CO2 double hydrate slurry for cold energy storage and transport: phase change characterization, rheological studies and L-Tryptophan modulation

Drivable by the demand for cleaner production and effective peak power management, CO2 double hydrate cold storage emerges as a promising enery storage solution. Characterizing the phase transitions and rheological properties of CO2 hydrate is essential for the advancement and optimization of CO2 hydrate-based cold storage technologies. This study investigates the phase transition properties of CO2 hydrates with various thermodynamic promoters using a stirred reactor under 2 MPa pressure and 9 K subcooling conditions. The thermodynamic promoters studied include tetrabutylammonium bromide (TBAB) and cyclopentane (CP). In all experiments, 3 mol % CP showed the highest gas consumption (35.05 mmol/mol), 1.81 times that of TBAB. L-tryptophan at low concentration increased CO2 + CP gas consumption by 6 %. To understand flow properties, rheological tests revealed that both pressure and temperature significantly impact the apparent viscosity of CO2 double hydrates across different additive concentrations: under same conditions, CO2+CP showed lowest viscosity, followed by CO2+TBAB with L-tryptophan addition, and CO2+TBAB alone had the highest. Adding CP and a small amount of L-tryptophan resulted in mild CO2 hydrate formation with the highest gas consumption, significantly enhancing the operational efficiency and safety of CO2 hydrate storage systems.