Electrical distribution networks face many operational challenges as various renewable distributed generation (DG), such as solar photovoltaic (PV) systems and wind, become part of their structure. Unlike conventional distribution systems, where the only unpredictable aspect is the load level, the intermittent nature of DG poses additional uncertainty levels for distribution system operators (DSO). The voltage quality problem considers the most restrictive issue that hinders high DG integration into distribution grids. Voltage deviates from the nominal grid voltage limits due to the excess power from the DG. DSOs are accustomed to improving the voltage profile by optimal adjustments of the on-load tap changers, voltage regulator taps and capacitor banks. Nevertheless, due to the frequent variability of the output energy from DG, these devices may fail in doing the needful. Battery energy storage systems (BESS) and smart PV inverter functionalities are regarded as promising solutions to promote the seamless integration of renewable resources into distribution networks. BESS are utilized to store the surplus energy during the high penetration of renewable DG that causes high voltage levels and discharge the stored energy when the distribution grid is heavily loaded, which leads to the low voltage levels. Smart PV inverters regulate the network voltage by controlling the reactive power injection or absorption at the inverter end. This dissertation proposes a management strategy that coordinates BESS and smart PV inverter reactive power capability to improve voltage quality in the distribution systems with high PV and wind penetrations. The proposed management method is based on a bi-level optimization algorithm consisting of upper and lower optimization levels. The proposed method determines the optimal location, capacity, numbers and BESS charging and discharging rates to support the distribution system voltage and to ensure optimal deployment of BESS. Case studies are conducted to evaluate the proposed voltage control method. The large size PV system and wind turbine impacts are studied and simulated on the modified IEEE-34 bus test feeder. In addition, the proposed method is applied to the modified IEEE low voltage test feeder to investigate the effectiveness of installing residential rooftop PV systems on the distribution system's voltage. Experimental results show promising outcomes of the proposed method in controlling the distribution networks' voltage. In addition, a day-ahead forecast of PV power output is developed in this dissertation to assist the DSOs to accurately predict the future amounts of PV energy available and reinforcing the decision-making process of batteries operation. Hybrid forecasting models are proposed based on machine learning algorithms, which utilize support vector regression and backpropagation neural network, optimized with three metaheuristic optimization algorithms, namely Social Spider Optimization (SSO), Particle Swarm Optimization (PSO) and Cuckoo Search Optimization (CSO). These algorithms are used to improve the predictive efficacy of the selected algorithms, where the optimal selection of their hyperparameters and architectures plays a significant role in yielding precise forecasting outcomes. / Doctor of Philosophy / The need for more renewable energy has grown significantly, and many countries are embracing these technologies. However, the integration of distributed generation (DG), such as PV systems and wind turbines, poses several operational problems to the distribution system. The voltage problem represents the most significant issue that needs to be addressed. The traditional voltage control equipment may not cope with the rapid fluctuation and may impact their service life. The continuous developments in the battery energy storage systems (BESS) and the smart PV inverter technologies result in increasing the hosting capacity of DG. BESS can store the excess power from the distributed generators and supply this energy to the grid for different operational objectives. On the other hand, the advanced PV inverter's reactive power capability can be exploited from which the grid can attain many benefits. This dissertation aims at providing a reliable control method to the voltage profile in distribution networks embedded with high PV and wind energy by optimal coordination between the operation of the BESS and the smart PV inverter. In addition, the solar forecasting can mitigate the uncertainty associated with PV system generation. In this dissertation, the PV power forecasting application is applied in the distribution system to control the voltage. Through utilizing PV power forecasting, the decision-making for battery operation can be upheld and reinforced. The BESS can store the surplus energy from the PV system as needed and supply it back in low PV power incidents. Experimental results indicate that proper coordination between the BESS and smart PV inverter is beneficial for distribution system operation that can seamlessly integrate PV and wind energy.

