Physical Modelling of the Interactions between Thermal Systems in Supermarkets

This thesis contributes to modelling and quantification of interactions between thermal systems in supermarkets. The main goal of this thesis is to describe the interactions between thermal systems in supermarkets by means of physical models and to quantify their energetic impact. To achieve this overall goal, three subgoals are defined. The first subgoal is the breakdown of supermarkets into control volumes and the definition of physical interactions between them. In this work, the usage of physical modelling implies two aspects: Firstly, using first principle models and secondly using physical connections between objects. The second subgoal refers to the analysis of the air exchange between refrigerated display cabinets and the sales area. The focus lays on the quantification of the air exchange and the determination of the influence of door openings on this air exchange. In laboratory tests, an adapted tracer gas method using CO2 is used to determine the air change rates between the display cabinets and the sales room for three typical supermarket cabinets. Additionally, a second way of quantifying the impact of door openings on the air infiltration uses the so-called thermal entrainment factor (TEF). The applications of the developed models are exemplified. The prediction of the energy consumption of a period of 28 days deviates only 3.3% from measurement data. The analysis of the effects of incoming and outgoing customers on fresh air change rates is the third subgoal of this thesis. This air exchange is highly relevant because it affects the thermal loads of the heating and air conditioning system as well as the need for ventilation. In order to estimate the air change rates, a model of the CO2 concentration in the sales room air is developed, which uses the air change rates of the closed and opened entrance door as parameters. The prediction of the CO2 concentration works very well: the mean absolute error of 26ppm is below the accuracy of the measurement data. Additionally, the entire supermarket is simulated with the objective to predict the humidity of the sales room air. The humidity ratio in the test supermarket for a period of 14 day could be predicted with a mean absolute error of 0.3g/kg, which again is below the accuracy of the measurement data. Finally, several of the interactions between thermal systems in a supermarket are quantified exemplarily. The energetic impact of the shopping of one customer is analysed for three scenarios.