THERMOSYPHONS FOR HIGH POWER LED LIGHTING PRODUCTS

Transcription

1 16th International Heat Pipe Conference (16th IHPC) Lyon, France, May 20-24, 2012 THERMOSYPHONS FOR HIGH POWER LED LIGHTING PRODUCTS Valery M. Kiseev, Victor G. Cherkashin Ural Federal University, Institute of Natural Science, Lenin av. 51, , Ekaterinburg, Russia, Phone: (+7) , Fax: (+7) ABSTRACT Extensive development of the light-emitting diode (LED) devices on the basis of LED matrix offers capabilities to occupy LED at the top place for the street and industrial lighting as one of the most reliable and energy-efficient devices. However, there are not enough experimental data on the optimization of the cooling systems for LED devices. Traditionally, the cooling systems for LED devices are designed with even pitch of LED locations on the radiator s surface. With the increase of LED power the radiator s surface and the distance between the LED locations grows correspondingly. It results in the increase of mass and size of the LED device. This paper presents some designs of two-phase thermal control systems for LED cooling on the basis of conventional thermosyphon (TS) and loop thermosyphon (LTS) and studies the experimental data, which were investigated for the two-phase systems considering the gravity influence. Additionally, the paper presents a comparative study between different working fluid, LED power and the radiators. KEY WORDS: Two-phase thermal control system, conventional thermosyphon, loop thermosyphon, heat pipe, LED lamp, radiator, working fluid, heat transfer 1. INTRODUCTION The main attractive quality of LED devices is its high level of luminous efficiency in comparison to alternative light sources. Therefore, employing LED technology may result in different economic and social effects. The most important effect among others is a considerable reduction in electrical energy consumption, used for illumination purposes, which, according to different estimations, is about 18-20% of overall produced electricity ( port_web1.pdf). The comparison of available light sources is presented in Table 1. Although LED lamps are more than 50 times as expensive as bright white lamps and about 7 times as expensive as compact luminous lamps, the price has been decreasing recently. Assuming that the characteristics improve and the price reduces, it is predicted that within a few years LED sources will be used in the majority of lighting systems. However, some of the factors that prevent LED from wide application should be taken into account. Thus, diode s parameters are very sensitive to working temperature, applied voltage, current etc. ( For example, when the temperature of a diode is more than 80 C the luminous efficiency reduces, while at 120 C it tends to zero (Staroverov K., 2008). Table 1. Characteristics of light sources Light source type (LS) Luminous efficiency of LS, lm/w Efficiency of lamps with respective LS, lm/w Life time, hours Glow lamp halogen lamp Compact luminous tube lamp metal-halide lamp luminous tube lamp Semiconductor LED (Cree XR-E) < > Sodium highpressure lamp <

