The flow chart of a fixed dry-bulb temperature economizer.

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Determination of free cooling potential: A case study for _ Istanbul, Turkey Hüsamettin Bulut a, Mehmet Azmi Aktacir a

a Harran University, Engineering Faculty, Department of Mechanical Engineering, Osmanbey Campus, 63190 Sanlıurfa, Turkey.

article info
Article history:
Received 11 May 2010
Received in revised form 26 August 2010
Accepted 31 August 2010
Keywords:
Air conditioning
Free cooling
Economizer
Energy saving
Istanbul
Turkey

appliedenergy

Abstract

A significant portion of energy consumed in buildings is attributed to energy usage by heating, ventilating

and air conditioning (HVAC) systems. Free cooling is a good opportunity for energy savings in air condi-
tioning systems. With free cooling, commonly is known economizer cycle, the benefits of lower ambient

temperatures are utilized for a significant proportion of the year in many climates. The detailed analysis
of local weather data is required to assess the benefits of economizer. In this study, free cooling potential
of  Istanbul, Turkey was determined by using hourly dry-bulb temperatures measurements during a per-
iod of 16 years. It is found that the free cooling potential varies with supply air temperature and months.

It is determined that although there are substantial energy savings during a significant portion of the year
especially in transition months (April, May, September and October), the high outdoor air temperatures
from June to August, made the system not beneficial for free cooling except at high supply air
temperature.

1. Introduction

The higher living and working standards, the adverse outdoor

conditions in urban environments and reduced prices of air-condi-
tioning units, have caused a significant increase in demand for air

conditioning in buildings. On the other hand, heating, cooling, ven-
tilating and air conditioning (HVAC) systems are major responsible

for energy consumption in buildings. As reported in the literature,
air-conditioning energy consumption shows an increasing trend
[1,2]. In recent years, many solutions have been suggested for
reducing energy consumption in buildings. Solutions are mainly

about initial stage of architectural design related to thermal perfor-
mance of building and the correct selection of HVAC system.

Reducing energy use for space cooling in buildings is a key measure
to energy conservation and environmental protection. The yearly

cooling load and the peak cooling demand of building can be re-
duced significantly in the thermally insulated buildings [3,4]. At

the same time, there has been a rapid change in the technology

of air conditioning. Energy conservative building design has trig-
gered greater interests in developing flexible and sophisticated

air conditioning systems capable of achieving enhanced energy-
savings potential without sacrificing the desired thermal comfort

and indoor air quality (IAQ) [5]. Various types of variable air vol-
ume (VAV) systems, air and water economizer, heat recovery, ther-
mal storage, desiccant dehumidification, variable-speed drives, and

direct digital control (DDC) devices have become more effective

and more advanced for energy efficiency [6]. A considerable

amount of energy can be saved if the HVAC system is properly de-
signed, operated and controlled. In all-air HVAC systems using an

economizer cycle can result in considerable energy savings [7].

Although economizer systems have existed for many years, in re-
cent years, many packaged unit manufacturers more extensively

offer air economizers to provide free cooling for energy savings
as well as to improve indoor air quality.

Free cooling application, commonly is known economizer cy-
cle, is used when outside conditions are suitable, that is, when

outside air is cool enough to be used as a cooling medium [8].
Two types of economizers are in use today. Those are water-side
economizer and air-side economizer. The air-side economizer
takes advantage of cool outdoor air to either assist mechanical
cooling or, if the outdoor air is cool enough, provide total cooling.
In an all air conditioning system, outdoor air is used as supply air.
The water-side economizer consists of a water coil located in the
self-contained unit upstream of the direct-expansion cooling coil.
ASHRAE Standard 90.1 addresses the application of water-side
economizer [8].

One method of improving the indoor air quality (IAQ) is to in-
crease the ventilation. Due to the fact that the outdoor air is used

directly in free cooling applications; a high indoor air quality can
be achieved. Providing high indoor air quality, compared with
the mediocre air that is present in many existing office building
worldwide, may increase productivity by an estimated 5–10%. An

annual loss of this magnitude caused by mediocre indoor air qual-
ity will often be much higher than energy costs, capital costs, and

the cost of operating the building [9].

 The location of _ Istanbul on the map of Turkey.

                                            Schematic of a typical all air conditioning system.

