| Resumo : |
Following market trends and environmental concerns the transportation sector has turned to electric propulsion. Electric vehicles (EV) are a promising alternative to internal combustion engine vehicles. However, EVs have their own design challenges that need study and optimization. Particularly, the battery is sensitive to temperature as to avoid thermal runaway and lifespan reduction. There are several technologies for battery thermal control, focusing on direct refrigerant cooling, a vapor-compression refrigeration system can be used to extract the battery heat and provide cabin temperature comfort through two distinct evaporators connected to the same compressor. Moreover, the usage of environmentally friendly refrigerant fluids is also envisaged. Considering all these aspects, this dissertation proposes an endoreversible thermodynamic modeling of a two-stage refrigeration system responsible for the battery and cabin cooling. Battery and cabin heat generations are considered as data inputs for the model. A positive displacement compressor with fixed volumetric and global efficiencies is modeled. The heat exchangers are discretized in zones related to the fluid phase. The mathematical model is solved numerically using the software Engineering Equation Solver (EES) with first guess data from an algorithm implemented in python. It is evaluated the effect of an intermediary heat exchanger on the system performance. It is analysed how the system respond to different operation conditions by considering varied configurations of heat demand and heat transfer effectiveness. The distribution of the total global thermal conductance between heat exchangers is evaluated as a manner to align good performance and system compactness. A second law analysis is performed by calculating the amount of entropy generated by each component and identifying the amount of exergy destruction associated with each component and with the refrigerant. All the results are compared for refrigerants R-134a, R-1234yf, R-1234ze(E) and R-32. It is concluded that a system with cabin and battery evaporators at the same pressure is more efficient that a system with evaporators at different pressure. |