# The Synergy of Renewable Energy and Desalination:
## 4 Energy Storage Systems
The hybrid energy storage systems consisting of different electrochemically composed batteries form hybrid energy storage systems (HESS). This type of ESS is one of the most promising energy storage systems. It combines the benefits of various chemical batteries, because some batteries are known for their high power density and rapid charging and discharging periods, but other batteries provide higher energy density and longer cycle life [66]. Therefore, for the high performance of desalination plants powered by renewable energy sources, a hybrid energy storage system that combines several kinds of electrochemical batteries may be a better choice than a regular one. Hybrid energy storage systems are especially useful for large desalination facilities because maintaining continuous operation requires the simultaneous long- and short-term storage of energy to overcome the intermittency of renewable energy sources. One of the main advantages of energy storage in desalination is that it ensures that desalination plants will continue to run even in the event that RES is not available, increasing their dependability. Optimizing energy utilization allows for more efficient management of energy resources, which lowers operating costs and boosts system performance overall. The application of HESS for desalination processes can help to lower carbon emissions because they increase the penetration of RES in the desalination sector, thus lessening reliance on fossil fuels. Hydrogen generated by electrolysis is another promising energy carrier and storage option for incorporating RES into the electrical grid that is presently being researched. Hydrogen energy storage offers distinct advantages over other systems, including exceptional energy density around 120 MJ/kg and suitability for long-duration or seasonal storage. Unlike batteries, hydrogen enables energy transport via pipelines or tankers and integrates with industrial processes requiring hydrogen feedstock. Although it already has a number of uses, it may also make it possible to create hydrogen-based energy storage systems that consist mostly of fuel cells, electrolyzers, and hydrogen storage systems [67; 68]. The technique of electrolyzing water to produce hydrogen using excess power from renewable sources is known as hydrogen storage. In [69], the authors discuss the possibility of hydrogen production with the help of renewable energy sources. Hydrogen storage systems are emerging as a pivotal technology for enhancing the efficiency and sustainability of RES-powered desalination facilities, helping to ensure a consistent water supply in both island and grid-connected modes. By integrating hydrogen energy storage, it becomes possible to produce hydrogen during periods of excess energy generation, which can then be stored and converted back into electricity or used directly for desalination processes when renewable output is low. In grid-connected scenarios, hydrogen energy storage offers similar benefits by facilitating load balancing and peak shaving. Desalination plants typically require substantial energy inputs, and hydrogen energy storage systems can help smooth out demand fluctuations by storing surplus energy during off-peak hours and releasing it during peak demand  
Figure 7: Comparison of NMC and LTO chemistries for HESS. times [70]. This flexibility can lead to lower operational costs and improved grid stability, making it easier to integrate higher shares of renewables into the energy mix (Figure 8). While the system in Figure 7 involves high upfront costs such as electrolyzers, compression, and storage tanks, its viability is context-dependent: regions with abundant renewable surpluses, existing gas infrastructure, or decarbonization mandates may prioritize it despite capital intensity. Advances in electrolyzer efficiency and economies of scale are expected to reduce costs, enhancing feasibility for grid resilience and multi-sectoral decarbonization.