تحقیقات تجربی و نظری انجماد و ذوب یخ برای طراحی و بهره برداری از فروشگاه های یخ

The Institute for Thermodynamics and Thermal Engineering (ITW) of the University of Stuttgart has developed a novel solar powered 10 kW absorption chiller. This chiller was implemented in the cooling system of the institute's building. To achieve a higher efficiency of the cooling system and to extend the hours of operation, a small ice store was designed, constructed and experimentally investigated. The experimental investigations include charging and discharging processes at different inlet temperatures and for different heat exchanger areas. For theoretical investigations a simulation program was developed and validated. Based on the results of these investigations a final heat exchanger design was established. In a further step the ice store was integrated into the building's cooling system. Long term measurements of the cooling system have shown good in-service behaviour of the ice store which fits the specific requirements of the cooling system in combination with the absorption chiller quite well.

Air conditioning and cooling of buildings are responsible for an increasing consumption of electrical energy. The main reason is a rising demand for comfort in commercial as well as private environments. Another reason is the growing number of buildings with a high glazing fraction in modern architecture. To decrease the electrical demand of such systems solar cooling could become a serious alternative due to the fact that demand of cooling energy and solar energy supply match quite well in time. Therefore, significant development work has started in the last few years. The Institute for Thermodynamics and Thermal Engineering (ITW) of the University of Stuttgart has focused some of its research activities on the development of an absorption chiller with a cooling capacity of 10 kW at standard conditions. At these standard conditions the heating inlet temperature at the generator is 100 °C. The inlet temperature of the condenser and the absorber is 27 °C. The outlet temperature of the evaporator is 15 °C (Zetzsche et al., 2007). The working pair ammonia/water allows evaporator temperatures lower than 0 °C and hence the formation of ice. The ice store was designed as a back-up system to support the absorption chiller mentioned above. From an exergetical point of view, the temperature level provided by the ice store is critical. There is a wide gap between cooling temperatures at approximately 16–18 °C and the ice temperatures equal or lower than 0 °C. But the advantages of the ice store are predominant. Cooling energy can be stored in a compact volume due to the phase change of water. Water is a cheap storage medium which is not at all harmful to the environment. With this arrangement it is possible to provide cooling energy even if the chiller is not working, for example because of insufficient radiation. It is also possible to run the absorption chiller when there is no cooling demand of the building, for example on weekends or public holidays. The hours of operation of the absorption chiller are thus extended. The cooling requirement of the rooms is met either by the absorption chiller or by the ice store. Finally, it is also possible to use the ice store to support the absorption chiller, if higher cooling requirements are necessary to cool the rooms than the absorption chiller is able to deliver. The aim of this project is to apply the cooling system to small offices and domestic buildings. Therefore, the components have to have a compact size; the ice store has been limited to a maximum volume of 0.5 m³. It is essential that the ice store achieves high charging and discharging rates, and a high volumetric capacity despite its small size. To reduce the electrical energy demand of the cooling system, no electrically powered equipment such as mixer or additional air injection was implemented. In general, such equipment is used to increase the turbulence of the liquid phase and, therefore, improve the heat transfer. The aim of the present experimental investigation was to show the heat transfer capacity of the ice store at inlet temperatures from −4 to −10 °C in charging mode and inlet temperatures from 20 °C at discharging mode. Also, the influences of heat transfer area and different operational discharging modes on the capacity were investigated. This study presents and discusses the measured data of the associated experiments. In additional to the experimental data, methods to calculate charging procedures are described. The calculations were validated with the experimental data. Ice stores use the phase change of water to ice and have therefore a high volumetric storage capacity. The specific volumetric storage capacity of ice stores is 40–53 kWh m−³ according to Bruder (1993). Phase change materials are an important application to store thermal energy and are topic of a large number of experimental and numerical studies. Tan et al. (2010) investigated experimentally the phase change of water for the cold energy recovery of liquefied natural gas refrigerated vehicles. Tay et al. (in press) investigated also experimentally the arrangement of phase change material system to optimize the storage density and thermal resistance of the heat exchanger. Another experimental study (Castell et al., 2011) investigates the heat transfer in a phase change storage system. Erek and Ezan (2006) investigated charging processes of an ice-on-coil thermal energy storage system experimentally and numerically. Helm et al. (2009) developed and constructed a latent heat storage tank and investigated the application of this latent heat storage tank. The latent heat storage supports a dry heat rejection of an absorption chiller to avoid the operation of a wet cooling tower. There are several papers concerned with the modelling of charging and discharging processes in ice-stores. Heat transfer coefficients for melting on a vertical cylinder were correlated by Kemink and Sparrow (1981). The measurements were performed with pure n-eicosane paraffin. Bareiss (1982) studied melting on plane horizontal and vertical surfaces, and on horizontal and vertical cylinders. He formulated correlations for Nusselt numbers to describe these kinds of boundary conditions. Betzel (1986) investigated the heat transfer for melting in a horizontal annulus and finned tubes. Riviere (1990) investigated the melting process for non-fixed ice elements in a horizontal tube. Zhu and Zhang (1999) developed a method to calculate the discharging process for a tank with horizontal heat exchanger tubes. Adam and Andre (2003) developed a TRNSYS model for latent energy storage. The model is compatible with other models for solar system components, such as heat pumps, collectors or heat stores. The model allows calculation of charging, and internal and external melting. None of these works, their methods and their results could be applied in the models described in this article. The investigated phase change materials, the shape and the orientation of the heat exchanger surface where to different to the present problem. For example they investigated horizontally adjustment of the heat transfer coils. Jekel et al. (1993) presented a theoretical model for ice storage in an ice-on-coil tank with a horizontal, spirally wound tube. They also developed a correction factor f which is used to modify the conductance of the ice when the ice cylinders are starting to contact each other at a critical radius rice, crit. This correction factor was also applied in the present work consider the influence of overlapping ice layers on the heat transfer during the charging process. Streit (1996) examined and calculated ice growing processes in a seasonal heat store. The storage mass was a packed bed of gravel and water. The heat exchanger tubes were installed horizontally. In the present article basic calculation methods of Streit (1996) were applied and enhanced to calculate freezing and melting of ice at vertical coils. Especially to calculate the discharging of the ice store new calculation methods were developed. For example a new Nusselt-Number was developed experimentally to reproduce the heat transfer mechanisms at free convection in a cylindrical, vertical gap.

The Institute for Thermodynamics and Thermal Engineering (ITW) of the University of Stuttgart has developed a novel solar powered absorption chiller which has been integrated into a cooling system for the institute's building. To complement the absorption chiller, an ice store with a nominal volume of 0.5 m³ was developed. For the design of this ice store several experimental and theoretical investigations have been completed. Part of the experimental work was the examination of the ice store behaviour under different charging and discharging temperatures. The effect on charging and discharging behaviour of different heat transfer areas was also examined. In case of discharging internal and external discharging were tested. For internal discharging higher and more constant discharging rates at approximately 5.5 kW are possible and the whole storage mass can be solidified. Therefore, internal discharging is used in the operation of the cooling system for the institute's building. The theoretical work produced a MATLAB simulation program which allows the calculation of different variables on charging processes. These variables are, for example, the heat transfer area, the mass flow, the inlet temperature and the kind of coolant. The validation with experimental data obtained under similar boundary conditions showed good agreement.