UPTEC ES 20027
Examensarbete 30 hp Juni 2020
Optimisation of electricity
usage during battery production
Emma Ulfsparre
Teknisk- naturvetenskaplig fakultet UTH-enheten
Besöksadress:
Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0
Postadress:
Box 536 751 21 Uppsala
Telefon:
018 – 471 30 03
Telefax:
018 – 471 30 00
Hemsida:
http://www.teknat.uu.se/student
Abstract
Optimisation of electricity usage during battery production
Emma Ulfsparre
Energy storage is an important key for future energy systems. A most common form of energy storage is the battery. However, producing a battery is not very efficient nor sustainable. Therefore, every part and every machine in the manufacturing process must be measured and analysed. The next step is to find solutions of how to make each part more effective.
The purpose of the thesis was to analyse the power consumption of a battery cycling machine and log the temperature changes. The quality of a battery cell is tested by charging and discharging the cell to different state of charge in this machine.
The results showed a lower efficiency during standby state, which is a state when the machine is not used yet is still running. The efficiency increased during charge and discharge of the cells.
Moreover, with enough cells discharging at the same time, the machine could produce electricity. This would also mean that the cells charge at the same time and lead to a volatile load profile.
The temperature increased slightly during charge and discharge but not above the upper limit.
In summary, by scheming the usage of the machines adapted to the number of cells, some machines can be turned off instead of being in standby state. All the machines should be connected to each other in order to exchange excess electricity between them. These solutions can lower the power consumption and make the process more efficient.
Ämnesgranskare: Uwe Zimmermann Handledare: Olivia Barragree
Populärvetenskaplig sammanfattning
De nationella och globala miljömålen kretsar kring att minska människans klimatavtryck.
Att lagra energi är en viktig faktor i problemlösningen, där batteriet är den vanligaste for- men av energilagring i dagsläget. Sveriges mål om att ha en fossiloberoende fordonsflotta om 10 år trycker även på utvecklingen av batterier till eldrivna bilar. Elbilar anses ofta vara en klimatsmart alternativ mot fossildrivna bilar, men vägen till ett färdigt batteri är inte helt miljövänlig. Tillverkningsprocessen av ett batteri kan medföra stora utsläpp av växthusgaser samt kräver hög effektförbrukning. Det är framför allt den här processen som behöver effektiviseras och kontrolleras för att batterier ska bli mer miljövänliga.
I samarbete med Northvolt AB utfördes den här studien i form av ett examensarbete som undersökte en del av sista steget i tillverkningsprocessen. Northvolt AB är ett svenskt företag som tillverkar litiumjonbatterier till främst biltillverkare. Sista steget av batteritillverkningen handlar om att säkerhetstesta battericeller samt undersöka om de uppfyller kvalitetskraven. Cellerna testas bland annat genom att laddas upp och laddas ur i ett antal cykler till olika laddningstillstånd. Maskinen som används för upp- och urladdning heter kammare. När fem kammare är ihopkopplade utgör de tillsammans ett torn. Det är tornen som har en hög effektförbrukning och dessutom genererar mycket värme.
Syftet med projektet var att analysera effektbehovet och mäta hur temperaturen ändras under testernas gång. Hur effektförbrukningen kan regleras och eventuell planering av användning diskuterades även. Att analysera effektbehovet för varje avdelning och maskin på anläggningen ger en bild av hur lastprofilen kommer att se ut. Efter att ha kartlagt effektbehovet kan förslag på åtgärder tas fram för att effektivisera varje avdelning.
Effektkvalitetsanalysatorer och temperaturloggare installerades för att mäta bland annat olika effekter, ström, spänning och temperaturen utanför en kammare. Inuti kammaren fanns temperatursensorer som också mätte temperaturen. Analysatorerna installerades både på ett torn och på en kammare i tornet. Alla tester registrerades i en mjukvara som kunde avläsa varje cells egenskaper. I mjukvaran kunde oönskade egenskaper hos celler upptäckas, för att sedan hanteras efter behov.
Tre olika laddningstillstånd mättes: uppladdning, urladdning och tomgång. Tomgång innebär att inga celler är i kammarna men att maskinen fortfarande är igång. Eftersom en stor del av resultatet skulle vara konfidentiellt visar resultaten en jämförelse mellan kammarens effektförbrukning och tornets effektförbrukning. Den här skillnaden utgör verkningsgraden. I ett ideal fall skulle kammarnas effektförbrukning motsvara tornets, men på grund av förluster i transformatorn och värmeförluster blir det skillnader. För temperaturerna jämfördes temperaturen under tomgång med temperaturen för upp- och urladdning. Även temperaturen utanför kammaren jämfördes med temperaturen inuti.
Resultaten visar att verkningsgraden är betydligt lägre under tomgång än jämfört med upp- och urladdning. Två lösningar för att effektivisera och minska effektförbrukningen är att byta ut transformatorn eller att minska tiden som maskinen går på tomgång. Att byta ur transformatorn är inget alternativ i det här fallet. Istället bör tornens användning schemaläggas utefter hur många celler som planeras att levereras dagligen. Dessutom, om tillräckligt många celler laddas ur samtidigt, kan elektricitet genereras. Det skulle även leda till att cellerna laddas upp samtidigt och resultera i en volatil lastprofil. I dagsläget utbyts överskottet av elektricitet mellan kammarna i ett torn men inte mellan
tornen. Genom att undersöka möjligheten att koppla ihop tornen går det att undvika att transformera överskottselen och utnyttja elen bättre.
Resultaten för temperaturloggningen visar en låg temperaturökning mellan tomgång och upp- och urladdning. Inomhustemperaturen får variera några grader och den här tem- peraturökningen var tillräckligt liten för att inte behöva implementera ett kylningssystem.
Temperatursensorerna på insidan av kammaren ändrades inte efter tiden för upp- och ur- laddning. Därmed gick det inte att beräkna förhållandet mellan temperaturen på utsidan och temperaturen på insidan. Det behöver inte betyda att det inte finns ett förhållande utan kan bero på att temperatursensorerna på insidan inte mäter lika noggrant som loggarna på utsidan gör.
Sammanfattningsvis går det att konstatera att effektförbrukningen kan minska om an- vändningen av maskinerna planeras mer noggrant utefter hur många celler som anländer till avdelningen. Ju längre i utvecklingen som cellerna kommer, desto mer homogent kom- mer flödet av celler att vara. Det underlättar planeringen och därmed kan de maskiner som inte ska användas stängas av. Effekttoppar kan även kapas genom att planera att celler i en kammare ladda upp samtidigt som celler i en annan kammare laddas ur. De tester som utfördes under projektets gång medförde relativt små temperaturökningar.
Dock är det viktigt att följa upp temperaturökningen om delar i processen ändras som kan generera mer värme. När alla avdelningar på Northvolt har kartlagt deras effektför- brukning, kan lastprofilen för hela anläggningen redogöras samt undersöka alternativ för förbättring.
