(367k) Rheological Characteristics of Tetra-n-Butylammonium Bromide Hydrate As a Thermal Energy Carrier

An increase in cooling efficiency is important to electricity consumption as the increasing demand for cooling of data centers as well as space in buildings. Thermal energy storage is an efficient approach to use wasted thermal energy to improve energy efficiency. Phase change materials (PCMs) are spotlighted for their potential to store the latent heat by utilizing the wasted cold energy for cooling for air-conditioning applications. Semi-clathrate hydrates are candidates for PCMs for cooling, which have suitable phase change temperatures (0 – 27 °C) and higher thermal energy storage density than chilled water. For the PCMs as cooling refrigerants, fundamental studies such as the rheological characteristics of semi-clathrate hydrate slurry needs to be studied to evaluate the pump power consumption and pressure drop in the transport pipeline. However, the rheological properties observed in preceding literature data showed a huge discrepancy and were mostly investigated with low hydrate(solid) fractions.

In this work, the viscosity characteristics of tetra-n-butyl ammonium bromide (TBAB) semi-clathrate hydrate, which is well known as semi-clathrate hydrate former, were investigated experimentally by using an in-situ rheometer up to 50wt% of hydrate fraction in slurries. The apparent viscosity of TBAB hydrate slurries exponentially increased with the increase of hydrate fraction in TBAB hydrate slurries. Type B TBAB hydrate had lower apparent viscosity than type A TBAB hydrate at a similar hydrate fraction and the difference increased with increasing hydrate fraction. Furthermore, the pressure drops and energy consumption for pumping for transport were evaluated by rheological data when using the TBAB hydrate slurries as a secondary refrigerant for the cooling process. The results were compared with that of chilled water. The results demonstrated that the energy consumption for TBAB hydrate slurries was lower than that of chilled water with up to a certain degree of hydrate fraction of TBAB hydrate slurries. However, the pressure drops and energy consumption for TBAB hydrate slurries increased with the increase of hydrate fraction in slurries due to high apparent viscosity. The knowledge gained in this work suggests useful information regarding the optimization of hydrate fraction and operating conditions for the further development of hydrate-based cooling systems.