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2 Defrosting for Air Source Heat Pump

production processes. It is easy to understand that the evaporator side heat exchanger
might be specially made.
The use of ASHPs is advantageous. Compared with other space heating methods,
an ASHP unit does not need a plant room, but could be placed on a roof or ground at
will to save floor area and reduce the construction and installation costs. As compared
to a boiler-based space heating system, ASHPs are safe and reliable and do not pro-
duce environmental pollution. However, the performances of ASHP units would vary
with the changes in outdoor weather conditions. When the unit extracts heat from low-
temperature air, frost may be formed on the surface of its outdoor coil. To maintain
normal operation of the ASHP unit, defrosting will be necessary once adequate frost
has been accumulated, which reduces the operating efficiency and output heating
capacity of the ASHP unit.
On the other hand, a GSHP unit converts low-grade shallow geothermal energy to
high-grade thermal energy by consuming a small amount of electricity. A GSHP unit
usually consumes 1 kWh of electrical energy to produce 4.4 kWh of thermal energy.
In fact, the heat drawn from soil is in most cases the stored solar heat, and hence should
not be confused with that in direct geothermal heating. In a geothermal heating sys-
tem, a circulation water pump but not a heat pump is required because the soil tem-
perature is higher than that in a space to be heated. Therefore, such technology relies
only upon convective heat transfer. As an efficient measure against rising energy
costs, GSHP technology has attracted worldwide attention since the 1980s and has
been a hot topic in China since the late 1990s. Earlier investigations on GSHP tech-
nology focused on the performance tests for experimental GSHP systems, and tech-
nical and economic comparisons with traditional ASHP units. Later, the complex heat
and mass transfer between ground heat exchangers used in a GSHP system and sub-
surface rock and soil was investigated in great detail, with a large number of heat
transfer models developed and reported. Recently, novel hybrid GSHP units and
the determination of soil thermal properties became hot research topics.
Furthermore, a WSHP utilizes energy resources in shallow water on the Earth’s
surface, such as the solar energy and geothermal energy absorbed by groundwater,
rivers, streams, and lakes. However, river/seawater heat pumps and wastewater heat
pump systems are the most widely used systems. When the river/seawater is used as a
heat source or sink, it is always stable with an almost unlimited capacity. For example,
several big data centers were built near rivers/seas, and released heat to the water using
a heat pump or direct sea/river water cooling. However, in recent years, more and
more environmental problems, such as the death of fish and water grass or the changes
and migration of microorganisms, emerged. These further destroyed local ecological
environments. On the other hand, wastewater heat pump systems take thermal energy
from treated domestic wastewater, and would thus have no adverse impacts on local
environments.
Hybrid heat pump systems have also been widely studied and reported. A solar-
assisted heat pump integrates a heat pump and thermal solar panels in a single inte-
grated system. Typically, the two technologies are used independently to produce
hot water. However, in this integrated system, the solar thermal panel functions as a
low-temperature heat source and the heat collected in the panel is fed to the heat