Defrosting sequences usually decrease the winter performance of air-source heat pumps. The drawbacks of most common defrosting techniques are a break in the heat production and a performance loss using the heat stock previously constituted as a source for evaporation. This article presents a heat pump for simultaneous heating and cooling (named HPS that faces frosting problems using an alternating operating sequence. Step 1: a subcooler connected to a water tank is located after the condenser in the refrigerant circuit to recover some energy by subcooling of the refrigeration during a normal heating mode. Step 2: the tank water at a higher temperature than ambient air is used as a source for a water evaporator. This strategy liberates the air evaporator for defrosting. Between the two evaporators at different temperatures (and local vapor pressures) a thermosiphon forms. A supplementary amount of vapor coming out from the water evaporator migrates towards the cooler inside surface of the air evaporator tubes in thermal contact with the frost layer. Vapor condenses when exchanging energy with the frost layer, and defrosting occurs. The liquid returns back to the water evaporator-compressor line by gravity. This system was tested experimentally on a HPS prototype and proved very efficient; defrosting time is short. Using this defrosting technique ensures:

• A continuous heat production with good performance while defrosting thanks to the higher evaporating temperatures,
• More frequent defrosting sequences because of easy activation, impacting on lower levels of frost layer and higher mean heat transfer coefficients.

An experimental study including infrared video recording describes the operation of the two-phase thermosiphon. Then a numerical model built to simulate the phenomenon using EES software was fitted by experimental results. Finally some paths for optimization of the two-phase thermosiphon defrosting are given.