Lithium-ion batteries for off-grid PV-systems

Abstract: This thesis provides a comprehensive and detailed analysis on the effect ofthe battery operation strategy on the lifetime of commercial lithium-ionbatteries and on the economics of off-grid photovoltaic (PV)-batterysystems.Lithium-ion batteries play a key role in the transition to a fossil-freesociety. Compared to electric vehicles, stationary energy storage hasdifferent requirements for the performance and lifetime of batteries.Although optimal battery design is critical to achieve high energy densityand longer lifetime, operation plays an important role in preventingpremature performance degradation. Understanding the effects ofsuppressed demand, geographical location, and application on system lifecycle costs also enables optimal system design.Load profiles for three applications were estimated and implemented in asimulation model, along with meteorological data for three locations andthe suppressed demand (SD) effect. Using the hourly state of charge (SOC)profiles, four battery operation strategies were designed using partialcycling with different cut-off voltages and two state of charge windows(ΔSOC). Commercial cells were used for the experimental tests. After over1000 cycles a post-mortem characterization was performed.The experiments revealed the cause of premature degradation at high SOCoperation to be a combination of impedance rise in the positive electrodeand loss of lithium inventory at the negative electrode leading to decreaseof capacity. Studies on the impedance spectra of the cells using physicsbasedmodeling revealed a loss of conductivity between particles in thepositive electrode. At system level, as the SD increases, so does theoperational ΔSOC width, while the reliability of the system decreases. Wedefined the reliability as loss of power supply probability. Finally,optimization of cost and reliability, revealed that an optimal system designfavors a battery operation strategy with wider ΔSOC instead of batterylifetime.