sustainability

Article
A Study on a Control Method with a Ventilation
Requirement of a VAV System in Multi-Zone
Hyo-Jun Kim 1 and Young-Hum Cho 2, *
1 Department of Architectural Engineering, Graduate School of Yeungnam University, 280 Daehak-Ro,
Gyeongsan, Gyeongbuk 38541, Korea; kimyo@ynu.ac.kr
2 School of Architecture, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk 38541, Korea
* Correspondence: yhcho@ynu.ac.kr; Tel./Fax: +82-53-810-3081

Received: 8 August 2017; Accepted: 31 October 2017; Published: 10 November 2017

Abstract: The objective of this study was to propose a control method with a ventilation requirement
of variable air volume (VAV) system in multi-zone. In order to control the VAV system inmulti-zone,
it is essential to control the terminal unit installed in each zone. A VAV terminal unit with
conventional control method using a fixed minimum air flow can cause indoor air quality
(IAQ) issues depending on the variation in the number of occupants. This research proposes a
control method with a ventilation requirement of the VAV terminal unit and AHU inmulti-zone.
The integrated control method with an air flow increase model in the VAV terminal unit, AHU,
and outdoor air intake rate increase model in the AHU was based on the indoor CO2 concentration.
The conventional and proposed control algorithms were compared through a TRNSYS simulation
program. The proposed VAV terminal unit control method satisfies all the conditions of indoor
temperature, IAQ, and stratification. An energy comparison with the conventional control method
showed that the method satisfies not only the indoor thermal comfort, IAQ, and stratification issue,
but also reduces the energy consumption.

Keywords: VAV system; terminal unit; TRNSYS; control method; multi-zone; ventilation requirement

1. Introduction
Buildings account for 25%, or more, of the entire amount of domestic energy consumption.
Especially, mid-sized and large buildings continue to increase in energy consumption. These days,
the reduction of greenhouse gases by energy saving is highlighted as an important issue.
Accordingly, building facility systems should be operated to make sure that energy is used and
saved efficiently.
The variable air volume system (VAV system) is adopted in many large buildings with increasing
interest in energy saving and various studies have been performed for its efficient operation [1].
The common air conditioning system is designed to perform air conditioning in a plurality of zones
using an air handling unit (AHU). A terminal unit is located in the end terminal that supplies air
conditioned air to each zone, which makes it necessary to control a terminal unit in each zone in order
to control a VAV system in multi-zone.
The systematic use of optimization techniques has been introduced in the heating, ventilation,
and air conditioning (HVAC) industry in recent years, with a substantial focus on VAV system optimal
control [2–4]. Yao et al. [5] compared three kinds of air conditioning systems (VAV, Constant Air
Volume, and fan coil system) in small office buildings in six different cities in China. The parameters in
the model were obtained from either the field-testing data or the performance data provided by the
product manual. The VAV system promised 17.0–37.6% potential energy savings, compared to the CAV
system, and 4.6–10.2% potential energy savings, compared to the fan coil system, depending on the
location of the city and climate. Engdahl and Johansson [6] investigated the potential energy savings

Sustainability 2017, 9, 2066; doi:10.3390/su9112066 www.mdpi.com/journal/sustainability

Sustainability 2017, 9, 2066 2 of 17

of a controlled supply air temperature of a VAV-based system by a comparison with a constant supply
air temperature. The model results showed 8–27% potential energy savings for the varying supply air
temperature case, compared to the constant supply air temperature case. Congradac and Kulic [7] used
genetic algorithms to optimize the return damper position such that the indoor CO2 concentration can
be kept as close to the desired level as possible and, at the same time, the lowest value of the valve
(the lowest energetic use) can be achieved. Nassif and Moujaes [8] proposed a new damper control
strategy for the outdoor, discharge, and recirculation air dampers of the economizer in a VAV system.
The strategy controlled the outdoor air by only one damper while keeping the others fully opened.
The simulation results showed that the annual energy savings in the supply and return fans of an
existing system, compared to the traditional strategy of three coupled dampers, were 12% and 5%,
respectively. Cho [9] examined the procedures for supply fan speed control, implementations and
detailed descriptions of the control sequence, and operation comparisons. The results show electricity
savings of 23% and gas savings of 19% over a six-month period. Cho [10] identified the relationship
between the supply air temperature and minimum air flow rate. In addition, the relationship between
the supply air flow rate and temperature that minimizes energy consumption was suggested through
proper supply temperature, stratification, and energy analysis according to the load that energy
consumption can be minimized.
CO2 -based DCV have been proposed to determine the minimum outside air intake based on the
number of occupants in the zone. The DCV for a single-zone have been studied by many researchers
over the past few decades. Subsequently, Int-hout [11] proposed a more detailed control method,
such as proportional control and PID control. Pavlovas [12] reported the results of a case study
over a Swedish multifamily building aimed at evaluating the DCV system with different strategies.
The outcome of the simulation shows that it would be possible to achieve energy savings using
occupancy- and humidity-controlled ventilation to reduce the average ventilation flow rate while
maintaining an acceptable indoor climate. Lu et al. [13] examined a new control strategy of CO2 -based
DCV for sports training arenas to address the challenge of control algorithms. The controlled
CO2 concentration using the new strategy was close to the CO2 set point at each training session;
therefore, the ventilation was optimized. Mysen et al. [14] inspected 157 Norwegian classrooms
to analyze the energy use over there different ventilation systems: CAV, DCV-CO2 , and infrared
occupancy sensor-based demand-controlled system (DCVIR). Their results show that DCV-CO2 and
DCV-IR reduce the energy use due to ventilation in an average classroom to 38% and 51%, respectively,
compared to the corresponding energy for a CAV system.
Cho and Liu [15] determined the relationship between the supply temperature and minimum air
flow rate at the terminal unit and presented a terminal unit air flow rate control method using step
control to control the indoor load and ventilation requirement. Kang [16] determined the relationship
between supply temperature, air flow rate, and energy by height and presented a terminal unit
air flow rate control method considering stratification and indoor air quality (IAQ). Kim et al. [17]
presented a method for controlling the supply temperature and air flow rate of a terminal unit to
control stratification. However, preceding studies are based on a single zone. Therefore, it is considered
to be additionally necessary to study methods for controlling a VAV terminal unit in a multi-zone.
Therefore, this paper proposed a VAV terminal unit control method in the multi-zone that meets
the ventilation requirement to control IAQ. The proposed method controls IAQ through air flow rate
control in case of the appearance of a space exceeding the IAQ control standard. The minimum outdoor
air intake rate was based on the CO2 -based model, which is determined depending on the indoor CO2
concentration. Moreover, this study composed a system using Trnsys17, a dynamic energy simulation
tool, to make a comparison between the existing and proposed algorithms, and analyzed thermal
comfort, IAQ, and energy consumption.
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2. Theoretical Considerations
2. Theoretical Considerations
2.1. VAV Terminal Unit
2.1. VAV Terminal Unit
The selection and control of a VAV terminal unit has a significant influence on energy
The selection
consumption and
in the control
HVAC of a VAV
system andterminal
on indoorunitcomfort.
has a significant
As seen influence
in Figure on energy
1, the consumption
single-duct VAV
in the HVAC system and on indoor comfort. As seen in Figure 1, the
terminal unit system consists of a controller, a temperature sensor, an actuator, a damper, single-duct VAV terminal unita
system consists of a controller, a temperature
reheating coil, and an air flow sensor [18]. sensor, an actuator, a damper, a reheating coil, and an air
flow The
sensorair[18].
flow sensor controls the damper to meet a required flow rate within the scope of the
The
minimum and air flowmaximum
sensor controls
air flowtherates
damper to meet
at the a required
terminal flow rate within
unit according to indoorthe scope of the
temperature
minimum
changes. Whenand maximum
the air flow air rate
flowofrates
theat the terminal
terminal unit according
unit reaches to indoor
the minimum andtemperature
the indoorchanges.
heating
When the air flow rate of the terminal unit reaches the minimum and the indoor
load is high, the reheating coil is operated to enhance the supply air flow rate and, thus, maintain heating load is high,
a
the reheating coil is operated to enhance the supply air flow rate and,
comfortable indoor temperature. The minimum air flow rate is calculated using the maximum air thus, maintain a comfortable
indoor temperature.
flow rate, which isThe minimumusing
calculated air flowthe
ratesensible
is calculated
load,using the maximum
indoor air flow rate,
set temperature, and which
supplyis
calculated
temperature. using the sensible load, indoor set temperature, and supply temperature.