The dissertation presents a generalized control structure for two-stage power converters operated in a renewable energy power system for smart grid and micro grid systems. The generalized control structure is based on the two-loop average-mode-control technique, and created by reconstructing the conventional control structure and feedback configuration. It is broadly used for both dc-dc and dc-ac power conversion based on the two-stage converter architecture, while offering several functionalities required for renewable energy power systems. The generalized control structure improves the performance and reliability of renewable energy power systems with multiple functionalities required for consistent and reliable distributed power sources in the applications of the smart grid and micro grid system. The dissertation also presents a new modeling approach based on a modification of the subsystem-integration approach. The approach provides continuous-time small-signal models for all of two-stage power converters in a unified way. As a result, a modeling procedure is significantly reduced by treating a two-stage power converter as a single-stage with current sinking or sourcing. The difficulty of linearization caused by time-varying state variables is avoided with the use of the quasi-steady state concept. The generalized control structure and modeling approach are demonstrated using the two-stage dc-dc and dc-ac power conversion systems. A battery energy storage system with a thermoelectric source and a grid-connected power system with a photovoltaic source are examined. The large-signal averaged model and small-signal model are developed for the two demonstrated examples, respectively. Based on the modeling results, the control loops are designed by using frequency domain analysis. Various simulations and experimental tests are carried out to verify the compensator designs and to evaluate the generalized control structure performance. From the simulation and experimental results, it is clearly seen that the generalized control structure improves the performance of a battery energy storage system due to the unified control concept. The unified control concept eliminates transient over-voltage or over-current, extra energy losses, power quality issues, and complicated decision processes for multiple-mode control. It is also seen that the generalized control structure improves the performance of a single-phase grid-connected system through increased voltage control loop bandwidth of the active ripple current reduction scheme. As a result of the increased loop bandwidth, the transient overshoot or undershoot of the dc-link voltage are significantly reduced during dynamic load changes. / Ph. D.

Lithium-ion based battery energy storage system has become one of the most popular forms of energy storage system for its high charge and discharge efficiency and high energy density. This dissertation proposes a high-efficiency grid-tie lithium-ion battery based energy storage system, which consists of a LiFePO4 battery based energy storage and associated battery management system (BMS), a high-efficiency bidirectional ac-dc converter and the central control unit which controls the operation mode and grid interface of the energy storage system. The BMS estimates the state of charge (SOC) and state of health (SOH) of each battery cell in the pack and applies active charge equalization to balance the charge of all the cells in the pack. The bidirectional ac-dc converter works as the interface between the battery pack and the ac grid, which needs to meet the requirements of bidirectional power flow capability and to ensure high power factor and low THD as well as to regulate the dc side power regulation. A highly efficient dual-buck converter based bidirectional ac-dc converter is proposed. The implemented converter efficiency peaks at 97.8% at 50-kHz switching frequency for both rectifier and inverter modes. To better utilize the dc bus voltage and eliminate the two dc bus bulk capacitors in the conventional dual-buck converter, a novel bidirectional ac-dc converter is proposed by replacing the capacitor leg of the dual-buck converter based single-phase bidirectional ac-dc converter with a half-bridge switch leg. Based on the single-phase bidirectional ac-dc converter topology, three novel three-phase bidirectional ac-dc converter topologies are proposed. In order to control the bidirectional power flow and at the same time stabilize the system in mode transition, an admittance compensator along with a quasi-proportional-resonant (QPR) controller is adopted to allow smooth startup and elimination of the steady-state error over the entire load range. The proposed QPR controller is designed and implemented with a digital controller. The entire system has been simulated in both PSIM and Simulink and verified with hardware experiments. Small transient currents are observed with the power transferred from rectifier mode to inverter mode at peak current point and also from inverter mode to rectifier mode at peak current point. The designed BMS monitors and reports all battery cells parameters in the pack and estimates the SOC of each battery cell by using the Coulomb counting plus an accurate open-circuit voltage model. The SOC information is then used to control the isolated bidirectional dc-dc converter based active cell balancing circuits to mitigate the mismatch among the series connected cells. Using the proposed SOC balancing technique, the entire battery storage system has demonstrated more capacity than the system without SOC balancing. / Ph. D.

