Elemental sulfur is a promising storage material for low to high temperature thermal energy storage (TES) applications due to its high chemical stability, high heat transfer rate, and low cost. In this study, we investigate the performance of sulfur-based TES systems (SulfurTES) in the temperature range of 50-600 oC for single-tank and multi-tank configurations, as well as heat transfer fluid options, and have found a wide range of system configurations that provide attractive performance and cost.

For a single-tank thermal battery configuration, a 2D, transient-state numerical model was developed and validated using experimental results obtained from the thermal cycling of a 10kWh SulfurTES battery. In general, the results show that a moderate shell aspect ratio (i.e., L/D ~ 4-7) and standard tube diameters (i.e., diameter ~ 2-6”) can be used to provide a range of high performance and low-cost systems.

Multi-tank configurations were investigated and found to have many attractive aspects. The proposed cascaded configuration with two shell passes boosts system’s energetic efficiency, providing a versatile system that accommodates wide selections of applications with low cost.

Effects of heat transfer fluid (HTF) properties on performance parameters were studied using sCO2 and HITEC Solar salt as HTFs. The large density of sCO2 leads to negligible compressor work that is beneficial to exergetic efficiency. High thermal conductivity and large energy density of HITEC salt provides promising thermal performance, but its prohibitive cost and limited operating temperature range make it undesirable to use with SulfurTES.

Finally, the cost analysis of SulfurTES system was conducted assuming integration with concentrated solar power plants. The results demonstrate significant cost advantage of SulfurTES over Molten-salt TES. The capital cost of SulfurTES achieves 2020 Sunshot TES cost target of $15/kWh, providing a levelized cost of energy of less than �5/kWh that satisfies the Sunshot cost target of 2030.

Recent developments in electronic and optoelectronic devices, especially in the highly power-sensitive areas of portable and wearable applications, have led to increased challenges in their thermal management. Conventional rotary fans are difficult to scale down and power-inefficient when miniaturized. Piezoelectric fans, which typically consist of flexible blades mechanically fixed to piezoelectric actuators, are promising alternatives to rotary fans because they have simpler structures, produce less noise, and require less power consumption [1]. Although piezoelectric fans have been commercially available for over a decade, systematic study of their power consumption mechanism has been limited. A combined experimental and modeling study is conducted to help elucidate the mechanism of power dissipation in piezoelectric fans, and correlate them with heat transfer performance.

Powers consumed by a commercially available piezoelectric fan and by its piezoelectric actuator (with the fan blade cut off) are measured separately as a function of frequency and bias input voltage. A mechanical model is next developed to theoretically partition the fraction of power transferred to the surrounding air and that portion consumed by the piezoelectric actuator. In parallel, the heat transfer coefficients over a heated flat surface are measured for a set of piezoelectric fans of different blade lengths and thicknesses to obtain insight into piezoelectric fans’ cooling enhancements normalized by power consumptions.

It is observed that the power consumption predicted by the proposed model for a piezoelectric actuator (with the blade cut off) agrees with that measured. In addition, the model estimated flow power (power transferred to surrounding air) matches the difference between the separately measured total power consumption of the piezoelectric fan and that of its piezoelectric actuator, indicating the validity of the proposed model in partitioning the power consumption of a complete piezoelectric fan. It is also confirmed that the heat transfer performance of the piezoelectric fan is closely related to its induced flow power rather than the total power consumed. As will be discussed in following chapters, the “power efficiency of piezoelectric fan cooling” should be characterized by its flow power rather than total power consumption.