ASHRAE Journal - September 2014 - 15

Monthly Energy Use (kWh)

TECHNICAL FEATURE

20,000
18,000
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
0

GSHP

VRF

DOAS

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

FIGURE 1 Total monthly energy use by each HVAC system.

to precool or preheat the outdoor air. The total cooling
capacity of the condensing units is 28.6 tons (100.6 kW).
Two years of data relating to the operation of the different HVAC systems have been collected and analyzed in
an attempt to evaluate the performance of the systems.
These data cover the time span from July 1, 2011 through
June 30, 2013. Data points that have been collected
include operating mode (off/heat/cool), zone temperature and discharge air temperature for each individual
FCU or heat pump. Ground loop supply and return
water temperatures and flow rate were also collected for
the GSHP system. For the DOAS, the flow rate of the supply air to each floor and the supply and return air temperatures and humidity levels were collected.
Metered energy used by each system was also collected. For the GSHP system, the power that is metered
and recorded includes the power for all 14 heat pumps
as well as the ground loop water circulation pumps. For
the VRF system, the power that is metered and recorded
is only the power for the two heat-recovery units and the
22 FCUs that are connected to them. The power for the
three dedicated split systems is metered through a different panel that also includes the power for computer
servers and other equipment in the computer room.
Figure 1 shows the monthly energy use by each system.
These raw data indicate that the VRF system used twice
as much energy as the GSHP system over the two-year
time span. However, it is of great interest to the HVAC
industry to know what caused such significant differences in the energy use of the two systems. The energy
consumptions are affected by several factors including:
* The heating and cooling loads of the conditioned
floor spaces;
* The control strategies of the two systems; and

The dedicated outdoor air system.

* The operating conditions and operational efficiencies of the two systems.
The characteristics and contributions of each of these
factors will be briefly discussed in this article. More
detailed information will be provided in successive
articles and in a final report.

Different Loads
The GSHP system serves 15,558 ft2 (1445 m2) of office
and meeting space primarily on the second floor with a
normal occupancy of 60 people. The VRF units for which
power measurements are available serve a total of 17,213
ft2 (1559 m2) on the first floor, which includes offices,
large meeting spaces and storage areas. The normal
occupancy of the area served by the VRF system is 43 people. The areas served by both systems had the same measured average combined lighting and plug load density of
0.45 W/ft2 (4.8 W/m2) for the two-year study period.
The DOAS, which conditions outdoor air to 55°F (13°C),
satisfied part of the cooling load in summer, but contributed to the heating load in winter. The average DOAS
airflow rate to the first floor was 2,560 cfm (1208 L/s),
which is significantly higher than the average flow rate
to the second floor of 1,480 cfm (699 L/s). In accordance
with ASHRAE/IES Standard 90.1, which requires supply
air temperature to be reset in response to building loads
or outdoor air temperature, the DOAS sequence of operations includes a provision for the supply air temperature to be reset to 60°F (16°C) if all space temperatures
are below their cooling setpoints and the outside air
enthalpy is below a minimum threshold. It also includes
a provision to raise the supply air temperature to 65°F
(18°C) if 80% of the zone temperatures are below their
heating setpoints.
SEPTEM BER 2014

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