Cooling of high-power LEDs by liquid sprays: Challenges and prospects
Introduction
Development in almost every field of critical technology such as defense, avionics, lightings, automobile industry, etc. is characterized by the integration of electronic components, wherein there is an absolute necessity of miniaturization of components. In view of this aggressive trend, there is continuous need for improved thermal management techniques, capable of dissipating high heat fluxes so as to bring the temperature of components within an acceptable range, prevent early component failure and increase reliability.
Over the past two decades, initial efforts for thermal management of high-power electronics focused on single-phase cooling techniques, relying on the sensible heating of the coolant. The development has moved from passive cooling techniques based on natural convection to improved active management with forced convection. However, with increasing demand, the cooling system developers shifted their focus towards two-phase active as well as passive technologies. Two-phase cooling exploits the latent heat as well as the sensible heat of the coolant. It removes a much higher wall heat flux than what is possible with single-phase cooling at lower temperature differences [44]. For example, in the laser diodes, there is a requirement to dissipate heat up to ~1 kW/cm2 at ~300 K; such a demand simply cannot be matched with a single-phase system [9].
Light emitting diodes (LEDs) based on solid-state lighting (SSL) technology are the most promising global lighting source due to their high efficiency and long life. While LEDs are replacing modern luminaires, it has additional applications in human health and well-being, connected lighting with internet of things and artificial intelligence, horticulture lighting for photosynthesis and designs for ecologically conscious lighting that limit lighting-related pollution. The colour of light emitted by LED is determined by the energy band gap (Eg) of the semiconductor used for the LED chip. In an LED, electrons from the n-region and holes from the p-region recombine at the junction due to the applied voltage. A part of the energy is released in the form of photons during their recombination, as shown in Fig. 1(a). The remaining portion of the released energy is in the form of heat, which needs to be effectively removed so that the device temperature remains below an acceptable operational limit [64].
LEDs offer several advantages over conventional lighting sources: (i) Energy saving: they require comparatively much less input energy for producing an equivalent light intensity. (ii) Warm-up time: the device does not require any heating or turn-on time and lights up quickly. (iii) Size: LEDs are small and compact. (iv) Long lifetime: LEDs have a long operational life of about 50,000 h or more, as compared to incandescent lamps, which have a lifetime of about 10,000 h. (v) Wide color temperature: LEDs offer a wide range of color temperatures (4500–12,000 K). (vi) High reliability: LEDs fail at a lower rate by dimming over a longer time, as compared to incandescent lamps, which have an abrupt failure [61]. Use of LEDs is expected to reduce the net world energy consumption by ~1000 TW·h·yr−1 [74].
Although LEDs possess the advantage of a higher percentage of visible light as compared to earlier light source technologies, Table 1, shows that the non-radiant heat energy component, which constitutes about 70–80%, can raise the junction temperature to ~120 °C, critically damaging the LED. The non-radiative recombination of holes and electrons occurs in the active region of chips and is of two types, namely, Shockley-Read-Hall (SRH) recombination and Augur recombination (AR) [61]. SRH recombination arises due to impurities present in the crystal structure of the semiconductor, creating a new energy level in the forbidden band gap. The probability of SRH recombination increases with increase in temperature. On the other hand, AR occurs when energy released by the electron in the conduction band is absorbed by another electron in the conduction band, or a hole in valence band, instead of releasing it in the form of a photon. The excess energy thus available is released during collision with the crystal lattice [124] and need to be eventually managed/dissipated to the ambient so as to safely maintain the junction temperature.
LEDs in the power range of 20–70 W are widely used in automotive headlights [92], advertising display, streetlamps, traffic lights, and stadium lighting (e.g. ~4 kW net capacity, Bee® LED company). Depending upon the electrical power input, the LEDs can be broadly classified into low power (generally 0.05 W to 1 W, micro/milli amps) and high-power (3 W or even tens of watts, several amperes) [45]. The low power LEDs does not require sophisticated thermal management solutions. The heat flux at chip level can be as high as 250–500 W/cm2 for the high-power LEDs; e.g., 300 W nominal power LEDs module by Huazhong University of Science and Technology, China, operating at 25–36 V and 0.1-7 A [19], [92], [123]. Hence, an effective cooling technology must be utilized to dissipate the heat power which will improve reliability, lifetime, and overall opto-electro-mechanical performance of high-power LEDs.
In this article, a comprehensive review of spray cooling design methodology and technological aspects, in the context of thermal management of high-power LEDs is presented. Several design equations and correlations of spray cooling are included, at global as well as droplet level interaction with the hot surface, which will be highly beneficial while designing of thermal management techniques for high-power electronic systems. This paper aims to reach to the audience of the engineering and industry community who are involved in applied thermal management of luminaire systems and their applications. In addition, thermo-physical properties of sprays and related physics of heat transfer, is explained with the help of elaborate figures, which meticulously and systematically represent features such as LEDs architecture, spray morphology, experimental setup and results of spray cooling through parametric studies, various phenomenological and qualitative mechanisms of heat transfer occurring during spray impingement phenomena at droplet levels, etc. Some emerging results from the respective laboratories of the present group of authors is also included to supplement the discussion and to provide support to the ongoing arguments.
Section snippets
LED packaging and thermal management
Electronic packaging entails the housing, enclosure and interconnection of integrated circuits to form electronic systems. Along with semiconductor manufacturing, packaging forms the core of the final product. Packaging provides several critical functions including, for example, circuit support and protection, heat dissipation, signal distribution, manufacturability, serviceability and power distribution. The effectiveness, reliability and cost, is strongly determined by the packaging strategy.
Cooling technology: Classification
Cooling technologies employed for thermal management of high-power LEDs are broadly classified into two categories, namely, passive cooling and active cooling systems, as shown in Fig. 2. In passive cooling, no external power input is required for heat removal; power input is required for removing heat in active cooling systems. In this section, a brief description of different cooling methodologies applied by researchers for real time high-power LEDs is presented.
Passive cooling techniques
Spray impingement cooling
Spray impingement cooling relies on the technique that when a liquid is forced through a small orifice, it breaks into small drops, which can be made to imping on the heated surface to remove heat effectively. Due to the high momentum impact of the drops on the heated surface, a thin liquid layer gets developed on the surface, which enhances the heated transfer. Rendering the liquid volume in the form of small drops creates a very high surface area to volume ratio. In the single-phase spray
Conclusions and outlook
Solid-state lighting and ancillary systems affect the performance and output of several core sectors, which together constitute a major portion of the world economic activity. An attractive feature of the LED technology is the combination of compact size, high power efficiency, and possibility of fast glow control. One of the most important consequences of large-scale adoption of the LED technology is a dramatic reduction of electricity costs. Hence, LEDs are used in diverse applications and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are thankful for research funding jointly provided by the Ministry of Education and Science of the Russian Federation (Agreement 14.613.21.0067, Project Identifier RFMEFI61317X0067); Department of Science and Technology, Government of India, under the International Multilateral Regional Cooperation Program [Project #DST/IMRDC/BRICS/ Pilot Call/HPCSLED/2017 (G)]; and the Ministry of Science and Technology of the People’s Republic of China (Project No. 2017YFE0100600). Author GS is
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