Operational performance of grid-connected microgrid with integrated solar photovoltaic (PV) electricity production and battery energy storage (BES) is investigated.  These distributed energy resources (DERs) have the potential to reduce conventionally produced electrical power and contribute to reduction of greenhouse gas emissions.  This investigation is based upon the DER’s techno-economic specifications and theoretical performance, consumer load data and electrical utility retail and distribution data.  Available literature provides the basis for DER specification and performance.  Actual consumer load profile data is available for residential and commercial consumer sector customers.  The electrical utility data is obtained from Mälarenergi, AB.  The aim is to investigate how to use simulations to specify a grid connected microgrid with DERs (PV production and a BES system) for two consumer sectors considering a range of objectives.  An open-source, MATLAB-based simulation tool called Opti-CE has successfully been utilized.  This package employs a genetic algorithm for multi-objective optimization.  To support attainment of one of the objectives, peak shaving of the consumer load, a battery operational strategy algorithm has been developed for the simulation.  With respect to balancing peak shaving and self-consumption one of the simulations supports specification of a commercial sector application with 117 kWp PV power rating paired with a lithium ion battery with 41.1 kWh capacity.  The simulation of this system predicts the possibility to shave the customer load profile peaks for the month of April by 20%.  The corresponding self-consumption ratio is 88%.  Differences in the relationship between the load profiles and the system performance have been qualitatively noted.  Furthermore, simulation results for lead-acid, lithium-ion and vanadium-redox flow battery systems are compared to reveal that lithium ion delivers the best balance between total annualized cost and peak shaving performance for both residential and commercial applications.

Battery energy storage systems are a key factor for enabling a continuous increase of the fraction of photovoltaics in the Swedish electricity grid. One big challenge is to utilise all potential services of such a storage system. The aim of this study was to improve the supervisory controller for an existing battery storage and photovoltaic solution marketed by the Swedish company Ferroamp AB. This has been done by developing a combined peak reduction and time-of-use bill management algorithm, together with a simulation and evaluation software for optimisation of algorithm parameters. The algorithms and tools were evaluated using an installation made by Ferroamp AB and Vattenfall Eldistribution AB as a case study. Sensitivity analyses has been performed on economic parameters and length of the algorithm training data set. Improvement of economic profit, in this case study, were 300 % compared to the currently used algorithm and 32 % compared to a conventional threshold peak reduction algorithm. Despite this improvement, the battery energy storage system is shown to be non-profitable, with the economic profit only covering 36 % of the investment costs, not taking interest rate into account. Like in many other studies, power storage was found more profitable than energy storage. An increase of the grid power tariff and the grid energy fee of 30 % to 40 % is found to make the system viable. One interesting finding is that by using the proposed optimal algorithm, 55 % of the cycle life of the battery storage is still accessible for other services when considering 10 years of economic depreciation time for the system.

The energy demand is steadily increasing, and the electricity sector is undergoing a severe change in this decade. The primary drivers, such as the need to decarbonize the power industry and megatrends for more distributed and renewable systems, are resulting in revolutionary changes in our lifestyle and industry. The power grid cannot be easily or quickly be upgraded, as investment decisions, construction approvals, and payback time are the main factors to consider. Therefore, new technology, energy storage, tariff reform, and new business models are rapidly changing and challenging the conventional industry. In recent times, industrial peak shaving application has sparked an increased interest in battery energy storage system (BESS).  This work investigated BESS’s potential from peak shaving and Time-of-use (TOU) applications for a Culture-house in Skellefteå. Available literature provides the knowledge of various BESS applications, tariff systems, and how battery degradation functions. The predicted electrical load demand of the culture-house for 2019 is obtained from a consultant company Incoord. The linear optimization was implemented in MATLAB using optimproblem function to perform peak shaving and time-of-use application for the Culture-hose BESS. A cost-optimal charging/discharging strategy was derived through an optimization algorithm by analyzing the culture-house electrical demand and Skellefteå Kraft billing system. The decisional variable decides when to charge/discharge the battery for minimum battery degradation and electricity purchase charges from the grid.   Techno-economic viability is analyzed from BESS investment cost, peak-power tariff, battery lifespan, and batter aging perspective. Results indicate that the current BESS price and peak-power tariff of Skellefteå Kraft are not suitable for peak shaving. Electricity bill saving is too low to consider TOU application due to high battery degradation. However, combining peak shaving & TOU does generate more profit annually due to additional savings from the electricity bill. However, including TOU also leads to higher battery degradation, making it not currently a viable application. A future scenario suggests a decrease in investment cost, resulting in a shorter payback period.  The case study also analyses the potential in the second-life battery, where they are purchased at 80 % State of Health (SoH) for peak shaving application. Second-life batteries are assumed to last until 70 % or 60 % before End of Life (EOL). The benefit-cost ratio indicates that second-life batteries are an attractive investment if batteries can perform until 60% end of life, it would be an excellent investment from an economic and sustainability perspective. Future work suggests integrating more BESS applications into the model to make BESS an economically viable project.

