Page 329 - Defrosting for Air Source Heat Pump

324 Defrosting for Air Source Heat Pump

This also reflects the dominant role of the running cost during economic analysis on an
ASHP unit. In addition, this figure shows that the additional first cost played minute
effects on the total cost.
In this section, a technoeconomic analysis study on frosting/defrosting operations
for an ASHP unit, with valves installed on the multicircuit outdoor coil, was con-
ducted. After the valves were installed, the economic performance of the new ASHP
unit was effectively improved. In 15 years’ service life, the total running cost
decreased as much as $798.87, or 7.67%, and the total cost about $776.36, or
6.68%. The running cost of the new ASHP unit in the heating season with frost for-
mation decreased $542.17, or 11.47%, and in heating season without frost formation
$256.72, or 7.64%. The valve effects on total cost were mainly shown at the first 5
operating years. When an ASHP unit worked after 10 years, the effects of the valves
on economic performance were not as obvious as the first several years. The payback
period for the additional first cost is less than 1 year. When the operating durations
were 5 or 10 years, the total costs saved were as much as $243.85 and $510.19, respec-
tively. For the new ASHP unit with valves installed, after about 2 years, the proportion
of first cost in total cost decreased to about 50%. For the new ASHP unit, if the pro-
portion of first cost needs to be decreased to 20%, the operating duration is about
7.8 years.

10.3 Refrigeration adjustment valve and water-
collecting tray

When an ASHP unit works at RCD mode, the frost accumulated on the surface of out-
door coil would melt, and thus flow downward. For a vertically installed multicircuit
outdoor coil, the melted frost from the upside circuit would flow into the downside
circuit. The melted frost would have negative effects on the system defrosting perfor-
mance. To avoid this negative effect due to downward-flowing melted frost, and thus
shorten the defrosting duration for a multicircuit outdoor coil in an ASHP unit, the
water-collecting trays could be placed under each circuit. Then, the melted frost could
be taken away before it flows into the downside circuit. In this section, based on the
valves installed in each circuit to adjust the refrigerant distribution, the installation of
water-collecting trays was further considered. That means that four working condi-
tions, with and without water-collecting trays and/or valves installed in an ASHP unit,
were investigated. Their economic performance on the frosting/defrosting operations
are analyzed.

10.3.1 Methodology
The methodology of this economic analysis is the same as the previous section men-
tioned. First, based on an experimental ASHP unit with a specially made three-circuit
outdoor coil, four typical conditions were designed, considering the installation style
of trays and valves. After the experimental procedures were confirmed, a series of
experiments was undertaken with frost accumulated at different FECs. Second,