Figure 1. Control sequence of the VAV terminal units (from the ASHRAE Application Handbook,
Figure 1. Control sequence of the VAV terminal units (from the ASHRAE Application Handbook,
Ch. 45 [18]).
Ch. 45 [18]).

The greatest value is selected as the minimum air flow rate of the VAV terminal unit with the
reheatinggreatest
The value is selected
coil in California Title 24as[19]theand
minimum
ASHRAE air Standard
flow rate 90.1
of the VAV
[20]: (1)terminal
30% of the unitmaximum
with the
reheating coil in California Title 24 [19] and
air flow rate; (2) 120 LPM/m × floor area; and (3) 142 L/s.
2 ASHRAE Standard 90.1 [20]: (1) 30% of the maximum air
flow In
rate; (2) 120 LPM/m 2 × floor area; and (3) 142 L/s.
general, it is most used at the HVAC system to set the minimum air flow rate at 30% of the
In
maximum general,
air flowit israte.
most Forused at the
an air HVAC system
conditioned to set120
floor area, theLPM/m
minimum 2 is a air
highflowair rate
flowatrate.
30%Theof
the maximum air flow rate. For an air conditioned floor area, 120 LPM/m 2 is a high air flow rate.
value is not desirable for the minimum air flow rate of the terminal unit, but is one of the values
The
oftenvalue is notThis
selected. desirable
method for selects
the minimum
too highairanflowair rate
flowofrate
the for
terminal unit, but air
the minimum is one
flow of rate,
the values
when
often
low heating load occurs in a space with a broad air conditioned area. The fixed value of when
selected. This method selects too high an air flow rate for the minimum air flow rate, low
142 L/s is
heating load occurs in a space with a broad air conditioned area. The fixed
selected in a zone that is in a special thermal state, as in a zone with a large north-facing glass value of 142 L/s is selected
in a zone or
window that is in a zone
a small special
withthermal
a highstate, as inload.
heating a zone with
It is notaan large north-facing
optimum value, glass
but iswindow
adoptedorby a
small zone with a high heating load. It is not an optimum value, but
some engineers as the minimum air flow rate in every zone irrespective of thermal condition. is adopted by some engineers as
the minimum
The existingair flow
fixedrate
airinflow
everyrate
zone canirrespective of thermal
cause discomfort condition.
among occupants and energy waste.
The existing fixed air flow rate can cause
Being higher than required to meet the indoor load, the minimum discomfort among occupants
air flow and rate energy waste.
unnecessarily
Being higher than required to meet the indoor load, the minimum
reheats the air cooled through the AHU, which consumes more fan energy than necessary. air flow rate unnecessarily reheats
the air cooled through the AHU, which consumes more fan energy than necessary.
2.2. Ventilation Requirement
2.2. Ventilation Requirement
The ventilation requirement can influence air conditioning and heating and fan energy in an air
The ventilation requirement can influence air conditioning and heating and fan energy in an air
conditioning system. ASHRAE Standard 62.1 defines how to calculate the ventilation requirement,
conditioning system. ASHRAE Standard 62.1 defines how to calculate the ventilation requirement,
which has a significant influence on the design and control of the ventilation system. As seen in
which has a significant influence on the design and control of the ventilation system. As seen in Table 1,
Table 1, it is a method for calculating the outdoor air flow rate using the minimum ventilation
it is a method for calculating the outdoor air flow rate using the minimum ventilation requirement by
requirement by zone type, occupancy density, and floor area. The outdoor air flow rate equation
zone type, occupancy density, and floor area. The outdoor air flow rate equation calculates the outdoor
calculates the outdoor air flow rate by considering the ventilation requirement enough for diluting
air flow rate by considering the ventilation requirement enough for diluting indoor pollutants from
indoor pollutants from occupants and pollutants from residents [21].
occupants and pollutants from residents [21].
Sustainability 2017, 9, 2066 4 of 17

Table 1. Minimum ventilation rates in office building (from ASHRAE Standard 62.1 [21]).