Diplomová práce se zabývá dopadem nabíjení elektrických vozidel a bateriových úložišť na schopnost distribučních sítí nízkého napětí absorbovat fotovoltaické systémy. Převážně venkovské, příměstské a převážně městské regiony s různými stupni penetrace nekontrolovaně i kontrolovaně nabíjených elektromobilů jsou analyzovány Monte Carlo simulacemi. Hostingová kapacita je také analyzována, jestliže jsou elektrická vozidla jak nahrazena, tak doplněna domácími bateriovými úložišti. Práce je zakončena krátkou analýzou využitelnosti BESS.

The Swedish electrical grid has historically been robust and reliable, but with increased electrification in numerous sectors, out-phasing of nuclear power and a high market diffusion of wind power, the system is now facing challenges. The rotational energy in the system is expected to decrease as a result of higher shares of intermittent energy sources, which can affect the stability of the grid frequency negatively. To manage increased frequency drops, the new Fast Frequency Reserve (FFR) market will be implemented by June 2020 in the Nordic power system. Simultaneously, it is expected that the demand of electricity will increase significantly in the transport and industry sectors in the coming years. Several DSOs already today indicate challenges with capacity and power security and have or will implement power tariffs as an economic incentive to prevent these problems. For energy intensive customers, such as industries, it will become important to reduce power peaks to avoid high grid fees. Several peak load shaving strategies can be utilized by industries to reduce their power peaks and thus the power tariff. The aim of this study is to economically analyze peak load shaving for Swedish industries. This is done using Li-Ion BESS and DSM, and to maximize the utilization of the BESS by including energy arbitrage and FFR market participation into the analysis. Firstly, a literature review is conducted within the topics of peak load shaving strategies, energy arbitrage and ancillary services. Secondly, data is gathered in collaboration with WSP Systems – Energy, the initiators of the project, to conduct case studies on two different industries. These cases are simulated in the modeling software SAM, for technical analysis, and then economically evaluated with NPV. Also, nine scenarios are created for the emerging FFR market concerning the number of activations per year and the compensation price per activation. The results from the case studies indicate that peak load shaving of 1 – 3 % with BESS provides a positive NPV for both case industries. However, higher percentages result in negative NPVs when no additional revenue streams are included. When considering energy arbitrage, it is concluded that the additional revenues are neglectable for both industries. Participating in the FFR market provides similar trends in the results as before. The exception is valid for scenarios with high numbers of FFR activations and compensation prices, where positive NPVs for all levels of peak load shaving can be concluded. The peak load shaving strategy DSM is implemented for one of the industries, where efficiency measures are concluded to have the most impact on the economic evaluation. If all efficiency measures would be implemented, the electricity consumption would be reduced by 17 %. Additionally, the power peaks would be reduced with 18 % and result in a significantly more positive NPV than peak load shaving using BESS. A sensitivity analysis concerning BESS capital cost and power tariff price concludes that the BESS price has a strong relation to the NPV, where a BESS price reduction of 60 % results in an NPV increase of at least 100 %. BESS prices have decreased the past years and are expected to keep decreasing in the future. Hence, investments in BESS can become more profitable and attractive in the coming years. Finally, for future research, it is recommended to combine the methodology from this study together with a load forecasting method. This combined methodology could then be practically applied to case specific industries with high peak loads. / Det svenska elnätet har historiskt sett varit robust och pålitligt, men i takt med ökad elektrifiering i flera sektorer, utfasning av kärnkraft samt ökad mängd installerad vindkraft ställs nu systemet inför nya utmaningar. Bland annat förväntas rotationsenergin i systemet minska som ett resultat av högre andelar intermittenta energikällor i systemet. För att hantera detta kommer den nya Fast Frequency Reserve (FFR) marknaden finnas tillgänglig från och med juni 2020. Samtidigt förväntas även efterfrågan på el inom transport- och industrisektorn öka markant de kommande åren. Redan idag är effektbrist ett problem i vissa regioner, vilket kan komma att förvärras. Många nätägare ska eller har redan infört effekttariffer för utnyttjande av deras