Page 304 - Defrosting for Air Source Heat Pump

298 Defrosting for Air Source Heat Pump

Fig. 9.42 DTTs and their corresponding average fin surface temperatures.

(3) Validation and discussions
Defrosting efficiency can be used to evaluate the performance of a defrosting opera-
tion, which is also used as the validation index for the best DTT in this study. As
defined, defrosting efficiency is the ratio of the actual amount of energy consumption
required to both melt the accumulated frost and vaporize the retained melted frost to
the total amount of energy available from an outdoor coil during an entire defrosting
operation, as follows:

E m + E v
η ¼ 100% (9.1)
d
Q comp + Q id, fan + Q id,air
where E m and E v are the total heat used for melting frost and vaporizing the retained
water, respectively, and they are evaluated by:

E m ¼ M f L sf L sf ¼ 334 kJ=kg (9.2)

E v ¼ M v L v L v ¼ 2443kJ=kgð Þ (9.3)
where M f and M v are the total mass of the frost formed on the outdoor coil and the mass
of vaporized melted frost, respectively, and L sf and L v are the latent heat of frost melt-
ing and the latent heat of the evaporation of water, respectively. Also in Eq. (9.1),
Q com , Q fan , and Q air are the energy consumptions by the compressor and supply
fan, and the thermal energy from the indoor air during defrosting, respectively.
Fig. 9.43 shows the calculated defrosting efficiency for N 1 –N 6 and the time point of
175 s into defrosting as well as the corresponding values of the energy supply for