Page 29 - Defrosting for Air Source Heat Pump

22 Defrosting for Air Source Heat Pump

Nguyen et al. [71] evaluated the thermal performances of a flash tank vapor-injection
cycle and that of a subcooler vapor-injection cycle using R-407C and reported their
heating COPs as 24% and 10% higher than those of a single-stage cycle, respectively.
Then, with outdoor temperatures of 20°Cto 15°C, Shao et al. [72] concluded that a
vapor-injection heat pump unit could provide enough heating capacity. At an ambient
temperature of 25°C, Ma and Zhao [73] experimentally investigated the operating
performance of an ASHP unit with a flash tank coupled with a scroll compressor
and demonstrated that the ASHP unit was more efficient than that with a subcooler
at 25°Cto 7°C.

2.2.4.2 Two-stage technique
For the two-stage technique, Wang et al. experimentally investigated a double-stage
heat pump heating system, which coupled an ASHP unit and a water source heat pump
unit [74]. Results indicated that it offered an average energy efficiency ratio up to 3.2
with a minimum of 2.5, and the average indoor temperature of 19.5°C with a minimum
of 18°C during tests. Compared with conventional ASHP systems, the operating char-
acteristics of the coupled system were greatly improved, so that it had considerable
application potential in cold regions. Li et al. [75] proposed and experimentally tested
a new frost-free ASHP system, concluding that the system could operate more effi-
ciently than a conventional ASHP unit in winter, and there was no need to periodically
defrost. At an ambient temperature of 15°C, the COP and heating capacity of a two-
stage vapor injection cycle were enhanced by 10% and 25%, respectively, as reported
by Heo et al. [76]. At an ambient temperature of 17.8°C, Wang et al. [77] found that
a maximum COP improvement of 23% for a two-stage heat pump system was
achieved. Bertsch and Groll [78] tested a specially designed R410A two-stage ASHP
unit, and a heating COP of 2.1 was observed at an ambient temperature of 30°C.
Although this technique may be employed for frost suppression in cold climates, it
makes the system more complicated than vapor injection.

2.2.4.3 Applying an external heating source
It is easy to understand that applying an external heat source could improve the system
operating performance at frosting conditions for an ASHP unit. Masaji qualitatively
demonstrated that the performances of an ASHP unit with a-kerosene fired burner
placed either near its indoor coil or under its outdoor coil could be improved at
low ambient temperatures [27]. Using the same method, Mei et al. [79] reported that
the heating capacity of an ASHP unit could be increased and the frost accumulation on
its outdoor coil suppressed by heating the liquid refrigerant in its accumulator. By
heating the liquid refrigerant, it was shown that the frequency of defrosting cycles
was reduced by a factor of 5 in Knoxville, Tennessee, in the United States, and the
indoor supply air temperature increased by 2–3°C because of the increased compres-
sor suction pressure. Different from preheating the inlet air of the outdoor coil, the
measure of applying an external heating source was to the heat refrigerant. However,
they both consumed a lot of energy. Therefore, to improve the economy of the ASHP