ASHRAE Journal - September 2014 - 22

2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0

Heating

Cooling

Average GSHP Power Use (W/ft2)

Average VRF Power Use (W/ft2)

TECHNICAL FEATURE

2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0

Heating

Cooling

Unallocated

17 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Ambient Dry-Bulb Temperature (°F)

FIGURE 11 Contributions of heating and cooling to VRF system power use.

FIGURE 12 Contributions of heating and cooling to GSHP system power use.

Different Operating Conditions and Operational Efficiencies

120

Ambient Air

Ground Loop Water Supply

Temperature (°F)

Another difference between the GSHP system and the
100
air-source VRF system lays in the heat sink and source-
ground vs. ambient air. The ground loop supply tem80
perature and the ambient air temperature are the key
60
parameters affecting the operational efficiency of the
GSHP and VRF systems, respectively. As shown in Figure
40
13, the ground loop supply temperatures are more favor20
able than the ambient air temperature for the operation
of the vapor compression cycle-lower when cooling is
0
JULY 1
DECEMBER 31
JULY 1
DECEMBER 31
JULY 2
needed and warmer when heating is demanded. The
2011
|
2012
| 2013
temperature differential between the ground loop supply temperature and the ambient air temperature is
FIGURE 13 Ground loop supply temperature vs. ambient temperature.
much larger in winter than in the summer, which indicates larger energy effiTABLE 2 Average operating source temperatures and catalog performance.
ciency advantages of the GSHP system in
VRF
GSHP *
the wintertime.
MID 90%
MEDIAN COP
MID 90%
MEDIAN
COP
Table 2 shows manufacturers' data for
SOURCE (AIR)
SOURCE (AIR)
SOURCE (WATER) SOURCE (WATER)
the cooling and heating efficiency of
TEMPERATURE RANGE, °F TEMPERATURE, °F
TEMPERATURE RANGE, °F TEMPERATURE, °F
the VRF system and the GSHP equipCooling
42 - 89
67 5.9
68 - 83
75 6.1 - 6.4
ment at source temperatures. While it
Heating
35 - 76
57 4.5
65 - 71
68 5.6 - 5.8
is difficult to directly compare systems
*GSHP COPs are for the first stage of operation; the range represents different units.
that use different sources, under the
conditions that they are operating at, the heat pumps
Note that these efficiencies are for manufacturers'
ran in a much narrower range of source temperatures
rated performance and do not take into account the
than the VRF system and have higher efficiency than
pumping power required for the GSHP system nor the
the VRF system at most conditions. While the cooling
part load effects on the VRF system. In contrast, the
efficiency of the GSHP equipment is only moderately
metered power data that this article has presented
higher than that of the VRF system, the GSHP equipinclude all of the operational power used by each
ment has much higher heating efficiency than the
system.
VRF system due to more favorable operating condiBy filtering the data to include only hours with no
tions supplied by the ground loop.
VRF units operating in heating mode, the effects of
22

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ASHRAE Journal - September 2014

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