ASHRAE Journal - May 2013 - 22

serve the AHU and a second set of 57°F (14°C)
chilled water pipes serve the chilled beams.

Energy Model
Each of the three designs was simulated with
EnergyPlus. EnergyPlus was chosen because it
has an explicit chilled beam module. Other simulation programs such as EnergyPro and eQUEST
do not have a chilled beam module and, therefore, typically approximate chilled beams with
induction units. EnergyPlus does not explicitly
allow chilled beams served by VAV reheat boxes.
Therefore, the hybrid design was approximated
using two systems serving the ACB zones: VAV
with HW reheat boxes and four-pipe fan coil
units with zero zone fan energy operating in sequence; whatever load the VAV box could not
meet was met by the four-pipe fan coil.
Lighting power density was modeled using the
prescriptive requirements of ASHRAE Standard
90.1-2007. Occupant density and receptacle power
density was modeled using the defaults in ASHRAE
Standard 90.1-2007 User’s Manual Table G-B (e.g.,
275 ft2/person and 0.75 W/ft2 for ofﬁces [26 m2/
person and 8.0 W/m2]). Schedules for HVAC, lighting, occupants and receptacles were modeled using
the defaults in User’s Manual Tables G-E to G-M.
The campus central cooling and heating plants
are modeled per the baseline modeling assumptions in Standard 90.1-2007 Appendix G for
chilled water and hot water plants.

VAV Reheat Box

Cooling Only
VAV Box

Figure 5: Typical HVAC floor plan for VAV reheat design.

VFD
CHW
Coil

VFD
Return

CH
Beam

Simulation Results

Supply
Fan
Array

Return
Fans

VAV
Box
RH
Coil

Conference
The results of the EnergyPlus simulations are
Rooms
shown in Figure 9. The VAV reheat design uses
40% less HVAC energy than the ACB design, and TWO 'A'
1 FLOOR PLAN LEVEL
the hybrid design uses 33% less HVAC energy T
than the ACB design. The VAV reheat savings relative to the ACB design are across the board: VAV
HEALTH SCIENCES GRADUATE ST
reheat has 28% FLOOR PLAN LEVEL TWOthan ACB,
less cooling energy 'A'
HEALTH SYSTEM design.
PROJECT 9558600
70% less heating energy, 60% less fan energy, etc. Figure 6: Schematic of hybrid ACB + VAV reheat
1/4" = 1'-0"

Understanding Simulation Results
Further analyses of the designs reveal that the simulation
results are not surprising.

Fan Power
Table 1 shows VAV reheat design has about 40% higher fan
power at design conditions than the ACB+DOAS design. However,
the ACB+DOAS fan power is constant at part load while the VAV
reheat fan power goes down very quickly at part load (thanks to
the near cube law relationship between fan speed and fan power).
As Figure 10 shows, the VAVR design uses less fan power
than the ACB+DOAS design whenever the part load ratio (airﬂow fraction required to meet the load) is less than about 83%,
22

ASHRAE Journal

which is almost all of the time. The annual average airﬂow
rate in the VAV reheat simulation was about 60% of peak ﬂow,
which explains why the VAV reheat design used less than half
of the fan energy of the ACB+DOAS design. Figure 10 also
shows that even if the ACB+DOAS design had been reduced
to 0.3 cfm/ft2 (0.14 L/[s·m2]) it would still use more fan energy
compared to a VAVR system with an average annual ﬂow rate
of 60%. A primary airﬂow rate of 0.3 cfm/ft2 (0.14 L/[s·m2])
is about the lowest possible with an ACB+DOAS system to
meet latent loads with the primary air and the sensible loads
with the chilled beams. Furthermore, there is reason to believe
that the 60% average part load ratio in the EnergyPlus model
is probably unrealistically high. The default ASHRAE schedashrae.org

May 2013

ASHRAE Journal - May 2013

Table of Contents for the Digital Edition of ASHRAE Journal - May 2013