Executive summary
The battery is the most used form of energy storage. The problem with the current batteries on the market is the manufacturing process, which requires a high-power con- sumption and emits greenhouse gases. Therefore, it is important to measure the power consumption and target the inefficient parts of the process. One of the most power consuming departments is the Formation & Ageing department. In this department, a battery cycling machine called chamber consumes the most power. When five chambers are connected to each other and together they form a tower. The purpose of the thesis was to analyse the power consumption of a prismatic chamber and a tower and measure the temperature changes.
There were three states considered when analysing the power consumption: charge, discharge, and standby state. The standby state is when the machine is not being used.
The results showed that the efficiency of the machine was 59% during standby state and 79% during charge. The results were calculated by comparing the chamber’s power consumption with the tower’s power consumption. There were two solutions to improve the efficiency of the machine. One was to improve the transformer and the other one was to schedule the usage of the towers. It is more efficient to use a tower’s full capacity before using other towers. The towers that are used should be turned off. By scheming how many cells will arrive each day, the usage of the towers can be scheduled.
Sometimes when battery cells in a tower discharged at the same time, the tower could produce electricity. Theoretically, the tower could produce 2,6 times more power compared to the power consumption during standby state. However, this would also lead to higher power peaks during charge. An uneven load profile is not pursued but if electricity would be produced, there should be a connection between the towers to exchange electricity. The current system only allows an exchange within a tower.
The temperature rose only +0,6°C during charge and discharge, compared to the room temperature. Even though it never exceeded the upper limit for the allowed room temperature, the charge rate was fixed and was at a low value during the measurements.
Hence, the temperature might rise even more if the charge rate increased. If it would rise beyond the upper limit, a cooling system should be considered but it is not needed now.
Acknowledgements
Throughout these last six months, I have had the pleasure to meet and work with brilliant people at a company I truly believe in. There are a lot of people who have been involved in one way or another of this journey but it is hard to mention all of you.
I will start from the beginning by thanking Lottis, a former colleague, who made me get in contact with Annika Werneman at Northvolt. We barely knew each other but she still wanted to help me. I want to thank Annika, who put her trust in me. She was the head of manufacturing when I started. Annika introduced me to everyone, including Olivia Barragree and Joakim Wahlund, and helped me get started with the master thesis when the original idea didn’t work out. Olivia was my thesis supervisor. She is an area manager and one of the team leaders of the Formation & Ageing team. Besides doing what a supervisor normally do, Olivia arranged so I could work from home and included me in social events. Thank you! Joakim was doing his master thesis at Northvolt when I started and then became a project engineer there. I based a major part of my methodology of how he had performed his master thesis. Also, he helped me understand and employ the measuring equipment used in this thesis. Thank you! I’d like to thank Dennis Song, who is a process engineer at Formation & Ageing. He discussed some of the results with me and helped me un. I also want to thank the Formation & Ageing team, who helped me collect data when I wasn’t there, taught me about the manufacturing process and made me feel like a part of the team. Besides Northvolt employees, I worked with Göran Larsson who works for Cavati AB. With his help, I measured the temperature changes from the machine and we discussed the results from the temperature logging. Thank you!
From Uppsala University, my subject reader was Uwe Zimmermann. He is an as- sociate professor at the Department of Materials Science and Engineering at Uppsala University. I want to thank him for giving me ideas and guidance, especially when my project did not go as planned. Also, he showed me how to present the results for a thesis with confidential information. Lastly, I am grateful for the support from my friends and family.
Emma Ulfsparre
Kungsängen, June 2020
Abbreviations
Terms at Northvolt
Chamber A machine which tests the battery cells through different conditions by charging and discharging the cells. A part of the formation equipment.
Cycler A machine which tests the battery cells’ quality by charging and discharging the cells through several cycles.
F&A Formation & ageing. The last department in the manufacturing process. It includes chambers, high temperature (room) and room temperature (room).
HT High temperature (room). The temperature can fluctuate ± 3°C from the ideal high temperature value.
NG No good. Describes the performance of a cell.
Rest state The state between charge and discharge of the cells.
RT Room temperature (room). The temperature can fluctuate ± 3°C from the ideal room temperature value.
Standby state The state before or after the formation process and without any cells charging or discharging in the chambers.
T&V Test & validation. A department not included in the manufacturing process. It includes cyclers and tests the quality of battery cells.
Tower Chambers stacked on top of each other and connected as one machine. A part of the formation equipment.
I
General abbreviations
AC Alternating current C-rate Charge rate
CT Current transformer DC Direct current
DOE Design of Experiment EVs Electric vehicles EOL End-of-line GHG Greenhouse gas
h Hour(s)
I Current [A]
LFP LiFePO4
Li-Ion Lithium-Ion LTO Li2TiO3
NCA LiNiCoAlO2
NCM LiNiCoMnO2
P Active power [W]
PF Power factor PFT Total power factor Q Reactive power [VAr]
S Apparent power [VA]
SEI Solid Electrolyte Interface SOC State of charge
T0 Ideal room temperature THD Total harmonic distortion
THD-I Total harmonic distortion for current THD-U Total harmonic distortion for voltage U Voltage [V]
Contents
Abbreviations I
Terms at Northvolt . . . I General abbreviations . . . II
1 Introduction 1
1.1 Background . . . 1
1.1.1 The global battery market . . . 1
1.1.2 Northvolt . . . 3
1.2 Purpose and goals . . . 4
1.2.1 Problem statements . . . 5
1.2.2 Constraints . . . 5
1.3 Disposition . . . 5
2 Theory 7 2.1 Lithium-Ion battery . . . 7
2.1.1 Characteristics and chemistry . . . 7
2.1.2 Manufacturing process . . . 8
2.2 Power . . . 11
2.2.1 Definition of active, reactive and apparent power . . . 11
2.2.2 Difference between power factor and total power factor . . . 12
3 Methodology 15 3.1 Literature survey . . . 15
3.2 Test outline . . . 15
3.3 Measuring equipment . . . 16
3.3.1 Power quality analysers and GridVis software . . . 17
3.3.2 Temperature loggers and Rotronic HW4-Lite . . . 19
3.4 Criticism of method . . . 21
4 Results 22 4.1 Results from the power quality analysers . . . 22
4.1.1 Current . . . 22
4.1.2 Power consumption . . . 23
4.1.3 Total harmonic distortion . . . 26
4.1.4 Power factor and total power factor . . . 27
4.2 Results from the temperature logging . . . 28
5 Discussion 30 5.1 Power consumption . . . 30
5.2 Temperature logging . . . 30
5.3 General sources of error . . . 31
5.4 Future projects . . . 31
6 Conclusion 32
7 References 33
A Appendix 35
A.0.1 How to navigate in GridVis . . . 35
A.0.2 How to navigate in Rotronic HW4-Lite . . . 35
List of Figures
1 Distribution of different battery cell chemistries for EVs in 2018-2019 [14] 2 2 Northvolt Labs in Västerås [1] . . . 33 An illustration of a battery’s components, where CC stands for current collector and V stands for voltage [27] . . . 7
4 Winding of a cylindrical cell [9] . . . 10
5 Winding of a prismatic cell [9] . . . 10
6 The relationship between P, Q, S and phase angle φ . . . 12
7 Total power factor during non-linear loads [20] . . . 13
8 Connections of the power quality analysers from Janitza . . . 17
9 The software GridVis . . . 18
10 The software Rotronic HW4-Lite . . . 19
11 Placement of temperature loggers T1 and T2 on the outlet of a chamber 20 12 Placement of temperature logger T3 on the inlet of a chamber . . . 20
13 Current variation for a chamber during charge, rest state and discharge of the cells and standby state . . . 22
14 Current variation for a tower during charge, rest state and discharge of the cells and standby state . . . 22
15 Current variation for both a tower and a chamber during charge, rest state and discharge of the cells and standby state . . . 23
16 Power consumption for a chamber with a half-full tray during charge, rest state and discharge of the cells and standby state . . . 23
