades increase as well and necessitate active cooling of these components. Experimental design work on such internal cooling schemes is carried out to find acceptable compromises between heat transfer and pressure losses. It is often carried out by using transient thermochromic liquid crystal techniques in combination with Plexiglas models. However, for real turbine blades this experimental technique is inappropriate due to the lack of optical access. Therefore, to study actual turbine blades there is need for development of noninvasive, nondestructive methodologies. This article describes a measurement technique that allows determination of internal heat transfer coefficients of real turbine blades experimentally. Thus, a test rig with a rapidly responding heater was designed to fulfill the requirement of a sudden increase in the air temperature within the cooling passages. The outer surface temperatures were measured using infrared thermography. To estimate the spatial distribution of internal heat transfer coefficients from transient surface temperatures the inverse heat transfer problem was solved. As optimization algorithm the Levenberg–Marquardt method was chosen. Outer surface temperature data was measured for a rectangular reference model with rib turbulators and compared with simultaneously acquired data using the thermochromic liquid crystal technique. It is concluded that the new experimental measurement technique could be used to quantitatively determine internal heat transfer coefficients.     

Liquid desiccant based two-stage evaporative cooling system using reverse osmosis (RO) process for regeneration

This paper presents preliminary findings of the energy analysis of a cooling system with two-stage evaporative coolers using liquid desiccant dehumidifier between the stages. The proposed evaporative cooling system utilizes air humidity for cooling in humid areas and requires no additional water supply. The major energy requirement associated with this cooling system is the energy for regenerating the weak liquid desiccant. Reverse osmosis process is considered for regeneration by mechanical energy and MFI zeolite membrane is proposed for separation of water from the weak desiccant solution. Energy analysis has been carried out for the proposed cooling system. The COP of the proposed cooling system is defined as the cooling effect by the mass rate of water evaporated in the system divided by the amount of energy supplied to the system, that is, the COP is independent of the energy source.     

基于液体介质的电动汽车动力电池热管理研究进展

电动汽车在节能减排上具有很大的潜力和优势,但其性能受动力电池的制约,而温度又会影响电池的安全和寿命。因此,为保证电动汽车的综合性能,需配置合理的电池热管理系统。由于液体冷却具有较好的降温效果,采用液体介质对电池进行热管理近年来逐渐引起重视。本文介绍了基于液体介质的电池热管理基本原理,综述了液体介质应用于电池热管理的研究进展,并重点介绍了新型热管在电池散热方面的应用,同时指出了目前液体介质冷却电池时存在的一些问题。     

Performance of serpentine channel based Li-ion battery thermal management system: An experimental investigation

Thermal management of large-scale Li-ion battery packs is of great significance to their safety and life cycle, which would impact their applicability in electric vehicles. Of the many strategies developed for this purpose, indirect liquid cooling has already demonstrated quite high potentials in thermal regulation of such battery systems. In this study, a compact lightweight serpentine wavy channel configuration was chosen to construct an indirect liquid cooling system for a battery module of cylindrical Li-ion cells. The serpentine channel has a number of six internal minichannels. Experimental test data were used to conduct a comprehensive thermal analysis to examine the highest temperature, the maximum temperature difference, and the heat accumulation percentages, and so forth within the battery pack. Results have revealed the ability of the cooling system to maintain the module temperature within appropriate working conditions for electric vehicle applications for most cycling tests including two driving cycles. Furthermore, the analysis insights raised by this study could be useful in understanding the cooling performance of the liquid-based thermal management systems for electric vehicles.     

Advanced thermal enhancement and management of LED packages

Thermal management of packages consists of external cooling mechanisms, heat dissipaters, and thermal interfaces. While keeping cooling condition constant, junction temperature of LEDs with higher thermal resistance increases more rapidly; hence the luminous efficiency decreases more obviously. This paper includes the discussion about the calculation methods of the lighting's heat transfer. The calculation process has been demonstrated by an example of cooling of LEDs lighting in this paper. In particular, the operation package heat transfer enhancement is required by most package manufacturers with a decrease of 20% ~ 30% of the thermal resistance over conventional package geometries.     

