18 October 2011

A two-phase thermosiphon defrosting technique for air-source heat pumps

Air-source heat pumps loose performance during winter because of low ambient temperatures and frosting. The most common defrosting technique is to reverse the cycle. Unfortunately it provokes a break in the heat production and the defrosting energy is drawn from the heat stock previously constituted. This article presents the design of a heat pump prototype for simultaneous heating and cooling (named HPS). Its refrigeration circuit involves a piece of refrigeration circuit that could be modified and implemented to standard air-source
heat pumps during retrofit or, maintenance in order to carry out defrosting with better performance. It uses a water tank that recovers the subcooling energy of the refrigerant at
first and is subsequently used as a cold source for evaporation. The second part of the sequence liberates the air evaporator for defrosting. Between two evaporators (air-torefrigerant and water-to-refrigerant) at different temperatures, a thermosiphon forms. A
supplementary amount of vapour flows out of the water evaporator and migrates towards the colder inside surface of the air evaporator tubes in thermal contact with the frost layer. The
vapour, while condensing, brings the defrosting energy. The liquid returns back to the water evaporator by gravity. This defrosting system was observed by means of infrared thermography and tested experimentally on a heat pump prototype. It proved very efficient as
defrosting time was short. Using this defrosting technique ensures:
– a continuous heat production with even better performance while defrosting thanks to the higher evaporating temperature
– more frequent defrosting sequences because more easily activated, impacting on lower frost thickness and higher mean heat transfer coefficients.
A numerical study was carried out to assess the performance improvement achieved on a heating sequence. Simulations show a COP and an exergetic efficiency improvement
respectively of 12 % and 18 %.