Waste heat recovery, thermal storage, and heat pumping in drying processes
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Drying is a ubiquitous process in a wide range of applications and industries, including residential and commercial clothes dryers, agriculture, chemical processing, and pharmaceutical industry. Clothes dryers have become widespread in North American households and commerce. More than 80% of the households in the US have a washer and dryer. Thermal drying of textiles is a very energy-intensive process coupled with the complexities of several transport phenomena (mass, momentum, and energy) occurring simultaneously, and their influence on material properties. Dryers reject ~ 58% of the total input energy as waste heat or losses. While system efficiency can be improved by optimizing system design and operational parameters, there is also a significant opportunity in harnessing the energy from the exhaust stream of the dryer and storing it for use in future drying cycles. In this work, a fundamental understanding of the heat and mass transfer processes in drying is sought with the goal of simultaneously reducing energy consumption and drying time. Transient thermodynamic, heat and mass transfer models are developed and validated experimentally. The validated models are used to optimize energy consumption and cycle time. Recirculation, waste heat recovery, thermal energy storage, and heat pumping are investigated as a means to achieve these goals. A commercial tumble dryer is used to implement these changes and demonstrate the effects of these enhancements. Implementation of these techniques is demonstrated to yield energy savings of up to 22% while simultaneously decreasing drying time by 23%. Adoption of these energy saving techniques in gas-fired tumble dryers across the U. S. market could reduce primary energy consumption by ~ 11.6 billion kWh annually.