The potential of free cooling represents a measure of the
capability of ventilation to ensure indoor comfort without using
mechanical cooling systems [10]. Free cooling is not alternative
of mechanical cooling, it must be thought as complementary
and supportive application for air conditioning system. Available
studies revealed that considerable energy savings could be

achieved using the free cooling under different climatic condi-
tions. Olsen et al. [11] showed that low-energy cooling systems

that maximize free cooling from outside air have the best energy
performance under mild UK climate conditions. Budaiwi [7]
investigated energy performance of the economizer cycle under
three climatic conditions in Saudi Arabia and presented signifi-
cant results for HVAC designers and operators seeking energy
efficiency in buildings through the economizer cycle. Wacker

[12] investigated the energy-savings potential and indoor com-
fort implications of economizer controls for packaged rooftop

HVAC equipment under weather conditions in Asheville, North
Carolina, USA. Karunakaran et al. [5] proposed a combined
variable refrigerant volume (VRV) and variable air volume

(VAV) air conditioning system that was controlled by the intelli

gent fuzzy controller. They analyzed and tested the VRV–VAV air
conditioning system for the summer and winter conditions of

Chennai, India under fixed ventilation, demand controlled venti-
lation (DCV) and combined DCV–economizer cycle ventilation.

The proposed system experimentally analyzed under fixed venti-
lation, demand controlled ventilation (DCV) and combined DCV

and economizer cycle ventilation techniques effectively con-
served 44% and 63% of per day average energy savings in sum-
mer and winter design conditions respectively, while compared

to the conventional constant air volume (CAV) air conditioning
system.

The control regions of a fixed dry-bulb temperature economizer on psychrometric chart.

The flow chart of a fixed dry-bulb temperature economizer.

Cooling energy requirements can be also reduced by using low-
energy technologies. Of the available technologies, night ventila-
tion, which is one of free cooling applications, is particularly suited

to office buildings because these are usually not occupied during
the night. Night ventilation works by using natural or mechanical
ventilation to cool the surfaces of the building fabric at night and

is more effective where a building includes a reasonably high ther-
mal mass, so that heat can be absorbed during the day [13]. The

effectiveness of night ventilation technique for residential build-
ings in hot-humid climate of Malaysia was investigated by Kubato

et al. [14].

The most important and characteristic parameter for free cool-
ing is local climatic features. Because free cooling potential is a

function of outdoor climate. So, only detailed analysis of weather
data reveals the free cooling potential of a region. Aktacir and Bulut

carried out some useful studies on the free cooling poten-
tial of different regions of Turkey. The free cooling potential of Kay-
seri, one of the main provinces of Central Anatolia, was determined

by using hourly outdoor temperatures by Aktacir and Bulut [15].
The result of their study showed that the free cooling potential is
high during cooling season (from 15 May to 30 September) and
especially in transit months (April and October) for Kayseri. The
free cooling potential of Antalya, located in Mediterranean region

of Turkey, was analyzed by Aktacir and Bulut [16]. It was deter-
mined that while the region has low free cooling potential during

cooling season (from June to August), the transitions months
(April, May, September and October) have the highest potential.
Aktacir and Bulut [17] also investigated the free cooling potential
of _
Izmir, one of the biggest cities of Turkey and located in Aegean

region. They found that the free cooling potential significantly var-
ies with supply air temperature and months in Aegean region.

Their study showed that the Aegean region has low free cooling po-
tential during cooling season (from June to August) and the transi-
tions months (April, May, September and October) have significant

potential.
The main purpose of this study is to determine and analyse the
free cooling potential in _

Istanbul (latitude: 40580

N, longitude:

29050
E and elevation: 39 m). The location of _

Istanbul is shown

on the map of Turkey in Fig. 1. _

Istanbul is the largest city of Turkey

and the third largest city in the world. _

Istanbul is located in the

north-west Marmara region of Turkey. _

Istanbul is the only city in

the world that spans over two continents. It encloses the southern
Bosphorus which places the city on two continents – the western
portion of _
Istanbul is in Europe, while the eastern portion is in Asia.
_
Istanbul has always been the center of the country’s economic life
and industry because of its location as an international junction of
land and sea trade routes. The city has a temperate climate with

hot and humid summers; and cold, wet winters. The city has a pop-
ulation of 12,573,836 residents according to the latest count as of 2007.

Variation of extreme and mean temperatures throughout the year and heating, cooling and transition periods for _ Istanbul.

