# The Synergy of Renewable Energy and Desalination:
## 4 Energy Storage Systems  
However, despite the fact that RES-powered desalination plant systems are attractive options to address the global freshwater deficit, there are some challenges and obstacles that slow down the adoption of the technology. In fact, these systems are mostly either part of the electric power grid or should have a reliable long-term energy storage system to guarantee a steady and uninterrupted supply of power to a desalination plant. Since the significant amount of electricity needed by the entire desalination plant comes from the conventional power grid, under the grid connection scenario, these systems could overload the grid, particularly in places with inadequate infrastructure for the delivery of electricity, at nighttime, and on cloudy days [59]. Effective energy storage and management systems are required to provide a consistent and reliable power supply and satisfy the varying energy needs of desalination operations because solar and wind energy are discontinuous [60]. Finding affordable and workable energy storage solutions for wastewater treatment facilities remains a challenge in this respect, particularly for large-scale applications. In addition to reducing power output variations, energy storage systems may also enhance supply quality, adjust frequency, and provide voltage dependability. Power can be stored using a variety of techniques, such as pumped hydro storage (PHS), compressed air energy storage (CAES), and battery energy storage systems (BESS). When there is excess generating capacity, energy storage devices are used to store the energy generated by renewable energy sources and release it to supply peak load demands [61]. Another advantage of ESS is that it allows the power distribution system to accommodate more RES [62; 63]. The storage capacities of battery systems normally range from 1 to 30 kWh for home use to MWh for commercial battery systems [64]. The charging and discharging operations also cause battery degradation, which shortens the battery's lifespan. The level of battery discharge, known as the depth of discharge (DoD), is also important. Deeper discharge cycles with around 80 percent DoD provide greater usable energy but accelerate degradation, reducing cycle life. Conversely, shallow discharges around 20-30 percent DoD prolong the lifespan, but require oversized storage for equivalent energy availability. The battery degradation process results in varying battery lifetimes, performance decreases, and cost losses under different operating situations. Therefore, in order to assess battery performance, the health prognostics must be examined. There are already many review papers available that cover different facets of battery energy storage devices. Stacks of cells transform chemical energy into electrical energy makeup batteries. There is currently a significant amount of study and investment in this area. The primary battery chemistries utilized in battery energy storage systems are illustrated in Figure 6.  
In [65], researchers showed that, to create a more dependable and effective energy storage solution, a hybrid energy storage system that integrates multiple electrochemical energy storage technologies is a good choice. Simultaneously, sophisticated control techniques are needed to integrate several energy storage systems into an HESS in order to guarantee the system's longevity and peak performance. Among the main conclusions is that the utilization of machine learning methods like deep reinforcement learning (DRL) or genetic algorithms (GA), as well as other recent advancements in HESS control systems, is very promising for hybrid energy storage systems for both small-scale and large-scale desalination plants. Figure 7 shows how different Li-ion battery technologies, such as lithium titanium oxide (LTO) and nickel manganese cobalt (NMC), can complement each other in HESS. Figure 6: Classification of electrochemical batteries for ESS [65].