Page 266 - Defrosting for Air Source Heat Pump

260 Defrosting for Air Source Heat Pump

as the experimental procedures. Then, five designed continuous cases were listed.
After these observed, measured and calculated results were presented, the system
energy and stability performances as well as the defrosting starting indexes were com-
paratively discussed and analyzed. The conclusions of this work were finally given.

9.2.1 Experimental cases
To investigate the frost accumulation influence on RCD performance, experimental
works were carried out using the described experimental ASHP unit. First, it was nec-
essary to ensure that the frost accumulations on the surface of the outdoor coil were
different to reach meaningful experimental results. It could be reached by two
methods. One is changing the relative humidity of the outdoor air in the experiment
because water vapor in the outdoor air is the source of frost. But the relative humidity
was always kept at a fixed value during frosting for comparison. Finally, another
method is changing the duration of the frosting mode, which was taken in this work.
In a real application, a time-based RCD is always started after frosting for 60–90 min.
However, the relative humidity of the ambient air is always at 40%–80%, which is
much lower than 90%. Therefore, in this study, the frosting durations were designed
to be a little shorter, at 50–70 min.
Second, for each circuit, frost accumulations over their surfaces should be similar
at different experimental cases. The FEC should be nearly the same in the different
experimental cases. For an ASHP unit with a multicircuit outdoor coil, the FEC is hard
to adjust due to many parameters affecting frosting performance, including the struc-
ture of the heat exchanger, the type of fin and its surface, the distribution of air and
refrigerant, etc. However, the refrigerant flow to each circuit could be varied by
adjusting the opening degree of the modulating valves, and thus adjusting the frost
accumulations and FEC. Therefore, in this study, a set of fixed valve opening degrees
was finally obtained by trial-and-error manual adjustments of the opening degrees of
the valves. With this operational method, the FEC was successfully controlled at
higher than 90%. For each circuit, the frost accumulation difference was less than
5%. After the opening degree was fixed, the water-collecting Cylinder D was placed
under Circuit 3 during defrosting. Then, a meaningful and effective experimental case
was conducted. Finally, the entire procedure of an experimental case is clearly illus-
trated in Fig. 9.2.
As shown in Fig. 9.2, the experimental ASHP unit was first operated at frosting
mode, at an air dry-bulb temperature of 0.5 0.2°C and a relative humidity of
90 3%. This ambient condition was jointly maintained by the use of experimental
ASHP unit, the LGUs, and humidifiers placed in the outdoor frosting space. Mean-
while, the indoor air temperature was maintained at 20 0.2°C by jointly using
the ASHP unit and the existing DX A/C system. Before frosting was terminated,
the opening degrees of valves could be adjusted as required. Before initiating the
defrosting operation, the FEC should be controlled at a value of higher than 90%.
In order to keep the system stable and safe, the four-way valve was switched to
defrosting mode after compressor shutdown for 1 min. It cost about 4 s before the
compressor was powered on again, and then a defrosting operation started. In this