Page 79 - Defrosting for Air Source Heat Pump

Modeling study on uneven 4

defrosting

4.1 Introduction

When the surface temperature of the outdoor coil in an ASHP unit is below both the air
dewpoint and the freezing point of water, frost can form and accumulate over the sur-
face of the outdoor coil, which is usually of a multicircuit structure in order to min-
imize its refrigerant pressure loss and enhance the heat transfer between the refrigerant
and outdoor air. However, frosting adversely affects the operational performance and
hence the energy efficiency of an ASHP unit; therefore, periodic defrosting is neces-
sary. Currently, the most widely used standard defrosting method for ASHP units is
RCD. During RCD, a space heating ASHP unit actually cools a space, degrading the
indoor thermal comfort while consuming electrical energy for melting frost. There-
fore, a defrosting period should be controlled to be as short as possible. In order to
better improve the defrosting performance for an ASHP unit, a number of previous
experimental studies were carried out to examine various ways for better defrosting
performances. These included optimizing the structure of an outdoor coil [1,2], fin
space adjustment [3], fin surface treatment [4], heating up the liquid refrigerant in
an accumulator [5], and providing heat for defrosting using PCM [6,7].
On the other hand, uneven defrosting was reported in limited previous experimen-
tal studies. O’Neal et al. [8] and Qu et al. [9] both investigated the transient defrosting
performances of ASHP units, each with a vertically installed four-parallel refrigerant
circuit outdoor coil. It was reported that when a defrosting process was terminated, the
surface temperature at the exit of the lowest circuit was much lower than that at the
exit of the highest circuit. In the study by Wang et al. [10], it was shown that for a
seven-circuit outdoor coil, at 6 min into defrosting, the surface of the down refrigerant
circuits, which accounted for almost 1/4 of the entire surface area, was still covered by
frost while that of the up-circuits was already free of frost.
An efficient alternative to experimentally studying the defrosting performance in
an ASHP unit is via a numerical approach and therefore, the last two decades saw a
growing number of modeling studies [11–21] on defrosting performance, although
most of them were on defrosting methods [17–23] rather than RCD. Krakow et al.
[13,14] developed an RCD model for an outdoor coil. In this model, a frost-melting
process on the outdoor coil surface was divided into four stages: preheating, melting,
vaporizing, and dry heating. Krakow et al. [15,16] later presented an idealized RCD
model for an ASHP unit with a receiver. Two parameters—system performance coef-
ficient and coil efficiency—were defined and used when evaluating defrosting perfor-
mance. Dopazo et al. [23] developed a detailed transient simulation model for hot-gas
bypass defrosting in an air-cooled evaporator. As compared to the model by Krakow
et al. [13,14], six stages were used in this model: preheating, tube frost melting, fin
Defrosting for Air Source Heat Pump. https://doi.org/10.1016/B978-0-08-102517-8.00004-7
© 2019 Elsevier Ltd. All rights reserved.