Page 349 - Defrosting for Air Source Heat Pump

Conclusions and future work 345

as the most fundamental reference parameter. This fundamental study is meaningful
for reaching an intelligent ASHP unit, which is reported in Chapter 9.
Based on the aforementioned five achievements, the last one was reached, which is
two technoeconomic analyses on frosting/defrosting operations for an optimized
ASHP unit. The first one considered two conditions with trays installed in its multi-
circuit outdoor coil. For the second one, four conditions, with trays and/or valves
installed in the outdoor coil, were considered. As analyzed, after the water-collecting
trays and/or valves were installed, the economic performance of an ASHP unit was
effectively improved. The initial and running costs over 1–15 years were calculated.
The effect of installing valves on the economic performance of the ASHP unit was
obviously much better than that of installing trays. After water-collecting trays were
installed, the total cost was decreased while the saved cost after the valves were
installed was increased. The payback period for the additional initial cost was also
calculated. This section is reported in Chapter 10.

11.2 Proposal for future work

Based on the results presented in this book, some potential areas for future research
related to defrosting for ASHP units are recommended as follows.

(1) System and component optimization [1]. When optimizing the original components in an
ASHP unit, more attention should be paid to the entire system’s operating performance.
Experimental or/and numerical studies on the frosting/defrosting performance for an ASHP
unit at different frost accumulations, circuit numbers, total heat exchanger areas, etc., have
still not been reported. When an ASHP unit with a horizontally installed multicircuit out-
door coil is applied to some special places, such as on the roof of a building or vehicle, its
system performance could be further studied.
(2) New methods and materials. New frost suppression measures [2] and defrosting methods
[3], taking their economic, energy and environmental performances into consideration,
should be studied. New hydrophobic materials with good durability, high thermal conduc-
tivity, and a self-cleaning function are meaningful for industry application.
(3) Model development. New defrosting models of an ASHP unit with PCM-TES, with indoor
thermal comfort at both daytime and nighttime or sleeping thermal comfort considered,
should be developed [4]. New frosting and defrosting models for a flat plate or a fin at
micro/nanoscales should be developed, for example, using the lattice Boltzmann method.
(4) Control strategy optimization. A defrosting control strategy of refrigerant distribution
adjustment should be studied so that the melted frost can have a positive effect on the
defrosting performance in an ASHP unit with a vertically installed multicircuit outdoor coil.
More indoor and outdoor environmental parameters could be considered to avoid mal-
defrosting [5]. An operation control strategy coupled with big data on climates can be
considered.
(5) Mechanism study. For some mechanical-based frost-suppression methods, such as ultra-
sonic vibration and air jet techniques, their mechanisms of frosting suppression on the
fin surface should be studied. In order to improve the FECs for a multicircuit outdoor coil,
and thus improve the system frosting/defrosting performance, the mechanism of multiphase
refrigerant distribution into each circuit during uneven heat transfer between the refrigerant
and outside frost, melted frost, and ambient air, should also be studied [1].