People Outdoor Area Outdoor Default Values

Occupancy Category Air Rate Air Rate Occupant Density Combined Outdoor Air Rate
m3 /h Person m3 /h m2 #/100 m2 m3 /h Person
Break rooms 2.5 0.6 50 3.5
Office space 2.5 0.3 5 8.5
Office Building
Reception areas 2.5 0.3 30 3.5
Main lobbies 2.5 0.3 10 5.5

2.3. Determined Minimum Air Flow Rate

2.3.1. Minimum Air Flow Rate for Indoor Load

The minimum airflow for a heating load is the airflow required by the room design heating load.
The minimum airflow needed to satisfy the building heating load requirement can be calculated with
Equation (1):
. Qh
V min,h = (1)
ρc p ( Ts − Tr )

2.3.2. Minimum Air Flow Rate for Ventilation

The minimum airflow rate that satisfies the ventilation requirements in multi-zone can be
calculated using Equations (2)–(6):


. R p Pz + R a Az
V min,v = (2)
Ez
.
∑ V oa
X= . (3)
∑ V da
. !
V oa
Z = max . (4)
V da
X
Y= (5)
1+X−Z
.
Voa = Y ∑ V da (6)

2.3.3. Minimum Air Flow Rate Requirement

The minimum airflow setpoint can be set so that it equals either the highest airflow rate required
by the room design heating load or the minimum rate required for ventilation:
. . . 
V min = max V min,h , V min,v (7)

3. Proposed Control Method of a VAV Terminal Unit

3.1. Simulation Conditions

This study used TRNSYS 17 [22–24], a dynamic energy simulation program to propose a method
for controlling the VAV terminal unit. The building was modelled using Google Sketch Up, and its
detailed information was entered using TRNBuild (2.0). The building’s HVAC system was modelled
using Simulation Studio (5.4).
The building was located in Lincoln, Nebraska, USA, and an AHU was selected that controlled
eight target spaces in which occupants resided. The spaces are used for office work and equipped with
Sustainability 2017, 9, 2066 5 of 17

single-duct VAV systems. The VAV system is operated 24 h year round, and the indoor temperature is
24 ◦ C. This
setSustainability
atSustainability
2017, 9,study
2017, 9, 2066 selected an AHU controlling eight indoor spaces to calculate the minimum
2066 5 of517
of 17
air flow rate of the VAV system in multi-zone. Figure 2 shows a space managed by the selected
thethe
AHU. minimum
minimum
Zone 2 isairusedflow
air flow rate
for rateofofthe
meetingtheVAV
VAV
oncesystem
system in
in multi-zone.
a week, multi-zone. Figure22shows
Figure
and is characterized shows
bya aspace
space
high managed
managed
occupancy byby the the
density.
selected
Theselected
terminal AHU.
AHU.
unit Zone Zone
controls 2 is used
2 isa used
supply for meeting
fordamper
meetingand once
once a week, and
week,airand
supplies is
flow characterized
is rate
characterized
on the basisby high
by of
high occupancy
the occupancy
set values of
density. The terminalunit unitcontrols
controlsaasupply
supply damper
damper and supplies air flow rate ononthe basis of of
the setset
indoor temperature and minimum and maximum air flow rates. The minimum air flow ratethe
density. The terminal and supplies air flow rate the basis is set at
values of indoor temperature and minimum and maximum air
values of indoor temperature and minimum and maximum air flow rates. The minimum air flow rates. The minimum air flowflow
approximately
rate is set
30% of the maximum air flow rate. Table 2 lists the basic information of the zone and
rate is set at at approximately30%
approximately 30%ofofthe
themaximum
maximum air air flow
flow rate.
rate. Table
Table22lists
liststhe
thebasic information
basic information of of
Figure 3 zone
shows the indoor load theschedule.
thethe
zone and and Figure
Figure 3 3shows
shows indoorload
the indoor load schedule.
schedule.

Figure 2. Schematic diagram of a single-duct VAV system and terminal unit.

Figure 2. Schematic diagram of a single-duct VAV system and terminal unit.
Figure 2. Schematic diagram of a single-duct VAV system and terminal unit.

(a)

(a)

(b)
Figure 3. Load schedule: (a) working days; and (b) weekends.
(b)
Figure 3. Load schedule: (a) working days; and (b) weekends.
Figure 3. Load schedule: (a) working days; and (b) weekends.
Sustainability 2017, 9, 2066 6 of 17

Table 2. Simulation conditions.

Lists Contents
Location Lincoln, NE, USA
Buildings
Use Office
Type Single duct VAV system
AHUs Design air flow rate 2420 m3 /h
System
Design Supply fam power 15 kW
VAV Terminal unit VAV terminal unit with reheat system
Schedule 24 h
Operating conditions
Set point temperature 24 ◦ C
Occupant [25] Seated, Light work, typing 150 W/person
Load conditions Light 13 W/m2
Equipment 16 W/m2
Thickness 0.2 m
Exterior wall
Thermal transmittance 0.31 W/m2 K
Thickness 0.09 m
Interior wall
Thermal transmittance 0.508 W/m2 K
Material properties
Thickness 0.3 m
Floor
Thermal transmittance 0.039 W/m2 K
Thickness 0.141 m
roof
Thermal transmittance 0.316 W/m2 K

Table 3 shows maximum and minimum air flow rate of VAV terminal unit. The terminal units
supply an air flow rate that based on the setpoint of the indoor temperature, and the maximum and
minimum air flow rate. The minimum air flow rate of the target zone is set at 30% of the maximum air
flow rate.

Table 3. Maximum and minimum air flow rates of a VAV terminal unit.

