Ultra-High Surface Area Architecture for Thermal Energy Storage

ABSTRACT: The Air Force is currently seeking improvements in capacity and responsiveness of Thermal Energy Storage (TES) systems. Current research and development with Phase Change Materials (PCM) in carbon foam and pelletization of metal (i.e. copper) encapsulated metal hydrides seek to overcome these limitations by increasing the thermal conductivity (i.e. copper, carbon) while also increasing the surface area-to-volume ratio (i.e. foam, micro-encapsulation). Unfortunately, these methods still have drawbacks. For PCM in carbon foam, the volume change during phase transition ruptures internal ligaments. For copper-encapsulated metal hydrides, the pelletizing of micro-encapsulated MH creates excessive interfacial boundaries. Both occurrences limit the overall thermal conductivity. Mainstream"s approach combines high burst pressure (104 atm) nano-cylinders, metal hydride deposition (yielding high surface area-to-volume ratio), and high thermal conductivity attributed to nano-structured carbon. The Phase I effort focuses on fabrication of the high surface area storage media and demonstration of storage capacity. The Phase II effort will focus on the TES system architecture. BENEFIT: Increasing the heat removal capacity for high-density variable heat loads, such as that found aboard avionic systems, remains a major technical challenge for NASA, military, and civilian applications. The raw storage heat capacity of metal-hydrides surpass that of most latent- and bond-based thermal storage materials except the latent heat of vaporization of water and in some cases ammonia. Furthermore, the heat input rate (i.e. thermal responsiveness) associated with absorption kinetics exceeds that of conduction-dominated latent heat of fusion. However, when the entire thermal energy storage (TES) system is compared, metal hydride TES cannot match paraffin based TES in capacity due to the required gas storage (for reversible systems) and large core structure (i.e. metal hydride heat exchanger). The current STTS effort is significant as it will overcome the remaining obstacles (i.e. low thermal conductivity and low surface area-to-volume ratio of the metal hydrides) for producing a field-deployable metal-hydride TES for avionic systems, such as directed energy weapons. The application of a thermal responsive, energy dense TES would be applicable to Directed Energy Weapons (DEW) such as High-Powered Microwaves and High Energy Lasers as well low duty cycle electronics.