elnät, vilket är ett ekonomiskt incitament för att hantera effektproblematiken där kunder med en mer flexibel elkonsumtion kommer gynnas. För större elförbrukare, som exempelvis industrier, kan det bli ekonomiskt betydelsefullt att sänka sina effekttoppar och därmed undvika höga nätavgifter. För att minska effekttoppar finns ett flertal så kallade peak load shaving-strategier, som kan utnyttjas av industrier för att minska kostnaderna för effekttariffen. Syftet med denna studie är att analysera peak load shaving för svenska industrier, med hjälp av ett Li-Ion batterilagringssystem och efterfrågeflexibilitet, samt maximera utnyttjandet av batteriet genom att inkludera energiarbitrage och deltagande i FFR-marknaden i analysen. Ett första steg i arbetet är att utföra en litteraturstudie för de berörda områdena. I ett andra steg insamlas data tillsammans med WSP, initiativtagaren av projektet, för att kunna göra en fallstudie på två industrier. För dessa fallstudier undersöks de tekniska förutsättningarna för att implementera peak load shaving-strategier genom modellering i simuleringsprogrammet SAM. Sedan utreds de ekonomiska förutsättningarna för fallstudierna, där NPV används som ekonomiskt nyckeltal. Dessutom skapas nio scenarion för den kommande FFR-marknaden för att uppskatta kostnader och inkomster. Resultatet av fallstudien visar att 1 – 3 % kapade effekttoppar med batterilagring ger ett positivt NPV för båda industrierna. Över 3 % blir resultatet negativt utan ytterligare inkomstströmmar inkluderade. Energiarbitrage konstateras att bidra med marginella positiva fördelar. Vid inkludering av FFR-marknaden i analysen erhålls liknande trender i resultaten, bortsett från scenarion med relativt högt antal avrop och pris. I dessa fall blir även 4 – 10 % kapade effekttoppar ekonomiskt attraktiva. För en av industrierna utvärderas efterfrågeflexibilitet, där effektivisering av elkrävande processer har störst inflytande på resultatet. Vid implementering av samtliga effektiviseringsåtgärder skulle elkonsumtionen minska med 17 %. Dessutom minskar effekttopparna med 18 %, vilket resulterar i ett signifikant mer positivt NPV, jämfört med användningen av batterilager. En känslighetsanalys gällande batteripris och effekttariffer, konstaterade att batteripriset har en stark påverkan på NPV. Vid en batteriprisminskning på 60 % ökar NPV med minst 100 %. Därmed kan batteriinvesteringar bli mer gynnsamma och attraktiva om batteripriser fortsätter att falla, vilket flera prognoser indikerar. Slutligen rekommenderas framtida studier att kombinera metodiken från detta arbete med en prognostiseringsmetod för elanvändning i industrier. Denna kombinerade metod kan sedan praktiskt tillämpas på fallspecifika industrier med höga effekttoppar.

The global energy system is under transformation. The energy transition from a centralized, fossil fuel based energy system to a more decentralized, renewable energy based system will challenge the balancing of electricity supply and demand. This stresses the importance of grid flexibility. In this challenge, energy storage will play a valuable role as it can provide flexibility and support the renewable energy integration. More specifically, lithium-ion battery energy storage systems (Li-ion BESS) demonstrate technological advantages and valuable application possibilities in the electricity grid. This thesis examines opportunities and barriers for deployment of grid-scale Li-ion BESS in the Swedish electricity market, and provides an overview of different perspectives of possibilities with BESS from several market actors. The purpose of the exploratory study is to gain an understanding of prospects for grid-scale BESS in Sweden. Through a comprehensive literature study and an empirical study, based on fourteen interviews with various actors in the electricity market, data was collected and analyzed. The analysis of the empirical findings resulted in two tables summarizing opportunities and barriers for implementation of BESS in Sweden. The opportunities and barriers are categorized into three hierarchical levels; contextual level, power system level and BESS level, referring to where in the system the benefits or hinders are localized. Also, key discussion points related to BESS (such as storage time perspective, ownership, grid services, cost, price signals and knowledge gap) are explored and evaluated. Furthermore, to understand the possibilities for grid-scale BESS in Sweden, a potential business setup for BESS is assessed and analyzed to identify preconditions for BESS to be attractive on the Swedish electricity market. The findings of the thesis indicate that grid flexibility is most likely going to be a considerable issue within 10-20 years. By the time of the potential nuclear phase out in Sweden, there will be major instabilities in the electricity grid if solutions are not in place. Therefore, keys