ASHRAE Journal - September 2014 - 16

TECHNICAL FEATURE

GSHP

0.30

VRF
Average Power Use (W/ft2)

Monthly Energy Use (kWh/ft2)

0.35
0.25
0.20
0.15
0.10
0.05
0.00
JUL. SEP. NOV. JAN. MAR. MAY JUL. SEP. NOV. JAN. MAR. MAY
2011
|
2012
|
2013
FIGURE 2 Normalized monthly energy use per square foot.

The measured power consumptions of the GSHP
and VRF systems were normalized with the floor space
conditioned by each system. As shown in Figure 2, the
normalized energy use (kWh/ft2) of the GSHP system
over the two-year period is 44% less than that of the VRF
system.
If the cooling and heating output that is provided by
each system could be measured, then a COP or EER
could be used to compare the systems. Unfortunately,
it was not feasible to install the amount of instrumentation (temperature, humidity and airflow sensors for
the discharge air, return air, and the outdoor air in
every zone) necessary to measure the cooling or heating provided by each of the systems. Estimation of the
cooling and heating output will be discussed in the
next article.

Different Control Strategies
Energy use for both the GSHP and VRF systems peaks
in the summer cooling season, but the VRF system shows
unexpectedly high energy use during the winter as well
as the fall and spring shoulder seasons, which in Atlanta
can still have days when a substantial amount of cooling
is needed.
Figure 3 shows instantaneous power usage for all three
systems during occupied building hours (7 a.m. to 6
p.m. on workdays) averaged for each 1°F (0.55°C) outdoor air temperature bin and normalized by the floor
area served by each system. The VRF system shows
unexpectedly high power use at times with mild temperatures. The normalized instantaneous power use
of the three systems was correlated to the coincident
16

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SEPTEM BER 2014

2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0

GSHP

VRF

35
55
75
Ambient Dry-Bulb Temperature (°F)

DOAS

FIGURE 3 Average power use vs. ambient temperature with change-point models.

The condensing units for the DOAS system.

ambient air dry-bulb temperature using a change-point
regression model.1
As shown in Figure 3, the minimum power use (i.e.
the horizontal portion of the change point regression model) for the GSHP system is 0.19 W/ft2 (2 W/
m2) over a temperature range of 44°F to 61°F (6.6°C
to 16°C). The minimum power use for the VRF system
is 0.67 W/ft2 (7.2 W/m2) over a much wider range of
47°F to 81°F (8°C to 27°C). The minimum power use
for the DOAS is 0.13 W/ft2 (1.4 W/m2), and the changepoint occurs at 46°F (7.7°C). Blower power use for the
GSHP system when all heat pumps are running in
ventilation mode is about 0.06 W/ft2 (0.65 W/m2). For
the VRF system, few data points are available when
all FCUs are running in ventilation mode, but blower
power use may be as high as 0.15 W/ft2 (1.6 W/m2).
The VRF system consumed more than three times
as much power as the GSHP system when the ambient air temperature was colder than 61°F (16°C). It
consumed 50% more power than the GSHP system

ASHRAE Journal - September 2014

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