Thermal management of hydrogen refuelling station housing on an annual level

A housing insulation of hydrogen refuelling station is vital from the aspect of safe operation of equipment in an environment that is installed. To secure hydrogen supply during the whole year, this work brings the solution for both cooling and heating insulation equipment inside of hydrogen refuelling station installed in Croatia, Europe. This hydrogen refuelling station was designed as an autonomous photovoltaic-hydrogen system. In the interest of improving its energy efficiency, an optimal thermal management strategy was proposed. To select the best technological solution for thermal management design which will maintain optimal temperature range inside the housing in cold and warm months, a detailed analysis of the system components thermodynamic parameters was performed. Optimal operating temperatures were established to be 25 °C in summer and 16 °C in winter, considering components working specifications. Insulation, type of cooling units, and heaters have been selected according to the HRN EN 12831 and VDI 2078 standards, while the regime of the heating and cooling system has been selected based on the station's indoor air temperature. The annual required heating and cooling energy were calculated according to HRN EN ISO 13790 standard, amounting to 1135.55 kW h and 1219.55 kW h, respectively. Annual energy share obtained from solar power plant used for the heating and cooling system resulted in 5%. The calculated thermal management system load turned out to be 1.437 kW.     

Absorption of water vapour in the falling film of water–lithium bromide inside a vertical tube at air-cooling thermal conditions

In the absorbers of air-cooled water–lithium bromide absorption chillers, the absorption process usually takes place inside vertical tubes with external fins. In this paper we have carried out an experimental study of the absorption of water vapour over a wavy laminar falling film of water–lithium bromide on the inner wall of a smooth vertical tube. The control variables for the experimental study were; absorber pressure, solution mass flow rate, solution concentration and cooling water temperature. Relatively high cooling water temperatures were selected to simulate air-cooling thermal conditions. The parameters considered to assess the performance of the absorber were; the mass absorption flux, the outlet solution degree of subcooling and the falling film heat transfer coefficient. The results indicate that in water cooling thermal conditions the mass absorption fluxes are in the range 0.001–0.0015 kg·m⁻²·s⁻¹, whereas in air-cooling thermal conditions the range of mass absorption values decreases to 0.00030–0.00075 kg·m⁻²·s⁻¹.     

闭式冷却塔内冷却盘管传热热阻分析

通过对闭式冷却塔内冷却盘管各热阻的数量级分析,认为在实际计算中,管壁导热热阻比其它热阻小一个数量级,计算中可忽略,但其余热阻均不可忽略。影响盘管总热阻大小的因素很多,从数学上分析了各热阻对总热阻的影响,找出影响盘管总热阻的主次因素,为冷却盘管的研究、设计及运行管理提供参考。     

Effect of orientation for phase change material (PCM)-based heat sinks for transient thermal management of electric components

Phase change material (PCM)-based heat sink, consisting of a conventional, extruded aluminum sink embedded with appropriate PCMs, can potentially be used for cooling of mobile electronic devices such as personal digital assistants (PDAs) and notebooks which are operated intermittently. During the use of such mobile devices, the orientation changes from time to time. A numerical investigation was carried out to study the effect of orientation of heat sink on the thermal performance of the combined cooling system to determine if it affects the thermal performance of a PCM-based cooling system significantly.     

柴油机冷却水套中无水冷却液的数值模拟分析

运用FLUENT软件对某型号柴油机冷却水套三种无水冷却液在不同温度下的冷却效果进行数值模拟分析.模拟结果表明:由于冷却液表面换热系数的变化受冷却液粘度及导热率的影响,而这两个因素随温度的变化而变化;温度较低时,冷却液的导热率的影响占主导地位,表面换热系数随温度的增大而减小;温度较高时,冷却液粘度的影响占主导地位,表面换热系数随温度的升高而增大.