Page 21 - Defrosting for Air Source Heat Pump

16 Defrosting for Air Source Heat Pump

inlet air was not economical in regions with long periods of very low outdoor air tem-
peratures, at 54°Cto10°C [27]. Therefore, to improve the economy of the ASHP
units, the energy used in preheating the inlet air should come from waste heat, such
as the heat in the exhausted indoor air or waste hot water.

2.2.1.3 Increasing the inlet airflow rate to an outdoor coil

Increasing the inlet airflow rate to an outdoor coil is also an external frost-suppression
measure. An experimental study on frost formation on a finned-tube heat evaporator
considering fan characteristics was conducted by Da Silva et al. [28]. The study results
demonstrated that airflow rate reduction was a dominant factor for the drop in the evap-
orator’s capacity. It was further suggested that the fan evaporator should be treated as a
coupled system under frosting conditions. To predict the performance of an outdoor coil
considering airflow reduction due to frost growth, a numerical model was developed
and validated by Ye and Lee [29]. In this study, the frost layer was assumed to be evenly
distributed on the surface of the heat exchanger. Results showed that the simulated heat
transfer rates and the accumulated frost mass agreed well with the experimental data by
7% and 9%, respectively. In practice, for better frost suppression, the above air param-
eters can be changed altogether. The changes in different ambient air parameters would
influence the rate of frost suppression. However, an experimental investigation on the
adverse effect of frost formation on a microchannel evaporator was undertaken by
Moallem et al. [30], suggesting that the air face velocity of the evaporator impacted less
significantly on the rate of frost growth. In addition, increasing the fan power input and
the noise level may be hardly avoided.

2.2.2 Removing frost with additional equipment
A further external frost-suppression measure is to destroy frost with additional equip-
ment, such as ultrasonic vibration or air jet techniques. Unlike the aforementioned
frost-suppression measures, no heat but only mechanical energy is used to destroy
the frost formation and growth.

2.2.2.1 Ultrasonic vibration technique
The ultrasonic technique was first used for frost suppression by Yan et al. [31] and Li
et al. [32]. As reported, the frost formation process on a flat surface was remarkably
restrained due to the effect of the ultrasound. Droplets with ultrasound are smaller than
those without ultrasound. After quantitative analysis of the sizes, the frost coverage
was all less than 52% of the coil surface with ultrasound, as compared to more than
65% without ultrasound. Frost crystals and frost branches on the ice layer could be
fractured and removed more effectively [33]. Using intermittent ultrasonic vibrations,
an experimental study on the defrosting performance of a finned-tube evaporator was
carried out by Tan et al. [34]. The study results indicated that the intermittent ultra-
sonic vibrations could effectively remove the frost accumulated on the fin surface. The
energy consumption for defrosting the ASHP unit decreased by about 3.14%–5.46%,