Page 122 - Defrosting for Air Source Heat Pump

Investigation of effect on uneven 5

defrosting performance

5.1 Introduction

With the growing demand for electricity worldwide, environmental aspects in connec-
tion with energy consumption, such as global warming, ozone layer depletion, and
high-levels of pollution, especially the PM 2.5 air pollution in Beijing in China, have
become a main concern while heavily influencing global energy policy. It is necessary
to emphasize the use of emerging and well-known renewable energy resources and
different energy conservation approaches. An ASHP unit, utilizing low-grade energy
in the air as a source, has the advantages of simple operation, high efficiency, no pol-
lution, ability to provide both cooling and heating, etc. [1]. Accordingly, as a key tech-
nology under the clean development mechanism (CDM) to mitigate climate change
and avoid global warming [2], it has become widely used as cooling and heating
sources for heating, ventilation, and air-conditioning over the recent decades [3].
However, when operated at heating mode under an extremely cold and high humidity
environment, frost would appear and accumulate over the outdoor coil’s surface in an
ASHP unit, which severely deteriorates the system operating performance. Therefore,
it is necessary to implement periodical defrosting to maintain its normal operation.
Currently, there are many defrosting methods investigated for ASHP units, and the
most widely used standard defrosting method is RCD. When an ASHP unit is operated
at RCD mode, its outdoor coil, which is usually installed vertically for space saving,
acts as a condenser and the indoor coil acts as an evaporator. On the other hand, in
order to minimize the refrigerant pressure loss along the tube inside and enhance
the heat transfer between the inside refrigerant and the outside ambient air via tubes
and fins, a multicircuit outdoor coil is usually used in ASHP units. The downward-
flowing melted frost helps form or reinforce a water layer between the frost and
the coil surface, which introduces a thermal resistance [4] and thus reduces the heat
transfer between the two [5]. It is reported that when defrosting at the top circuits is
terminated, the bottom ones are still covered with frost [6]. Also, when the tube sur-
face temperature at the exit of the top circuit reaches the preset defrosting termination
temperature, the temperature of the bottom circuit is much lower [7–10]. Thereafter,
the negative effects of downward-flowing melted frost due to gravity are demon-
strated [7, 11, 12], with water-collecting trays installed between circuits to improve
defrosting efficiency.
However, for a vertically installed multicircuit outdoor coil, it is hard to avoid the
uneven defrosting phenomenon. This means it is nearly impossible for each circuit to
reach the preset defrosting termination temperature at the same time. Thus, the energy
consumption due to uneven defrosting can hardly be saved. Consequently, avoiding
uneven defrosting for a multicircuit outdoor coil in an ASHP unit becomes a technical
Defrosting for Air Source Heat Pump. https://doi.org/10.1016/B978-0-08-102517-8.00005-9
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