17 Power consumption for a tower with three chambers with a half-full tray each during charge, rest state and discharge of the cells and standby state 24 18 The active power consumption for a chamber and for a tower, with a half- full tray in one chamber and three half-full trays in total for the tower . . 24
19 An example of how THD-U could behave during charge and discharge of the cells for a) the tower and b) the chamber . . . 26
20 An example of how the value of the power factor could alter in a chamber for a) PF and b) PFT during charge and discharge of the cells . . . 27
21 Temperature logging from loggers the outside a chamber, where T0 was the ideal room temperature . . . 28
22 Temperature logging from loggers outside, and sensors inside, a chamber, where T0 was the ideal room temperature . . . 29
List of Tables
1 Lithium-Ion battery parameters [13, 14] . . . 8 2 Conclusion of the advantages and disadvantages of the two methods . . . 16 3 A comparison in mean active power consumption between the chamber
and the tower, with a half-full tray in a chamber and three half-full trays in total . . . 25 4 Active power consumption in the chamber and the tower during full charge
and discharge, where P stower is the tower’s consumption during standby state . . . 26 5 Temperatures under normal circumstances, with T0 as the reference tem-
perature . . . 28
1 Introduction
Sweden is heading for a net-zero carbon economy by 2045. So far, the country has the second-lowest CO2 emissions per GDP and per capita among the member countries of the International Energy Agency (IEA) [11]. The Swedish Environmental Protection Agency’s (EPA) vision is to tackle the environmental problems now and not pass it on to the next generation. Even so, among the 16 national environmental quality objectives Sweden has set, barely two of them will be achieved. The other 14 goals will not be achieved, based on policy instruments that have been either decided already or planned.
One of the goals, which appears to not be achieved, is Reduced Climate Impact. This goal is about reducing the risks of increasing the average global temperature. [28]
A solution to achieve the Reduced Climate Impact-goal is to increase the electricity generated by renewable energy sources because they release less CO2 to the atmosphere than fossil fuels. A big problem in electricity generation is to match electricity demand with electricity supply, since the electricity generation from renewable energy sources is intermittent. The largest baseload power source in Sweden is nuclear power, which currently produces around 40% of the country’s electricity supply [29]. Nuclear reactors are shutting down due to political policy and a larger portion of electrical energy comes from renewable energy sources. Renewable energy production leads to more supply vari- ation and power peaks within the grid. There are three main ways to solve this: flexible generation, flexible loads, and energy storage. [26]
The interest of energy storage has increased rapidly in the last few years. Its deploy- ment almost doubled between 2017 and 2018 and it reached over 8 GWh in the end of 2018. In the technology mix in energy storage installations, the Lithium-Ion (Li-Ion) batteries are the most widely used. The market for electric vehicles (EVs) is driving the manufacturing capacity for Li-Ion batteries and this capacity is predicted to be increased by three times by 2022. [10]
For the batteries to become more sustainable, beside improving the battery itself, the manufacturing process must improve. A Swedish report was conducted by IVL Swedish Environmental Research Institute in 2019 [5], to target the greenhouse gas (GHG) emis- sions associated with the car battery manufacturing process. It showed that the battery manufacturing process is energy demanding and has large GHG emissions, but it has improved compared to their older report from 2017. The newest calculations showed that the Li-Ion battery production on average emits 61–106 kilos of CO2-equivalents per kWh battery capacity produced. The GHG emissions range primarily depends on production methods and the type of electricity used in the process. [12]
1.1 Background
1.1.1 The global battery market
The Li-Ion battery is one of the most dominating battery types on the market today [27]. A detailed description of what a Li-Ion battery is and how it works, can be found in section 2.1. In 2018, Li-Ion batteries had a capacity of 150 GWh worldwide, where batteries for EVs accounted for 65% of that. Among the batteries for EVs, there are four main battery cell chemistries: LiFePO4 (LFP), Li2TiO3 (LTO), LiNiCoAlO2 (NCA) and
LiNiCoMnO2 (NCM). These cell chemistries represent the cathode structure of the cell, except for LTO which represents the anode structure. [14]
Figure 1: Distribution of different battery cell chemistries for EVs in 2018-2019 [14]
The NCM cell chemistry is becoming more common compared to the other Li-Ion cell chemistries. In November 2019, NCM stood for 50% of all the batteries used in EVs, which can be seen in figure 1. The numbers 811, 622, 523 and 111 represent the ratio between nickel, cobalt, and manganese in the cathode material. NCM 111 has the same amount of all three elements and NCM 811 has eight-parts nickel, one-part cobalt and one-part manganese. This ratio must not be confused with the total percentage of the cathode since the cathode also contains lithium, oxygen, and other additives. Figure 1 shows that NCM 811 has increased lately since both car and battery manufacturers want to decrease their dependency on cobalt. There are economical, energy and ethical reasons for using less cobalt. [14]
Firstly, the economical reason for this is because cobalt is an expensive metallic ele- ment whilst nickel is cheaper in comparison. Secondly, the energy reason for it is because the battery’s capacity will increase. More nickel in a battery means more reactivity, both on the inside and outside of the cell. Increased reactivity means increased energy density, which results in a longer range for EVs, where more than 300 miles are common today. On the other hand, increased reactivity also means lower stability and leads to a more complicated cell to handle. Since a cell with high nickel content is more reactive, cobalt and manganese are used to stabilise the cathode structure and decrease the risk of degradation of material, thermal runaway and fire. According to IVL Swedish Envir-
2
onmental Research Institute in their latest battery report [5], another energy reason is that the energy usage in the manufacturing process can potentially decrease by 7% and can have 14% less GHG emissions when comparing NCM 811 with NCM 111. Hence, less energy usage is needed and less GHG emissions are released when manufacturing a battery cell with less cobalt in it. [14]
Finally, the ethical reason for decreasing the amount of cobalt in the cell brings up the relation between cobalt, poor working conditions and Congo-Kinshasa. In 2018, approx- imately 70% of the total mining of cobalt came from Congo-Kinshasa. The lack of proper personal protective equipment for the miners and unstable tunnels have caused breath- ing problems, chronic diseases, and deaths. Amnesty investigated this work environment in 2019 and requested the battery industry to make the first ethical battery within five years. Like other battery manufacturers, the start-up company Northvolt invest in the NCM 811 chemistry. [18]
1.1.2 Northvolt
Northvolt AB is a Swedish-founded company whose vision is to produce the world’s greenest Li-Ion battery. Their first large-scale battery factory, a Gigafactory, will be in Skellefteå in Sweden and it is called Northvolt Ett. Before the industrialisation of Li-Ion batteries can start, a blueprint for battery cell production will be developed in Northvolt’s demonstration plant in Västerås called Northvolt Labs, see figure 2. Together with customers, Northvolt Labs is going to produce Li-Ion battery cells and industrialise the manufacturing process to set a standard operation for the mass production. [22]
Figure 2: Northvolt Labs in Västerås [1]
Northvolt is going to secure the manufacturing process with long-term agreements and finance it with investors and loans. To enable the access of lithium hydroxide, Northvolt has signed an agreement with Tianqi Lithium Corps, who is one of the largest lithium producers in the world [24]. Car companies, such as BMW and Volkswagen, support Northvolt by investing and ordering battery cells in advance and the production of these cells will start in 2021 [24]. European Investment Bank (EIB) is the world’s largest universal financial institution of the European Union and lends money for projects in both
the private and the public sectors [8]. EIB financed the construction of the demonstration plant in Västerås and Northvolt Ett is currently under appraisal [23].
