Many design choices affect the performance of generators. These include the size, number of magnets, the materials used for the coils and geometric features such as the diameter of the rotor/stator and air spacing. The performance of the generator must be assessed on a range of criteria including the electrical yield, cost of materials, weight per megawatt, ease of manufacture and dynamic performance. The design is further complicated by the interconnected nature of electromagnetism, rotational- and structural-dynamics. The optimisation of the novel and complex generator design using trial and error physical prototyping and testing is incredibly expensive and time-consuming. Instead, Xi developed a series of numerical simulations using COMSOL Multiphysics to rapidly trial different design choices allowing an assessment of output power and cost-benefit analysis. The models fully-couple electromagnetics, rotating machinery and electric circuit to allow a design modification to be assessed within hours – compared to week or months if a similar change was made with a physical prototype. The models can also be coupled to thermal dynamics, structural mechanics or acoustic finite element modules to allow the assessment of heat, vibration and sound output. These models provided a powerful tool for assessing the output power, CAPEX and OPEX costs of the Green Spur generator.