Page 305 - Defrosting for Air Source Heat Pump

Defrosting control strategy 299

Fig. 9.43 Energy analysis and defrosting efficiency calculated at six nodes and 175 s.

defrosting (E m + E v ) and energy consumption during defrosting (Q com + Q fan + Q air ).
It is obvious that, at 175 s, the defrosting efficiency reaches its peak, at about 60.6%.
Clearly, it is demonstrated that, at 175 s, the corresponding DTT is the best value.
When the RCD process for the experimental ASHP unit terminates at this temperature,
the defrosting efficiency will reach the highest value. And thus, the system will save
the most energy during defrosting, as compared to terminating the RCD at other
temperatures.
Here, the standard of the DTT chosen work is the defrosting efficiency reaching the
highest value. However, a frosting/defrosting cycle should be comprehensively con-
sidered. That means there is a best time for the best comprehensive energy perfor-
mance for a frosting/defrosting cycle, which may not be the same as the fixed time
in this study. Also, the DTT would be different from the value in this study. In fact,
it is hard to evaluate the whole cycle because the frosting and defrosting processes are
affected by too many factors. As shown in Fig. 9.32, the different durations contained
in a frosting/defrosting experimental cycle are more problems in accurately analyzing
it. Therefore, it becomes the limit for our previous work, and is a fundamental and
interesting point for further study.
In conclusion, an experimental study on an ASHP unit with a vertically installed
three-circuit outdoor coil for its RCD termination temperature was undertaken and the
study results are reported. To find the most suitable DTT for an ASHP unit during its
RCD, a three-circuit outdoor coil was tailor-made and carried out. Basing on the even
frosting status, the defrosting parameters were collected and effectively analyzed. The
methodology for a suitable DTT or its range was first proposed and conducted by
using the tube surface temperature curve as the baseline and analyzing all other
defrosting parameters. The found suitable DTTs were validated by defrosting