Application of gas hydrates in cold storage technology : experimental study and thermodynamic modeling.

Abstract

The ever-increasing demands for air conditioning technology especially in residential areas results in a severe imbalance between power generating utilities, especially during peak usage hours. To provide the required peak capacity, expensive peak-generators are compulsory. Hence, researchers are compelled to find an effective air conditioning system which can be utilized during peak-hours with the minimum of electrical consumption. One solution to this problem is to shift the electrical consumption from peak-hours to off-peak hours by a combination of a cold storage technology with the air conditioning system. In the cold storage system, the cold energy can be stored by a medium during off-peak hours (e.g. ice or water) and it can be released for use at peak hours. An air conditioning system which operates with cold storage technology usually consists of a storage medium, a storage tank, a coolant circulator, a pump and a condenser. Due to the fundamental role of storage medium in cold storage systems, various investigations have been performed in order to design an applicable storage tank. Ice, eutectic salt, and water are the most common materials being used as storage materials in cold storage applications. However, the application of these conventional materials as storage medium is not practical, due to their formation at low temperatures, their low enthalpy of dissociation and their low density of cold storage. It was found that most refrigerant hydrates can be utilized as a suitable cold storage medium in air conditioning systems due to their significant properties such as high enthalpy of formation/dissociation close to that of ice, and temperatures of formation above the freezing point of water. In this study an extended review on the application of clathrate/semiclathrate hydrates in cold storage systems is performed. The gas hydrate dissociation conditions of refrigerants R23, R134a, R125a, R22, R116, R410A, R407C, R408A, R508B, R404A, R406A, R427A and R507C have been measured experimentally using an isochoric pressure search method. From the measured experimental dissociation data, the enthalpies of hydrate dissociation are evaluated. Results indicate that R507C has the highest enthalpy of dissociation amongst the other refrigerant blends. R134a, R410A, R407C and R427A with low pressure of hydrate formation/dissociation, show the most suitable behaviour to be used in cold storage applications. A thermodynamic model with the ability to correlate dissociation conditions of refrigerant hydrates in the different phase equilibrium boundaries (Hydrate-Ice-Vapour, Hydrate-Aqueous solution-Vapour, Hydrate- Hydrate-Aqueous solution-Liquid refrigerant) has been proposed. The difference between model predictions and experimental data is reasonable. Furthermore, in order to examine the rate of the refrigerant hydrate formation, an experimental study has been performed on the kinetics of the hydrate formation of the refrigerant blends namely R407C, R410A, R507C, R404A, R406A, R408A and R427A. The induction time of hydrate formation, apparent rate constant of the hydrate reaction, water to hydrate conversion during hydrate nucleation and growth, storage capacity and the rate of hydrate formation of above mentioned refrigerants at different initial conditions (pressures and temperatures) have been calculated using a kinetic model. The results demonstrate that in the presence of pure water R407C has the maximum apparent rate constant, appropriate induction time, and highest storage capacity at temperate pressures and temperatures amongst the eight refrigerants studied. The effect of sodium dodecyl sulfate (SDS) with different concentrations of 400, 500 and 600 ppm on hydrate nucleation and growth rate was investigated. In contrast to the refrigerants R406A, R404A, R408A and R427A an inhibition effect of SDS on gas hydrate nucleation rate was found for the refrigerants R407C, R410A and R507C. The most relevant kinetic results were found for the system R406A + 400 ppm SDS solution.