There are losses along the manufacturing line which should be decreased to reduce the power peaks and get more efficient energy usage throughout the whole process. Heat losses are a problem when a machine is running since they reduce the efficiency in the device. The three largest energy consuming departments at Northvolt Labs, which also generate a lot of heat, are coding, upstream calcination, and the formation equipment [25].
The focus in this paper is on the formation equipment in the Formation & Ageing (F&A) department. Among the formation equipment, a battery cycling machine called chamber generates the most heat. The chamber performs a safety test for the first cycle of the battery cells where the cells are charged and discharged. Depending on the battery type, the first cycle is done before entering the F&A department. The chamber also charges and discharges the cells to different state of charge (SOC) and with various charge rates (C-rates). Another part of the department is ageing, where the cells are aged in two different temperatures, high temperature (HT) and room temperature (RT).
Both HT and RT values are confidential, but the temperatures can fluctuate ±3°C. In the future, the cells that operate correctly can be sold to a customer. A chamber belongs to a larger machine called tower. The tower has a total of five chambers, and it is connected as one machine. [25]
1.2 Purpose and goals
The purpose of this thesis is to determine the specifications needed for the formation equipment. The focus will be on the chambers and the tower for the prismatic battery cells. By measuring the energy usage and power peaks along with the power factor, the power consumption of the formation equipment can be analysed. The aim is to calculate the load profile for the F&A department in Västerås in order to use these results and scale them for Northvolt Ett. Hopefully, the results can be used to improve the manufacturing line at Northvolt by adapting the planned power consumption to the actual energy usage.
It can also be used when ordering more formation equipment.
Similar tests have been done in other departments and for other power consuming machines. When every machine in each department has been measured and analysed, the total energy demand and load profile can be set for the Northvolt Labs. Afterwards, every department can work on energy-efficient solutions for their machines in order to decrease the electricity consumption and increase the general efficiency. The equipment’s manual showed the maximum power consumption but how much power the chambers and the tower actually consume when running was unknown before this project started.
In this report, it is irrelevant to know how much the maximum power consumption is for the formation equipment.
4
1.2.1 Problem statements
To achieve the purpose of the report, the following problem statements have been set up:
1. What is the power consumption and the energy usage for prismatic chambers and towers? How can it be improved?
2. How much is the indoor temperature affected during charge and discharge of the cells? Is there a correlation between it and the environment temperature inside the chamber?
3. During full capacity, how much electricity could be produced when discharging cells? Can it be used as in-house electricity?
1.2.2 Constraints
One of the biggest constraints of this thesis was how far Northvolt had come in their own manufacturing process. The battery cells that Northvolt is going to produce in the future, were not available during the tests. Instead, the cells in the tests were not completely manufactured at Northvolt Labs. These cells are called DOE cells. DOE stands for Design of Experiment, which means the cells were designed after results from different experiment. Another way of describing it is "learning by doing". They were used in order to test both the formation equipment and the cells characteristics but would not be sold to the costumer. Since the manufacturing process was not running properly, the cells entering the F&A department varied in quantity. Therefore, the formation equipment might not have been functioning correctly since they were not full during the tests. This means the actual losses can differ from the measured losses. Also, the reason why this paper only focuses on the prismatic battery cells is because the manufacturing process of prismatic cells has advanced further than it has for cylindrical cells.
Besides the limitation with how far Northvolt had come in their manufacturing pro- cess, time was limited. The thesis had a deadline in the beginning of the summer 2020.
Also, the thesis took place during a pandemic. This caused not only a delay, but also fewer tests. In a longer period, the real cells can be manufactured and tested.
The report itself also had constraints since there are two versions of it. One that could be published by Uppsala University and one that contained confidential information. The full version of the report, with confidential information, was only shared with Northvolt.
Since some calculations and results could not be shared in the published version, it might be harder for the reader to follow.
1.3 Disposition
The paper started with background information of the thesis: the current situation of Li-Ion batteries, what Northvolt is and explained why it is important to investigate certain parts of the manufacturing line. Afterwards, a theory chapter. It is mainly about Li-Ion batteries and the parts of the production process, which is important to know about in order to understand the thesis itself. This chapter helps the reader understand what a Li-Ion battery is and how it is produced. The exact characteristics of the Li- Ion battery cell produced at Northvolt is confidential and is not discussed in the report.
The theory also contains general information about power to introduce the terminology, which is discussed throughout this paper. After that, the methodology presents how the results were gained. It describes how a literature survey was used to collect helpful information before starting the tests, such as doing a research of the studied machines.
It also introduces the measuring equipment used for the tests. The next chapter is the results and it includes the result from both the power quality tests and the temperature logging. The chapter is followed by discussion and conclusion to analysing the results further and summon the results together with the problem statements. In the end of the report, an appendix is included but tables will be missing in the non-confidential version of the paper.
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2 Theory
2.1 Lithium-Ion battery
Every battery contains an anode, cathode, separator, and electrolyte. A simple illustra- tion of how these materials are connected to each other is shown in figure 3. Anode and cathode consist of active material particles with different particle sizes, binders and elec- tronically conductive additives. They are electrodes and the flow of the current defines them. The current flows out of the cathode and into the anode. Anode is usually seen as the positive side and cathode is the negative side. A separator is between the anode and cathode and all components is moistened in electrolyte. The separator and electrolyte promote the ions’ movements between the anode and cathode material. [27]
Figure 3: An illustration of a battery’s components, where CC stands for current collector and V stands for voltage [27]
In general, the purpose of a battery is to convert chemical energy to electric energy, and this is called electrochemical storage. A Li-Ion battery cell is a rechargeable cell type, hence both a galvanic and an electrolytic cell, based on lithium and other elements.
