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Hybrid lithium-ion capacitors (HyLICs) have received considerable attention because of their ability to combine the advantages of high-energy lithium-ion batteries and high-power supercapacitors. State of charge (SOC) is the main factor affecting the practical application of HyLICs; therefore, it is essential to estimate the SOC accurately. In this paper, a partition SOC-estimation method that combines electrochemical and external characteristics is proposed. The discharge process of the HyLICs was divided into three phases based on test results of electrochemical characteristics. To improve the estimation accuracy and reduce the amount of calculation, the Extended Kalman Filter (EKF) method was applied for SOC estimation at the interval where the capacitor energy storage characteristics dominated, and the Ampere-hour (Ah) method was used to estimate the SOC at the interval where battery energy storage characteristics dominated. The proposed method is verified under different operating conditions. The experimental results show good agreement with the estimation results, which indicates that the proposed method can estimate the SOC of the HyLICs accurately.

With more renewable energy sources (RES) which are inherent intermittent and unpredictable connecting with power grid, various stability problems occur, among which the peak load regulation is the most prominent. Energy storage systems (ESSs) are essential for buffering the electricity grid. Selecting the most suitable energy storage technology among various alternatives is of great importance. In this work, the sustainability of typical energy storage technologies was studied with respect to four aspects for peak shaving scenarios, including technical (i.e. maturity, energy density, round-trip efficiency, duration ranges, life cycles, lifetime and position flexibility), economic (levelized cost of energy, net present value), environmental (i.e. global warming, damage to human health, damage to ecosystems, damage to resource availability) and social (public acceptance) based on the full life cycle. This study evaluated the soft criteria including maturity, position flexibility and public acceptance by Analytic hierarchy process (AHP). Life cycle assessment (LCA) and life cycle cost (LCC) methods were combined to study the life-cycle environmental and economic performance. Technique for order preference by similarity to an ideal solution (TOPSIS) was applied for determining the sustainability prioritization of energy storage technologies. The sensitivity analysis was carried out to investigate the effects of control and economic input parameters on environmental performance and economic performance. In addition, the effects of criteria weights, electricity sources and number of daily cycles were conducted on sustainability ranking of ESSs. The results showed Lithium iron phosphate battery (LIPB) and pumped hydro storage (PHS) had good sustainability performance, which could be the most suitable energy storage technologies for peak shaving scenarios.