Elanvändningen i Sverige förväntas fördubblas till 2045 på grund av ökad elektrifiering av det svenska samhället. För att ställa om till ett elsystem som är beroende av mer förnyelsebara elproduktionsslag föreslår Svenska kraftnät (2021) att Sverige kommer att behöva öka sin flexibilitet i elnätet. Ett sätt att göra detta på är genom ellagring, där batterisystem är ett alternativ. Någon konkret plan för hur och när detta skulle genomföras har inte gjorts av Svenska kraftnät. Därför syftade detta projekt till att ta reda på om ett litiumjärnfosfat (LFP) batterienergilagringssystem skulle vara en lönsam investering att använda för energiomställning i det svenska elmarknadsområdet SE3. Detta mål uppnåddes genom att modellera ett batterilager över ett år och extrapolera dessa resultat till en investeringskalkyl genom annuitetsmetoden. Sammanfattat så konstaterades det att det inte är en lönsam investering. Men det fullständiga svaret är mer komplicerat än så. Batteriets storlek och livslängd påverkar batteriets investeringskostnad, som vidare påverkar batteriets lönsamhet. Batteriets livslängd är beroende av egenskaper som batteriets upp och urladdningstid (s.k. C-rate) samt i vilken utsträckning batteriet laddas och laddas ur (s.k. state of charge), som i sin tur influerar totala antalet laddningscykler batteriet kan genomföra. Exakt hur mycket dessa egenskaper påverkar livslängden för LFP:erna är oklart eftersom resultaten från tidigare studier av LFP-batteriers prestanda skiljer sig något. Till exempel är det känt att state of charge för ett batteri påverkar livslängden för ett LFP-batteri, men mer exakt hur mycket varierar beroende på studie, vilket visas i detta examensarbete. Det som gör detta ännu mer komplicerat är det faktum att investeringskostnaden kommer att förändras beroende på till exempel vilken state of charge som används. Dessutom varierar uppskattningar av investeringskostnaden för LFP-batterier i olika källor. Allt detta leder till flera typer av osäkerheter för att bestämma den exakta investeringskalkylen. Det huvudsakliga bidraget denna studie kan ge är dock att den kan ge en första inblick i hur ett batterilagringssystem för kortvarig energilagring (energy shifting) skulle fungera i Sverige. Med fler studier liknande detta projekt skulle en mer konkret plan kunna göras för genomförandet av det statliga klimatmålet om netto noll utsläpp av växthusgaser till år 2045 (Naturvårdsverket, 2023). / The electricity use in Sweden is expected to double before 2045 due to increased electrification of the Swedish society. In order to transition into an electrical system that is dependent of more sustainable renewable energy sources, Svenska kraftnät (2021) is suggesting that Sweden will need to increase their flexibility in the power grid. One of the main ideas on how to do that is through energy storages, where battery systems could play an important part. However, a concrete plan of how and when this would happen was not made clear by Svenska kraftnät. Therefore, this project aimed at finding out whether a Lithium iron phosphate (LFP) battery energy storage system would be a worthwhile investment to use for energy shifting in the Swedish SE3 electricity market area. This aim was reached through modelling a battery storage over a year and extrapolating these results into an investment calculation using the annuity method. In short, it was found that it is not a profitable investment. But the full answer was found to be more complicated than that. The battery’s size and lifetime affect the battery’s investment cost, which further affects the battery’s profitability. The battery’s lifetime is dependent on battery characteristics such as the charge/discharge time of the battery (C-rate) and the extent to which the battery is charged and discharged (state of charge), which in turn influences the total amount of charge cycles a battery can perform. Further, how significant these characteristics affect cost and lifetime of the LFP’s is unclear as the results from previous studies on LFP batteries differ somewhat. For example, it is known that the state of charge range of a battery affects the lifetime of an LFP battery, but by exactly how much varies with different studies, which is explained in this master thesis. What makes this even more complicated is the fact that depending on the state of charge used, the investment cost will change. Additionally, the assessed investment cost also changes depending on the source used. Therefore, the exact cost is difficult to determine. However, the main contribution this study has is that it can give a first insight into how a battery storage system for energy shifting would work. With more case-like studies similar to this project, a more concrete plan could be made about how to realise the Swedish governmental climate goal of net zero greenhouse gas emissions by the year 2045 (Naturvårdsverket, 2023).

<p dir="ltr">Batteries have become essential for a wide range of applications in the field of energy storage and electrification, from portable gadgets to electric cars and renewable energy systems. But effectiveness, performance, and lifespan of a battery pack are closely related to each of the individual cells of which it is composed. The phenomenon of cell voltage imbalance, which can result in a variety of problems ranging from decreased capacity and efficiency to safety concerns and premature failure, poses a significant challenge in managing battery systems.</p><p dir="ltr">Therefore, battery cell balancing plays a crucial role in improving the overall performance of battery packs. To guarantee uniform charge and discharge characteristics, balancing is the process of equalizing the charge of individual cells inside a battery pack. Battery cell balancing seeks to prolong the operational life of packs, improve the efficiency of its energy use, and ensure the safety of the overall system.</p><p dir="ltr">The methods used for battery cell balancing encompass a wide range of approaches, from passive methods that release extra energy as heat, to active methods that move energy across cells. The particular battery chemistry, application requirements, and required level of balancing precision are only a few examples of the variables that influence the choice of balancing technique.</p><p dir="ltr">Lithium Iron Phosphate (LiFePO4) rechargeable batteries are widely used by electric utility companies in battery storage applications. Battery cells are combined to form a battery module. Each module is constantly monitored with sensors and controlled by a Battery Management System (BMS). The BMS performs balancing of the cells. Each cell in the battery stack is monitored to maintain a healthy battery state of charge (SoC). The motivation for this work is to develop an active balancing system to replace a passive system currently being performed manually on an existing battery storage system consisting of LiFePO4 cells. An active cell balancer was designed using the LT8584 active cell balancer, which is based on a flyback DC-DC converter design. An LTspice simulation of the design was created for a single cell. It demonstrates critical parameters of the flyback converter cycle time. A PCB board, designed using KiCAD, was implemented. It is anticipated that the proposed design could be used to restore the health of SoC of faulty modules in lieu of removing and replacing them with a new module, resulting in potential cost savings. The proposed design is scalable in that it could be used for <i>n</i> number of cells in a battery module consisting of LiFePO4 battery cells.</p>