Elektrikli araçlarda kullanılan 18650 Li-iyon pillerin soğutma analizi ve kontrolü
Cooling analysis and control of 18650 Li-ion batteries used in electric vehicles
Yazar: OSMAN TAHA KÜTÜK
Danışman: PROF. DR. METİN GÖKAŞAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Mekatronik Mühendisliği Ana Bilim Dalı / Mekatronik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Mekatronik Mühendisliği = Mechatronics Engineering

Özet:
Elektrikli araçlarda prizmatik ve silindirik li-iyon piller kullanılmaktadır. Bu piller birbirine seri ve parallel bağlanarak pil paketi haline getirilir ve araçların yakıt enerjisi yerine geçerek tahrik enerjisini sağlar. Bu tahrik enerjisini sağlarken doğal olarak pil paketinden dinamik olarak akım çekilir. Sonrasında ise bu pillerin şarj edilmesi gerekir. Pil paketi şarj ve deşarj esnasında pilin iç direncinden dolayı ısınmaya maruz kalır. Bu ısının pil ömrüne ve anlık olarak pil paketine zarar vermemesi için dışarıya atılması gereklidir. Pillerin olması istenen sıcaklık aralığı 20oC ila 40oC arasındadır. Bu aralıkta tutulamadığında öncelikli olarak pil ömrüne ve verimine negatif etki olur sonrasında daha yüksek sıcaklıklara çıkılması halinde piller yanma ve patlama gibi tesirlere yol açabilirler. Son yıllarda araçların park halinde iken ve ya şarj esnasında ikne yanmaya başlaması batarya termal sistemlerinin verimli olmadığını göstermiştir.  Elektrikli araçların kullanımın artması bu soğutma konusunun verimli hale getirilmesi gerektiğini gün yüzüne çıkarmış ve bununla ilgili çalışmalar yapılmıştır. Bu çalışmada ise silindirik 18650 li-iyon pil incelenmiş ve bu pillerden oluşturulan 16'lı pil paketinin belirlenen akımlarda oluşturduğu ısılar hesaplanmıştır. Belirlenen akımlarda yüzey sıcaklık değerlerini elde edecek deneyler yapılmış termal görüntüler elde edilmiştir. ANSYS/Fluent kullanılarak belirlenen hacimsel debilerde ve hava, su, ATF şanzıman yağı ve batarya soğutma sistemi için üretilmiş AmpCool yağı gibi farklı akışkanlarda analizler yapılmış, bu analiz sonuçlarından hangi akışkanın soğutma açısından daha verimli olduğu ve kullanılabilirlik açısından en uygun olduğu yorumlanmıştır.Yine analiz sonuçlarından alınan verilerle sistemin çalışması gerekli pompa debi aralığı belirlenmiştir. Deney düzeneği tasarlanmış gerekli kontrol kartı, pompalar, motor sürücü kartları sıcaklık sensor seçimi yapılmış ve deney düzeneği kurulmuştur. Basit kontrol algoritması geliştirilerek pil paketinin üzerine yerleştirilen sıcaklık sensörlerinden veriler okunmuş maksimum sıcaklığa göre sistemin belirlenen debiyi vermesi sağlanarak soğutma süreci incelenmiştir. Belirli şartlarla birlikte yapılan analiz ve deneyler öngörülen sonuca ulaşmış ancak daha kullanıma uygun bir tasarımla ve şartların daha genişletilmiş haliyle sistemin oluşturulması gelecek planı olarak belirlenmiştir.

