# The Synergy of Renewable Energy and Desalination:
## 4 Energy Storage Systems
In order to meet the typical electrical load of a residential building, a hybrid photovoltaic power-to-hydrogen, storage, and fuel cell system is suggested in [71]. The productivity of alkaline electrolysis cells is evaluated with a two-switch buck-boost converter included, recognizing alkaline as a necessary electrolysis technology. The equilibrium condition at the DC and AC buses is established after a thorough formulation with regard to each unique system component. In [72], the authors proposed a novel synchronized sizing model for a solar or wind powered reverse osmosis desalination plant and used a follower-leader optimization framework to optimize the design both with and without demand-side water management (DSWM). The system was made up of PV panels and wind turbines to generate electricity, a battery bank to store extra power, an RO unit to produce freshwater and desalinate saltwater, and freshwater storage tanks. As a long-term storage solution, a hydrogen storage system was added to store extra electricity at the end of each day. The RO system was mostly powered by PV and WT, with any extra energy being stored in a battery-hydrogen storage (BHS) system. Energy storage systems can significantly support the penetration of renewable energy source-driven desalination, but at the same time, some considerations regarding the challenges of ESS should be taken into account. During the charging process, the storage acts as load, while during discharge, it acts like a generator. For example, batteries in grid applications shift from load mode during charging when there is an excess of RES to discharging at peak demand, operating as generators when there is a lack of available RES. The dual role of the battery storage systems can be described by the following equations:  
\[P_{Charge}=VI \tag{1}\]  
\[P_{Discharge}=P_{Charge}\eta_{rt} \tag{2}\]  
where \(V\) and \(I\) are charging voltage and current, respectively, and \(\eta_{rt}\) is a roundtrip efficiency that indicates the ratio of energy provided during discharging to the energy received during charging including charging losses. Also, it is necessary to consider self-discharge and parasitic losses as limiting factors for ESS. In [73], the author showed that commercially available Li-ion batteries, in contrast to the traditional nickel-metal hydride or lead-acid  
Figure 8: Application of hydrogen energy storage system. batteries, are characterized by high round-trip efficiency, preferable energy/power characteristics, low maintenance, and sustainability. Li-ion batteries have very low self-discharge in the range of 2 to 5 percent. It is also necessary to consider parasitic losses that occur due to the conversion of stored energy into usable electrical energy. Inverters and other power electronics can introduce inefficiencies. Also, energy can be lost as heat due to internal resistance in batteries and other components, especially during charging and discharging cycles, or as energy consumed by peripheral systems such as cooling systems that support the ESS operation. Batteries are indispensable not only for grid-connected applications, but also in off-grid settings or areas with unstable grid infrastructure. In remote locations such as rural communities and mining operations, battery energy storage systems paired with renewables ensure uninterrupted power by compensating for solar/wind intermittency. Without grid backup, batteries manage load fluctuations, prevent system shutdowns during low generation, and provide surge capacity for high-demand equipment like high-pressure pumps.