engleski

Load Shifting and Storage of Cooling Energy through Ice Bank or Ice Slurry Systems - modelling and experimental analysis

Ice based Cool Thermal Energy Storage (CTES) systems have attracted much attention during last few decades. The reason are mainly of eco-nomical and environmental nature. Compared to conventional refrigera-tion and air-conditioning systems without cool thermal energy storage, implementation of CTES will increase environmental standards and overall efficiency of the energy systems as it contributes to the phase-out of CFC-s and HCFC-s and reduces peak loads in electricity grids. For the application of a cool thermal energy storages in refrigeration in-stallations and HVAC systems in industry and building sector, it is neces-sary to have appropriate design tools in order to sufficiently accurate pre-dict their performance. In this thesis theoretical and experimental inves-tigations of two ice based cool thermal energy storage systems, namely static, indirect, external melt, ice-on-coil, i.e. ice bank system and dy-namic, ice slurry cool thermal energy storage system are carried out. An ice bank storage technology for cooling purposes is known for a long time. The main drawbacks which are hindering its wider use are the sys-tem complexity, high first costs, system efficiency which is highly de-pendant on design, control and monitoring of the system, etc. On the other hand, ice slurry technology was not well studied until recently, while in the current scientific literature there are still differences between results and conclusions reported by different investigators. The aim of the present thesis is to extend the knowledge in field of ice based CTES systems, thereby contributing in the development and wider utilization of those systems. In the first part of the thesis a computer application, named “BankaLeda”, which enables simulation of an ice bank system perform-ance is presented. In order to verify developed simulation model an ex-perimental evaluation has been performed. Field measurements have been conducted on a two module silo which was installed as a part of the refrigeration system in dairy and cheese factory “Antun Bohnec” in the city of Ludbreg in Croatia. Experimental findings were compared to the simulation model. The software „BankaLeda“ presents a strong optimi-zation tool for designers and engineers in the field by providing a high degree of freedom in defining particular system design and operating pa-rameters. It offers a basis for assessment and testing of a new energy ef-ficient system arrangements and measures. Besides it will give decision-makers the ability to test potential solutions in the process of CTES sys-tem design. In the second part of the thesis ice slurry pressure drop and heat transfer in horizontal straight tubes have been experimentally investigated. In particular a mixture of 10.3 % of ethanol and water with an initial freez-ing point of -4.4 °C was considered. It was found that the behaviour of ice slurry flow is changing with time and that ice slurry pressure drop is generally higher than for single phase flow. However for ice concentra-tions of 15 % and higher, for certain velocities ice slurry pressure drop is found to be of similar value as for single phase fluid. Moreover, if ice slurry is to be used as a energy transport media it is recommended to keep the ice mass fraction at a level of 20 %. With tube geometry and thermophysical properties of a carrier fluid the heat transfer of ice slurry is generally a function of ice mass fraction and velocity. The imposed heat flux has no or has just minor influence on the heat transfer coefficient. Up to ice mass fraction between 10-15 % the mean heat transfer coefficient shows only slight (laminar flow) or no in-crease (turbulent flow) in comparison to single phase flow. Beyond that ice mass fraction the heat transfer coefficient is increasing significantly. The test data for pressure drop and heat transfer in laminar and turbu-lent regime was compared to several correlations from the literature. A new correlations for ice slurry pressure drop and heat transfer in the laminar flow regime, for 10.3 % ethanol and water mixture, was derived based on the present experimental data. The correlation for pressure drop predicts 82 % of the experimental data with 15 % accuracy, while the correlation for heat transfer predicts 75 % of the data with the same accuracy. In order to investigate advantages and disadvantages of a dynamic, ice slurry system over a static, indirect, external melt, ice-on-coil CTES sys-tem and to assess their differences from economical aspects, a theoretical simulation model of an ice slurry CTES have been developed. It was found that the ice slurry based CTES systems posses higher economic and energy saving potential than static type systems. In the best case sce-nario the total energy consumption of dynamic CTES system was found to be approximately 25 % lower than for a static CTES system.

refrigeration systems; cool thermal energy storage system; ice; modelling; simulation; field measurement; experiment; indirect system; ice-on-coil system; external melt; ice slurry; homogeneous storage; heterogeneous storage.

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