Maximum Air Flow Rate Minimum Air Flow Rate

Zone Type
m3 /h m3 /h
Zone 1 600 166
Zone 2 350 92
Zone 3 160 37
Zone 4 190 42
VAV Terminal unit with reheating coil
Zone 5 178 38
Zone 6 890 252
Zone 7 220 50
Zone 8 210 47

3.2. Supply Air Flow Rate Reset of the VAV System

The ventilation requirement of indoor zone calculated Equation (3). Table 4 shows the ventilation
requirement of each zone. A selection was made of coefficients to decide the required outdoor air flow
rate as 9 m3 /h·person, 1.08 m3 /h·m2 in an office space presented by ASHRAE Standard 62.1.
Sustainability 2017, 9, 2066 7 of 17

Table 4. Outdoor air flow rate for ventilation (ASHRAE Standard 62.1).

People Outdoor Area Outdoor Zone air Distribution Outdoor Air

Occupants Floor Area
Zone Air Rate Air Rate Effectiveness Flow Rate
m3 /h·Person Person m3 /h·m2 m2 - m3 /h
Zone 1 9 4 1.08 28 0.8 108
Zone 2 9 8 1.08 14 0.8 141
Zone 3 9 1 1.08 4.2 0.8 22
Zone 4 9 1 1.08 7.56 0.8 28
Zone 5 9 1 1.08 4.68 0.8 23
Zone 6 9 6 1.08 49.36 0.8 174
Zone 7 9 1 1.08 8.96 0.8 30
Zone 8 9 1 1.08 6.48 0.8 26
AHU 552

The outdoor air intake ratio can be calculated using the outdoor air intake ratio of the critical zone
in multi-zone with Equations (3)–(7). Table 5 shows the outdoor air intake ratio of each zone and AHU.

Table 5. Determined outdoor air flow rate using ASHRAE Standard 62.1.

Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone 6 Zone 7 Zone 8 Total

Supply air flow rate (m3 /h) 564 487 124 141 129 841 169 158 2613
Outdoor air flow rate (m3 /h) 108 141 22 28 23 174 30 26 552
Zi 0.19 0.29 0.18 0.2 0.18 0.21 0.18 0.16 -
X 0.21
Z 0.29
Y 0.23
OA (m3 /h) 601

The minimum air flow rate of the VAV terminal unit is calculated using Equation (8). Table 6
shows the calculated maximum and minimum air flow rate and reset conditions. The minimum air
flow rate is changed depending on the number of occupants and decreased up to 80% of the existing
minimum air flow rate.

Table 6. Comparison of air flow rate between the existing and reset conditions.

Existing Air Flow Rate Air Flow Rate Reset

Zone Maximum Air Flow Rate Minimum Air Flow Rate Maximum Air Flow Rate Minimum Air Flow Rate
m3 /h m3 /h m3 /h m3 /h
Zone 1 600 166 564 27~108
Zone 2 350 92 487 21~84
Zone 3 160 37 124 12~22
Zone 4 190 42 141 10~28
Zone 5 178 38 129 15~23
Zone 6 890 252 841 80~174
Zone 7 220 50 169 16~30
Zone 8 210 47 158 11~26

3.3. Control Method for a Multi-Zone VAV Terminal Unit

If an AHU controls multi-zone, the minimum air flow rate of a terminal unit is very important,
and the terminal unit should be controlled in accordance with the circumstances in each zone. For the
control for the terminal unit, air flow rate is controlled according to changes in the absolute temperature.
At the existing terminal unit, air flow rate is controlled for indoor temperature alone, not for IAQ.
In this case, IAQ is based on CO2 concentration. The existing CO2 -DCV controls only the outdoor air
flow rate of the AHU to control the indoor CO2 concentration. However, if the outdoor air flow rate is
increased to control the indoor CO2 concentration, excessive AHU coil energy will be used according
to the outdoor air. Therefore, this study aims to propose a method for controlling the air flow rates of
 Sustainability 2017, 9, 2066 8 of 17

Sustainability 2017, 9, 2066 8 of 17

a terminal unit and an AHU, delaying the outdoor air inlet time, and reducing the outdoor air flow
rate, in order to control the indoor CO2 concentration. If there appears a zone in which indoor CO2
The VAV terminal unit is controlled using the minimum air flow rate set at 30% of the
concentration is higher than 1000 ppm:
maximum air flow rate according to the unit’s existing control method. The method is to control the
(1) Control indoor CO concentration by increasing the air flow rate of a terminal unit in the zone
valves in the terminal unit2 and the reheating coil. Figure 4 shows the existing control algorithm of
concerned. Do not increase outdoor air flow rate.
the VAV terminal unit.
(2)If ventilation CO2 concentration is low, control the indoor CO2 concentration by increasing the
return
(1) Cooling air flow rate. Do not increase outdoor air flow rate.
Mode
(3) Control indoor CO2 concentration by increasing outdoor air flow rate from the AHU.
Cooling mode begins when the indoor temperature is higher than the indoor cooling set
Therefore, this study proposed an integral control algorithm after conducting a simulation
temperature. Air flow rate is determined by the cooling load, and a damper is operated to maintain
comparison between air flow rate increase models at the terminal unit and the AHU.
the set indoor temperature between maximum and minimum air flow rates.
(2) 3.3.1.
HeatingExisting Control Method of the VAV Terminal Unit in Multi-Zone
Mode
The VAV terminal unit is controlled using the minimum air flow rate set at 30% of the maximum
Heating mode begins when the indoor temperature is lower than indoor heating set
air flow rate according to the unit’s existing control method. The method is to control the valves in
temperature. Air flow rate is determined by the heating load and the minimum outdoor air flow
the terminal unit and the reheating coil. Figure 4 shows the existing control algorithm of the VAV
rate,terminal
and theunit.
reheating coil has a valve operated to maintain an air supply set temperature.
Figure 5 shows the results from the application of the existing method for controlling the
(1) Cooling Mode
existing VAV terminal unit in winter. Indoor temperature remained at 24 °C, the set indoor
Cooling
temperature. As mode beginswere
occupants when the indoor
deployed temperature
around 9:00 AM,is higher
indoorthan
CO2the indoor cooling
concentration set
increased,
temperature.
but the minimum Airair
flow raterate
flow is determined by theThis
was supplied. cooling load, and a damper
is considered is operated
to be because the to maintain of
deployment
the set indoor temperature between maximum and minimum air flow rates.
occupants caused the reduction of the indoor heating load and the ensuing minimum supply of the
air flow rate. When
(2) Heating Mode applying the existing VAV terminal unit control method, the problem of IAQ
occurs, ifHeating
indoormode
CO2begins
generation is indoor
when the increased by a sudden
temperature is lowerincrease in heating
than indoor the number occupants in
set temperature.
winter.
Air flow rate is determined by the heating load and the minimum outdoor air flow rate, indoor
This makes it necessary to establish a control method considering the and the CO2
concentration.
reheating coil has a valve operated to maintain an air supply set temperature.

