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38 Defrosting for Air Source Heat Pump

questionable. For an ASHP unit with a multicircuit outdoor coil, as summarized, ter-
minating an RCD operation based on the tube surface temperature at the exit of the
lowest circuit is the most widely used. Also, it is still questionable in practical appli-
cations due to the randomness of the DTT setting at a wide temperature range. More
defrosting start and termination control strategies should be further studied.

2.6 Concluding remarks

Recent studies on frost/defrosting for ASHP units are reviewed in this chapter based
on papers published in 2000–2017. The current research status is summarized, and the
finished work, unsolved problems, potential practical applications, and their barriers
identified. It is demonstrated that the 11 listed frost-suppression measures, preheating
the inlet air with waste heat, and a fin surface coating treatment with new coating
materials are highly recommended. To resolve the frosting problem, five typical
defrosting methods are classified and discussed. Being the most widely used
defrosting method, RCD is extensively evaluated and its four enhancement methods
are thereby listed. A defrosting operation starts based on time duration, at 60–90 min,
and one or two other parameters. It is terminated when the tube surface temperature at
the exit of the lowest circuit in a multicircuit outdoor coil reaches a preset value within
the range of 10–50°C.

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