## 9 Conclusions  
This work presents an innovative approach for the techno-economic assessment of coaxial HVAC transmission cables made with HTS tapes and cooled with liquid nitrogen using a double-sided cooling option with two concentric coolant channels inside the cable. The proposed method is based on the OSCaR optimization tool, adapted here for high-voltage transmission lines rated for extremely high power values. Despite the necessary insulation thicknesses required to handle the voltage levels, the coaxial configuration is believed to offer advantages over other cable designs, maintaining a limited radial size compared with the powers involved. To the authors' knowledge, this paper is the first to present a techno-economic analysis model specifically applied to this type of superconducting cable. This work aimed to identify the cable structure which minimized the cable system's costs, allowing users to select the operating conditions of the line and conduct comprehen  
Figure 17: Total costs per kilometer of cable, varying the line length. The cases considered had \(P_{max}=2\) GW, \(V_{rms}=220\) kV, \(cos\varphi=0.9\). Figure 18: Number of intermediate cooling stations (blue bars, referenced on the left axis) and distance between adjacent cooling stations (red bars, referenced on the right axis), varying the line length. The cases considered had \(P_{max}=2\) GW, \(V_{rms}=220\) kV, and \(cos\varphi=0.9\). sive and time-efficient parametric analyses. Mathematically, this was achieved through a constrained multi-variable minimization process for the cable system's cost solved via a genetic algorithm in MATLAB software v2022b. Ample space was given in the article in detailing the methodological approach so as to allow readers to understand the relationships between the variables involved and potentially adapt the tool to their specific needs. The thermo-hydraulic and electro-magnetic models were included in the model via numerical and analytical correlations with proven effectiveness, ensuring the tool comprehensively addressed the key technical factors which must be considered when sizing a cable system. A grading approach for the critical current of the HTS tapes used in various phases was introduced in the tool, a technique which, to the authors' knowledge, has not yet been applied to superconducting cables. The cost per unit length of the coated conductors was optimized according to the thermo-electro-magnetic conditions to which they were subjected in the individual concentric layers. This enabled optimization of both the quantity and quality of the superconducting tape used in the cable, a factor which is known to significantly impact the total costs. The selected system variables included the number of parallel HTS layers per phase, the winding angle of the tapes in each layer, the radius of the central former, the thickness of the annular gap for the liquid nitrogen counterflow, the nitrogen mass flow rate valid in both channels, the number of intermediate cooling stations to maintain the temperature and pressure limits in the coolant, and finally the ratio \(\sqrt{2}\Big{|}\frac{I_{tapK_{i}}}{I_{tC}\Big{(}T_{HTS_{max}},\ |B|_{i,1},\ \theta_{i,1} \Big{)}}\), which in turn determined the characteristics of the tapes used in each phase (in terms of the critical current at 77 K and self-field \(I_{c77K_{i}}\)) and the corresponding cost per unit length. The constraints imposed on the optimized geometry concerned the temperature and pressure levels of the nitrogen within the two channels, ensuring it remained in a liquid state in each modular cable segment. Additionally, the geometry of each superconducting layer was adjusted to ensure a uniform current distribution between the layers of the same phase and among the phase currents. Then, the \(I_{c77K_{i}}\) value of the tapes in each layer was kept within reasonable limits based on the estimated current capabilities of tape manufacturers. Overall, the cost function considered here distinguishes between operating costs and capital costs.