Page 260 - Defrosting for Air Source Heat Pump

Energy transfer during defrosting 255

Outdoor space Indoor space Outdoor space Indoor space

Outdoor coil EEV Indoor coil coil TES-out EEV coil Indoor

Compressor Outdoor Compressor TES-in

(A) (B)

Outdoor space Indoor space Outdoor space Indoor space

EEV EEV coil
coil coil Indoor
Outdoor Compressor Indoor coil TES-in-out Outdoor Compressor TES-in-out

(D) (C)

Fig. 8.26 Illustrations of two coils and the TES unit in an ASHP unit. (A) Two coils. (B) Two
coils and two TES units. (C) One TES unit in series. (D) One TES unit in parallel.

Here, the design method for a PCM-TES could be introduced. As an ASHP unit
takes Q from the outside air, the total energy input to the TES is COP Q. The effi-
ciencies of energy storage and release are n1 and n2, respectively. The two values are
percentages, in the range of 0%–100%. During defrosting, the total energy consumed
from the indoor coil is Q d . There is the balance of Q d ¼ COP Q η 1 η 2 . So, there are
four directions to limit the volume of TES: (1) improve the energy storage efficiency,
(2) improve the energy release efficiency, (3) decrease the defrosting energy con-
sumption, or improve defrosting efficiency, and (4) increase the energy storage per
unit volume. Clearly, when we consider optimizing the ASHP unit, the fundamental
study could guide us to find the solution.

8.5 Concluding remarks

In this chapter, experimental studies on the energy transfer mechanism in an ASHP
unit and the effects of indoor and outdoor coils’ MES on defrosting performance were
carried out, with the following conclusions: (1) Four types of heat supply and five
types of energy consumption were quantitatively analyzed. The heat supplies during
defrosting included the thermal energy of the indoor air, the MES of the indoor coil,
the power input to the indoor air fan and compressor, and the heat consumption during
defrosting, including heating the ambient air, heating the melted frost, heating the out-
door coil metal, and vaporizing the retained melting frost; (2) The MES effects on