Galvanic process is another word for discharging and electrolysis means charging. A rechargeable cell is also called a secondary cell, whilst a primary cell is only a galvanic cell which means it cannot be recharged. A battery has a closed system, which implies that the materials in the anode and cathode undergo reduction-oxidation (redox) processes.
For a battery to be considered having "good" quality, the two most important factors are high voltage and high capacity. To get a high voltage, the electrode potential between anode and cathode material must have a large difference. [19, 27]
2.1.1 Characteristics and chemistry
The chemical reaction for Li-Ion batteries can be seen in equation 1, 2 and 3 below.
The reaction that goes in the right direction (−→) shows the charging state and the left direction (←−) implies discharging. [27]
Anode : Cn+ xLi++ xe− CnLix (1) Cathode : LiCoO2 Li1−xCoO2+ xLi++ xe− (2) T otal reaction : LiCoO2+ Cn CnLix+ Li1−xCoO2 (3)
Table 1: Lithium-Ion battery parameters [13, 14]
Battery type Li-Ion
Specific energy 90–300 [Wh/kg]
Specific power 500 [W/kg]
Energy density 200–700 [Wh/L]
Charge/discharge efficiency ∼100 [%]
Self-discharge 5–10 [%/month]
Lifetime >1000 [cycles]
Nominal cell voltage 3,0–4,2 [V]
Table 1 displays general specifications for Li-Ion batteries. The exact parameters of Northvolt’s own battery cells are strictly confidential and are not shared in this paper.
Specific energy is the nominal battery energy per unit mass, Wh/kg, and the battery chemistry and packaging determine the characteristic [21]. This way of determining the characteristics also applies to energy density and specific power [21]. Specific power is the maximum power available per unit mass, W/kg [21]. To achieve a given performance of the battery, specific power determines the required battery weight [21]. Energy density is the nominal battery energy per unit volume, Wh/L, and another word for it is volumetric energy density. It is calculated by multiplying voltage with specific capacity, where specific capacity is the number of electrons transferred per mole. [19]
2.1.2 Manufacturing process
In the manufacturing process for Li-Ion batteries, there are three main steps: electrode manufacturing, cell assembly and cell finishing. In cell assembly, the final structure of the cell is selected between three cell designs: pouch, cylindrical and prismatic. Whatever cell design is selected, the cell has two electrodes and one separator between the electrodes.
Northvolt has invested in making both cylindrical and prismatic cells. They are also going to make their own cathode active material to have better control over the manufacturing process. How the active material for cathode is made is not mentioned in this paper since it is not important for this thesis. The following parts will speak in a general tone how the manufacturing process works and not specifically for Northvolt.
Electrode manufacturing contains several steps: mixing, coating, first drying, cal- endering, slitting and vacuum drying. It has two process lines for cathode and anode material flow. To avoid cross-contamination between anode and cathode, two different mixers are used. A combination of a rotating mixing tool and at least two different raw
8
material get a mixture called slurry. Active material, conductive additives, binders, and solvents are combined in a slurry mixture called distinction. The mixing process must be in a clean room, e.g. under vacuum, to avoid gas inclusions. When the material is mixed, the slurry goes through a pump to a tank in the coating department. By using an application tool, a foil can be coated with the slurry from the tank and is often coated on one side at the time. Some examples of application tools are a slot die, a doctor blade or an anilox roller. Usually, copper foil is used for anode and aluminium foil is used for cathode. Since Northvolt is going to make cylindrical and prismatic cells, the foil is coated continuously and not intermittently. The foil proceeds into a dryer for the first drying process and is fed back to the coating process for the other side to be coated. The thickness of the coated foil is dependent on the cell design and varies between 5–25 µm.
Another coating technique is to coat both sides at once by using two opposite application tools. [9]
The first drying process removes the solvent from the material by supplying heat. The foil goes through temperature zones, normally a chamber system, with a temperature profile of 50–160 °C. A roller system is common to use to transport the foil. After the temperature zones, the foil cools down to room temperature by passing cooled rolls and get rolled up. A double-coated foil will, in the calendering process, be statically discharged and cleaned before getting compressed by two rotating rollers. Afterwards, the foil is cleaned, the thickness is measured and then rolled up. This roll is called the mother roll. The line pressure defines the porosity which in turn defines the energy density of the material. A mother roll is a wider electrode coil. The mother rolls are normally fed to the slitting process manually. In the slitting process, the mother roll is rolled out and cut into smaller electrode coils. The cutting is usually done by rolling knives which can result in released particles. Therefore, the coils must be cleaned by brushes or suction.
Then the coils are rolled up and the new rolls are called daughter rolls. The last step of electrode manufacturing is the vacuum drying process. The daughter rolls go into a vacuum oven and dry for 12–30 hours. After the vacuum drying process, solvents and residual moisture are removed from the rolls and the rolls can be transferred into a dry room. [9]
Cell assembly for prismatic and cylindrical cells has the following steps: winding, packaging, electrolyte filling. Winding has a different approach depending on the chosen cell type. For the cylindrical, a centre pin is the core of the cell which is wrapped in electrode foils with a separator foil in-between the electrode foils and another separator foil to enclose the electrode foils, see figure 4. The result is called jelly roll. To secure the jelly roll, an adhesive tape is wrapped around it. [9]
Figure 4: Winding of a cylindrical cell [9]
The winding of the prismatic cell is similar to the cylindrical. In this case, the electrode foils and separator foils are wrapped around a winding mandrel and secured with a piece of adhesive tape. Figure 5 illustrates the winding process for prismatic cells. [9]
Figure 5: Winding of a prismatic cell [9]
The next step is the packaging process. For the cylindrical cells, the jelly roll is inserted into a robust cylindrical housing made of metal which has an insulator in the bottom. Normally, the anode current collector is welded into the bottom of the cylindrical housing and the cathode current collector is welded to the lid. Between the lid and the jelly roll, an insulation ring is inserted before the welding process. The prismatic cell has another process. The cell housing is rectangular instead of cylindrical and an insulation foil is used to protect the jelly roll from getting damage during insertion. The edges of the jelly roll are compressed and welded through ultrasound into the contact terminals attached in the lid. A laser welding process will seal the prismatic battery cell. The last step of cell assembly is electrolyte filling. The electrolyte filling is divided into two sub- processes: filling and wetting. The filling process consist of a high-precision dosing needle that helps to fill the cell with electrolyte under vacuum. The wetting process activates the capillary effect in the cell by applying a pressure profile to the cell. Depending on the manufacturer and the desirable cell type, evacuation and partial filling are repeated a couple of times. The cells are later sealed by for example beading or welding. [9]
Cell finishing is the last process and it has the following steps: formation, ageing and end-of-line (EOL) testing. The formation process, which this thesis focuses on, contains the first charge and discharge processes for the cells. The cells are put into trays and are connected through small pins on the surface of the cells. The cells will be charged and discharged with defined current and voltage, where the first charge has a C-rate between 0,1C–0,5C and a SOC of 20–80%. A C-rate of 1C implies one hour of charge
10
or discharge whilst 0,5C is two hours of charge or discharge. Solid Electrolyte Interface (SEI) is formed in the formation process and it is the interface layer between the electrode and electrolyte. [9]
A tray contains the battery cells and a chamber only has room for one tray at a time.
