Page 12 - Defrosting for Air Source Heat Pump

8 Defrosting for Air Source Heat Pump

Chapter 6 introduces the results of experimental studies on unevenly distributed
frost on the surface of each circuit in a multicircuit outdoor coil during both frosting
and defrosting. First, the even and uneven frosting statuses are comparatively inves-
tigated, and thus the effect of uneven frosting on COP demonstrated. Then, the inves-
tigations on defrosting performance are carried out with even and uneven frosting and
the considerations of whether the melted frost is locally drained. The investigation
results are used to evaluate the effects of uneven frosting on defrosting performance.
Chapter 7 investigates the influence of refrigerant distribution on defrosting per-
formance in an ASHP unit having a vertically installed multicircuit outdoor coil. Sim-
ilar to that in Chapter 6, whether the melted frost is locally drained away is considered.
The results of defrosting durations and energy analysis are also used to evaluate the
uneven refrigerant distribution effect on defrosting performance.
Chapter 8 analyzes the energy transfer mechanism during RCD in an ASHP unit,
where all types of heat supply and energy consumption are quantitatively compared.
While the roles of indoor and outdoor coils are changed during defrosting, the thermal
energy stored in the metal of the two coils is also changed. The net energy transfer in
two coils is analyzed, with the influence on defrosting performance evaluated.
Chapter 9 reports the optimization of defrosting initiation and termination control
strategies for an ASHP unit having a multicircuit outdoor coil. First, a time-based ini-
tiation of defrosting control strategy is experimentally investigated, with different
total frost accumulations on the outdoor coil surface. Then, the condition of melted
frost locally drained during defrosting is considered, and thus the corresponding
experiments carried out. Third, a defrosting termination strategy is optimized with
a more suitable defrosting termination temperature suggested, based on extensive
experimental results. In this chapter, the defrosting initiation and termination control
strategies are evaluated based on defrosting performance.
Chapter 10 reports the technoeconomic performances of optimized ASHP units by
installing valves to adjust the refrigerant distribution and water collecting trays to
locally drain away the melted frost during defrosting. Both frosting and defrosting
are considered, and their operational conditions at three typical seasons analyzed.
Finally, the effects of installing valves and trays on economic performance, the var-
iation of total running costs, and a payback analysis for the additional initial costs
are given.
Chapter 11 draws conclusions from the research work presented in this book and
recommends possible future work on the development of defrosting technologies for
ASHP units.
In addition, Appendices A and B are provided to calculate the defrosting efficiency
and its error, and the metal energy storage effect on defrosting performance. Appen-
dices D and E list the calculation methods for frosting evenness coefficient and
defrosting evenness coefficient, respectively. Appendix F presents the program listing
of Model 1 in Section 4.2.