Page 16 - Defrosting for Air Source Heat Pump

Previous related work: A review 13

on frost suppression. These studies are meaningful for accurately controlling the oper-
ating performances of an ASHP unit at a fixed climate environment.

2.2.1.1 Reducing inlet air humidity
Because water is the source of frost, the relationship between frosting and reduced
inlet air humidity [4] was investigated, and the leading role of inlet air humidity
among other parameters affecting frosting was demonstrated. Also, the use of a des-
iccant to significantly reduce the frosting rate was experimentally and comparatively
investigated, with and without placing a solid desiccant upstream of the inlet air [13].
Furthermore, using a solid desiccant, a novel frost-free ASHP unit was presented, and
its frost-suppression characteristics were experimentally and numerically investigated
[14–16].
Using this method, not only was inlet air humidity reduced, but also the air tem-
perature increased by absorbing the heat from the solid desiccant, so that less frost was
formed on the outdoor coil’s surface at a given frosting duration. Thereafter, there
have been increasing research interest in developing alternative dehumidifying
methods and medium. For example, liquid desiccant was used for dehumidifying inlet
air, due to a number of advantages including low air pressure drop, air cleaning effects,
and lower regeneration temperature. Table 2.1 lists 10 typical studies on solid and liq-
uid desiccants for frost suppression for ASHP units from 2000 to 2017. Clearly,
related studies are still ongoing using both experimental and numerical approaches.
For an ASHP unit, reducing inlet air humidity is the most fundamental measure to
suppress frosting. However, its initial cost is high, and the floor space of the additional
desiccant facility will be increased. In addition, desiccant regeneration costs thermal
energy, thereby increasing the running cost. These economic issues limit the practical
application of this frost-suppression measure.

2.2.1.2 Preheating inlet air
Preheating the inlet air to an outdoor coil is a simple but effective technique to reduce
or prevent frosting for an ASHP unit. Certain heating elements can be placed in the air
duct for the inlet air, so that when the outdoor air temperature drops below the frosting
point, the heating elements are activated. To prevent frosting, the air temperature at
the inlet of an outdoor coil must always be higher than the frosting point [7]. This heat-
recovery technique was first coupled with this frost-suppression method by Liu et al.
[23]. The exhausted indoor air and ambient air were mixed before entering the outdoor
coil, and thus the frosting duration was prolonged and the rate of frost growth reduced.
Then, to experimentally investigate the frost-suppression effect, an electric heater was
fixed upstream of the outdoor coil in an ASHP unit [24, 25]. It was shown that the
ASHP unit’s heating capacity was increased by 38.0% and the COP by 57.0% when
the electric heater was turned on for heating the inlet air at the outdoor air condition of
below 2/1°C (dry-bulb/wet-bulb temperature). However, the disadvantage of
preheating the inlet air in very cold regions was the high energy consumption [26].
A comparison of different frost-suppression measures suggested that preheating the