When the tray with cells has entered the chamber, the process will alter between charge, rest and discharge. It is common to refer how many times cells charge and discharge with cycles. One cycle means one charge and one discharge. For example, if a cell has charged two times and discharged two times, it has gone through two cycles. The cells always have to rest between charge and discharge, which is referred as the rest state in this report. Depending on what recipe the process follows, the cells will charge and discharge between different SOC.
The final step is ageing which is divided into HT ageing and RT ageing. This step is necessary in order to assure the quality of the cells. It is done by measuring the open circuit voltage (OCV) of the different cells over a longer period. Also, cell properties are measured. For instance, the cells should have constant capacity, low internal resistance, and low self-discharge rate. Normally, the cells first go into the HT ageing and then RT ageing, where they are stored in ageing shelves. Depending on how the cell properties change over time, they can be sold to the customer. If the change is too significant, the cells are not good enough to be sold but instead used for research. Before shipping the cells to the customers, shorter tests will be performed to make sure they have the appropriate cell properties. This step is called EOL testing. A final grading divides the cells into different classifications depending on their performance. [9]
2.2 Power
2.2.1 Definition of active, reactive and apparent power
Power can be divided into three categories: active (P), reactive (Q) and apparent power (S). The mathematical definitions of these are shown in equations 4, 5 and 6 below. [31]
S = U I [V A] (4)
P = U I cosφ = S cosφ [W ] (5)
Q = U I sinφ = S sinφ [V Ar] (6)
The relationship between the three different powers and the phase angle is also illus- trated in figure 6 below.
Figure 6: The relationship between P, Q, S and phase angle φ
As shown in figure 6, S is the total amount of power required. The unit is Volt- amperes (VA) since S is also calculated by multiplying the voltage and the current. P is the actual work of light, heat, and motion with the unit Watt (W) whilst Q is the nonworking power and has Volt-amperes reactive (VAr) as unit. Even though Q does not provide active energy, it is important because it sustains the magnetic or the electric field in machines and devices. Therefore, S and P differ because of the inductance in the system, also known as the reactive element Q. The power factor (PF) depends on the phase angle φ and a greater angle shows how inefficient the electricity is being used. Q increases if φ increases, which implies that S will also increase but P will stay the same.
[3] Q can be both positive and negative and it depends on if the circuit is inductive or capacitive. A positive Q value means the reactive power is consumed whilst a negative Q value means the circuit delivers reactive power [31]. P is normally positive, but it can also be negative if the PF is negative. A negative PF occurs when a machine or device generates power instead of consuming it and the electrical power flows in reverse direction. In other words, the electrical power flows back towards the source. [7]
2.2.2 Difference between power factor and total power factor PF is usually defined as:
P F = P
S = cosφ (7)
Equation 7 shows that PF is the ratio of P and S whilst φ is the phase angle between the voltage and the current. If the current is lagging the voltage, then PF also lags and if the current is leading, PF is leading. The current lags when the load is inductive, and it leads when the load is capacitive. Figure 6 shows a lagging power factor. When φ is 0°, the voltage and the current have the same phase placement following a pure and undistorted sinusoidal wave. Otherwise, the phase angle φ represents the displacement in phase between the current and the voltage. The range of PF is between 0 and 1 because
12
cosine cannot be greater or less than these values. This is the conventional description of PF but in reality, PF and cosφ can differ. [3]
To separate these two power factors in this paper, the power factor which includes distortion is called total power factor (PFT). PFT becomes a three-dimensional quantity when power semiconductors are used. It adds another factor called harmonic component or harmonics because of the non-linear loads. Anything that is transformed from altern- ating current (AC) to direct current (DC), is a non-linear load. Q and phase control combined is called displacement component. The vector sum of the displacement com- ponent, P and the harmonics gives the total S. An illustration is shown in figure 7 below.
[20].
Figure 7: Total power factor during non-linear loads [20]
The non-linear loads can be connected to a normal sinusoidal voltage, but the current gets distorted and its waveform gets non-sinusoidal. This is expressed by using harmonics.
Harmonics are a waveform of varying amplitudes which appear from frequencies multiples of the fundamental frequency f of the voltage. The harmonics can be expressed as 2f, 3f, 4f, etc., where f is 50 Hz or 60 Hz, and these create the total current waveform.
A summation of all harmonic components of the current waveform who are compared to the fundamental component of the current wave is called the current total harmonic distortion. This is also applicable for voltage. Since the total harmonic distortion (THD) is applicable for both current and voltage, they are separated in this report as THD-I for current and THD-U for voltage. [3]
T HD = r∞
P
n=2
In2
I1 (8)
Equation 8 shows the definition of the total harmonic distortion of the current [4]. In
is the amplitudes of the harmonics. In the context of power systems, THD should always be compared to the fundamental component instead of the signal’s root mean square [4].
Electrical equipment can get detrimental effects from harmonic distortion. For example, the current in power systems can be increased by the THD which can result in higher temperatures and in worst case, lead to thermal runaway. Moreover, additional core loss in motors is caused by higher frequency harmonics and it results in excessive heating of the core. [2]
Current harmonics create voltage distortion. Currently, there is no national standard THD value to limit systems but there are some recommendations for voltage harmonics.
The recommended limits for these harmonics are set to 5% of the fundamental THD- U and 3% for a single harmonic. The limits are not requirements but lower THD in a system will ensure proper operation of equipment and the equipment life span will be longer compared to high THD. High THD can lead to subtle malfunctions of the machines and cause serious severe consequences. [6]
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3 Methodology
3.1 Literature survey
A literature survey was conducted in order to perform the tests in an efficient way and get the best possible test outline. To begin with, looking at other methodologies for similar tests was a good starting point of the project. Especially other master theses written for Northvolt. By studying them, it was easier to distinguish what information could be shared and what should be described in a general form. At the T&V department, Joakim Wahlund wrote his master thesis about the performance of the cyclers and how much energy those machines needed [15]. Through meetings and visits at this department, the foundation of how to perform the tests for this thesis could be set.
The purpose of the cyclers in T&V department differs from the chambers in F&A, which leads mainly to an operation time difference. Tests in cyclers can run for weeks whilst tests in chambers can take between less than an hour and a day, depending on the C-rate. A test with a lower C-rate takes longer time than with a higher C-rate.
How many times the cells charge and discharge and how long they rest, also affect the operation time.
Another part of the literature study was to determine what equipment would be used to measure the chambers. As for Wahlund, he used power quality analysers from the German company Janitza Electronics GmbH. The power quality analyser is preferable since it does not take too much space and it can calculate e.g. P, Q, and S along with the power factor and the different loads between the three phases.
3.2 Test outline
Due to lack of current transformers (CTs), there were two discussed methods to perform the test. The first method was to measure one tower and then measure one chamber through two separate tests. The second method was to measure both the tower and a chamber in the same test. It was interesting to measure both the tower and the chamber to see how big the losses were in the chambers that were not operating. In an ideal case, when only one chamber is used, it would be the same energy usage as for the whole tower.
