LIGHTWEIGHT HIGH-EFFICIENCY ENERGY-STORAGE TRANSFORMERS

Lightweight high-efficiency energy-storage transformers require stiff external structures. The conductors used to minimize resistive losses have an inherently low modulus of elasticity and are therefore useless as a structure. This generalization includes new "hyperconductors" such as high-purity aluminum in an aluminum-iron-cerium matrix. Although the aluminum material has high yield strength, the useful strength is limited by the strain induced in the pure aluminum filaments during magnetic loading. To maintain low resistivity at 20 K (boiling hydrogen), filament strain must not exceed 0.2% All aluminum alloys at all temperatures have essentially the same elastic modulus, 70 GPa. The useful strength is thus limited to 140 MPa. Self-supporting transformers or inductors operating at this stress store only about 15 kJ/kg. Improving the specific energy requires adding a lightweight stiff structure. Boron or graphite might be used to make such a structure. We estimate that transformers or inductors can store energy at 50 to 100 kJ/kg using a boron structure and hyperconducting windings. In this paper we describe the scaling relationships for energy-storage transformers and how external structures improve specific energy. We compare transformer mass and efficiency to several transformer designs, each storing 5 MJ and 1 MA.