Solid-Solid Cryogenic PCM

The Navy seeks to develop a cryogenic temperature retaining material that can maintain its cryogenic temperature, around 30 Kelvin, for a time period of 2-4 hours without substantially increasing the final system weight. The application is to maintain cryogenic temperatures on superconducting DC power cables in the event that cooling is interrupted. Most materials lose their specific heat capacity at low cryogenic temperatures and have poor heat storage capability. Materials that undergo a phase change involving latent heat offer a better heat storage option than materials that use sensible heat storage. Nitrogen, in a solid-to-liquid phase change, is sometimes used, but nitrogen is a gas at temperatures above 77 Kelvin, which adds to system complexity. Nitrogen gas also presents an asphyxiation risk when released in a confined space, making it undesirable on a Navy ship. Nickel-titanium, or Nitinol, alloys experience a solid-solid phase change behavior with the transition temperature dictated by the ratio of nickel and titanium and by the presence of other alloying elements. Nitinol alloys have demonstrated martensitic-to-austenitic phase change behavior at temperatures lower than 77 Kelvin. Triton proposes to develop a Nitinol alloy with martensitic-to-austenitic phase change at 30 K. The latent heat of phase change for Nitinol alloys varies from about 4 kJ/kg to about 35 kJ/kg with many alloys around 15 kJ/kg. Nitinol with the typical latent heat characteristic can manage 100 W of thermal load for 4 hours with about 100 kg of material. A material sample will be created and demonstrated in Phase I to confirm the phase change temperature and the latent heat of phase change. In Phase II we will demonstrate a product format to provide continuous thermal energy storage along the length of a superconducting DC power cable.