## 2 Battery and Hydrogen Energy Storage System
### BESS, HESS, and Other Energy Storage Systems
HESSs, particularly hydrogen-based systems, boast high energy density and the ability to store large amounts of energy for extended periods, making them potential replacements for fossil fuel-based energy generation. However, HESSs face challenges such as high costs, technical complexity, and safety concerns, especially regarding hydrogen's flammability. Ongoing research aims to address these issues and improve system efficiency. HESS applications in photovoltaic power generation demonstrate benefits like improved power quality, flattened intermittence, and enhanced frequency and voltage regulation in microgrid operations [104; 105]. Battery energy storage systems (BESSs) have evolved significantly, with various technologies available for different applications. Traditional lead-acid batteries, while reliable and cost-effective, generally have lower energy density, shorter lifespans, and lower efficiency compared to modern alternatives. Lithium-ion batteries (LIBs) have emerged as the current leading technology due to their decreasing costs and high performance, despite safety concerns. Other technologies like vanadium redox flow batteries (VRFBs) offer advantages such as longer lifespans, better high-temperature performance, and lower environmental impact. The selection of battery technology depends on specific application requirements, including peak shaving, load leveling, power reserve, renewable energy integration, and voltage and frequency regulation [106; 107]. Ongoing research aims to improve battery efficiency, lower costs, and optimize energy storage for various built environment applications [108]. Mechanical energy storage systems, particularly pumped hydro storage (PHS) and flywheels, play crucial roles in grid stability and renewable energy integration. PHS offers large-scale storage capacity and long operational lifespans, while flywheels excel in high-power, short-duration applications [109]. Flywheels provide advantages such as high cycle life, long operational life, high round-trip efficiency, and low environmental impact [110]. They are suitable for load leveling, frequency regulation, and peak shaving in power systems [111]. PHS and compressed air energy storage (CAES) are preferred for long-duration storage, with PHS offering higher efficiency and CAES providing faster start-up. The choice of mechanical storage system depends on specific application requirements, with a series connection recommended when combining these systems with solar or wind energy to enhance stability and control [112]. In comparing BESSs, HESSs, and other energy storage systems, each offers unique advantages and challenges depending on the application. BESSs stand out for their high efficiency, rapid response times, and scalability, making them ideal for applications such as grid stabilization and renewable energy integration. However, challenges like capacity degradation, high costs, and safety concerns persist. On the other hand, HESSs, particularly hydrogen-based systems, excel in long-term energy storage and offer versatility across various sectors, though they face higher costs and technical complexity. Traditional battery technologies like lead-acid and mechanical systems such as pumped hydro and flywheels provide reliable, cost-effective solutions but are limited by lower energy density and specific geographical or application constraints. Ultimately, the choice of energy storage technology hinges on factors such as efficiency, cost, lifespan, and application requirements, with ongoing research aiming to optimize these technologies for a more resilient and sustainable energy future.