Page 14 - Defrosting for Air Source Heat Pump

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Previous related work: A review

2.1 Introduction

A heat pump unit is an environmentally friendly and reliable means to maintain an appro-
priate thermal comfort level in an indoor space, and can be used for both space heating and
cooling at a high operating efficiency. During a cooling season, it transfers heat from an
indoor space to a heat sink, in the same way an air conditioner does. During a heating
season, it extracts thermal energy from a heat source, and delivers the extracted thermal
energy to a heated indoor space. From a global point of view, 90% of the worldwide pop-
ulation resides in regions where heat pump units can be suitably used for indoor environ-
mental control [1–3]. Compared with traditional space heating and/or heat generation
methods using coal or electricity, studies have shown the potential of using heat pump
units to help drastically reduce greenhouse gases, in particular CO 2 emissions. With
the rising cost of energy at the forefront of world attention, there has been a growing inter-
est in using heat pump technology as an energy-saving means.
A number of heat sources are available for space heating heat pump units such as
air, underground water, and soil. Among these heat sources, air and water are the most
common ones for space heating heat pump units. Therefore, air-to-air heat pump units,
air-to-water heat pump units, water-to-air heat pump units, and water-to-water heat
pump units are commonly found in buildings or industry. Among them, ASHP units
are relatively easy and inexpensive to install, and have therefore been the most widely
used types of heat pump units for many years. An ASHP unit actually consists of vapor
compression refrigeration, a condenser or an indoor coil, and an evaporator or outdoor
coil to transfer heat from one place to the other.
Heat pump technology is based on the Carnot cycle, which was presented by Carnot in
1824. With nearly 200 years of history, this technology is very mature. However, when an
o
ASHP unit works at the heating mode in winter, at an ambient air temperature of 7 Cto
o
5 C and relative humidity (RH) higher than 65%, frost is likely deposited and a frost layer
is formed on the surface of its outdoor coil [4]. Frosting increases both heat transfer resis-
tance and air flow passage resistance during the heating operation, and thus adversely
degrades the system performance, or even results in an undesired shutdown. Therefore,
extensive experimental and theoretical investigations have been carried out to study
ASHP units’ operating performances under frosting and/or defrosting conditions.
Related studies have become very hot in recent years. In addition, the server air
pollution in winter and the corresponding coal to electricity policy in China have also
motivated related interests and research work in the applications of ASHP units.
Review articles have been published on the topics of domestic heat pumps [5, 6],
air-to-air heat exchangers [7–9], frost-suppression methods [10], defrosting methods
[11, 12], etc. To provide an overview of available studies for researchers, product
developers, and policy makers, a review and a comparative analysis of the available
literature from 2000 to 2017 on frosting/defrosting studies for ASHP units are
Defrosting for Air Source Heat Pump. https://doi.org/10.1016/B978-0-08-102517-8.00002-3
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