ASHRAE Journal - December 2014 - 20

TECHNICAL FEATURE

FIGURE 7 Estimated monthly system heating COP.

FIGURE 8 Estimated monthly system cooling EER.

6.0
GSHP

Cooling System EER

Heating System COP

5.0

VRF

4.0
3.0
2.0
1.0
0.0
JUL. SEP. NOV. JAN. MAR. MAY JUL. SEP. NOV. JAN. MAR. MAY
2011
|
2012
|
2013

Several factors contribute to the large difference in
loads between the two systems. First, the DOAS provided nearly twice as much cooling to the first floor
(58 MWh/year average during the study period) as to the
second floor (33 MWh/year average). This reduces the
cooling load, but increases the heating load for the VRF
system. As noted in our first article,2 at times zones on
the first floor are overcooled by the outdoor air, causing the FCU for those zones to operate in heating mode
to effectively provide reheat. The first floor has lower
regular occupancy than the second floor, and the meeting rooms are used infrequently, so it is unclear why
the DOAS airflow to the first floor is higher. Also, the
temperature control scheme of the VRF system causes
the FCUs in adjacent zones in the open office environment to, at times, operate in conflicting modes simultaneously. The loads from this conflicting operation
are a larger part of the total heating loads than the total
cooling loads because the heating loads due to envelope
losses that are not counterbalanced by solar and internal heat gains are relatively small for this building and
climate. The conflicting operations can occur in both
summer and winter, but the heating loads in summer
are small compared to the loads in winter, so they do not
show in the scale of Figure 5.
To quantify system efficiency, it is necessary to know
how much energy was used for each mode of operation
(heating or cooling) but only total system power measurements are available. When all units in a system are
running in the same mode, the energy used can be allocated accordingly. When individual units were running
in different modes simultaneously, system energy use
was allocated to heating and cooling based on the total
20

ASHRAE JOURNAL

ashrae.org

D ECEM EBER 2014

20
18
16
14
12
10
8
6
4
2
0

GSHP

VRF

JUL. SEP. NOV. JAN. MAR. MAY JUL. SEP. NOV. JAN. MAR. MAY
2011
|
2012
|
2013

capacity of the units that were running in each mode
at the particular time. Allocating the energy use in this
way, total system heating COPs and cooling EERs can
be estimated, as shown in Figures 7 and 8. The error bars
reflect the +14/-11% uncertainty in the estimates of cooling provided and the ±7% uncertainty in the estimates
of heating provided for the GSHP system and the ±5/4%
uncertainty for the VRF system. These system COPs
include all of the energy used by each system including fan power for units that are running in ventilation
mode, standby power for unit control boards when the
building is unoccupied, and pumping power (for the
GSHP system).
During the winter of 2011 through 2012, the estimated
GSHP system heating COP was 3.3±0.2 and the estimated
VRF system heating COP was 2.0±0.1. The following winter the estimated GSHP system heating COPs increased
by 18% to 3.9±0.3, in part because the differential pressure setpoint on the ground loop had been decreased
from 20 psi to 8 psi, which reduced pumping power.
Another contributing factor to the increased COP during
the winter of 2012 through 2013 is colder weather, which
increased the runtime of the heat pumps and thus proportionately decreased the "overhead" system power use
associated with ventilation blowers and pumps. During
a May 2014 site visit, a power meter was installed on the
pumps for a short time, and power was recorded at differential pressure setpoints of 15 psi and 8 psi. Figure 9
shows the effect of the differential pressure setpoint on
the pumping power. VRF system heating COPs could
not be estimated during the winter of 2012 through 2013
because of the equipment modifications in the VRF
system.



ASHRAE Journal - December 2014

Table of Contents for the Digital Edition of ASHRAE Journal - December 2014

Contents
ASHRAE Journal - December 2014 - Intro
ASHRAE Journal - December 2014 - Cover1
ASHRAE Journal - December 2014 - Cover2
ASHRAE Journal - December 2014 - 1
ASHRAE Journal - December 2014 - 2
ASHRAE Journal - December 2014 - Contents
ASHRAE Journal - December 2014 - 4
ASHRAE Journal - December 2014 - 5
ASHRAE Journal - December 2014 - 6
ASHRAE Journal - December 2014 - 7
ASHRAE Journal - December 2014 - 8
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