Enhancing the energy efficiency of land vehicles air conditioning units through a new evaporative membrane cooler

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Led by the growing need for energy efficiency in the air conditioning systems on land vehicles, this paper proposes an original approach that integrates evaporative cooling technologies into the existing systems. In detail, the new evaporative membrane system adds to the traditional one an auxiliary kit composed of a water reserve, a circulation pump, a heat exchanger, and an innovative capillary crossflow membrane cooler. The evaporation process is used to both pre-cool the air flow to the condenser, thus lowering the refrigerant condensation temperature, and also to supply refreshed water for pre-chilling the renewal air. The proposed architecture is particularly feasible for vehicular applications where a water reservoir already exists or where it is possible to install and maintain one. The designed membrane cooler grants high mass transfer efficiency, compactness (membrane surface-to-volume ratio around 1000 m2/m3) and manages high air flow rates while minimizing pressure loss. It is composed by organized layers of polypropylene capillaries where water flows, and plastic spacers for the airflow.First, the vapor and heat transfer mechanisms involved are investigated and the cooler’s mathematical model is described. Numerical results show that a wet-bulb effectiveness exceeding 0.85 can be reached.Subsequently, an assessment of energy performance indicators for a motorhome application is conducted within the MATLAB© Simulink environment. This involves a comparative analysis between the proposed system and the conventional standalone one. The considered contactor has a total membrane area of 8.32 m2 and handles 1200 m3/h of air and 150 kg/h of water. Results indicate that the efficiency ratio and the energy savings of the two systems decrease as the external air humidity increases even though the average energy efficiency ratio of the new system remains significantly higher. Mechanical energy savings are very interesting and decrease as the external air humidity increases: they are about 36–38 % within the humidity range [40–50 %], 26–30 % within the range [50–60 %], and 17–21 % within the range [60–70 %]. The corresponding water consumptions are between 2 kg/h and 6.5 kg/h, and they increase as air humidity decreases.Due to its simple architecture and high performance, this system is very promising in applications for motorhome, city buses, trains, and trucks.