Page 342 - Defrosting for Air Source Heat Pump

Technoeconomic performances 337

To clearly compare the running costs in the heating season in four typical cases,
Fig. 10.20 shows their running costs in a year. It is obvious that the running cost in
the heating season without frost formation was much more than that in the heating
season with frost formation. From highest to lowest, the running costs in the heating
season with frost formation were 1416.76 CNY in Case D5, 1396.83 CNY in Case D6,
1250.57 CNY in Case D7, and 1247.41 CNY in Case D8. Their biggest difference was
169.35 CNY between Case D5 and Case D8. For the running costs in the four typical
seasons in the heating season without frost formation, their values were at 995.74
CNY in Cases D5 and D6, and 919.65 CNY in Case D7 and D8. Their biggest differ-
ence was also between Case D5 and Case D8, at 76.09 CNY, which was smaller than
the difference in the heating season with frost formation. This is because the water-
collecting trays installed had no effect as there was no defrosting operation during the
heating season without frost formation. In addition, the total difference showed that
after the trays and valves were installed, the total running cost during heating seasons
could decrease by 245.44 CNY, or 10.17%.
Furthermore, in order to clearly analyze the running cost consumed in the heating
season with frost formation, Figs. 10.21 and 10.22 show the running costs at frosting
and defrosting stages, respectively, in the four typical cases over a year. As shown in
Fig. 10.21, the two curves’ trends are nearly the same. This shows that the total run-
ning cost was mainly decided by the electricity cost on the frosting operation stage. It
is reasonable because the operation duration frosting is 60 min during a 70 min frost-
ing/defrosting cycle. However, the total running cost shows at D5>D6>D7>D8, but
the electricity cost on frosting at D5¼D6>D7¼D8. This is because the trays and
valves had no effect at this stage. As shown in Fig. 10.22, these effects were shown
on the running costs of electricity on defrosting, and the indoor air thermal energy
consumed during defrosting. Therefore, from highest to lowest, the running cost
was 1416.76 CNY in Case D5, 1396.83 CNY in Case D6, 1250.57 CNY in Case
D7, and 1247.41 CNY in Case D8. Their biggest difference was 169.35 CNY between
Case D5 and Case D8. Here, the fact that the running cost could save a lot after trays
and valves were installed was further confirmed. Also, from the big difference
between Case D6 and Case D7, as shown in Fig. 10.21, we can find that the economic
effects of the valves are bigger than that of the trays. Due to the installation of trays and
valves, in Fig. 10.22, the trends of the two curves show that, from Case D5 to Case D8,
their running costs became steadily smaller. Their biggest difference was also shown
between Case D5 and Case D8, at 29.53 CNY in indoor air thermal energy consumed,
and 12.83 CNY in electricity cost during defrosting. It is interesting that the value of
the indoor air thermal energy consumed during the defrosting operation is always
higher than the electricity cost of defrosting. This is because the cold refrigerant takes
more thermal energy from the indoor air during defrosting. Clearly, the defrosting
duration in Case D8 was the shortest. Therefore, the running cost in this case was
always the lowest.
In order to analyze the proportions of the three running costs, the indoor air thermal
energy consumed, and the electricity cost on the defrosting and frosting stages, the
data showing the running cost of a frosting/defrosting cycle in the heating season with
frost formation is available in Fig. 10.23. In the four typical cases, their total running