Page 308 - Defrosting for Air Source Heat Pump

10
Technoeconomic performances

10.1 Introduction

To further improve the RCD performance for an ASHP unit, different studies were
conducted globally, such as heating and/or dehumidifying the inlet air of the outdoor
coil, structure and dimension adjustments for the outdoor coil, fin type and surface
treatment, additional defrosting energy supply with a PCM-TES system, FEC
improvement on the surface of the outdoor coil, control strategy optimization via
refrigerant distribution adjustment, etc. For an outdoor coil in an ASHP unit, a mul-
ticircuit structure is usually used in order to enhance its heat transfer and minimize its
refrigerant pressure loss. To save the floor space, the multicircuit outdoor coil is
always vertically installed in its practical application.
For an ASHP unit with a vertically installed multicircuit outdoor coil, it was easy to
find the uneven defrosting phenomenon in the open literature. When the other circuit
was waiting for the lowest circuit to terminate its defrosting process, the heat transfer
between the hot refrigerant tube and fins and the ambient cold air would consume a lot
of energy [1]. Not only would defrosting efficiency be degraded, but also the
defrosting duration would be prolonged and the indoor thermal comfort level affected.
As demonstrated by previous experimental and numerical studies, the downward
flowing of melted frost along the surface of the outdoor coil was one of the important
reasons for uneven defrosting [2, 3]. At the same time, a lower FEC would also affect
RCD. It was reported that when the FEC was increased from 82.6% to 96.6%, the
defrosting efficiency could increase from 42.0% to 48.7%. After the negative effects
of melted frost flowing down were eliminated, the defrosting efficiency was increased
by about 5.7% as the FEC was increased from 79.4% to 96.6%. Furthermore, it was
proved that frosting COP was increased from 4.10 to 4.26 as the FEC was increased
from 75.7% to 90.5% [4]. Among all the previous experimental studies, a series of
valves was used to adjust the refrigerant distribution, according to the tube surface
temperatures at the circuit exits.
However, for a new technology or innovation, a technoeconomic analysis is very
important and should always be carried out before its wide application [5–7]. The total
cost of technoeconomic proposed new ASHP unit is increased by the additional
investment of valves, and a longer payback period is expected. To solve this problem,
the economy has to be improved based on the characteristics of the ASHP unit.
Although many methods were used to improve the operating performance of an ASHP
unit [8, 9], especially to optimize its RCD performance [10, 11], only a few studies
with a technoeconomic analysis were reported [12]. Horton et al. gave an economic
analysis when evaluating a high-performance cold-climate heat pump [13].As
reported, the maximum additional cost of the system changes for the Minneapolis
location in the United States was $430 for a vapor-injected system and $391 for an
oil-flooded system. These estimates were based on an assumed 3-year simple payback
Defrosting for Air Source Heat Pump. https://doi.org/10.1016/B978-0-08-102517-8.00010-2
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