###### Abstract
Based on the different optimization designs, the vertical TES with axial flow direction was found to perform the best in terms of aptitude value and lower risk value, as evaluated by Quality Function Deployment (QFD) and Failure Mode and Effect Analysis (FMEA), respectively. Since the conclusive decision and techno economic optimum was not made, further work is needed to understand the usability of such low-cost waste slag material for high-end CSP TES applications. [PERSON] et al. [2] successfully developed and employed an enthalpy-based multiple relaxation time (MRT) lattice Boltzmann method (LBM) to calculate the transient heat characteristics of phase change material (PCM) roofs of green buildings in both hot summer and cold winter areas of China. The authors analyzed the life cycle energy savings of the PCM roof under an intermittent energy utilization condition by comparing it with the performance of a roof filled with sensible insulating materials (SIM). Paraffin was chosen as the PCM, whereas perlite was considered for the SIM since both of these materials have similar thermophysical properties and hence it is fair to compare them for roofing performance and energy savings purposes. Overall, the PCM roof was found to exhibit a better thermal performance than the SIM for energy consumption when the appropriate melting point and layer thickness of the PCM were selected. According to the authors, since the current paraffin PCM is expensive, new low-cost, high-chemical stability PCMs are yet to be explored for viable green building applications. [PERSON] et al. [3] reported a seasonal thermal energy storage material based on bio-glass such as Xylitol. The major drawback of utilizing Xylitol as a viable TES is due to its difficulty in the nucleation triggering (energy discharge triggering) process and hence its subsequent low crystallization rates (discharge power delivery). The authors devised a method to address the abovementioned obstacles by developing air-lift reactors that have sequential processes--namely, air bubble generation, the transportation of nucleation sites, and subsequent crystallization. The nucleation and crystal growth of Xylitol was successfully monitored at various times such as the induction period, the recalescence period, and the end period via thermal analysis measurements. The authors concluded that the new air-lift reactor and its underlying techniques are very promising to discharge the TES system based on pure Xylitol at the required power when needed. Further optimization of the bubbling conditions and air injection ratios will enhance the capabilities of Xylitol as a compatible phase change material for TES applications. Electrochemical Energy Storage: An electrochemical energy storage device or a rechargeable battery is comprised of two electrodes (a cathode and an anode) and an electrolyte (liquid or solid). The secondary batteries mentioned above primarily convert the chemical energy contained in the active materials deposited on the electrodes into electrical energy by electrochemical reduction-oxidation (redox) reactions. There are many types of batteries based on the nature of the elements or compounds of the electrodes, such as nickel-cadmium (Ni-Cd), nickel-metal hydride (Ni-MH), lithium-ion (Li-ion), lithium-polymer (Li-pol), silver-zinc (Ag-Zn), nickel-hydrogen (Ni-H\({}_{2}\)), sodium-sulfur (Na-S), zinc-air (Zn-O\({}_{2}\)), etc. The energy density and power density are the two governing factors for the selection of the appropriate battery type for the desired applications. Additionally, the life cycle testing and charge/discharge rates, as well as the associated memory effects, must be considered for day-to-day applications in automotive operations, mobile phones, laptops, etc. In this Special Issue, one review paper on Li-ion batteries and three research articles on various EES systems were included, and are briefly described below. [PERSON] et al. [4] extensively reviewed the nanocomposite materials used in rechargeable Li-ion batteries.