Both these two methods would calculate how much the real values differ from the ideal values. For the first method, the issue would be to continuously install and uninstall the measuring equipment on the chamber and on the tower. It would mean more time planning and more labour including other people. The problem with the second method was that more CTs were needed, hence it would cost more, and the first tests would be delayed on account of purchasing and shipment time.
From the literature survey, it was found that the cell performance differs between the cells because they were not the final products. Especially when some cells are classified as no good (NG) cells. They are considered NG cells if the voltage or the resistance are not within the acceptable range but also if they do not behave as expected. Hence, the results will differ if a tray contains NG cells. Since the battery cells’ characteristics varied, the results from running the tests with different cells will probably vary as well.
Another problem is that more cells are needed when doing the same test twice, which the first method would have meant. The second method was chosen for this project.
Table 2: Conclusion of the advantages and disadvantages of the two methods First method
Advantage Disadvantage
· Earlier start of tests, leads to more tests · More cells needed due to more
collected required tests
· No money spent on CTs · Less accurate results, comparing cells with various quality
· More installations occasions Second method
Advantage Disadvantage
· Fewer cells needed · Later start of tests due to purchasing
· More accurate results, measurements done and shipping
at the same time with the same cells · Money spent on CTs
· Fewer installation occasions
Table 2 summarise the benefits of each method and also their disadvantages. For safety reasons, only half-full trays were used for the formation process. The manufacturing process was still in the development phase during the measuring period, but completely full trays will be charged and discharged in the future. In the beginning of the measuring period, the lack of cells made it complicated to always have half-full trays. The idea was to make a regression analysis from the test results and use the analysis to calculate how the parameters, such as power factor and power consumption, would be for a full tray. Since a tray did not contain more than half of its capacity, the regression analysis became the foundation of the calculations. It would also work as a comparability for the real measurements and see how accurate the regression analysis would be. The measurements took place between March and June 2020.
3.3 Measuring equipment
The equipment used in order to measure the different electrical parameters are the same as for the T&V department. The measuring equipment bought from Janitza were two power quality analysers called UMG605 and UMG104 [17, 16]. UMG605 was connected to the tower and UMG104 to one of the chambers in the tower. For simplicity, the power quality analysers will be called UMG605-tower and UMG104-chamber in the report. Each cell’s capacity was measured after every charge and discharge state. Since the cells were still in progress and not the final product during this thesis, this step was important in order to follow the performance of the cells. It was also for safety reasons to avoid e.g.
thermal runaway.
To get a closer look on the heat losses, a test programme was developed by Göran Larsson to measure and analyse these losses. By attaching temperature loggers on the inlet and outlet air flow of the chambers, the correlation between the temperature differ- ences and heat losses can be set. The temperature loggers were bought from the Swiss company Rotronic AG [30]. Inside the chamber, temperature sensors that measure the environment temperature of the chamber were already installed before the tests. This is
16
not the temperature of the cell itself. Using the instruments from Janitza and Rotronic together with the temperature sensors, the losses were measured through three different ways, which increased the certainty of the results. Janitza was using GridVis software whilst Rotronic was using a software called Rotronic HW4-Lite.
3.3.1 Power quality analysers and GridVis software
Figure 8: Connections of the power quality analysers from Janitza
How the two power quality analysers were connected is seen in figure 8. The analysers are the blue devices in the figure, where UMG605-tower is on the left and UMG104-chamber is on the right. The black cables were connected to the three phases for both voltage and current. The blue cables were connected to a neutral point whilst the grey cable was connected to the computer. Once the analysers were connected to the tower, the chamber, and a socket, they were always running and collecting data. There was no start or stop button for the test on the analysers.
In this project, six sets of parameters were chosen for the tower: PFT & PF, SPQD, THD-U & THD-I, current, energy and voltage. SPQD includes apparent, active, reactive and distortion power. Distortion power is not discussed in this report, only THD. For the chamber, four sets of parameters were chosen: PFT & PF, SPQ, THD-U & THD-I and voltage & current. All the chosen parameters were found under data exports in the folder-tree in GridVis, see figure 9. In the software, PF is called cosφ and PFT is called PF but in this report, they are referred as PF and PFT.
Figure 9: The software GridVis
Some parameters, such as P, could show negative values depending on how the power quality analysers were installed in the machine. The current could be in the opposite direction, which would cause a negative P value. This must be adjusted in GridVis to get proper results before the test starts. The adjustment was done in configuration settings in the software. For more information on how to navigate the software, see A.0.1 in Appendix.
The storage depth of the power quality analysers depends on how many electrical parameters were examined. UMG605-tower had a bigger storage depth from the begin- ning compared to UMG104-chamber. With the chosen parameters, UMG605-tower could store the last 72 hours whilst UMG104-chamber stored the last 12 hours. This was done by connecting a computer to an ethernet cable attached to the analysers. The next step was to make sure the IP-address was correct, which was changed manually. To make sure the connection between the power quality analysers and the computer was correct, a connection test was performed in GridVis for both the tower and the chamber. Once the connection test was approved, the synchronisation could start. Synchronisation was usually done after a test. The test lengths varied, and UMG104-chamber had to be synchronised every 12th hour, or else the new data would overwrite the old data. This means the first data values in the test could be lost if synchronisation does not happen frequently. After the test had finished and after synchronisation, every data export had to be executed and saved as Excel spreadsheets. In the Excel spreadsheets, diagrams could be created, and the results could be analysed.
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3.3.2 Temperature loggers and Rotronic HW4-Lite
During a period of three weeks, the temperatures from the inlet and the outlet air flow of the examined chamber were recorded. The test was logging how the temperature changes in order to understand how the indoor temperature will be affected when battery cells are charging and discharging. It was done by attaching one of the temperature loggers from Rotronic on the inlet and two loggers on the outlet. The loggers were attached on the inlet and outlet with zip ties. The accuracy of the temperature loggers is ± 0,3°C for a temperature range of 18–28°C and the precision is 0,1°C [30].
Figure 10: The software Rotronic HW4-Lite
Figure 10 shows the software Rotronic HW4-Lite, which was used together with the temperature loggers. Before the test, the temperature loggers were connected to the software through a USB cable. The pressure is not interesting for these tests and hence not recorded. The lower temperature limits was set to 10 °C, the upper limit to 50 °C and hysteresis was 3 °C. Log interval was 60 seconds and Stop if full was the recording mode.
An interval log of 60 seconds would give a recording time of 22 days, since the device could log 32 000 times before the memory storage was full [30]. Before the recording started, the old memory was cleared and after three weeks, the memory was downloaded to the software. The reference value for the temperature was set to T0, which was the ideal room temperature in this paper. A further explanation of the settings is found in A.0.2 in Appendix.
Figure 11: Placement of temperature loggers T1 and T2 on the outlet of a chamber Figure 11 shows the outlet of a chamber and the placement of the temperature loggers.
T1 was placed on one of the smaller fans whilst T2 was placed on an opening with a bigger fan behind the doors. Figure 12 shows the inlet of a chamber and how T3 was placed.
