Cold Storage With A New Type Of Gas Hydrate And Its Growth Characteristics Experimental Study

Recently, the increasing demand of electric power for residential air-conditioninghas been more urgent in many countries. The hydrate cold storage system, which canshift the demand to off-peak period and contribute to the load leveling of powergeneration, has been developed commonly as the suitable new-type energy-savingsystem for its larger cold storage density and higher chiller efficiency compared to otherstorage systems(i.e. water, ice, eutectic salt storage system). The refrigerants gashydrates, usually formed above273.15K and under moderate pressure, whose formationheat is comparable to the fusion heat of ice, can be considered as promising energystorage materials in air-conditioning systems.Nowadays, Chlorofluorocarbons (CFCS) and Hydrochlorofluorocarbons (HCFCS)substances will be phased out gradually based on The Montreal Protocol on Substancesthat Deplete the Ozone Layer2007, and the research on the development of newenvironmentally friendly refrigerants, which has more potential and superiority forapplication, has become a new era. The application of the basic theory involved inmechanism and calculation of gas hydrate phase equilibrium, the dynamiccharacteristics of hydrate formation, thermal properties, etc. For the economicalfeasibility of hydrate cold storage technique has been determined by hydrate phaseequilibrium conditions, formation characteristics and hydrate growth speed and so on,further and intensive research is necessary to help to promote the application.Experimental research and mechanism analysis on new-type gas hydrate growthcharacteristics (phase-equilibrium, dynamics and thermal properties) for cold storagehad been carried out and the conclusion is listed as follows:1) The direction of new-type gas hydrate cold storage media selection has beenpointed out. The selection from the alkane series, Hydro Fluorocarbons (HFCs),water-soluble materials, hydrate slurries and other types of categories is presented andsummarized, and some promising media has been listed. Based on the superiority ofbinary gas hydrates to that of simple ones (such as shoter induction time, largerformation rate, azeotropic melting property and non-azeotropic melting property), anddue to the better characteristics of property compensation for mixtures, new-type binarygas hydrates are presented and summarized.2) The visual experimental apparatus for the study of growth characteristics of gas hydrates used in cold storage was originally designed. The established experimentalsetup consists of four main subsystems: transparent reactor with magnetic stir, waterbath, image collection system and data acquisition system.3) The equilibrium data of isobutane hydrates above and below the freezing pointhad been tested comprehensively, and the data coincided with the calculated data fromequilibrium theoretical model well—fitting out the equilibrium curvilinear equation ofisobutane hydrates (258K<T<276K), which may provide physical parameters for coldstorage applications.4) The dynamics of isobutene gas hydrate had been studied under different watervolumes, stirring rates, initial coolant temperatures, water samples and sodium dodecylsulfate (SDS) concentrations with initial pressure of0.38MPa in the reactor. The speedand dense degree for hydrate formation had been discussed through the length ofinduction time in the formation process and residence time of phase transitiontemperature point in the decomposition process respectively.Results show that more dense hydrate could be obtained in the water volume of200ml than others for larger gaseous-liquid space ratio. Larger stirring rate (250rpm)could increase disturbance and improve the gaseous-liquid mass transfer, which canlead to quick hydrate formation. But too large stirring rate could also lead to thedecomposition of hydrate. With lower initial coolant temperature, the super coolingdegree could be lager, and the more hydrate could be obtained. Historical water candecrease the induction time for its “memory effect”. The induction time was decreasedby18min with SDS when compared with the condition without SDS, and the gasfraction in formed hydrate increased for surfactant SDS can reduce the interfacialtension of gas-water contact effectively and accelerate the rate of gas molecules’entering into gas-water interfacial layer. Besides, the induction time was very close(about20min) under different SDS concentrations (800ppm，1000ppm and1200ppm),and the amount of formed hydrate tended to coincide. It can be recognized that the SDSdensity of this experiment is above the Critical Micelle Concentration (CMC).5) Based on the superiority of binary gas hydrates for cold storage to that of singleone, on the condition that the mass fraction of guest materials was fixed to31.4%, theformation decomposition of tetrabutylammonium bromide (TBAB) tetrahydrofuran(THF) gas hydrate mixture for cold storage was studied by changing the concentrationsof TBAB and THF. The variations of induction time, decomposition temperature, cold storage capacity and discharged cold capacity during the reaction process werecompared. It is revealed that the decrease of TBAB concentration can prolong theinduction time of crystallization, shorten the transit jump temperature and lower thehydrate decomposition temperature, and along with the decreased cold storage capacityand shorter completion time of hydrate decomposition. With TBAB in the solutionrange (25.7%~27.4%), the decomposition temperature of the samples was in the range(6.3～8℃), which is close to the temperature of air conditioning system, and itsdischarged cold capacity was higher than that of other TBAB concentrations.Accordingly, the TBAB solution with25.7%and27.4%concentrations may be apromising cold storage material for air conditioning.6) Thermal property tests was carried out by Differential Scanning Calorimetry(DSC) comprehensively on the phase-change temperature and fusion heat of TBABhydrate, THF hydrate and TBAB-THF hydrate mixture. Results show a good trend that,TBAB-THF hydrate has the superiority for more proper phase-change temperature andincreased fusion heat. More broad and developed view is that adding appropriateamount of hydrate with lower phase change temperature to hydrate with higher one canmake the mixture hydrate more suitable for cold-storage (5~8℃); some hydrates withlower phase-change temperature can even make the fusion heat of mixture hydrateincreased greatly.Generally speaking, through a large number of experimental tests and theoreticalanalysis, the preliminary growth characteristics of new-type cold storage gas hydrateshave been revealed, and the corresponding mechanism analysis and experimentalverification has been carried out, making certain contributions on the furtherdevelopment of gas hydrate cold storage technology. Nevertheless, further researchesare still needed to be carried out from both domestic and foreign experts to help topromote the application of the new technology.