Energy Storage Appl., Volume 1, Issue 1 (December 2024) – 5 articles

Stability studies remain a crucial aspect of power systems dynamic analysis, and are typically explored in three main categories: numerical methods, linearization techniques, or direct methods, which utilize Lyapunov energy functions. This paper belongs to the third category, and highlights the usefulness of the Popov stability criterion in the analysis of nonlinear power system models. The main advantage of this criterion is that it provides conditions for global asymptotic stability of an equilibrium point, for a nonlinear dynamic system. We show a general method to apply this stability criterion, and examine its uses in several specific applications and case-studies. The results are demonstrated by analyzing the stability of a system that includes a grid-connected storage device and a renewable energy source. Full article

The precise determination of the specific heat capacity of lithium-ion cells is essential for thermal management design. Though, varying influences and insufficient parametric analyses are found in the literature. Therefore, a simple and inexpensive measurement setup is utilized to measure the specific heat capacity of cells independent of their format and dimensions. A comprehensive parametric analysis is performed assessing the effect of cell casing, cell chemistry, temperature, state-of-charge, and state-of-health. For the first time ever, a predictive analysis on material level is conducted allowing for understanding the individual factors in detail. Thus, an analytical approach for calculating the specific heat capacity can be validated by comparing predictive values to experimental data for the first time. It is found that the cell format has a significant influence on the specific heat capacity due to varying mass fractions and housing materials. Furthermore, the cell chemistry and corresponding layer thicknesses are of high importance, too. By selecting specific heat capacities for individual materials from the general literature, the analytical prediction matches the experimental data and is thus validated for the first time ever. Moreover, temperature has a positive linear effect on the specific heat capacity which can increase by up to 15% over the operating range. Furthermore, the positive temperature dependency improves the charging performance. Finally, neither SOC nor SOH significantly affect the specific heat capacity of lithium-ion cells. Full article

In recent years, electric vehicles (EVs) have gained significant traction within the automotive industry, driven by the societal push towards climate neutrality. These vehicles predominantly utilize lithium-ion batteries (LIBs) for storing electric traction energy, posing new challenges in crash safety. This paper presents the development of a mechanically validated LIB module simulation model specifically for crash applications, augmented with virtual short circuit detection. A pouch cell simulation model is created and validated using mechanical test data from two distinct out-of-plane load cases. Additionally, a method for virtual short circuit prediction is devised and successfully demonstrated. The model is then extended to the battery module level. Full-scale mechanical testing of the battery modules is performed, and the simulation data are compared with the empirical data, demonstrating the model’s validity in the out-of-plane direction. Key metrics such as force-displacement characteristics, force, deformation, and displacement during short circuit events are accurately replicated. It is the first mechanically valid model of a LIB pouch cell module incorporating short circuit prediction with hot spot location, that can be used in full vehicle crash simulations for EVs. The upscaling to full vehicle simulation is enabled by a macro-mechanical simulation approach which creates a computationally efficient model. Full article

Developing operation strategies for district cooling systems with chilled water storage is challenging due to uncertain fluctuations of cooling demand in actual operations. To address this issue, this paper developed an adaptive operation strategy and performed its validations by modeling and simulating a commercial cooling system in Shanghai using OpenModelica. Firstly, the originally designed operation strategy of the cooling system was evaluated by simulation but was found unable to meet the statistically averaged ideal cooling requirements due to the early exhaustion of stored chilled water at about 5:30 PM. Then, to build foundations for adaptive operation strategy development, a newly designed operation strategy was established by increasing the operation time of base load chillers in the valley and flat electricity price periods. The new strategy proved numerically sustainable in satisfying the ideal cooling demand. Moreover, to realize the strategy’s adaptability to actual cooling load fluctuations, an adaptive operation strategy was developed by tracking the target stored chilled water mass curve that was calculated by implementing the newly designed strategy. The simulation results verify that the adaptive operation strategy enables good adaptability to representative cooling load fluctuation cases by automatically and periodically adjusting the operation status of base load chillers. The adaptive operation strategy was then further widely numerically tested in hundreds of simulation cases with different cooling load variations. The time-lagging problem resulting in strategy failures was found in numerical tests and was addressed by slightly modifying the adaptive strategy. Results indicate that the adaptive operation strategy enables adaptability to deal with cooling demand fluctuations as well as allowing low cooling supply economic costs and power grid-friendly characteristics. This study provides theoretical support to strategy design and validations for district cooling system operations. Full article