2 Moreover, the lifetime decreases significantly. In order to deal with all the above mentioned problems it is required to develop and employ an efficient method of heat removing, i.e. new LEDs or LED matrices which possess the maximal heat diffusion ability are needed. Similar problems with thermal control exist in other fields such as space and aircraft industry, semiconductor industry, etc. In order to deal with these problems two-phase thermal control systems heat pipes and devices based on them are often used (Dan P., Ray D., 1979; Faghri A., 1995) than the radiator, as well as, it allows to spatially separate the heat input and output. The lack of moving parts as well as the benefits mentioned above positively affect LED devices, allowing to easily arrange the radiator at places with the most intensive heat transfer and where natural or forced convection takes place. This results in lower power consumption, illumination efficient increase of LED matrix and lifetime expansion alongside the overall rise of reliability. Therefore, the goal of this work was to develop and study cooling technologies of LED matrices that have the output of more than 30 W using effective heat sinks loop heat pipes (Gerasimov Yu.F. et al., 1975), including the most simple of them: loop thermosyphons (Kiseev V. M. et al, 1992) and conventional thermosyphons (Pioro A. S., Pioro I. L., 1988;). 2. DEVELOPMENT AND STUDY OF TWO- PHASE THERMOSYPHONS FOR LED MATRICES COOLING 2.1. The schematic diagram of a loop two-phase thermosyphon (LTS). The schematic diagram of a loop two-phase thermosyphon is shown in Figure 1. When voltage is applied on diode 2 it begins to glow and produces heat energy, which then is passed through substrate 9 to the liquid located at liquid cavity 7 of evaporator 1. Depending on the density of heat flux, the liquid boils or evaporate and becomes vapor, consuming latent heat of vaporization. During this process the vapor s pressure in a vapor cavity 8 is higher than the one in condenser because of the temperature difference between them, so the vapor travels through the vapor line 3 to the condenser 4, where it condenses with release of latent heat of condensation and becomes a liquid. Then the released heat is transferred to the condenser 4 and eventually by means of the radiator 6 to the environment. The liquid, influenced by gravity or capillary forces, drains through liquid line 5 back into the liquid cavity 7 and, therefore, encloses the evaporationcondensation heat transfer cycle. The proposed LTS design allows to transform the heat energy from light emitting diodes compactly situated on metal substrate, which has smaller size Figure 1. The schematic diagram of loop twophase thermosyphon (LTS) for LED cooling. 1 evaporator, 2 diodes; 3 vapor line; 4 condenser; 5 liquid line; 6 radiator; 7 liquid cavity of evaporator; 8 vapor cavity of evaporator; 9 LED matrix Figure 2. Loop thermosyphon (LTS No 1) with LED matrix (on the right) Figure 2 shows the photograph of developed experimental LTS for cooling LED matrices (on the left) and a LED matrix with 20 to 40 diodes (Osram. Germany) located on the aluminum plate, which is 1.7mm in thickness and 80mm in the diameter (on the right).to reduce the number of light emitting diodes and correspondingly the overall price of the device, the nominal load per diode has been increased up to 3.6 W, whereas in

3 most cases 1 Watt per diode is used. The reduction of luminous efficiency, caused by the increase of nominal load from 0.35 A to 1 A, were about 24% of the nominal. the inner wall surface back into the bottom part of chamber 1, thereby enclosing the evaporationcondensation heat transfer cycle. The evaporation chamber of the studied LTS was made of copper, tablet-shaped with 60mm inner diameter and 25mm wall thickness respectively. The vapor line and the coil condenser (4 loops with step 25mm) were made of copper tube with inner diameter of 4, 1mm thickness. For the condenser line a copper 2x0.5mm tube was used. Radiator 6 is composed of two standard finned radiators, which are 125x98 mm and 125x196 mm with the fin s length of 39mm and 46mm for LTS No 1 and LTS No 2 respectively. The total finned surface of radiator was 0.20 m 2 for the LTS No 1 and 0.48 m 2 for the LTS No 2 respectively. On the finless surfaces of radiators grooves for the coil condenser were made. Two halves of the radiator with applied thermal interface were fastened together by bolts so that the coil condenser was between the halves, inside the radiator as it is shown in Fig 1. The material of LTS No 2 s radiator is aluminum. In order to test thermal characteristics of heatconducting composite plastic with thermal conductivity of 8 W/m K (Krivatkin A., Sakunenko Yu., 2010) (Teplostok Т6-E5-7, SpecPlast-M ltd, Russia), the comparison of the aluminum radiator and the radiator made of the plastic have been conducted. The dimensional parameters of the radiators were the same The schematic diagram of the conventional two-phase thermosyphon (CTS) The schematic diagram of the conventional twophase thermosyphon is presented in Figure 3. The illuminating device works in the following way. When voltage is applied on diodes 4, they begin to glow and produce heat energy, which then is passed through substrate 3 to liquid 2 located at the bottom part of chamber 1. Depending on the density of heat flux, the liquid boils or evaporates and becomes vapor, consuming latent heat of vaporization that for most liquids exceeds the thermal conductivity for hundred times. After that, the vapor travels to the above part of chamber 1, where it condenses and becomes a liquid with release of latent heat of condensation, which is then transferred to environment by means of radiator 5. The originated condensate 6 drains on Figure 3. The schematic diagram of conventional thermosyphon (CTS) 1 evacuated hermetic chamber two-phase closed thermosyphon, 2 working fluid, 3 heatconductive metal substrate, 4 LEDs, 5 radiator s fins, 6 condensate, 7 optically transparent cover, 8 power supply with functional sensors The heat-transfer system, from which the air has been evacuated, is filled with working fluid that has a freezing point lower than the lowest climatic temperature specific for the region. For instance, in southern regions, where the air temperature in winter time is not lower than +5 0 С, pure distilled water might be used as a working fluid, while in northern regions, where the air temperature could possibly be below +5 0 С, the working fluid might be methanol, acetone and ethanol, the freezing temperature of those are below 60 0 С. The proposed CTS design allows to transform the heat energy from light emitting diodes compactly situated on metal substrate, which has smaller size than the radiator, as well as to spatially separate the heat input and output. The lack of moving parts as well as the benefits mentioned above positively affect LED devices, allowing easily arrange the radiator at places with the most intensive heat

