Heat transfer, energy conversion, and efficiency during cold discharge of a novel tetrabutylammonium bromide hydrate cold storage system

Abstract

• External melting by internal heat exchange was used to study hydrate cold discharge. • Thermal buffering occurred due to energy balance caused by heat transfer. • Forced convection from gas disturbance reduced cold discharge time by 3 times. • Cold discharge efficiency can be maintained above 83 % under different conditions. • Time-efficiency number was proved to quantify the total cold discharge efficiency. Cold storage technologies using clathrate hydrates as working media have recently attracted more attention in the energy storage field because they undergo phase transitions with large latent heats and suitable temperatures for cold storage. However, the application of hydrate cold storage technologies is limited by the fact that the cold discharge mechanism is still unclear. Thus, this paper adopted the method of external melting by internal heat exchange to investigate the heat transfer, energy conversion and efficiency of a novel tetrabutylammonium bromide (TBAB) hydrate cold storage system during cold discharge. The system consisted of an internal circulating gas disturbance device and a metal spiral hydrate-on-coil heat exchanger. The heat transfer mechanism and cold discharge capacity, rate, and efficiency for different cold charge temperatures, flow rates and disturbance modes were compared and analyzed. A novel time-efficiency number was introduced to evaluate the comprehensive performance of the time, energy conversion and efficiency of cold discharge. The results showed that the heat exchange balance during the cold charge and discharge stages had a thermal buffering effect on the storage unit. Forced convection caused by gas disturbance resulted in a larger cold discharge capacity and reduced the discharge time by a factor of 3 compared to operation without gas disturbance. In addition, the cold discharge capacity gradually increased with decreasing cold charge temperature and increasing flow rate. The cold discharge efficiency was more than 83% under different conditions. A comparison showed that the time-efficiency number tended to be larger with gas disturbance. This study showed that the time-efficiency number can be used for quantitative comparison of the cold discharge time and efficiency in hydrate cold storage systems.