Page 7 - Defrosting for Air Source Heat Pump

Introduction 3

pump’s evaporator. A hybrid system can produce thermal energy in a more efficient
and less expensive way. A further typical hybrid system is an air/water-brine/water
heat pump. Unlike other hybrid systems, this system usually utilizes both conven-
tional and renewable energy sources. For example, it uses air and geothermal heat
in a single compact device. There are two evaporators in an air/water-brine/water
heat pump: an outdoor air evaporator and a brine evaporator. Both evaporators are
connected to the heat pump cycle to allow the use of the most economical heating
source according to the actual operating conditions, air, geothermal heat, or both.

1.2 Frosting and defrosting

As previously mentioned, when an ASHP unit works at heating mode in winter to take
thermal energy from the ambient air, the surface temperature of its outdoor coil can be
much lower than both the dewpoint of air and the freezing temperature of water.
Therefore, frost would form and accumulate on an outdoor coil surface. In fact, the
formation of frost on the surfaces of plant leaves is a well-known phenomenon in
nature. This phenomenon was recorded in an ancient Chinese poem, The Reed.
The second verse of this love poem reads, “Dew and frost gleam,” meaning that
the dew on the surface of reed leaves was changed into frost. The fourth verse reads,
“Beyond the stream,” meaning that the reed was near a river where the air humidity
was high. The poem may describe the natural phenomenon on an autumn day at dusk,
when air temperature was low. Clearly, the following three conditions–cold surface,
high air humidity, and low air temperature–are the prerequisites for frost to be formed.
Therefore, it can be easily understood that frosting is commonly observed in refrig-
eration fields because of operating conditions [1].
Frost deposition is inevitable once moist air is exposed to a cold surface having a
temperature below the water triple point and the air dewpoint [2]. To clearly understand
the frosting mechanism, the process of frost formation on a cold flat plate surface is
shown in Fig. 1.1 [3]. It could be divided into four periods according to the growth time-
line: (1) the droplet condensation period, (2) the solidified liquid tip growth period, (3)
the frost layer growth period, and (4) the frost layer full growth period. During the drop-
let condensation period, the condensing droplets at a subcooling state are formed on the
cold surface. The droplet nucleation occurs first, followed by the coalescence of the
droplets. As the vapor-liquid and liquid-solid phase changes take place, the droplets
merge and solidify; the diameters of the solid droplets increase significantly.

h Droplet Crystal Tip Vertically Horizontally Tips Hollow frost Frost layer Ice layer
nucleation Coalescence Droplet growth growth
d
(1) Droplet condensation (2) Solidified liquid tip- (3) Frost layer growth period (4) Frost layer full growth
period growth period period
t c t t
t
Fig. 1.1 The four periods in a frost growth process on the surface of a cold plate.