Figure 4. Existing
Figure control
4. Existing controlmethod
method algorithm
algorithm ofofthe
theVAV
VAV terminal
terminal unit.unit.

Figure 5 shows the results from the application of the existing method for controlling the existing
VAV terminal unit in winter. Indoor temperature remained at 24 ◦ C, the set indoor temperature.
 Sustainability 2017, 9, 2066 9 of 17

As occupants were deployed around 9:00 a.m., indoor CO2 concentration increased, but the minimum
air flow rate was supplied. This is considered to be because the deployment of occupants caused
the reduction of the indoor heating load and the ensuing minimum supply of the air flow rate.
When applying the existing VAV terminal unit control method, the problem of IAQ occurs, if indoor
CO2 generation is increased by a sudden increase in the number occupants in winter. This makes it
necessary
Sustainability 2017, 9,to establish a control method considering the indoor CO2 concentration.
2066 9 of 17

Figure 5.Figure
Simulation result
5. Simulation using
result existing
using existingcontrol method
control method of of
thethe
VAVVAV terminal
terminal unit in unit in winter.
winter.

3.3.2. Proposed
3.3.2. Proposed Control
Control MethodMethod
of aofVAV
a VAVSystem
System
For a terminal unit and AHU, a method for increasing the air flow rate was established with respect
For a terminal unit and AHU, a method for increasing the air flow rate was established with
to the minimum fresh outdoor air flow rate and outdoor air flow rate for operating an economizer.
respect to the minimum
However, freshindoor
when controlling outdoor air flow rate
CO2 concentration and
using theoutdoor airflow
increased air flow raterate for
of the operating an
terminal
economizer. However,
unit and the AHU, when
returncontrolling indoormay
CO2 concentration CObe2 concentration
increased by the using theinincreased
increase the number airofflow rate
of the terminal unit
occupants, andincreases
which the AHU, returnCO
the supply CO 2 concentration
2 concentration, making may be increased
it impossible by the
to control the increase
indoor in the
number CO of2 concentration.
occupants, whichIn this case, the indoor
increases theCOsupply
2 concentration should be controlled
CO2 concentration, by increasing
making the
it impossible to
outdoor air flow rate.
control the indoor CO2 concentration. In this case, the indoor CO2 concentration should be
Figure 6 shows the relationship between outdoor air and outdoor air flow rate by simulation using
controlled
the COincreasing
by the outdoor air flow rate.
2 integration control method. Minimum outdoor air alone was inhaled and the economizer
Figure 6 showsinthe
was operated relationship
other sections when between
the indooroutdoor air and
temperature outdoor
exceeded airtemperature
a set flow rate (24 by ◦simulation
C)
◦
using theandCO 2 integration
outdoor temperaturecontrol
was 5 Cmethod.
or lower inMinimum outdoor
the case of the existing air alone
outdoor wasrate
air flow inhaled
control.and the
However, when applying CO integration control method, outdoor air was
economizer was operated in other sections when the indoor temperature exceeded a set temperature
2 inhaled more than the
minimum outdoor air flow rate even in the minimum outdoor air flow rate section to control the
(24 °C) and outdoor temperature was 5 °C or lower in the case of the existing outdoor air flow rate
indoor CO2 concentration. The more outdoor air inhaled, the more energy will be consumed in the
control. AHU
However, when applying CO2 integration control method, outdoor air was inhaled more
and the reheating coil. However, the control of the indoor CO2 concentration is considered to
than therequire
minimum outdoor
an increase air flowairrate
in the outdoor flow even
rate. in the minimum outdoor air flow rate section to
control the indoor CO2 concentration. The more outdoor air inhaled, the more energy will be
consumed in the AHU and the reheating coil. However, the control of the indoor CO2 concentration
is considered to require an increase in the outdoor air flow rate.
(24 °C) and outdoor temperature was 5 °C or lower in the case of the existing outdoor air flow rate
control. However, when applying CO2 integration control method, outdoor air was inhaled more
than the minimum outdoor air flow rate even in the minimum outdoor air flow rate section to
control the indoor CO2 concentration. The more outdoor air inhaled, the more energy will be
consumed 2017,
Sustainability in the AHU and the reheating coil. However, the control of the indoor CO2 concentration
9, 2066 10 of 17
is considered to require an increase in the outdoor air flow rate.

Figure 6. Relationship between outdoor climate and outdoor air flow rate by resetting of the
Figure 6. Relationship between outdoor climate and outdoor air flow rate by resetting of the outdoor
outdoor air flow rate.
air flow rate.

Calculated with the air flow rate increase method, the air flow rate is greater than that to eliminate
indoor load and may consume too much air conditioning and heating energy without controlling the
indoor temperature. To solve this problem, this study proposed a VAV terminal unit CO2 integration
control method that resets the outdoor air flow rate and supply temperature on the basis of the
above-mentioned air flow rate increase method. When increasing the air flow rate to control CO2
concentration, the air flow rate supplied is more than required and air conditioning and heating
energy is wasted. Therefore, when a VAV terminal unit system works in a ventilation mode due to the
non-control of CO2 concentration, the supply temperature should be reset using Equations (9) and (10)
to ensure that set indoor temperature is maintained:
.
Qh = ρVC p ( Ts − Tr ) (9)

