Offshore wind power is trending towards larger capacities and deep-water applications. To accommodate floating platforms in deep water and to avoid reliability issues related to gearboxes, it is essential to enhance the power density of direct-drive wind generators thereby reducing their weight and volume. This paper presents a conceptual design for a 20 MW low-temperature superconducting (LTS) wind generator. A thermosiphon helium piping cooling system, which rotates with the LTS excitation coils as the rotor, is proposed to keep the coils at 4.2 K by circulating helium through gravitational force while rotating without any pump. Using NbTi wires for the excitation coils, the designed sample generator features a radial magnetic flux density of 3 T in the air gap at a rated speed of 10 rpm. The rated voltage and current are 3300 V and 3500 A respectively. By a 2D finite element model, the basic electromagnetic structure is designed that has an air gap diameter of 6 m, an axial length of 1.1 m with 48 poles and 576 slots. The maximum magnetic field of 4.3 T locates inside the NbTi excitation coils. The cryogenic cooling system employs 8 cryostats, each containing six NbTi coils with a carbon fiber reinforced plastic suspension system and a GM cryocooler. The estimated heat leaks for a single cryostat are 0.382 W at 4.2 K and 39.8 W at 50 K.
