# Hybrid Renewable Systems for Small Energy Communities: What Is the Best Solution?
## 1 Introduction
Enhanced storage capacity can also be achieved through the construction of new reservoirs, re-habilitation of existing dams, and the use of aquifer recharge methods to increase groundwater reserves. These efforts will ensure a steady water supply for agricultural, industrial, and domestic needs while reducing the environmental impact. Hence, pumped-storage hydropower has expanded significantly [38]. Pumped-storage hydropower, which functions by moving water between reservoirs at different elevations, acts as a giant battery that stores energy by pumping water uphill when electricity demand is low and releasing it to generate electricity when demand is high. This technology plays a critical role in grid balancing, particularly as more renewable energy sources are integrated into power systems. It is also adaptable for micro-scale systems within urban settings, enabling cities to enhance their energy resilience and optimize local water management. Integrating such systems into urban infrastructure encourages water managers to embrace smart city innovations to optimize water management, energy efficiency, and the integration of renewable energy sources [39]. Digital twins, which are virtual representations of physical assets, allow for real-time monitoring, simulation, and optimization of water and energy systems within a city. By integrating renewable energy sources such as solar and wind with pumped-storage hydropower, urban areas can achieve higher energy efficiency and reduce their carbon footprint. This holistic approach aligns with smart city initiatives that aim to improve quality of life, enhance sustainability, and manage resources more effectively. The cost of electricity for large-scale hydropower remains competitive when compared with fossil fuels [40]. This cost advantage is due to the low operating and maintenance expenses associated with hydropower plants and their long operational lifespans, often exceeding 50 years. Large-scale hydropower projects can generate electricity at prices ranging from 0.03 to 0.05 \(\upmu\)kWh, making them among the most affordable renewable energy options. When combined with other renewable energy sources, such as solar and wind, hydropower helps lower the overall LCOE, further incentivizing the transition to a cleaner energy system. By combining renewable energy systems like solar, wind, and pumped-storage hydropower, the levelized electricity costs can be lower [41]. This integrated approach takes advantage of the complementary characteristics of different renewable energy sources. For example, solar energy is more abundant during the day, while wind energy can often provide power during nighttime or cloudy periods. Pumped-storage hydropower adds a layer of reliability by storing excess energy and releasing it when needed, thus ensuring a continuous and stable power supply. This synergy between renewables reduces the need for fossil fuel-based power generation, cuts emissions, and provides a pathway toward achieving net-zero energy systems. The global energy transition towards greener systems has become a central focus of climate policy worldwide [42]. Governments and international organizations are increasingly committing to reducing greenhouse gas emissions, phasing out coal and other fossil fuels, and investing in clean energy technologies. Policies such as carbon pricing, subsidies for renewable energy projects, and targets for zero-emission vehicles are being implemented to accelerate the shift towards a sustainable energy future. It is urgent to replace fossil fuels with clean energy alternatives to achieve global climate goals and meet various sustainable development targets [43; 44]. The shift from fossil fuels to renewable energy sources like wind, solar, and hydropower is essential for limiting global temperature rise to 1.5 \({}^{\circ}\)C above pre-industrial levels, as outlined in the Paris Agreement.