Qh
Ts = . (10)
ρVC p
Figure 7 shows a VAV system CO2 integration control algorithm.
(1) Cooling Mode
Cooling mode begins when the indoor temperature is higher than the indoor cooling set
temperature. Air flow rate is determined by the cooling load, and a damper is operated to maintain a
set indoor temperature between the maximum and minimum air flow rates.
(2) Heating Mode
Heating mode begins when the indoor temperature is lower than the indoor heating set
temperature. Air flow rate is determined by the heating load and minimum outdoor air flow rate,
and the reheating coil has a valve operated to maintain an air supply set temperature.
(3) Ventilation Mode
When the indoor CO2 concentration is higher than 1000 ppm, an increase occurs in the air flow
rates in the terminal unit and the AHU. The air flow rate of the terminal unit increases by 10% of the
maximum air flow rate, and the air flow rate of the AHU is increased by 10%, when the damper opening
ratio of terminal unit is 100% and indoor CO2 concentration is higher than 1000 ppm. When the supply
CO2 concentration exceeds 1000 ppm, an increase in the outdoor air flow rate occurs. When operating a
control CO2 concentration, the air flow rate supplied is more than required and air conditioning and
heating energy is wasted. Therefore, when a VAV terminal unit system works in a ventilation mode
due to the non-control of CO2 concentration, the supply temperature should be reset using
Equations (9) and (10) to ensure that set indoor temperature is maintained:
Sustainability 2017, 9, 2066 11 of 17
= ( − ) (9)

ventilation mode, the air supply set temperature of=the reheating coil is reset according to Equation(10)
(10).
When the ventilation mode is finished, a heating mode is followed by the air supply set temperature of
Figure 7 coil.
the reheating shows a VAV system CO2 integration control algorithm.

Figure 7.
Figure 7. VAV
VAVsystem
systemCO
CO22 integration
integration control
controlalgorithm.
algorithm.

(1)Results
4. Cooling
andMode
Discussion
Cooling
The mode
indoor begins
thermal when
comfort, theand
IAQ, indoor temperature
stratification is higher
issue were than tothe
estimated indoorthe
evaluate cooling set
proposed
temperature.
control methodAirofflow rate is
the VAV determined
terminal unit. by
Anthe coolingofload,
analysis and a damper
the indoor thermaliscomfort
operated
wasto conducted
maintain a
set indoor temperature between the maximum and
using the standards of the indoor set temperature (24 C). ◦
minimum air flow rates.
An evaluation of the IAQ was conducted based on the indoor CO2 concentration. The standard of
the indoor CO2 concentration was 1000 ppm. Stratification was considered to have occurred when the
temperature difference between 0.1 m and 1.1 m from the floor was more than 3 ◦ C [26].

4.1. Indoor Environment Comfort

4.1.1. Indoor Temperature

Figure 8 shows the indoor temperatures in the case of peak load in summer and winter in
the proposed VAV terminal unit control method. In every zone, indoor temperatures are 23–25 ◦ C,
which meets the 24 ± 1 ◦ C indoor set temperature.
4.1. Indoor Environment Comfort

4.1.1. Indoor Temperature

Figure 8 shows the indoor temperatures in the case of peak load in summer and winter in the
proposed VAV
Sustainability 2017, 9, terminal
unit control method. In every zone, indoor temperatures are 23–25°C,
2066 12 of 17
which meets the 24 ± 1 °C indoor set temperature.

Figure 8. Thermal comfort analysis data of the proposed control method.

Figure 8. Thermal comfort analysis data of the proposed control method.

4.1.2. Indoor CO2 Concentration

4.1.2. Indoor CO2 Concentration
Figure 9a shows an indoor CO2 concentration in the case of peak load in winter in the
FigureVAV
proposed 9a shows
terminalan indoor CO2 concentration
unit control algorithm. Indoorin theCOcase of peak load in winter in the proposed
2 concentration remained at 1000 ppm or
VAV
lowerterminal
under most unit control algorithm.
conditions, whichIndoor
meets CO concentration
the2IAQ standards.remained
In Zone at 2, 1000
thereppm or lower
appeared under
a section
most conditions,
in which CO which
2 concentration
meets the IAQ standards. In Zone 2, there appeared a
exceeded 1000 ppm due to an increase in the number of occupants.section in which CO2
Sustainability 2017, 9, 2066 12 of 17
concentration exceeded 1000 ppm due to an increase in the number of occupants. However, indoor
CO 2 concentration
However, indoor CO is 2kept the same is
concentration as,kept
or below,
the same the as,
standard
or below,valuethewith the operation
standard value with of the
the
ventilation mode according to the proposed VAV terminal unit control
operation of the ventilation mode according to the proposed VAV terminal unit control method. method. Figure 9b shows
an indoor
Figure 9b CO 2 concentration
shows an indoor in COthe case of peak load in summer. Indoor CO2 concentration
2 concentration in the case of peak load in summer. Indoor was kept
CO2
at 1000 ppm, or lower, under most conditions. In Zone 2, there appeared
concentration was kept at 1000 ppm, or lower, under most conditions. In Zone 2, there appeared a2 a section in which CO
concentration
section in which exceededCO21000 ppm with anexceeded
concentration increase in1000
the number
ppm with of occupants.
an increase However,
in thethe proposed
number of
VAV terminal unit control increased the air flow rate or outdoor air flow
occupants. However, the proposed VAV terminal unit control increased the air flow rate or outdoor rate, thus keeping the
indoor
air flowCO 2 concentration
rate, thus keeping below the standard
the indoor value. Indoor below
CO2 concentration CO2 concentration
the standardwas higher
value. in winter
Indoor CO2
than in summer. If outdoor air temperature falls below a specific
concentration was higher in winter than in summer. If outdoor air temperature falls below atemperature with the operation
of the economizer,
specific temperaturethe withAHUthe minimizes
operation ofthetheoutdoor air flow
economizer, the rate,
AHUthus increasing
minimizes the indoor
the outdoor CO2
air flow
concentration in winter. If the outdoor air temperature rises above a specific
rate, thus increasing the indoor CO2 concentration in winter. If the outdoor air temperature rises temperature, the AHU
minimizes the outdoor
above a specific air flow rate,
temperature, the but
AHU indoor CO2 concentration
minimizes the outdoor is considered
air flow rate, to bebut
keptindoor
low due CO to2
the high air flow rate at the terminal unit.
concentration is considered to be kept low due to the high air flow rate at the terminal unit.

