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6 – Modelling 6.2 Modelling of Cylindrical Single- Shell DHW Tanks 6.2.1 Introduction The extensive theoretical and experimental work of Abdoly and Rapp (1982) has been an important contribution to the understanding of the general heat transfer mechanisms and transient temperature development in cylindrical single-shell tanks filled with hot water or with hot and cold water. Oppel et al. (1986), Homan et al. (1996), Rodriguez et al. (2002), Yoon et al. (2003) and Shin et al. (2003) developed different computer models in order to study the thermal stratification mechanism in storage tanks, and thereby determine the optimum design and operating condi- tions. However, since the aim of their work was to investigate thermal storage tanks used in thermal solar systems, the results were not directly applicable when analysing the hot water system for a residential CO2 heat pump. A transient two-dimensional heat conduction model, also termed the tank model, was therefore developed in order to enable tailor-made calculations to be done for actual DHW tank designs, temperature levels, hot water demands and gas cooler heating capacities. Most of the calculations were carried out for a static system without inlet and outlet water flows, and the DHW tank was either filled with hot water or with equal volumes of hot water and cold city water. The outlet water temperature from the bottom of the tank was also simulated during operation in the charging mode. The computer model was verified by means of experimental data from a test rig. The design and operation of the test rig as well as the experi- mental results are presented in Sections 4.2 and 5.2, respectively. 6.2.2 The Computer Model 6.2.2.1 Reasons for Model Simplifications The transient computer model was limited to calculate the impact of heat conduction inside the tank and between the water and the ambient. Conse- quently, the simulation results represented the minimum temperature change in the tank, and with that the minimum exergy loss. If convection as well as mixing caused by the inlet and outlet water flows had been taken into account, the model would have become considerably more complex. Also the results would have been inevitably linked to specific 192PDF Image | Residential CO2 Heat Pump System for Combined
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