Page 235 - Defrosting for Air Source Heat Pump

230 Defrosting for Air Source Heat Pump

using a flow hood that had a 16-point velocity grid located at the center of a
400 400 mm air duct that was 600 mm long. Air temperature and humidity down-
stream of the outdoor coil were measured by a hygrosensor located inside an air duct
900 mm downstream of the outdoor coil.
In Eqs. (8.1), (8.2), Q i, a is the thermal energy transferred from the indoor air, which
was evaluated by:

Z
t d
X
ð
Q i,a ¼ c i,a m i,a dT ¼ c i,a ρ V i,a Δt T ind,in T ind,out Þ (8.8)
i,a
0
where c i, a is the specific heat of the indoor air and m i, a the mass rate of the indoor air.
ρ i, a is the density of air in the indoor heated space, and V i, a the volumetric flow rate of
air passing through the indoor coil. T ind, in and T ind, out are the measured air temper-
ature at the inlet and outlet of the indoor coil, respectively.
All the system operating parameters, such as temperature, pressure, relative humid-
ity, refrigerant mass flow rate, voltage, current, etc., were measured in real time. All
sensors and measuring devices were able to output direct current signals of 4–20 mA
or 1–5 V to a DAS for logging and recording. All the measured data throughout both
the frosting and defrosting periods were collected and recorded by the DAS at an inter-
val of 5 s. During defrosting, photos of surface conditions of the outdoor coil were also
taken at an interval of 10 s. The measuring accuracy for various sensors/instruments
used in the experimental ASHP unit was summarized in Chapter 3. Experimental pro-
cedures and conditions, the calculated relative standard errors for the four calculated
parameters, the total energy supply for defrosting, and the total energy consumption
during defrosting were also detailed.
(2) Experimental cases

A series of experimental works using the experimental ASHP unit was carried out to
investigate the energy transfer process and the effect of MES on system defrosting
performance. In order to obtain meaningful experimental results, first it was necessary
to ensure that the MES was different during RCD. MES is decided by the metal tem-
perature difference, specific heat, and total mass. In this study, frosting/defrosting
modes fixed the lowest/highest metal temperature of the outdoor coil and the
highest/lowest value of the indoor coil, respectively. That means the metal tempera-
ture difference is unchangeable. Specific heat is also constant, decided by the type of
material. Therefore, only the total metal mass of the indoor or outdoor coil could be
adjusted. For an ASHP unit with a multicircuit outdoor coil, it could be reached by
changing the working circuit number, with the help of the solenoid valves installed
at the outlet refrigerant pipe of each circuit. Total refrigerant mass flow quality is con-
stant when different numbers of circuits are working at defrosting mode.
Second, for each circuit, frost accumulation over their surfaces should be similar at
different experimental cases. In this study, this was carried out by adjusting the open-
ing degrees of the manual stop valves installed at the inlet of each refrigerant pipe, and
thus adjusting the refrigerant mass flow rate into each circuit. With this operational
method, the FEC was controlled at higher than 90%. For each circuit, the frost accu-
mulation difference was less than 5%.