4 transfer and where natural or forced convection takes place. This results in lower power consumption, illumination efficient increase of LED matrix and lifetime expansion alongside the overall rise of reliability. Figure 4 shows the photograph of developed experimental CTS for cooling LED matrices and a LED matrix with 20 to 40 diodes (Osram. Germany) located on the aluminum plate, which is 1.7mm in thickness and 80mm in the diameter. To reduce the number of light emitting diodes and correspondingly the overall price of the device, the nominal load per diode has been increased up to 3.6 W, whereas in most cases 1 Watt per diode is used. The reduction of luminous efficiency, caused by the increase of nominal load from 0.35 A to 1 A, were about 24% of the nominal. environmental temperature T env was measured as well. All thermocouples were connected to data acquisition system (Owen TRM-148). The experimental LED illuminating device based on a conventional thermosyphon and the locations of thermocouples is shown in Figure 5. For this series of experiments the device were made of aluminum and the total external surface for heat exchange with environment was S р = (0.336 ± 0.004) m 2, while the CTS s mass was m cts = (2.40 ±0.05) kg. Figure 5. Experimental LED illumination device and the thermocouples locations Figure 4. The conventional thermosyphon with LED matrix and the LED matrix (left-bottom) 3. EXPERIMENTAL APPARATUS AND PROCEDURE The experimental setup was designed to measure temperature on different points of thermosyphon versus nominal power load N applied directly to LED matrix or versus a heat load Q. The local temperatures on LTS characteristic points were measured by thermocouples or the infrared imager (Fluke Ti32). Thermocouples were located as follows. One thermocouple was placed on the heat input zone, on the middle part of the LED matrix or on the middle part of the evaporator s shell between the electric heater and the evaporator. (label T h ). Two thermocouples were mounted on the vapor line and the liquid line, one on the outlet of the evaporator (label T v ) and one on the inlet of the evaporator (label T l ) respectively. The 4. RESULTS AND DISCUSSION Figure 6 represents a typical experimental curve for LTS No 2 with water as a working fluid. From the graph it is seen, that under a low heat load the oscillations of measured temperature with the magnitude up to 3-4 degrees occur. However, the magnitude considerably decreases with a rise of the heat load. One of the goals of this work was to compare the thermal characteristic of heat conductive plastic against the ones of traditional materials, such as aluminum. To do so, two geometrically identical radiators, one of the plastic and the other of traditional aluminum, were made. Figure 7 below shows the results of the comparison.