to grid flexibility need to be evaluated and planned for well in advance, and the findings indicate that BESS could be a possible part of the solution. Until now, the regulatory framework has been perceived as rather unclear when it comes to energy storage, which has led to uncertainties among the market actors. These unclarities are about to be clarified with new laws and regulations, which will enable potential businesses for BESS. With the changes in the regulatory framework in place, we see an opportunity with new actors on the market. Our analysis shows that the BESS owner will most likely be a commercial actor, to enable utilization of a BESS for combined applications. An important factor, affecting the possibilities of implementing BESS on the Swedish electricity market, is the cost of BESS. We consider the cost aspect as vital for the likelihood of deploying BESS in Sweden. Based on our main findings, we conclude several preconditions for the deployment of BESS in Sweden. These are; decreased costs of BESS, acceptance from market actors, increased knowledge, a trading platform for grid services provided by a BESS, coordination between markets and electricity load forecasts. We believe that if these preconditions are fulfilled, Li-ion BESS has a chance to affect and have an impact on the Swedish electricity market. / Dagens energisystem är under förändring. Det sker en omvandling där energisystemet går från att vara centraliserat och fossilbaserat till att bli mer decentraliserat och baserat på förnybar energi. Detta kommer att utmana balanseringen av elproduktion och elkonsumtion, vilket betonar vikten av flexibilitet i elnätet. I den stundande utmaningen kommer energilagring att spela en viktig roll eftersom det kan bidra med flexibilitet och samtidigt stödja integrationen av mer förnybar elproduktion. Mer specifikt har energilagersystem med litiumjon-batterier flertalet tekniska fördelar och värdefulla användningsområden i elnätet. Det här examensarbetet utforskar möjligheter och hinder för en framtida implementering av nätanslutna litiumjonbatterilager på den svenska elmarknaden och ger en överblick över perspektiv på utsikter för batterilager från flertalet marknadsaktörer. Syftet med den utforskande studien är att få en ökad förståelse för framtidsutsikterna för storskaliga batterilager i Sverige. Genom en omfattande litteraturstudie och en empirisk studie, baserad på fjorton intervjuer med olika aktörer på elmarknaden, samlades data in och analyserades. Analysen av de empiriska resultaten resulterade i två tabeller som sammanfattar möjligheter och hinder för implementering av batterilager i Sverige. Möjligheterna och hindren kategoriseras i tre hierarkiska nivåer; kontextuell nivå, kraftsystemnivå och batterilagersystemnivå, med hänvisning till var i systemet fördelarna eller barriärerna är lokaliserade. Dessutom utvärderas flera betydande diskussionsteman relaterade till batterilager (såsom lagringstid, ägande, nättjänster, kostnad, prissignaler och kunskapsluckor). För att förstå möjligheterna för att etablera batterilager i Sverige har en möjlig affärsuppställning utvärderats och analyserats. Detta för att identifiera förutsättningar för att batterilager ska vara attraktivt på den svenska elmarknaden. Examensarbetets resultat tyder på att nätflexibilitet sannolikt kommer att bli ett betydande problem inom 10-20 år. Den troliga avvecklingen av den svenska kärnkraften kommer att resultera i instabilitet i elnätet om inte lösningar finns på plats. Därför behöver lösningar för att uppnå flexibilitet i elnätet utvärderas och planeras i god tid och uppsatsens resultat visar på att batterilager kan vara en möjlig del av lösningen. Fram till nu har det funnits oklarheter i regelverket gällande energilagring, vilket har lett till osäkerheter hos marknadsaktörerna. Nya lagar och förordningar kommer att klargöra flertalet osäkerheter och möjliggöra potentiella affärer med batterilager. När det förändrade regelverket är på plats, ser vi potential för nya aktörer på marknaden. Vår analys visar på att batterilager med största sannolikhet kommer att ägas av kommersiella aktörer för att möjliggöra kombinerade användningsområden av batterilager. Möjligheterna till implementering av batterilager på den svenska elmarknaden påverkas i hög grad av kostnaden för batterilager. Vi anser att kostnadsaspekten är avgörande för sannolikheten att utnyttja batterilager i Sverige. Vår slutsats är att det finns flertalet förutsättningar för att batterilager ska bli attraktivt och lönsamt i Sverige. Dessa är; minskade kostnader för batterilager, acceptans från marknadsaktörer, ökad kunskap, en handelsplattform för nättjänster som tillhandahålls av batterilager, samordning mellan marknader samt lastprognoser. Om dessa förutsättningar uppnås anser vi att litiumjon-batterilager har en chans att påverka den svenska elmarknaden.

