The research presented in this thesis focuses on the use of hydrate slurries in the air conditioning and refrigeration areas. Both experimental and mathematical methods have been used. Hydrate slurries have been suggested as promising cold storage materials that can be used in air conditioning systems due to their high latent heat (193 kJ/kg and 387 kJ/kg for the hydrates studied in this thesis) and positive phase change temperature (12.5 °C and 8.0 °C for the hydrates studied in this thesis). However, large scale industrial applications of hydrate slurries are still very limited. This suggests that more research efforts should be devoted to the demonstration of its advantages.@en

CO2 hydrate slurry is a favourable direct coolant of fresh products due to its large latent heat and phase change temperature around 7 °C. Continuous production of this slurry is, however, difficult to realise due to the high rate of hydrate formation. The use of additives is proposed with the purpose of decreasing the formation rate so that the controllability of the process is improved. Type-III Antifreeze Proteins (AFPs) are non-poisonous additives which, at low dosage, have proven to be efficient limiters of gas hydrate formation. The effect of these additives on the CO2 hydrate formation rate is experimentally investigated in this study.

The concentration of the AFPs investigated in this research is 10 ppm. Experimental results show that the supercooling degree of the solution is only slightly affected by the addition of AFPs. Results also show that the addition of AFPs slows down the dissolution rate of CO2 gas into the aqueous solution which is the first step of gas hydrate formation. A hydrate growth equation has been used, from which the experimental mass transfer coefficient of CO2 through the solution has been derived. Results show that the decrease of hydrate growth rate with the addition of AFPs can be related to a decrease of the CO2 mass transfer coefficient which gives a lower mass transport rate from bulk liquid phase to the crystal interface.@en

The objective of this paper is to investigate the pressure drop and heat transfer characteristics of a coil heat exchanger in which TBAB hydrate slurry is being formed from a 36.5 wt % TBAB solution. The coil heat exchanger has a 5.6 mm internal diameter. First tests were carried out with water in order to validate the experimental method. TBAB hydrates are formed by cooling the mixture of TBAB and water at temperatures from 20 °C to 11 °C. Experimental heat transfer coefficients and pressure drop of TBAB hydrate slurry have been obtained with solid concentrations in the range of 10-45 wt %. The results are compared with values for TBAB solution and water. Existing pressure drop prediction methods can reasonably predict the experimental data. The experimental data cannot be predicted with existing heat transfer prediction methods. A method is proposed for the prediction of heat transfer under hydrate formation conditions.

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Latent heat thermal storage (LHTS) systems can be applied to minimize the discrepancy between energy supply and demand in cold applications. Combining this system with night time generation of TBAB hydrate slurry can significantly reduce the investment cost of systems for cooling purposes. TBAB hydrate slurry generation requires temperature around 12 °C, operates at atmospheric pressure and, from an energetic point of view, application of this slurry can contribute to significant energy savings. In this paper, a numerical model of a LHTS system is coupled with a hydrate crystal growth model, which has been validated with experimental data collected with a coil heat exchanger for TBAB hydrate generation. The model has been used for the estimation of the production of TBAB hydrate slurry in an air-conditioning system. Results indicate that with a certain volume of the LHTS system, it is possible to fulfil the considered 3 kW peak cooling demand for a specified office building without extra external energy input during the day. The presented work provides guidelines for the thermal performance and design optimization of a TBAB hydrate slurry based LHTS system for an air-conditioning system. @en

Hydrate slurry has been reported to be a suitable secondary fluid for refrigeration and air-conditioning systems. The latent heat of CO2 hydrate is 387 kJ/kg under phase equilibrium condition of 7 °C and 30 bar. The utilization of CO2 hydrate slurry in air-conditioning systems is promising in improving the energy efficiency and shifting energy supply and demand load as well as relieving greenhouse effect caused by normal refrigerants like CFCs, HCFCs etc. The production of CO2 hydrate slurry in a coil heat exchanger is investigated in this study. Crystals are supposed to firstly form on the wall of the tube, generating a solid layer. The appearance of the solid layer increases the heat resistance from liquid to the refrigerant. Type-III antifreeze proteins have been added to the solution to better control the crystallization process of hydrate formation since AFPs have proven to be an effective hydrate formation preventer which is environment friendly. A kinetic model is developed based on the gas hydrate growth model of Skovborg and Rusmussen (1994), taking the mass transfer process to be the rate-control step. The influence of pressure, temperature and concentration of the hydrate formation preventer on the diffusion coefficient of dissolved gas into the solid interface is investigated and mainly the concentration of hydrate formation preventer appears to have a large impact. Results show that the growth rate decreases with the increase of the concentration of AFPs. Higher concentrations of AFPs move the equilibrium line to slightly higher temperatures. @en

CO2 hydrate slurry has a high latent heat and has been proposed for heat storage applications. This paper reports experimental data obtained during the generation of CO2 hydrate crystals in a fluidized bed heat exchanger (FBHE) which is cooled by an auxiliary primary refrigeration cycle. The operating range and performance of the system are investigated. A heat and mass transfer model is proposed for the prediction of the performance of the CO2 hydrate fluidized bed heat exchanger based on an existing heat and mass transfer correlations for this type of equipment. The model can predict the experimental trends. The maximum solid fraction in the slurry attained during the experiments was 34 wt %.The production of hydrates in the FBHE can reach ca. 32 kg/h in an open system with continuous CO2 feed. @en

CO2 hydrate slurry is a newly developed secondary cooling fluid. In order to use CO2 hydrate as a direct coolant of fresh products, a new CO2 hydrate slurry production system (a fluidized bed heat exchanger, FBHE) is under construction and it has been optimized for the expected operating conditions. A sensitivity analysis has been used to identify the optimum design for the specific operating conditions. The model includes the growth of CO2 hydrate on the fluidized bed wall as a function of the operating conditions. The heat of formation is removed by a falling film of evaporating R134a outside the fluidized bed. The maximum capacity of the set-up is 5 kW. The parameters optimized include the internal diameter of the fluidized bed heat exchanger, bed porosity, inert particles inside the FBHE, as well as the refrigerant temperature. Under specific operating conditions: pressure inside the FBHE is 35 bars and solution initial temperature is 6°C, the optimized parameters according to simulation are: internal diameter is 85 mm, bed porosity is 0.7, particle diameter is 5 mm, and refrigerant temperature is 0°C. Under these conditions the apparatus produces 13.2 kg hydrate in one hour. @en