Abstract:
The 18650 type lithium-ion batteries are commonly used in notebook computers, or electric devices as power sources. It refers to a kind of batteries with a diameter of 18mm, length of 65mm. Lithium-ion batteries are the main power sources of electric vehicles and electronic devices. They have are capacity and high power density. However, lithium-ion batteries are vulnerable to excessive heat generation during fast charging. The excessive heat generation leads to high temperature of the battery, which challenges the safety of the battery. Statistics shows that most of lithium-ion battery safety accidents are caused by the overheating of the battery. Therefore, the thermal safety of lithium-ion batteries is becoming more and more important. The temperature field of the battery under different conditions is the basis of the thermal safety evaluation of batteries. Battery performance, cycle life and system safety are all dependent on temperature distribution in the battery, which depends on heat generation rate within the battery and on heat removal rate at the battery surface. The methods of solving the temperature distribution include analytical method , experimental method , simulation method  and so on. The analytical method is usually used to solve simple models while the cost of experimental method is too high. Therefore, the simulation method becomes the main method of thermal analysis. Prismatic and cylindrical Li-ion batteries are used in electric vehicles. These batteries are connected to each other in series and parallel to form a battery pack and provide the propulsion energy by replacing the fuel energy of the vehicles. Naturally, current is drawn dynamically from the battery pack while providing this drive energy. Afterwards, these batteries need to be charged. The battery pack is exposed to heat during charging and discharging due to the internal resistance of the battery. This heat must be discharged in order not to damage the battery life and the battery pack momentarily. The desired temperature range of the batteries is between 20oC and 40oC. If it cannot be kept within this range, it will primarily have a negative effect on battery life and efficiency, and later on, if higher temperatures are reached, the batteries may cause effects such as burning and explosion. In recent years, when the vehicles are parked or during charging, the ignition of the battery has shown that the thermal systems of the batteries are not efficient. The increase in the use of electric vehicles has brought to light the need to make this cooling issue efficient and studies have been carried out on this. In this study, the cylindrical 18650 li-ion battery was examined and the heat generated by the 16 battery pack formed from these batteries at the determined currents was calculated. Experiments were made to obtain surface temperature values at the determined currents and thermal images were obtained. Analyzes were made at volumetric flow rates determined using ANSYS/Fluent and in different fluids such as air, water, ATF transmission oil and AmpCool oil produced for the battery cooling system. In addition to the air and water fluids investigated depending on the system, transmission oil used in vehicles and AmpCool oil produced for battery cooling were selected. Later in this booklet, it is explained why AmpCool oil is the best refrigerant, but also compared with other fluids. Again, with the data obtained from the analysis results, the required pump flow range for the system to operate was determined. Pump selection and supply were made according to the flow range However, research on three-dimensional heat transfer models, particularly the temperature field of 18650 lithium-ion batteries with various charging rates and natural cooling circumstances, is uncommon. Using the finite element analysis program ANSYS, this article will model the heat generation and heat dissipation characteristics of 18650 type lithium-ion batteries under natural cooling circumstances. Active and passive cooling systems are the two types of thermal management system for battery systems. Active cooling, on the one hand, consists of air and liquid cooling systems that require an external source of energy. Passive cooling, such as phase change materials (PCM), on the other hand, does not require any external energy. Because of its cheap production cost, easy layout requirements, and excellent dependability, air cooling is one of the most appropriate thermal management system. Natural air cooling and forced air cooling are the two types of cooling systems. The impact of natural and forced air cooling on cylindrical cells was studied experimentally and statistically by a number of researchers.The correlation between the pump and the flow sensor was calculated and the mathematical expressions required for calibration were reflected on the electronic card on which the algorithm will be used. Before the experiments started, experiments were carried out on the smooth flow of the fluid in the system and the correct acquisition of data from the sensors. The experimental setup was designed, the necessary control card, pumps, motor driver boards, temperature sensors were selected and the experimental setup was set up. While the experimental setup was being set up, the fluid inlet and outlet parts were carefully placed in the system and discharge valves were added. The temperature data of the currents determined through the temperature sensor placed on a Li-Ion battery were obtained and the highest temperature time graph was drawn. Based on this graph and the analysis results, the most appropriate and simple control algorithm has been considered. By developing a simple control algorithm, the data were read from the temperature sensors placed on the battery pack, and the cooling process was examined by ensuring that the system gave the determined flow rate according to the maximum temperature. The analysis and experiments carried out with certain conditions have reached the predicted result, but it has been determined as the future plan to create the system with a more suitable design and more expanded conditions.