The battery energy storage system (BESS) is crucial for the energy transition and decarbonisation of the energy sector. However, reliability assessment and capital cost challenges can hinder their widespread deployment. Reliability and cost-benefit analysis help address these challenges and assess BESS adoption's feasibility and viability, which is the aim of this project. A BESS contains various components such as battery packs, inverters, a DC/DC converter, a Battery Thermal Management System (BTMS), electrical protection devices, a transformer, and an Energy Management System (EMS). All these fundamental components must be considered to obtain a complete reliability prediction. Most previous studies focused on the reliability analysis of individual components, but few consider all the abovementioned components in collective reliability analysis. In this thesis, each component is mathematically modelled to estimate failure rates and then used to predict the reliability of the overall BESS system. The model accuracy is verified by comparing the computed reliability indices with the values from standards/references, showing that the proposed reliability prediction methods provide reasonable outcomes. Different scenarios to enhance BESS reliability through component redundancy are explored in this project. It is proved that applying component redundancy can boost the overall BESS reliability at the price of an increased capital cost. However, the enhancement in reliability and lifespan due to component redundancy can also curtail maintenance costs. A cost-benefit analysis assesses each scenario's profitability, considering manufacturers' and owners' perspectives. It helps determine the optimal balance between reliability and profitability. Redundancy applied to components with higher failure rates and lower costs improves the reliability and profitability of the BESS. The finding highlights the importance of strategic component selection for enhancing BESS reliability. Careful reliability and cost analysis should be performed simultaneously to find the most optimised BESS scenario. / Batterienergilagringssystemet (BESS) är avgörande för energiomställningen och avkarboniseringen av energisektorn. Tillförlitlighetsbedömning och utmaningar med kapitalkostnader kan dock hindra deras utbredda användning. Tillförlitlighet och kostnads-nyttoanalys hjälper till att hantera dessa utmaningar och utvärdera BESS-antagandets genomförbarhet och genomförbarhet, vilket är syftet med detta projekt. Ett BESS innehåller olika komponenter som batteripaket, växelriktare, en DC/DC-omvandlare, ett Battery Thermal Management System (BTMS), elektriska skyddsanordningar, en transformator och ett energiledningssystem (EMS). Alla dessa grundläggande komponenter måste beaktas för att få en fullständig tillförlitlighetsförutsägelse. De flesta tidigare studier fokuserade på tillförlitlighetsanalys av enskilda komponenter, men få beaktar alla ovan nämnda komponenter i kollektiv tillförlitlighetsanalys. I denna avhandling modelleras varje komponent matematiskt för att uppskatta felfrekvensen och används sedan för att förutsäga tillförlitligheten hos det övergripande BESS-systemet. Modellens noggrannhet verifieras genom att jämföra de beräknade tillförlitlighetsindexen med värdena från standarder/referenser, vilket visar att de föreslagna metoderna för tillförlitlighetsprediktion ger rimliga resultat. Olika scenarier för att förbättra BESS-tillförlitligheten genom komponentredundans utforskas i detta projekt. Det är bevisat att tillämpning av komponentredundans kan öka den övergripande BESS-tillförlitligheten till priset av en ökad kapitalkostnad. Förbättringen av tillförlitlighet och livslängd på grund av komponentredundans kan dock också minska underhållskostnaderna. En kostnads-nyttoanalys bedömer varje scenarios lönsamhet, med hänsyn till tillverkarnas och ägarnas perspektiv. Det hjälper till att bestämma den optimala balansen mellan tillförlitlighet och lönsamhet. Redundans som tillämpas på komponenter med högre felfrekvens och lägre kostnader förbättrar tillförlitligheten och lönsamheten för BESS. Resultatet belyser vikten av strategiskt komponentval för att förbättra BESS-tillförlitligheten. Noggrann tillförlitlighets- och kostnadsanalys bör utföras samtidigt för att hitta det mest optimerade BESS-scenariot.