Modelling and Power Optimization of a Hybrid Vehicle Refrigeration System
Abstract: This thesis is concerned with the modelling and power optimization of a vapor compression refrigeration system in a hybrid vehicle. The system consists of a compressor, a condenser, two expansion valves, an evaporator and a chiller. The achieved model describes the states of the system and the total power consumption of the components when the system is in balance, i.e. there is no time dependencies of the variables. The model is based on thermodynamic laws and properties together with empirical parameters which are estimated from given data of a refrigeration system. A comparison is made between the model and the given data, which shows good conformance. The model is optimized with respect to the system power consumption for different cooling demands at the evaporator and chiller, and the optimal solutions describes the values of system state variables and the control variables. Three different variations of the optimization problem have been examined, the first one simulates a stationary vehicle, the second one a moving vehicle and the third one simulates a stationary vehicle with fixed speed of the evaporator fan. It has also been tested what impact the ambient air and glycol temperatures has on the optimal solution. The results of the optimization problems shows that when increasing the demanded cooling power at the evaporator, both the compressor frequency and the evaporator air flow increases, but the air flow increases faster to its maxi-mum capacity since evaporator fan consumes less power than the compressor. When increasing the cooling demands of the chiller we see the same behavior, but when the evaporator cooling demands are much higher than the chiller’s, the compressor frequency are determined solely by the evaporator cooling demand. For the case of the moving vehicle, the air flow through the condenser is higher without requiring any power from the condenser fan. This results in a lower total power consumption, thanks to the zero condenser fan power and a reduced compressor power as a result of the lower condenser pressure. For the case when fixing the evaporator fan effect, the system produces an evaporator cooling power of a minimum 3 kW even though it is not demanded. Apart from that, the results looks similar to the normal case. The results also showed that the values of the ambient temperatures have a great impact on the optimal solutions.
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