Page 46 - Defrosting for Air Source Heat Pump

36 Defrosting for Air Source Heat Pump

Based on the aforementioned direct and indirect types of frost accumulation sens-
ing technologies, recent defrosting start control strategies include: (1) measuring the
ice thickness using a holographic interferometry technique [114], (2) measuring the
frost surface temperature by an infrared thermometer [115], (3) sensing refrigerant
flow instability [116], (4) sensing frost using a photocoupler, photooptical systems,
or fiberoptic sensors [12], (5) modeling the amount of frost on the coil surface by
applying neural networks [27], and (6) calculating the effective mass-flow fraction
by the fin surface temperature [117]. Notably, the principle of photoelectric technol-
ogy for frost detection using a photocoupler was systematically investigated by Wang
et al. using experimental, numerical, and theoretical approaches [118–121]. Based on
a frosting map for ASHP units, a novel temperature-humidity-time defrosting control
strategy was further proposed by them [122]. In their given frosting map, there are
three typical regions: (1) the frosting region, (2) the condensing region, and (3) the
nonfrosting region. The frosting region was further divided into three frosting zones:
(1) the server frosting zone, (2) the moderate frosting zone, and (3) the mild frosting
zone. In addition, each zone established two subzones, I and II. The six subzones were
divided and surrounded with Curves A to E and Line 1. Therefore, the working con-
dition of an ASHP unit will be clearly presented in this frosting map, and thus the
corresponding control strategy could be suitably made. This frosting map is funda-
mental and meaningful for the intelligent control design work of the ASHP units.

2.5.2 Defrosting termination
Mal-defrosting was found and reported in many studies, which was described by
Wang et cal. [123] as follows: a defrosting operation is carried out either a long time
after a “critical” level of frost has been reached or when it is not necessary. Clearly,
this description focuses on the start of defrosting, but the conditions where a defrosting
operation is terminated earlier or later than a “critical termination time” are not con-
sidered. With respect to defrosting termination, related research is much less seen. It
should be noted that for defrosting an ASHP unit, a complete defrosting process covers
both melting the frost and drying the coil surface. Otherwise, once the ASHP unit
returns to heating operation, the melted frost retained on the outdoor coil surface
would become ice. This may change the structure of a frost layer, increasing the den-
sity and enhancing the thermal conductivity of the frost layer [124]. During reverse
cycle defrosting, not only is a great deal of energy for melting the frost and vaporizing
the melted frost off the outdoor coil surface consumed, but also the occupants’ thermal
comfort may be adversely affected [125]. Therefore, shortening a defrosting period is
always one of the defrosting control purposes for ASHP units. For example, Chinese
Standard GB/T 7725-2004 specifies that the defrosting duration for an ASHP unit
should not exceed 20% of its total working hours. Therefore, it is meaningful to accu-
rately terminate a defrosting operation.
In practical applications, an RCD operation can be terminated based on the tube or
fin surface temperature of an outdoor coil, the refrigerant pressure difference across an