Axial Flux PMSM Power Take-Off for a Rim-Driven Contra-Rotating Pump-Turbine

Pumped hydropower storage (PHS) is a cost-effective and mature energy storage technology. However, it has inherently been limited to locations with suitable topographies. Therefore, the ALPHEUS project aims to implement PHS for shallow seas and coastal environments with the goal to support grid stability. To ensure optimal efficiency and fast switching times in these low-head applications, a 10 MW Contra-Rotating (CR) axial Reversible Pump-Turbine (RPT) is designed, which has a rim diameter of 6.4 m and a high efficiency-maximum 84% and 90% for pump and turbine, over a large operating range. Furthermore, the RPT is rim-driven, which averts the hydraulic impact caused by shaft-driven systems. To ensure optimal efficiency at variable speed operation, the runners are driven by two separate Axial-Flux Permanent Magnet Synchronous Machines (AF-PMSMs). This paper proposes a Power Take-Off (PTO) design, where the electric machines and bearings are integrated in the rim of the CR RPT. The AF-PMSM dimensional design results in an outside diameter of 6.87 m. Next, active hydrostatic bearings are proposed due to their higher lifetime compared to roller bearings and better start-stop behaviour compared to hydrodynamic bearings. Finally, the advantages of the rim-driven PTO design with AF-PMSMs are elucidated by means of a comparison with the PTO of a shaft-driven CR RPT with similar power ratings. Next to the differences in hydraulic impact and bearing and sealing complexity, the active mass of both PTOs is examined. It is shown that the rotary mass and the total mass the rim-driven PTO is respectively three times and six times lower than that of the shaft-driven PTO, while the permanent magnet usage for both PTOs remains similar, with the total permanent magnet mass being 1.1% lower for the rim-driven PTO.