Use of Pressure as an Unconventional Dynamic Control Variable on Desorption-Based Thermal Energy Storage

Thermal energy storage (TES) materials operate by serving as a thermal reservoir that can be charged and discharged on demand, thereby improving system energy efficiency and decreasing or delaying peak load for a system. As an example, TES incorporated into buildings can serve as a ‘thermal’ battery for the power grid, helping to offset intermittent renewable power generation resources such as wind and solar. In general, TES materials have long been defined by the quest for higher energy and power densities, especially for mobile applications (i.e., automotive, aerospace). While these characteristics are still critical for many applications, novel materials systems are desired which also: 1) afford opportunities for external dynamic control and tunability, and 2) allow for rapid athermal recharging for applications with fast response times required.<br/><br/>Here, we describe hybrid desorption and evaporation-based TES materials systems composed of zeolite beds supported on thermally-conductive metal lattice structures forming a composite wick structure. Importantly, the systems under investigation are open system concepts, wherein evolved water vapor is not collected in a secondary bed or fluid reservoir, but rather is allowed to leave the system. These systems offer a wide range of potential attributes of interest for many thermal management applications: (1) the use of the open concept system concept increases energy and power densities dramatically relative to either two-bed evaporative or desorption based systems, or relative to condensed phase transitions (e.g., solid-liquid, or solid-solid), (2) the dependence of both evaporation and desorption processes on the local partial pressure of water vapor in the system introduces a key control variable (pressure) which can be used to dynamically control either the temperature at which heat is absorbed, or the rate of heat absorption at a particular temperature, and 3) the use of liquid recharge allows for rapid on-demand recharging.<br/>Here, we present and discuss material attributes of the proposed zeolite hybrid wick structure, and their impact on the performance of the open TES system. These include (1) the selection of a particular zeolite phase optimized for low-temperature (~100 C) energy storage, (2) the partitioning of water between adsorbed water which is contained within nanoscale porosity within the zeolite particles, and liquid water which occupies pore space between zeolite particles, (3) the incorporation of hydrophilic binders to permit rapid imbibition of liquid water into the structure, and (4) the meso-scopic thermal design of the composite structure to enable fast absorption of heat into the hybrid zeolite TES material. We will present experimental observations correlating the TES material performance with its overall structure. Transient thermal response of the material is used to calibrate finite volume numerical thermal model, which is used for iterative design of the composite material structure.<br/>Finally, we introduce pressure as a control variable and will evaluate the response of the material under quasi-isobaric heating conditions, and under quasi-isothermal decompression experiments. In this series of experiments, we will report (1) the tunability of energy storage temperature in response to pressure changes, and (2) the dynamic thermal response of the system to an abrupt change in pressure. These observations will elucidate the nature of thermal energy storage under unconventional, but technologically relevant ranges of operational pressure.