Theoretical and experimental analysis of the transient behaviour of refrigerated transport systems

Refrigerated transport systems are employed in the cold chain to supply the consumer with safe, high-quality perishable freights. Differently from static refrigeration systems, refrigerated transport equipment is required to perform reliably in a wide range of ambient temperatures and under extremely variable weather conditions (solar radiation, cloudiness, rain etc…). Refrigerated vehicles include vans, rigid trucks and semi-trailers. The refrigerating system has to ensure a precise temperature control of the internal cargo space: given a certain perishable freight requiring a certain set-point temperature, a lower temperature might damage it while a higher temperature might reduce its shelf life or result in unsafe or low quality product for the consumer. Close temperature control systems for chilled goods require continuous, modulated refrigeration combined with high rates of air circulation. This thesis contains the results of a study carried out during three years of PhD activity, conducted at the Construction Technology Institute (ITC) of the National Research Council (CNR) in Padova, where an official ATP test station is located. The ATP (Agreement on the International Carriage of Perishable Foodstuffs and on the Special Equipment to be Used for such Carriage) is an international agreement regulating the transport of perishable foodstuff. Starting from this experience, dedicated experimental activity was set up to collect data: ATP K-value measurement tests have been performed, along with step tests intended to evaluate the characteristic time constant of the insulated body of a brand new refrigerated van, designed to the carriage of chilled products (0°C-class refrigerated vehicle). The experimental data collected during the ATP test and step test of the brand new refrigerated van were used to develop and validate a 0-D, unsteady, lumped capacitance, simple numerical model of its insulated box. The numerical model presented in this thesis introduces a novel approach to model the average transient response of the insulated body of a refrigerated vehicle: the model is able to reproduce the actual performance of the structure without the need for the detailed drawing of the structure or the knowledge of the actual properties of the material used for the construction. Under the stimulus of environmental sustainability and in order to offer the market with a natural alternative to F-gases, CO2 was considered as a possible working fluid for refrigerating units serving refrigerated vehicles. A new lay-out for the cooling unit, developed for refrigerated transport application, which utilizes carbon dioxide as the working fluid, was developed. Starting from the experience developed in stationary refrigeration, but taking care of constraints specific of refrigerated transport, the cooling unit was designed to operate according to three different configurations: traditional low-pressure receiver cycle configuration, ejector cycle configuration and ejector cycle configuration utilizing an auxiliary evaporator located in the line between the ejector outlet and the low-pressure receiver. Numerical simulations of the refrigerating system operation with an internal cargo space temperature varying between -5°C and +15°C and an external ambient temperature spanning between 15 °C and 45 °C demonstrated that the operation of the refrigerating system with the ejector cycle configuration was convenient in hot climates and internal air temperature between +5 °C and -5 °C. For ambient temperature lower than 30 °C the operating range of the ejector could be extended with an auxiliary evaporator at the outlet of the ejector. Taking the coefficient of performance of the system as a thermal performance indicator, the ejector cycle configuration provided a maximum advantage by 15.9 % over the traditional cycle configuration at 42°C ambient temperature and -5°C internal cargo space temperature. On the other hand, when the auxiliary evaporator was employed in the refrigerating system, the maximum advantage of 14.2 % on the COP was identified for an ambient temperature of 27 °C and an internal cargo space temperature of 5 °C. After the numerical models of the cooling unit and the insulated body were developed, they were coupled and a typical delivery mission was defined. The selected delivery mission was characterized by a total of 12 door opening operations where doors are kept completely opened and heat enters directly from the external environment. Numerical results gave the input to insight on the correct dimensioning and sizing of a refrigerated transport cooling unit, but were also utilized to compare the thermal performance of the state of the art (i.e. cooling unit operating with R-134a as the working fluid) with the innovation proposed in this thesis. Moreover, numerical simulations were able to assess the main heat fluxes that influence the performance of the refrigerating system during operation.