(a) (b)
Figure 9.
9. Indoor CO2
Indoor COconcentration analysis data of the proposed control method: (a) winter; and (b)
Figure 2 concentration analysis data of the proposed control method: (a) winter;
summer.
and (b) summer.

4.1.3. Indoor Vertical Temperature Difference

Stratification was considered to have occurred when the temperature difference between 0.1 m
and 1.1 m from the floor was more than 3 °C. “To analyze the stratification, the space was separated
into 17 horizontal nodes with 0.2 m intervals from the bottom. This divided the internal heating
 (a) (b)
Sustainability
Figure 2017, 9, 2066CO2 concentration analysis data of the proposed control method: (a) winter; and (b)
9. Indoor 13 of 17
summer.

4.1.3. Indoor Vertical Temperature Difference

4.1.3. Indoor Vertical Temperature Difference
Stratification was considered to have occurred when the temperature difference between 0.1 m
Stratification was considered to have occurred when the temperature difference between 0.1 m
and 1.1 m from the floor was more than 3 ◦ C. “To analyze the stratification, the space was separated into
and 1.1 m from the floor was more than 3 °C. “To analyze the stratification, the space was separated
17 horizontal nodes with 0.2 m intervals from the bottom. This divided the internal heating elements
into 17 horizontal nodes with 0.2 m intervals from the bottom. This divided the internal heating
among each horizontal node” [16]. Figure 10 shows the indoor vertical temperature difference in the
elements among each horizontal node” [16]. Figure 10 shows the indoor vertical temperature
proposed control algorithm at the peak load in winter. The proposed algorithm satisfied the indoor set
difference in the proposed control algorithm at the peak load in winter. The proposed algorithm
temperature without stratification in any zone.
satisfied the indoor set temperature without stratification in any zone.

Figure 10. Vertical room air temperature data of the proposed control method.
Figure 10. Vertical room air temperature data of the proposed control method.

4.2. Energy Consumption

The VAV terminal unit control method of the existing case is a fixed minimum air flow rate
control. The minimum air flow rate is 30% of the maximum air flow rate. As the existing terminal unit
minimum air flow rate is set at a fixed value, the fixed air flow rate is supplied even in the case of
low indoor load and causes the unnecessary consumption of fan and reheating energy, which leads
to an increase in total energy consumption. Accordingly, this study proposed a control method for
making the minimum air flow rate float according to the load and stabilizing the IAQ even in the
case of sudden load change. The method satisfied both the indoor thermal comfort and the IAQ.
By analyzing energy consumption, this chapter evaluated the VAV terminal unit control method and
the CO2 integration control method to which the existing fixed minimum air flow rate was applied.
Energy analysis of the existing VAV terminal unit control method and the proposed control method
presented in this chapter was conducted. Table 7 shows the simulation case.

Table 7. Simulation case.

Case Control Logic Classification

Existing control method of the VAV terminal unit in
multi-zone
Existing case Fixed minimum air flow rate
Constant minimum air flow setpoint: 30% of the
maximum air flow rate
CO2 integration control method of the VAV system
Proposed case Variable minimum air flow rate
Variable minimum air flow setpoint
 Table 7. Simulation case.

Case Control Logic Classification

Existing control method of the VAV terminal unit in
multi-zone
Existing case Fixed minimum air flow rate
Sustainability 2017, 9, 2066 Constant minimum air flow setpoint: 30% of the maximum air 14 of 17
flow rate
CO2 integration control method of the VAV system
Proposed case Variable minimum air flow rate
Variable minimum air flow setpoint
Figure 11 shows AHU component-specific energy consumption by case. Reheating coil energy
was approximately 10.4 AHU
Figure 11 shows GJ and 8.3 GJ in the existing
component-specific control method
energy consumption Case
by case. 1 andcoil
Reheating theenergy
proposed
control
wasmethod Case 2, respectively:
approximately 10.4 GJ and 8.3 energy
GJ inconsumption decreased
the existing control by approximately
method Case 1 and the 20% in Case 2,
proposed
control method
as compared to CaseCase 2, the
1. In respectively: energy
cooling coil, consumption
energy decreased by 82,000
was approximately approximately
MJ and20%
69 GJin Case
in Case 1
2, as compared
and Case to Caseenergy
2, respectively: 1. In thedecreased
cooling coil,
byenergy was approximately
approximately 82,0002,MJ
15% in Case asand 69 GJ in to
compared Case
Case 1.
1 and Case 2, respectively: energy decreased by approximately 15% in Case 2, as compared
In the supply fan, energy was approximately 17 GJ and 15 GJ in Case 1 and Case 2, respectively: to Case
1. In the supply fan, energy was approximately 17 GJ and 15 GJ in Case 1 and Case 2, respectively:
energy decreased by approximately 15% in Case 2, as compared to Case 1. Total energy consumption
energy decreased by approximately 15% in Case 2, as compared to Case 1. Total energy
decreased by approximately 17 GJ, 16% at the AHU, including the reheat and cooling coils and the
consumption decreased by approximately 17 GJ, 16% at the AHU, including the reheat and cooling
supply fan.
coils and the supply fan.

Figure 11. Comparison of the AHU energy consumption between the existing and proposed control
Figure 11. Comparison of the AHU energy consumption between the existing and proposed
methods.
control methods.
Figure 12 compares the reheating coil energy consumption of each zone. Annual reheating coil
energy 12
Figure consumption
compares the wasreheating
reduced atcoil
least 34%, consumption
energy and up to 94%.of This
eachresult
zone.was caused
Annual by the coil
reheating
energy consumption was reduced at least 34%, and up to 94%. This result was caused by the each
resetting of the minimum air flow rate and supply temperature. The supplied air flow rate of resetting
of theSustainability
minimum 2017,
air9, flow
2066 rate and supply temperature. The supplied air flow rate of each14zone of 17 was

decreased by applying a floating minimum air flow rate and energy was saved by resetting the supply
zone was decreased by applying a floating minimum air flow rate and energy was saved by
air temperature lower than the fixed temperature.
resetting the supply air temperature lower than the fixed temperature.

