Outdoor cabinets containing power electronics components need to be cooled effectively and at the same time protected from outside air, which may contain moisture and various kinds of dirt that would reduce the reliability of the electronics. Air-to-air heat exchangers are widely used in the industry as they are cost-competitive and easy to install and maintain. On the other hand, they are inefficient and bulky. ABB holds a patent on a cost-effective modular compact thermosyphon-based air-to-air heat exchanger for power electronics cabinets. This technology uses numerous multiport extruded tubes with capillary-sized channels disposed in parallel to achieve the desired compactness. The heat exchanger is made of a stack of thermosyphon units to cope with the required heat loads. The experimental performance of this novel power electronics cooling system with R134a was measured for a single unit and a stack of thermosyphons. The influence of different parameters such as the heat load, fluid filling ratio, air temperature, and flow rate were investigated. A numerical model was developed in order to predict the performance of the thermosyphon unit and stack for various and changing operating conditions. Prediction shows good agreement with the experimental results.