# Optimization of DC Energy Storage in Tokamak Poloidal Coils
## 3 Need for Energy Storage in Tokamak PF Coils
### Characteristics and Advantages of SCs  
SCs have emerged as a promising energy storage technology, particularly suitable for applications requiring rapid charge and discharge cycles [34]. Key characteristics and advantages of SCs include high power density, long cycle life, wide temperature range, and low maintenance. SCs can indeed deliver and absorb power quickly, making them ideal for buffering short-term power demands, and, unlike traditional batteries, they can undergo millions of charge/discharge cycles with minimal degradation. Furthermore, they operate effectively across a broad temperature spectrum, enhancing their suitability for various environmental conditions. From a maintenance point of view, SCs require less maintenance than batteries, reducing operational costs. For these reasons, they were considered for mobility and renewable sources, but with lower power and energy levels and shorter time scales with respect to the DTT PF PSs [35; 36].  
The integration of SCs into the DC storage stage between the AC/DC converters and the DC/DC converters can significantly improve the performance and reliability of the PSs  
Figure 3: Power delivered to the PF coils in the DTT reference scenario.  
for PF coils. Moreover, SCs can be also utilized in other PSs within tokamaks and other fusion systems.  
Of course, SCs do have some drawbacks too. Notably, their equivalent capacitance decays with frequency, becoming almost negligible at frequencies around 100 Hz, while the internal losses increase [37; 38; 39]. This limitation necessitates the use of additional filters to interface SCs effectively with power electronics, ensuring that they can still meet the demands of the system without introducing unwanted harmonics or instability. These filters are crucial for maintaining the PSs' overall performance and reliability, as they mitigate the effects of the SC frequency-dependent characteristics [19].  
Modeling an SC with precision in simulations is a complex and non-trivial task due to the intricate behavior involving numerous parameters and complex phenomena [40]. Even though the authors are experimentally and theoretically characterizing such phenomena [40], the time scale of the analysis is compliant with the adoption of a simplified model for the purposes of this work in order to focus on the energy balance analysis, without considering transients associated with individual components. The entire SC bank was simulated using a single capacitance, representing the total bank value, in series with a resistance reflecting the equivalent series resistance (ESR) of the bank. This approach streamlined the simulation process while still capturing the essential dynamics of the energy storage system.  
The Ragone plot is a typical tool used to compare energy storage technologies based on their power and energy densities [41; 42]. A Ragone plot with the essential information for this paper is sketched in Figure 4. The ellipses in Figure 4 denote the most relevant families of storage technologies. This representation clarifies that SCs cover a zone complementary to other technologies, offering a trade-off between the high specific energy of batteries and the high specific power of traditional capacitors. Lithium-ion capacitors (LICs) are SCs with a sort of battery-like electrode to extend the energy capabilities and then the covered zone.  
The dotted lines in Figure 4 summarize the characteristic durations of the operations having the corresponding powers and energies. According to this tool, SCs operations should be limited to a few tens of seconds.  
Figure 4: Ragone plot comparing the main energy storage technologies and some typical durations of their operations. To support the analysis of this paper, some characteristic times of the DTT PF PS system are also reported, to be discussed in the following paragraphs.