| A new concept of thermal design to optimize the operating temperature of HTS magnets is developed, aiming simultaneously for compactness and efficiency. The optimization procedure seeks the operating temperature to minimize the power consumption in steady state. This procedure includes the modeling of the critical properties of HTS conductors, the dimensions of HTS windings, the heat transfer analysis for cooling load estimate, the thermal interface between the HTS windings and cryocooler, and the thermodynamic evaluation of the required refrigeration. Finally, this method is applied to two specific cooling systems for HTS transformers: a liquid-cooled system with pancake windings and a conduction-cooled system with solenoid windings. The optimum temperature turns out to be slightly above 77 K, the normal boiling temperature of nitrogen, for both the liquid-cooled system and the conduction-cooled system, but could vary considerably by the magnitude of AC loss in the HTS conductors. Operation at a temperature below 77 K can be justified, if the amount of AC loss is substantially reduced or the savings in capital investment by the compactness is significant in comparison with the operational cost.; A new cryogenic design for cooling HTS transformers, the so called natural convection system, is proposed in accordance with the results of an optimization study and the considerations of liquid nitrogen as cooling media. In the natural convection system, HTS windings are immersed in a liquid nitrogen bath where the liquid is cooled simply by copper sheets vertically extended from the coldhead of a GM cryocooler above the windings. Liquid nitrogen in the gap between the windings and the copper sheets develops a circulating flow by the buoyancy force in the subcooled state. Such a system based on cooling by natural convection with subcooled liquid nitrogen could be an excellent option for HTS transformers, when considering all aspects of compactness, efficiency, and reliability. (Abstract shortened by UMI.)... |
