Optimized design of thermal batteries to enhance the performance of space conditioning systems

Abstract

The recovery and reuse of waste heat to enhance the overall performance of a thermodynamic system is dependent on effective heat exchangers to absorb or reject thermal energy. Thus, improvements to the heat exchanger design can often be beneficial in the creation of compact or more energy efficient systems. One such advancement is the insertion of an energy storage medium into the heat exchanger to act as a thermal battery. Such technology delays the thermal exchange between the hot and cold fluid streams, allowing the exchange to occur on demand or when more expedient for an energy efficient application. Phase change materials (PCMs) are often used as the storage medium due to their high energy density combined with a nearly isothermal storage process corresponding to the phase-transition temperature. However, their low thermal conductivity significantly limits the rate of thermal charging and discharging. As such, increasing the thermal performance of PCMs is crucial to the widespread adoption of thermal energy storage technologies. This dissertation studies the design of an advanced heat exchanger with a thermal energy storage medium, specifically a PCM thermal battery, with engineered thermal properties to enhance charging and discharging rates. To control the thermal properties of the storage materials, PCM composites enhanced with aluminum and graphite foams are characterized and tested under various charging conditions to guide the design of thermal batteries. Experiments are performed to verify the salient features of the foams that control thermal charging rates along with thermal conductivity, density, and latent heat of the composite. Additionally, numerical models are developed and validated to predict the time to fully charge the battery. This fundamental work is used to guide the optimized design of a thermal battery for integration as the condenser in a vapor compression refrigeration cycle to minimize heat released to the ambient during operation. Component-level performance studies of the thermal battery are completed.