5 the ability to work under temperatures below 0 o C, is of interest as water is non-toxic and easy-towork with. Figure 6. Working temperatures (Т) of LTS No 2 (working fluid - water) for various heat loads (Q) Fig. 8. The temperature field as a function of time and power. Working fluid acetone. Below, in Fig. 9 a typical experimental dependence of the heat zone temperature on time t is shown. In this experiment the working fluid was frozen and the heat load on thermosyphon Q was 50 W. When the temperatures are about 0 o C the phase transition ice-water occurs, which is seen as a small step on the curve. Then, with the rise of temperatures the device s working regime changes to the normal two-phase one. Figure 7. Working parameters for LTS No 1. Comparison of aluminum (Al) radiator and heatconductive plastic (TP) radiator. From the data above it is followed that the heat conductive plastic, having a relatively low thermal conductivity of 8 W/m K compared to aluminum and its alloys ( W/m K), is suitable for heat sink within a natural convention condition. During these series of experiments, when the aluminum radiator was replaced by the TP radiator, the temperature on heat input zone increased by 4-8 per cent depending on the heat load (other conditions were kept the same). Figure 8 represents the results of test for the conventional thermosyphon with acetone as a working fluid depending on an input power. In addition, a possibility to employ water as a working fluid, supposing its chemical compatibility with thermosyphon s materials and Fig. 9. Startup diagram T 1 = f ( t) from frozen state of working fluid (water) under nominal heat load Q = 50 W and the environmental air temperature T env = (24 ± 1) o C This study has shown that all working parameters of thermosyphon filled with water remained the

6 same for both normal conditions and when the phase transitions occurred. Additionally, no mechanical damages or changes in the geometry of the thermosyphon were observed. This can be explained by a relatively small portion of the water in the internal volume of the device, about 2.5 per cent of the volume. Considering all the factors, water may be recommended as a working fluid for two-phase thermosyphons; however, some of limits such as chemical compatibility, relatively small volume of liquid, should be taken into account. 5. CONCLUSIONS liquid. Inzhenerno-Fizicheskii Zhurnal. v. 6. p. 957 (in Russian) Kiseev V. M., Pogorelov N.P., Menkin L. I.( 1992) The study on two-phase thermosyphon application for mock-up fuel elements temperature regime modeling. Proc. of the 8 th IHPC. Beijing. p.673. Pioro A. S., Pioro I. L. (1988) Two-phase thermosyphons and its applications in industry. Naukova dumka, Kiev (in Russian) Krivatkin A., Sakunenko Yu. (2010) Heat conductive plastics - challenge to aluminum. Solid state lightning. v. 1. p. 54. (in Russian) 1. A thermal control system for LED cooling based on the loop thermosyphons has been proposed. The advanced study of loop thermosyphons has been performed and it has been shown that these systems might be applied in the LED device s cooling. 2. A thermal control system for LED cooling based on the conventional thermosyphons has been proposed. The diverse study of conventional thermosyphons with flat end heat input has been performed and it has been shown that these systems might be applied in the LED devices cooling. 3. The comparative analysis of aluminum radiators and heat-conductive plastics has been performed. It was shown that the heat conductive plastics, lowcost, low-weight and easy-to-process material, have a great potential in thermal applications. 4. Using of thermosyphons allowed to raise the nominal load per diode to 3.5 W (as a rule 1 W) by decreasing the number of diodes more than tree times, while the reduction of luminous efficiency was no more than 24%. REFERENCES ort_web1.pdf. Staroverov K. (2008) Cooling systems for LED. Novosty Elektroniky. v. 17. p (In Russian) Dan P., Ray D. (1979) Heat pipes. Energia, Moscow, (in Russian) Faghri, A. (1995) Heat pipe science and technology. Taylor & Francis, London Gerasimov Yu.F., Maydanik Yu.F, Shegolev G. T., Filippov G. A., Starikov L. G., Kiseev V. M., Dolgirev Yu. E. (1975) Low temperature heat pipes with separated channels for vapor and