# Research Needs for Realization of Zero-Carbon Power Grids with Selected Case Studies
## 1 Introduction
### Definition and Research Needs  
When focusing on carbon neutrality on a national scale, it is first necessary to define a zero-carbon power grid. Renewable energy sources can be categorized into two types [11]: weather-dependent sources, such as wind turbines and photovoltaic (PV) arrays, often termed variable renewable energy (VRE) sources, and weather-independent sources including nuclear, hydroelectric, biomass, and geothermal power plants. The first- and second-type sources use power electronics converters and synchronous machines, respectively, to convert non-electric energy to electric energy [12]. A zero-carbon power grid is not simply a power grid including only VRE sources (VRE share of total power generation is 100%). For example, in 2019, the United States acquired approximately 10% of its electricity from non-VRE sources; this is expected to grow by 2050 [13]. The current power grids of Quebec, Norway, Iceland, and Austria rely heavily on hydroelectric, biomass, geothermal, and solar power, and already operate at or very close to 100% renewability [14; 15]. However, the implementation of zero-carbon power grids using VRE sources, primarily wind power and solar PV, is receiving greater attention, given their global abundance and the lack of geographical constraints. In other words, VRE will account for a large proportion of future power. Rapid VRE integration will be the primary pathway toward zero-carbon grids worldwide [4]. An understanding of the challenges posed when moving toward a zero-carbon grid is essential when identifying and developing a research agenda that lays out what is and is not known, and the first and the best solutions. The understanding will enable the establishment of a research roadmap, assisting participants in prioritizing and focusing effort and investment to achieve the right solutions of many issues associated with zero-carbon power grids in a reliable and sustainable manner. For example, a power grid with instantaneous VRE penetration of 100% will take considerably shorter to achieve that of a grid with an annual penetration of less than 80% [15; 16]. This is principally because existing technologies can assist penetration to 80% but the final 20% requires new research. System operators then need to understand what challenges will become critical during power grid decarbonization and have a plan on how and when to resolve them. Technologies of Interest  
Every power grid differs technically, socially, and economically; different pathways toward decarbonization are required. Most grids with high proportions of VRE will undergo intensive renovation in terms of energy storage, sector coupling, demand response (DR), distributed control and estimation, and power electronics installation [2, 4, 14, 15, 17]; these are not required by grids with abundant renewable energy that is less variable. Although the technologies are very powerful, their synergistic contributions are yet to be revealed; it is essential that they be well integrated to ensure reliable and cost-effective grid decarbonization. Recently, battery costs have declined dramatically, facilitating VRE use in short-term (e.g., daily) grid operations. Battery storage is unlikely to be viable in longer term (e.g., seasonal) operations [18]. Hydrogen tanks can be a good alternative for seasonal storage [19], although power-to-gas technologies remain very costly, particularly when operating in the classical electricity-in/electricity-out modes; however, the costs are decreasing. Thermal energy storage also can meet longer-term storage needs [20]. In power grids, the demand side will be subject to major changes as the economy and industry continue to be electrified. Existing loads can be better managed using communication and command-and-control systems, or can be replaced by new loads evidencing better operational flexibility.