Page 22 - Defrosting for Air Source Heat Pump

Previous related work: A review 17

whereas the heating capacity was increased by 2.2%–9.03% and the COP by 6.51%–
15.33%. As reported, the indoor thermal comfort level was improved. However, the
effect of using intermittent ultrasonic vibrations for frost suppression is limited
because a basic ice layer on the surface could not be removed with ultrasonic vibra-
tions. It was believed that the mechanism of ultrasonic frost suppression was mainly
attributed to high-frequency ultrasonic mechanical vibrations that could break up frost
crystals and frost layers, then frost would fall off by gravity, but not due to the ultra-
sonic cavitation effect or heat effect. These continued studies promoted the use of the
ultrasonic vibration technique for frost suppression, but its application is limited in
practice due to its high initial cost and complex control system.

2.2.2.2 Air jet technique

The air jet technique may also be an effective frost-suppression measure, and no thermal
energy is needed to melt the frost [27]. It was first applied to a horizontal single-row array
of cooled tubes immersed in a gas-solid fluidized bed. The heat transfer and defrosting
characteristics of the cooled tubes were experimentally investigated, and the fluidized
bed produced gas-solid particle impinging jets that effectively removed frost layers on
the tube surface. It had been verified that frost-free running of the cooled tubes was pos-
sible under an operating condition of inlet air temperature of 7°C, inlet air RH of 80%,
and a tube surface temperature of 17°C. Fei and Mao [35] experimentally investigated
the use of compressed air for frost suppression, and indicated that this measure could
remove frost in a timely manner. Hence, it could be applied where compressed air
was available. Furthermore, the measure of frost suppression on heat exchangers using
solid particles accelerated by an air jet impinging on the heat exchanger surfaces was stud-
ied by Sonobe et al. [36]. The study was motivated by the development of a cryogenic heat
exchanger for a hypersonic aircraft engine. However, as with the ultrasonic vibration tech-
nique, not much literature about frost suppression using air jet techniques has been iden-
tified. In addition to the disadvantages of high initial and running costs and the
inconvenience of use, the technique’s effect on frost suppression for an ASHP unit is
not well understood, which limits its practical applications.

2.2.3 Optimizing outdoor coil structure
Apart from the external frost-suppression measures described in Sections 2.2.1 and
2.2.2, a number of internal measures to suppress frosting have been developed through
optimizing the structure of an outdoor coil so as to alleviate the negative impact of
frosting on the operating performance of an ASHP unit.

2.2.3.1 Adjusting fin and tube geometry

The use of an outdoor coil having a wider fin space was first recommended to slow
down frost growth by Young and Watters et al. [37]. Then, it was experimentally
investigated by Yan et al. [38] and Sommers and Jacobi [39]. As reported, at air-side
Reynolds numbers between 500 and 1300, the air-side thermal resistance was reduced