ASHRAE Journal - March 2022 - 34

ASHRAE AWARD OF ENGINEERING EXCELLENCE
FIGURE 4 Hybrid HVAC system by AHU and TWU.
Return Duct/Bypass Duct
Supply Duct
Plenum Chamber
SA
Railway
Noise
EA
EA
SA
RA EA
Exhausted Through the
Baseboard Slit
Through-Wall Units (TWU)
(Perimeter AC)
(PMV) of each air-conditioning mode in each season was
well within the comfort zone.
In such a hybrid air-conditioning system, in terms of
thermal comfort and energy savings, the temperature
setting of the AHUs and TWUs were investigated under
different conditions in each season to determine the
appropriate setpoint. As an example, operative temperature
and absolute humidity within the offi ce were plotted
to compare with the comfort zone of ANSI/ASHRAE
Standard 55-2013 , Thermal Environmental Conditions for
Human Occupancy. In summer, the TWU 24°C (75°F)
condition was cold underfoot, and the distributions are
outside of the comfort zone in the perimeter zone, and
lowering the temperature setting lowers the operative
temperature. Therefore, the TWU 26°C (79°F) condition
was more desirable, because interior/perimeter temperatures
were within the comfort zone for the 0.5 clo occupant,
and the PMVs were within the comfort range. Even
during heating conditions, the vertical air temperature
difference between fl oor level (FL) +0.1 m (0.33 ft) and
FL+1.1 m (3.61 ft) both in the interior and in perimeter
was less than 2°C (3.6°F). It was confi rmed that both the
interior and perimeter were within the comfort range of
" Category A " defi ned by ISO.
The perimeter zone is a challenging thermal environment,
but showed higher values from the results of thermal
comfort and sensation when compared to the interior
zone. The TWUs are located at the perimeter, and
the increased self-effi cacy due to the freely adjustable
temperature environment may have had an effect on the
34
ASHRAE JOURNAL ashrae.o rg M A R C H 2 0 2 2
RA
SA
Slight Airfl ow During
Cooling
Natural Ventilation from
Common Areas
OA Section
AHU With a High Performance Filter (MERV 13) That Separates the OA
and RA Sections for Good Air Quality, Dehumidifi cation and Humidifi cation
satisfaction of those who had seats near the perimeter,
compared to those who had seats in the interior.
Innovation
A signifi cant innovation was to develop a new hybrid
AC system of a central AHU for the interior zone and the
perimeter TWUs, which reinforced resiliency in both
daily energy use and emergencies (Figure 4). A new TWU
was developed by adding an exhaust operation function
that uses heat recovery of indoor exhaust air to the heat
exchanger of the compressor in place of the conventional
type, which is unable to bring outside air into the
room or exhaust indoor air. The innovations compared
to the conventional unit are as follows:
1. Conditioned air is supplied to the offi ce by the AHU,
and indoor air is exhausted by the TWUs, thus exhausting
indoor air across the entire façade. The TWUs
recover heat from the indoor exhaust air by a compressor
for enhancing AC effi ciency. Exhausting indoor air
improved the effi ciency by approximately 30% compared
to a standard unit.
2. The TWU's operating pattern can be organized as
shown in Figure 5. The operation pattern of the TWU
changes so that the CO2 concentration in the room
becomes the control value. During the fresh air cooling
mode by the AHU and TWU, the fresh air introduced is
exhausted from the outdoor fan of the TWUs, which is
in exhaust mode. The ventilation system performance
produced by these TWUs achieved unidirectional airfl ow
to the windows and does not stir or mix the indoor air.
RA
SA
VAV
SA
Air-Handling Unit (AHU)
RA
Supply Fan
RA Section
Motor Damper for
Fresh Air Cooling
OA
RA
2022
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ASHRAE Journal - March 2022

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