Page 11 - Defrosting for Air Source Heat Pump

Introduction 7

7. Technoeconomic performance analysis on the optimization adjustments of the ASHP unit,
based on the previous frosting and defrosting experimental results. After a series of optimi-
zation adjustments is taken in the previous experimental study, including adjusting the
refrigerant distribution by installing valves and local drainage of melted frost with water col-
lecting trays, their practical applications are considered, and thus their economic perfor-
mances during frosting and defrosting are quantitatively analyzed. The outcomes are also
expected to be useful in the pricing or governmental subsidy policy for ASHP units.

1.4 Book outline

This book consists of 11 chapters and 6 appendices, as follows.
Chapter 1 introduces the background, the classifications and advantages of heat
pump technology, and frosting and defrosting issues for an ASHP unit; states the moti-
vation to develop the new methods or technologies to improve the frosting and
defrosting performances of an ASHP unit; and elaborates the objectives and scopes
of the research work presented, followed by the outline of this book.
Chapter 2 reviews the published research works on frost-suppression measures,
defrosting methods, improvements for reverse cycle defrosting, and the defrosting ini-
tiation and termination control strategies for ASHP units, respectively.
Chapter 3 presents experimental studies of the uneven defrosting on the outdoor
coil in an ASHP unit, having both two working circuits and three working circuits.
In each set of studies, both the conditions of with and without water collecting trays
installed to locally drain the melted frost away during defrosting were considered. The
effects of melted frost downward flowing due to gravity during defrosting along the
multicircuit outdoor coil on the operating performance of the ASHP unit are qualita-
tively and quantitatively investigated.
Chapter 4 develops two sets of semiempirical mathematical models for RCD for an
ASHP unit having a multicircuit outdoor coil, with and without the consideration of
melted frost downward flowing along the surface of the outdoor coil. A defrosting
process is divided into four stages according to the defrosting physical timeline,
and the models are validated with the experimental results reported in Chapter 3.
Using the validated models, the heat supply and energy consumption during defrosting
could be analyzed, and some parameters that are difficult or even hardly possible to be
measured are predicted. The two models are further applied to alleviating uneven
defrosting for ASHP units with different proposed control strategies.
Chapter 5 reports the investigations of the effects of the elimination and retention of
melted frost due to surface tension on uneven defrosting in an ASHP unit having a
horizontally installed multicircuit outdoor coil. For a horizontally installed multi-
circuit outdoor coil, the flow path and direction of melted frost are changed, so that
uneven defrosting is expected to be alleviated and the defrosting performance thus
improved. In addition, some melted frost can also be retained on the downside of
the outdoor coil surface due to surface tension during defrosting, which would con-
sume some thermal energy.