Page 45 - Defrosting for Air Source Heat Pump

Previous related work: A review 35

2.5 Defrosting control strategy

Frosting would seriously affect the operating performance of an ASHP unit by reduc-
ing its COP and output heating capacity. Hence, periodic defrosting is necessary.
Defrosting initiation and termination control strategies impact system reliability
and energy efficiency [111]. Studies on selecting suitable control parameters for
defrosting initiation and termination were therefore carried out.

2.5.1 Defrosting initiation
There are two types of control strategies to start a defrosting operation: time-based and
demand-based. For the former, the defrosting start is controlled by a preset timer. Due
to the advantages of simplicity and low cost, many of the earlier ASHP units employed
the time-based defrosting start method. Usually, for every 60–90 min of frosting oper-
ation, a defrosting operation would be initiated [12, 96, 110]. However, the perfor-
mances of these earlier ASHP units could well suffer from some unnecessary
defrosting operations, resulting in a degraded operational efficiency. A time-based
defrosting start results in two typical mal-defrosting problems: unnecessary defrosting
when no or little frost has accumulated on the surface of the outdoor coil, and no
defrosting when frost presents. As reported from a field test experiment, no mal-
defrosting was initiated when more than 60% of the outdoor coil surface area was
frosted after 5 days of operation [112]. During this 5-day frosting period, the system
COP was significantly degraded at only 2.3 under an environmental temperature of
7.9°C. Comparing the test data before and after frosting, it was found that such a
mal-defrosting phenomenon decreased the system COP by up to 40.4% and the
heating capacity by up to 43.4%. To avoid mal-defrosting, and thus to improve the
energy efficiency of the system, the temperature and/or pressure parameters were con-
sidered in the time-based defrosting start method. When the parameters reach preset
values, the timing starts. Currently, these time-based defrosting start methods are
widely used in applications.
A further control strategy applied to initiate a defrosting cycle was first proposed by
Eckman, and called the demand defrosting start [27]. As the name suggests, it could
start a defrosting operation only when needed. A demand defrosting start control strat-
egy defrosts the display cabinet when sufficient frost is formed, adversely affecting
the operating performance. The use of the demand-based defrosting start method
would lead to: (1) better temperature control, (2) increased product quality and life,
(3) reduced product losses, and (4) significant energy savings. Therefore, when apply-
ing this strategy, an ASHP unit would start defrosting only when an adequate frost
buildup was detected. Thus, it was important to accurately detect the presence and
growth of frost. A number of frost-detecting techniques have been developed over
the years, including: (1) measuring the thermal conductivity of ice, (2) calculating
the air pressure differential across an evaporator, (3) calculating the degree of refrig-
erant superheat [113], (4) sensing the temperature difference between the air and evap-
orator surface, and (5) sensing the outdoor air fan power [12, 27].