Page 47 - Defrosting for Air Source Heat Pump

Previous related work: A review 37

outdoor coil, or the defrosting operation time duration [12]. Although the control strat-
egy of a time-based defrosting start is widely used, a defrosting operation is not always
terminated based on time duration. Currently, the most-used method for terminating a
defrosting operation is based on the tube surface temperature of an outdoor coil.
A temperature sensor is usually placed on the tube surface at the exit of the lowest
liquid-line circuit of a vertically installed multicircuit outdoor coil [96].
A defrosting operation will be terminated once a preset temperature is reached. It
is obvious that when the preset temperature is higher or lower, a defrosting duration
would be prolonged or more residual water is left, respectively. Both result in potential
energy waste for an ASHP unit, or even degrading the indoor thermal comfort. How-
ever, there is no standard defrosting termination temperature (DTT) or even a fixed
range given in the application, due to the diversity of equipment and operating cli-
mates. Different DTT settings were used and reported in 2000–2017, from 10°C
[126] to 50°C [127], as summarized in Table 2.10. Obviously, the temperature range
covering 40°C is too big. However, in the open literature, no relative study on
defrosting termination temperature was reported, or was this fundamental problem
even pointed out.
Different strategies to start and end defrosting were studied to improve the ASHP
system operating performance at an entire frosting-defrosting cycle. Currently, devel-
oped time-based control strategies are widely used to start a defrosting cycle, due to
their advantages of simplicity and reliability, although the accuracy remains

Table 2.10 DTT settings for ASHP units (2000–2017)
Circuit Capacity
Item DTT (°C) number (kW) Year Author

1 10 12 55 (Cooling) 2009 Huang et al. [126]
2 12 2 8.82 (Cooling) 2004 Ding et al. [128]
3 15 4 55–350 2013 Wang et al. [129]
(Heating)
4 18 4 6.8 (Heating) 2010 Qu et al. [90]
5 20 / / 2005 Cho et al. [130]
6 20 12 16 (Cooling) 2011 Choi et al. [88]
7 22 12 50 (Cooling) 2004 Huang et al. [131]
8 24 12 55 (Cooling) 2007 Huang et al. [132]
9 24 4 6.8 (Heating) 2012 Qu et al. [96]
10 25 2 37.5 (Heating) 2015 Dong et al. [98]
11 26 2 4.8 (Heating) 2012 Dong et al. [110]
12 30 4 0.88 2003 Liu et al. [23]
(Compressor)
13 33 2 2.8 (Cooling) 2012 Dong et al. [133]
14 35 2 2.5 (Heating) 2011 Dong et al. [102]
15 50 2 2.5 (Heating) 2011 Hu WJ [127]