Sophie Wang, PhD, University of Illinois Urbana-Champaign

Frost accumulation on heat-transfer surfaces poses serious challenges for air-source heat-pump systems. The low-thermal-conductivity frost layer reduces heat transfer and blocks air flow, increasing air-side fan power requirements. For most heat-pump systems, a routine reverse-cycle defrosting operation is used, which not only adds power consumption but also interrupts the heat supply and undermines indoor thermal comfort.  In this talk, I will present solutions from two different angles: (1) Delaying or slowing frost formation via surface features - Passive icing control through surface design techniques, such as hydrophobic coatings and roughness, has been proved effective in delaying frost formation. However, most prior work has used a recipe-based approach, without careful recognition of the condensation and freezing dynamics. I will describe the underlying mechanisms by which ice initiation and propagation can be promoted or impeded using surface micro/nano-features along with wettability. Together with a general model of frost growth that includes these surface effects, this work can provide guidelines for surface design; and (2) Sensing real-time frost properties with flexible sensors for defrost control - Given enough time or severe enough conditions, frost will eventually nucleate and build up, and how to control the defrosting process is important for overall system efficiency and indoor thermal comfort. In this part, I will introduce different versions of capacitance sensors developed in our lab. The first-generation sensor, with careful calibration, can precisely capture frost growth dynamics after initiation, including how parameters such as frost thickness, frost density, and frost growth rate evolve as the frost layer grows. The second-generation sensor can capture the whole process from condensation and ice nucleation to mature frost growth. These flexible sensors can serve for both model validation and defrost control for high-efficiency heat-pump systems.