# Hybrid Renewable Energy Systems--A Review of Optimization Approaches and Future Challenges
## 3 HRES Design
### Grid Connection  
Not only energy storage solutions but also grid connection type influence the optimization model, so Figure 4 shows the number of articles published annually from 2017 to 2024, focusing on grid-connected (\"on\") and off-grid (\"off\") energy systems.  
The growing interest in grid-connected systems has been captured, particularly in recent years. It indicates a shifting research focus towards the integration of energy systems with existing grids, gradually moving away from islanded grids and reflecting the global expansion of electricity networks. Grid integration offers several advantages, such  
Figure 4: Distribution of grid connection type in cited references, each number represents the amount of the identified grid connection types in HRES in a given year (developed by the authors).  
Figure 3: Distribution of energy storage system in cited references, each number represents the amount of the identified energy storage components in HRES in a given year (developed by the authors).  
as improved access to renewable energy resources, reduced reliance on localized energy generation, and enhanced system efficiency through centralized management and load balancing. Successful examples include the European Union's push for cross-border electricity markets [23] and the integration of large-scale renewable plants in China and the United States, which leverage grid connectivity to meet national energy targets [24; 25]. These advancements demonstrate how grid-connected systems support energy demand and foster technological innovation, economic growth, and environmental sustainability, making them a cornerstone of modern energy infrastructure development [26].  
The practical applications of HRESs extend far beyond residential power supply. Some telecommunication towers, which require continuous and reliable energy to maintain connectivity, all around the World have been powered using core renewables, such as sun and wind. Hydrogen and methanol fuel cells are incorporated in a grid-connected and islanded HRES [27]. A notable real-world example is the microgrid on Kodiak Island. In remote areas, HRESs are crucial for rural electrification, ensuring access to clean energy where grid extension is economically unfeasible [21]. In industrial contexts, HRESs stabilize the energy supply for manufacturing plants and support green hydrogen production, as seen in large-scale projects in Queensland, Australia [28]. This example also provides pelagic discrete energy trading system between Australia and Japan [29].  
Additionally, HRESs are increasingly integrated into urban microgrids to improve grid resilience and support demand-side management, leveraging real-time optimization algorithms [20; 30]. These diverse applications highlight the flexibility of HRESs in addressing global energy challenges while promoting sustainability and energy equity.  
Many grid-connected systems also support island mode operation, ensuring flexibility and reliability during grid outages or disruptions (see Table 1). This dual-mode capability highlights the importance of resilience in modern energy systems while emphasizing the benefits of integration with broader electricity networks.  
The synergy between renewable diversity, storage solutions, and grid integration helps optimize system cost and energy utilization in a diverse geographical area and paves the way for developing resilient and adaptable energy infrastructures from remote areas to urban environments. These trends emphasize the dynamic nature of energy research as it adapts to emerging challenges and opportunities in the energy transition to a sustainable future.