Figure 12. Comparison of the reheating coil energy consumption between the existing and proposed
Figure 12. Comparison of the reheating coil energy consumption between the existing and proposed
control methods.
control methods.
Figure 13 compares energy consumption, including those of the AHU and reheating coil. The
total energy consumption at the reheating coil of the terminal unit showed approximately 118 GJ,
according to the existing terminal unit control method, but decreased by approximately 68%
according to the proposed terminal unit control method. This result was caused by the resetting of
the minimum air flow rate and supply temperature. The supplied air flow rate of each zone was
decreased by the application of a floating minimum air flow rate, and energy was saved by
 Figure 12. Comparison of the reheating coil energy consumption between the existing and proposed
control2017,
Sustainability methods.
9, 2066 15 of 17

Figure 13 compares energy consumption, including those of the AHU and reheating coil. The
total Figure
energy 13 compares energy
consumption consumption,
at the reheating coil ofincluding
the terminal those
unitofshowed
the AHU and reheating
approximately 118coil.
GJ,
The total energy consumption at the reheating coil of the terminal unit
according to the existing terminal unit control method, but decreased by approximately showed approximately 11868%
GJ,
according to
according to the
theexisting
proposed terminal
terminal unitunit
control method,
control method.but decreased
This resultbywas approximately
caused by the68% according
resetting of
to the proposed terminal unit control method. This result was caused by the
the minimum air flow rate and supply temperature. The supplied air flow rate of each zone was resetting of the minimum
air flow rate
decreased byandthe supply temperature.
application The supplied
of a floating minimum airair
flow raterate,
flow of each
and zone
energywaswas
decreased
saved by by
the application
lowering of a floating
the supply minimum
temperature air flow
below rate, and
the fixed energy was
temperature. saved
Total by lowering
energy the supply
consumption was
temperature below the fixed temperature. Total energy consumption
approximately 228 GJ and 135 GJ at Case 1 and Case 2, respectively. Energy consumption was approximately 228 GJ was
and
135 GJ at Case
decreased 1 and Case 2, respectively.
by approximately Energy consumption
41% by the proposed terminal unitwas decreased
control by approximately
method, as compared to41% the
by the proposed
existing one. terminal unit control method, as compared to the existing one.

Figure 13. Comparison of the total energy consumption between the existing case and the proposed
Figure 13. Comparison of the total energy consumption between the existing case and the
case.
proposed case.

5. Conclusions
The minimum air flow rate of the VAV terminal unit is a chief factor that has a direct influence on
indoor heat comfort, IAQ, and energy consumption. This makes it very important to select an optimal
minimum air flow rate. According to the existing VAV terminal unit control, indoor temperature,
alone, is controlled using a fixed minimum air flow rate, and IAQ can be affected by the change in the
number of occupants. Therefore, this study proposed a VAV terminal unit control method considering
an indoor ventilation requirement in a multi-zone, and evaluated a terminal unit control method
proposed in comparison with the existing one.

(1) The target space selected eight zones that are controlled using an AHU and applied the fixed
minimum air flow rate of the VAV terminal unit. The simulation results showed that the indoor
set temperature was maintained. However, when the number of occupants increased suddenly,
only the indoor temperature was checked and controlled according to the existing terminal unit
control method, which created IAQ problems. To solve the problem of the existing terminal unit
control method, the present study proposed a terminal unit control method by the application of
a variable minimum air flow rate and the increase in the terminal unit air flow rate, AHU air flow
rate, and outdoor air flow rate.
(2) IAQ control was improved by the terminal unit and AHU air flow rate increase method,
as compared to the existing control method. However, when the outdoor air flow rate was
low in winter, the IAQ was not maintained even by the supplied maximum air flow rate of
the terminal unit. When the IAQ was not controlled, even at the maximum air flow rate of the
terminal unit, the air flow rate was supplied, with its CO2 concentration lowered by the increase
Sustainability 2017, 9, 2066 16 of 17

in the air flow rate of the AHU. This increased the air flow rate in zones with stable indoor
thermal comfort and air quality, thus exceeding the set indoor temperature.
(2) This study proposed a VAV terminal unit CO2 integration control method for increasing the
outdoor air flow rate according to the increase in air flow rate at the terminal unit and AHU on
the basis of indoor CO2 concentration. Any CO2 problem led to the operation of the ventilation
mode, which increased the air flow rate and solved the IAQ. Indoor thermal comfort was also
stable, even when the air flow rate was increased with the resetting of the supply temperature by
the increase in the air flow rate. The proposed VAV terminal unit control method satisfies all the
conditions of indoor thermal comfort, IAQ, and stratification. An energy comparison with the
existing control method showed that the method not only satisfies the indoor thermal comfort,
IAQ, and stratification, but also reduces energy consumption.

Acknowledgments: This research was supported by a grant (17 CTAP-C115251-02) from the Technology
Advancement Research Program (TARP) funded by the Ministry of Land, Infrastructure, and Transport of
the Korean government.
Author Contributions: All authors contributed to this work. Hyo-Jun Kim performed the results analysis and
wrote the majority of this article. Young-Hum Cho was responsible for this article and gave conceptual advice.
Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature
Qh Room heating road, kw
ρ Standard air density, kg/m3
cp Specific heat capacity, kj/kg ◦ C
Ts Supply air temperature, ◦ C
Tr Room air temperature, ◦ C
.
V min,v Minimum air flow rate for ventilation, m3 /h
.
V min,h Air flow rate for heating load, m3 /h
Rp Outdoor airflow rate required per person from ASHRAE Standard 62.1–2010 m3 /h·person
Pz Zoon population, person
Ra Outdoor airflow rate required per unit area from ASHRAE Standard 62.1–2010 m3 /h·m2
Az Zoon floor area, m2
Z Critical zone outdoor air flow rate ratio
X Uncorrected AHU outdoor air flow rate ratio
.
V oa Air flow rate for outdoor air (ventilation) requirement of AHU, m3 /h
.
V da Supply air flow rate for each zone, m3 /h
Y Corrected AHU outdoor air flow rate ratio
.
V min Minimum air flow rate, m3 /h

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