Yüksek güçlü LED (ışık yayan diyot) ışık kaynaklı armatürlerin soğutma sistemlerinde ısı borularının kullanım analizi
Analysis of heat pipe usage in cooling systems of luminaires with high power LED (light emitting diode) light source
Yazar: SEHER ATEŞ
Danışman: PROF. DR. SERMİN ONAYGİL
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Enerji Bilim ve Teknoloji Ana Bilim Dalı / Enerji Bilim ve Teknoloji Bilim Dalı
Derece: Doktora, 2021
Konu: Enerji = Energy ; Makine Mühendisliği = Mechanical Engineering ; Mühendislik Bilimleri = Engineering Sciences

Özet:
LED ışık kaynakları günümüzde genel aydınlatma uygulamalarında sıklıkla kullanılmaktadır. Işık akısı, etkinlik faktörü ve ömür gibi kriterler dikkate alındığında LED ışık kaynakları geleneksel ışık kaynaklarının alternatifi olabilecek performanslar ortaya koyabilmektedir. Ancak çalışma sırasında yüksek ısıl güçlerin açığa çıkması sebebiyle LED ışık kaynaklı armatürlerin ısıl yönetimleri bir diğer deyişle soğutmaları önemli bir sorun olarak ortaya çıkmaktadır. Sürekli ve etkili bir soğutma sistemi olmadan LED'lerin beklenilen etkinliği sağlamaları mümkün değildir.
Bu tezde öncelikle LED ışık kaynaklarının özelliklerine sıcaklığın etkisinin belirlenebilmesi amacıyla ölçümler gerçekleştirilmiştir. Sonuç olarak LED'lerin özelliklerinin sıcaklıktan farklı oranlarda etkilendiği gözlemlenmiştir. Bununla birlikte ölçülen tüm LED'lerin özellikleri artan sıcaklıktan olumsuz etkilenmiştir.
Literatürde mevcut soğutma sistemleri ile ilgili araştırma gerçekleştirildiğinde, aktif ve pasif pek çok sistemle karşılaşılmıştır. Üstünlük ve zayıflık analizleri yapıldığında ise sadece soğutucu kanatların kullanıldığı sistemler ile soğutucu kanatlar ve ısı borularının birlikte kullanıldığı sistemler ön plana çıkmıştır.
Bu kapsamda noktasal ısıl yüklerin yüksek, dolayısıyla sıcaklık etkileşiminin kritik olduğu COB LED'lerin kullanıldığı bir aydınlatma armatürü prototibinin sadece kanatlı ve ısı borulu kanatlı iki farklı soğutma sistemine sahip versiyonlarının ısıl simülasyonları yapılmış, üretimleri gerçekleştirilmiş ve deneysel analizleri yapılmıştır. Deneysel ölçümler ile ısıl simülasyon sonuçları doğrulanmış ve iki sistem birbiriyle karşılaştırılmıştır. Belirlenen noktalarda iki sistem arasında ortalama sıcaklık farkının yalnızca 3,3°C olduğu gözlemlenmiştir. Işık akısı olarak 253 lümene karşılık gelen bu değer armatür toplam ışık akısında sadece % 0,6'lık bir farklılığı göstermektedir. Değerlendirilen armatür prototibi için iki soğutma sistemi arasında yüksek sıcaklık ve ışık akısı farkı ortaya çıkmamıştır.  
Çalışmada, iki sistem için gerçekleştirilen ek ısıl simülasyonların parametrik irdelemesi istatistiksel olarak analiz edilmiştir. Öncelikle seçilen parametreler için faktöriyel analizler gerçekleştirilmiştir. Faktöriyel analizlerden elde edilen ön bilgiler de göz önüne alınarak daha fazla simülasyon sonucu içeren bir regresyon analizi yapılmıştır. Soğutucu malzemesinin, termal macunun (gres) ısıl direncinin ve ısıl gücün etkili parametreler olduğu belirlenmiştir. Ayrıca sistemin maksimum sıcaklığına etki eden parametre etkileşimleri de belirlenmiştir. Detaylı regresyon analizleri sonucunda, soğutucu sistem tasarımının yani sadece kanatlı ya da ısı  borulu kanatlı sistem ayrımının tek başına etkili olmadığı görülmüştür. Diğer yandan tasarımın diğer parametreler ile ikili ve üçlü etkileşimlerinin olduğu gözlemlendiğinden, soğutucu sistem farklılığının regresyon denklemlerinde bulunması gerektiği sonucuna varılmıştır.
Tez kapsamında yapılan ölçümler ile ortaya çıkan bir diğer önemli sonuç, LED ışık kaynağı özelliklerinin sıcaklık ve sürüş akımından farklı oranlarda etkilenebildiğinin görülmesidir. Bu durum tasarım aşamasına geçilmeden önce LED özelliklerinin ölçüm ile belirlenmesinin gerekliliğini ortaya çıkarmaktadır. LED üreticilerinin kataloglarında LED özelliklerini farklı sıcaklık ve farklı sürüş akımı değerleri için vermesinin armatür üreticilerine tasarımda yol gösterici ve doğru yönlendirici olacağı açıktır.

Abstract:
Nowadays, LED light sources are used frequently in general lighting applications. Considering the criteria such as luminous flux, efficacy factor, and life, LED light sources can exhibit performances that may be a good alternative to conventional light sources. However, due to the high thermal powers being radiated during the operation, the thermal management of LED luminaires is a very crucial issue. In other words it is important to cool luminaires which has LED light sources, by means of appropriate system design. Without a steady and effective cooling system, it is not possible for LEDs to achieve the expected performance.