The displays on the loggers are censored in the figures, otherwise the current temperature would be shown.
Figure 12: Placement of temperature logger T3 on the inlet of a chamber
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3.4 Criticism of method
The factory was still in the development phase when the tests were established. Some tools were missing, and the measuring equipment were not installed in the beginning.
There are some factors to be considered as criticism of the chosen method.
How many cells the department received during the measuring period differed from the approximated number of cells. The approximation was done by the area manager of the F&A department and it assumes how many cells the F&A department will receive on a weekly basis. This assumption was mainly based on surveillance of cells. There is a great possibility that the actual flow rate of cells will differ from the approximated flow rate.Also, the cells will probably not enter the chambers at the same time. The calculations will probably show larger peaks and troughs for the power consumption than the actual peaks and troughs, since charge and discharge will happen simultaneously according to the calculations. If the cells start the formation process at different times, the load profile will be less volatile since charge and discharge can happen at the same time for different chambers.
The placement of the loggers has an impact of the result. Out of the three loggers, T1 had the most stable air flow because it was attached very close to a fan. T2 and T3 might have had a bit unstable air flow, since T2 was further away from a fan and T3 barely had an air flow. If T2 and T3 were attached on the inside of the chamber, there would have been less disturbance from the surrounding. This was not possible for T3 without disassembling the chamber. T2 could have been attached differently but would also have been in the way if someone needed to fix a problem inside the chamber. The placement might cause disturbance from external factors, such as opening the doors into the chamber where T1 and T2 were attached. However, the results can still illustrate a good overview of the temperature change.
Most of the results were measured without the third phase for the tower. When analysing the first results, one of the three phases gave abnormal values for the tower.
The third phase showed a current near zero, which meant the power consumption was uneven between the three phases. The results showed that two phases carried all the load whilst the third phase barely consumed any power. It was due to an installation error. It took over a month to find the error and the error was eliminated near the deadline of the thesis. To avoid more delays and losing large amount of data, only a few tests were done after the problem was fixed. Instead, the results from the last tests showed the correlation between the three phases and from those tests, the third phase could be approximated for the earlier tests. Even though the data for the third phase was approximated, it was validated to the real correlations between the phases and can therefore give valid results.
All tests were not registered in the internal data base. Normally, all tests are following a recipe for all the different steps in the F&A department. It is easy to go back and find old results or make follow ups when something went wrong for a tray of cells. The process is automatic once the trays have entered the system. Sometimes, a test without a recipe is done manually. It is more difficult to follow the steps for these trays and there the time schedule can only be approximated. The lack of information from these tests made it difficult to analyse without making a lot of assumptions and were for that reason not included in the calculations.
4 Results
4.1 Results from the power quality analysers
4.1.1 Current
Figure 13: Current variation for a chamber during charge, rest state and discharge of the cells and standby state
An example of how the current behaved in the chamber is illustrated in figure 13. The curve starts in the middle of a discharge, goes into a charge state, then a discharge again and finally a charge before exiting the chamber. Between charge and discharge, there is always a rest state. How many cycles the cells go through was not relevant for this paper and the values on the x-axis and y-axis cannot be shared, due to confidentiality constraints.
Figure 14: Current variation for a tower during charge, rest state and discharge of the cells and standby state
During the same period as for figure 13, figure 14 also illustrates how the current varies, but for the tower instead. As seen in the result, only one chamber was used here.
Otherwise, the current would not follow the exact same pattern as in figure 13. The third phase, L3, could not be measured due to an installation error. By analysing results from
22
before and after the error, L3 was approximated to behave as the first phase L1 with similar values. In this case, L1 and L3 were assumed to be the same.
Figure 15: Current variation for both a tower and a chamber during charge, rest state and discharge of the cells and standby state
Figure 15 illustrates the current’s correlation between the tower and the chamber. The values differ between the phases, especially for the tower. The current for the chamber was almost half the size of the current of the tower when there were no cells in the chambers. This is called standby state in this paper. Standby state is before the cells enter the chambers and the formation process starts or after the cells exit the chambers and the formation process ends. The discharge state also had similar result, but the charging behaved differently. In the charge state, the tower’s current was 3,5 times larger than the chamber’s current.
4.1.2 Power consumption
Figure 16: Power consumption for a chamber with a half-full tray during charge, rest state and discharge of the cells and standby state
Figure 16 illustrates the consumption of P, Q and S for one chamber. The black line indicates where the y-axis is zero. When the cells are in rest state, the chamber consumes
the same amount of power as when the chamber is in standby state. During discharge, the chamber produces electricity instead of consuming it. Throughout the formation process, Q was almost at a constant negative value for the chamber.
Figure 17: Power consumption for a tower with three chambers with a half-full tray each during charge, rest state and discharge of the cells and standby state
Figure 17 shows the power consumption of the tower when three trays entered three different chambers almost at the same time. In the end of the diagram, there are three smaller peaks that indicate the last charge before exiting the chamber. Hence, the peaks reveal how close to each other the trays started the formation process. Figure 16 has the same time scale as figure 17.
Figure 18: The active power consumption for a chamber and for a tower, with a half-full tray in one chamber and three half-full trays in total for the tower
Figure 18 compares the active power consumption for the chamber in figure 16 and the tower in figure 17. When comparing the two figures, it can be seen that the second tray entering the formation process was the same tray shown in figure 16. Since the trays were charging and discharging almost at the same time, the amplitude will increase between the charge and discharge state. During a part of the discharge, P was negative and hence the tower produces electricity.
24
During standby state, the active power consumption for the chamber was rounded to 12% of the total power consumption of the tower. It was calculated by summing all the measured standby state values. Assuming all five chambers consume the same amount of power, together they would consume 59% of the total power consumption during standby state. It means 41% of the power is lost, most likely in the transformer and through heat losses.
Table 3: A comparison in mean active power consumption between the chamber and the tower, with a half-full tray in a chamber and three half-full trays in total
Standby state (Ps) Full charge (Pc) Full discharge (Pd) P schamber
P stower = 11, 87% P cchamber
P ctower = 23, 87% P dtower
P dchamber = 30, 47%
11, 87% · 5 = 59% 23, 87% · 3 = 71, 61%
P schamber P ctower
= 3, 65% 3, 65% · 2 = 7, 3% 71, 61% + 7, 3% = 78, 91%
Table 3 summarise the calculations for standby state, full charge and full discharge and it is the specific case from figure 18. The mean power consumption that one chamber reached during full charge was 24% of the total mean power consumption during charge.
This means that all three chambers with the same number of cells stood for 72% of the total mean power consumption during charge. Three out of five chambers in the tower were used, which means two chambers were empty during the test. The two empty chambers were assumed to consume the same amount active power as during standby state. Two chambers in standby state would then consume twice as much. In comparison with the total mean power consumption during charge, the consumption for two empty chambers stands for 7%. Together, all five chambers consumed 79% of the total mean power consumption during charge. When discharging, the chamber had a higher absolute value than the tower and both had negative mean values. 30% of the absolute value for the chamber’s electricity production, was the total electricity production during discharge.
