Page 24 - Defrosting for Air Source Heat Pump

Previous related work: A review 19

be reduced significantly by means of cross-linked hydrophilic polymeric coatings.
The frost thickness was decreased in the range of 10%–30% when compared to using
an uncoated metallic surface. Wu and Webb [47] investigated both frosting and
defrosting processes on hydrophilic and hydrophobic surfaces, showing that a hydro-
philic coating was preferable, with less frost and retained water on it. Cai et al. [48]
experimentally studied the frosting conditions on a normal copper surface, a hydro-
phobic coating (car wax coating) surface, and a hygroscopic coating (glycerol coating)
surface. Frost growth could be restrained by using both a hydrophobic coating and a
hygroscopic coating at the initial stage of its formation, and the thickness of the hydro-
philic coating was directly proportional to the frost-suppression effect. Similar results
have also been reported by Jhee et al. [49] and Liu et al. [50]. It was quantitatively
reported that the use of surface hydrophilic polymer paint could suppress frost forma-
tion by up to 3 h and reduce frost thickness by at least 40%, and the frost layer formed
on the coated surface was loose and could be easily removed [50].
In addition, there have been studies in an attempt to understand the mechanism of
surface treatment on frosting/defrosting. For example, Chen et al. [51] reported a hier-
archical surface that allowed interdroplet freezing wave propagation suppression and
efficient frost removal. It was demonstrated that the enhanced performances were
mainly because of the activation of the microscale edge effect on the hierarchical sur-
face, which increased the energy barrier for ice bridging as well as engendering the
liquid lubrication during defrosting. It was believed that the concept of harnessing
the surface morphology to achieve superior performances in two opposite phase tran-
sition processes might shed new light on the development of novel materials for var-
ious applications. As summarized in Table 2.2, related studies on the mechanism of
surface treatment on frost suppression at regular/nanoscales were widely published in
2000–2017.
Although optimizing the structure of outdoor coils by way of adjusting the fin space
and alignment could effectively suppress frost, changing the fin types and coating
treatment on the fin surface would make the design and manufacture of outdoor coils
more difficult, and increase the initial cost of an ASHP unit. Hence, more and more
related research work is being carried out.

2.2.4 System adjustment and optimization
Adjusting and optimizing the structure of ASHP systems may also be viewed as exter-
nal frost-suppression measures for ASHP units.

2.2.4.1 Vapor-injection technique

The vapor-injection technique has been marketed for use in room air conditioners
since 1979, but its application to ASHP units only received more attention recently,
as it can help suppress frosting in cold climates [69]. Zhnder et al. [70] tested an air-
water vapor-injection heat pump unit at an inlet air temperature of 7°C and reported
an increase in heat output of 28% and a COP improvement of 15%, respectively, as
compared to a unit without injection. Also, at an ambient temperature of 7°C,