In this dissertation, firstly, measurements were made to determine the effect of temperature on the properties of a wide range of high power LED light sources. In these measurements a one meter diameter temperature-controlled Ulbricht Sphere was used. Selected high power LED light sources were measured for nominal driving currents stated at datasheets, at six different temperature levels between 25°C - 75°C.   As a result, it was observed that the properties of the LEDs were affected at different rates from the temperature. However, the properties of all LEDs measured were negatively affected by the increasing temperature. This situation has indicated that high power LED light source based luminaires must have an efficient cooling system in order to meet the targeted criteria at their design stage. 
In the literature, there are many types of active and passive cooling systems encountered. Active systems have an additional power requirement for cooling purposes, whereas passive systems do not. Some examples of active systems are fans, liquid-cooling systems, synthetic jets, piezoelectric fans, etc. As examples of passive cooling systems, various cooling fins and heat pipes can be shown. The properties of active and passive cooling systems were evaluated mutually. It has been observed that passive cooling systems have advantages such as simple structure, ease of production, application flexibility and low cost compared to active systems. Considering the usage areas of luminaires with high luminous flux and hence high powers (road lighting, industrial high-bay lighting, etc), the use of passive cooling systems comes to the forefront in a lot of ways.
In this dissertation, it has been decided to use COB LEDs where regional thermal loads are high and therefore heat transfer is crucial. Firstly, LED-based high-bay luminaires on the market have been examined. Then, the COB LED which has used in the luminaire prototype designed in this study was selected. The properties of the selected COB LED was measured by means of the temperature-controlled Ulbrich Sphere. The number of COB LEDs which must be used in the luminaire prototype was calculated according to aimed luminous flux, by considering these measurement results.
Afterward, thermal simulations, production and experimental analyses of the luminaire prototype for two different cooling systems (only with fins and heat pipe together with fins) were made. 
Considering the production conditions, technique and capability, it is decided that the plate dimensions of the prototype, which will be designed for comparison of cooling systems, are 290 mm × 221mm × 14 mm and the height of the fins should be 76 mm. The prototype has 26 numbers, 1 mm thick fins. For both reducing the weight and material usage as much as possible, the fin thickness was selected very thin taking into account the conditions in production. In the choice of the number of fins, in addition to the thermal performance and production possibilities, the equal length of the fins gaps was also considered. The cooling system, in which heat pipes and fins are used together, also includes four 200 mm long heat pipes. 
The prototypes were created in order to compare the effects of heat pipe cooling systems in high power and high luminous flux COB LED luminaires will consist of the mechanisms where only heat accumulation and flow paths are formed, not all components of a luminaire. In both prototypes, especially the surfaces on which the LEDs will be mounted (heat transfer surfaces) were cleaned with isopropyl alcohol to remove any residues that may have occurred during the production. In order to improve heat transfer between COB LEDs and heat sink, thermal grease was applied between the contact surfaces of the PCB and the heat sink.
Experimental measurements of luminaire prototypes were made for both cooling systems. During the experimental analysis of both cooling systems, the prototypes were kept in the air by the lanyard mechanism to avoid contact with any surface, as in thermal simulations. Throughout the experimental analyses, ambient temperature and air velocity were continuously controlled with probes.  It was observed that the ambient temperature was within tolerances close to 25 ° C. 
In both experimental measurements, two constant-current drivers were used to drive COB LEDs. Temperature measurements were carried out using a Keithley 2700 Digital Multimeter device and K type thermocouples  connected to the prototype operating at 1400 mA driving current using.
Time-dependent measurements were collected from the points determined on the luminaire prototype until the temperature stabilized. Experimental measurements and thermal simulation results were verified and the two cooling systems were compared with each other.
At the determined points, it was observed that the average temperature difference between the two cooling systems was only 3.3 ° C. This value, which corresponds to 253 lumens as the luminous flux shows only a difference of 0.6%. For the luminaire prototypes  evaluated, there was no huge differences between the two cooling systems in terms of temperature and in luminous flux.
Thermal simulations were validated by experimental measurements. By considering the differences between simulations and experimental analyses it was decided that thermal simulation results were reliable.  Then parametric examinations were carried out according to additional thermal simulation results. Additional thermal simulations performed for two cooling systems and the results were statistically analyzed. 
Firstly, initial factorial analyses were carried out for the variables thermal power, heat sink thermal conductivity, air temperature, thermal grease resistance, and cooling system design. By considering the initial information obtained from factorial analyses,  regression analyses involving more simulation results was performed. It is determined that the thermal resistance of grease, thermal power and heat sink thermal conductivity are effective parameters. In addition, parameter interactions that affect the maximum temperature of the system were also determined. As a result of detailed regression analysis, it has been observed that the cooling system design, that is, only the fin or heat pipe-fin system separation is not effective alone. On the other hand, since it was observed that the cooling system design has double and triple interactions with other parameters, it was concluded that the cooling system difference should be found in the regression equations.
It is another important result that comes out with the measurements made in the thesis study that the properties of the LED light source can be affected at different rates from the increasing temperature and driving current. This reveals the necessity of determining the LED properties with measurements before proceeding to the design phase. It is clear that the LED manufacturers should give the LED properties such as luminous flux, efficacy, and efficiency for different temperatures and driving current values in their product catalogs. This systematic approach will be a helpful and correct guide in the design process for the luminaire manufacturers.