Monthly total Nbin values (h/month) for _ Istanbul.

Monthly total Nbin values (h/month) for _ Istanbul.

2. Economizer cycle and economizers in HVAC systems
An economizer cycle is an air conditioning cycle that utilizes
the free cooling capacity of outdoor air either directly or to cool

the condenser water in a cooling tower (or an evaporative cool-
er), and then to cool the air indirectly, instead of using refriger-
ation to provide cooling/dehumidification so as to maintain a

required space temperature. The component and devices used
in the operation of an economizer cycle are collectively called

an economizer, and the type of control used to operate the econ-
omizer cycle effectively and energy-efficiently is called econo-
mizer control. There are two types of economizers, typically

referred to as air-side and water-side economizers. An air econ-omizer consists of outdoor, exhaust, relief, and recirculating ducts and dampers in the air handling unit or packaged unit,

as well as a control system to operate them. Air economizer con-
trol can be subdivided into enthalpy-based differential enthalpy,

fixed enthalpy, and electronic enthalpy economizer controls and
also temperature-based fixed dry-bulb and differential dry-bulb
economizer controls. Water-side economizers use the outdoor
air to cool the condenser water or cooling water in the cooling
tower or evaporative cooler first, and then to cool the mixture
of outdoor and recirculating air through a precooling coil. A
water economizer consists mainly of a cooling tower (or an
evaporative cooler), a water precooling coil in the air handling
unit or packaged unit, a circulating pump to circulate cooling

water, and the associated control system [6]. This article dis-
cuses the use of free cooling potential in air-side economizers

for an all-air HVAC system.
Fig. 2 shows a schematic of a typical all-air HVAC system which
consists of an air handling unit, a refrigeration (chiller) system,
dampers, fans and air ducts. A significant energy saving is possible
when the system is properly switched over to an economizer cycle.
At the onset of economizer operation, return dampers are closed,
outside dampers are opened, and the maximum possible outside

air is supplied to cooling coils. The control algorithms for switchover are typically classified into dry-bulb temperature based and en-
thalpy based [8]. With the economizer cycle, the need for mechan-
ical cooling can be eliminated completely (free cooling) or reduced

partially (partial free cooling) whenever outdoor air conditions al-
low. The energy cost savings can be very significant, depending on

the economizer load and the hours of operation.

        Variation of monthly total Nbin values for _ Istanbul.

Many control methods and applications which determine set

points and the control strategy for an economizer are recom-
mended to maximize energy saving [8]. The enthalpy econo-
mizer control yields lower overall energy consumption.

Simulation results by Wacker [12] show that enthalpy econo-
mizer saves 5–50% more compressor energy compared to dry-
bulb economizer with switchover set point of 24 C in the six

different cities of USA. Comparing the enthalpy of outdoor air
with that of recirculating air requires wet-bulb temperature or

dry-bulb temperature and relative humidity measurement. In ac-
tual practice, humidity sensors may demonstrate considerable

errors (sometimes up to 10%) and have extensive maintenance
requirements. Dew point sensors are delicate and expensive
and cause maintenance difficulties. In spite of the superiority
of enthalpy economizer, its application is greatly impeded by

the so-far notoriously unreliable humidity measurement of out-
door air [19]. Therefore, it is simpler and more convenient to

use only temperature sensors and to compare the outdoor air

temperature to with the recirculating temperature (or a prede-
termined set point) instead of sensing and comparing enthalpies.

This method of control is called temperature economizer control
[6]. Outside weather zones using temperature-based economizer
control are shown in Fig. 3. As shown in Fig. 3, in fixed dry-bulb
temperature economizer, the psychrometric chart can be broken
down into three regions by selecting an appropriate supply air
temperature (typically between 13 and 18 C) and indoor
temperature (typically set point is 24 C). When outdoor air
temperature is greater than indoor air temperature (region 1:
mechanical cooling mode), the outdoor and exhaust air dampers
will be at their minimum opening and the mechanical cooling is
needed. When outdoor air temperature is located in region 2
(partial free cooling mode), bounded by the indoor and supply
air dry-bulb temperature, 100% outdoor air is used since the

outdoor air dry-bulb temperature is less than the room temper-
ature and the mechanical cooling can be required. But, the oper-
ation hours of the refrigeration system are less than that of

region 1. In region 3 (free cooling mode) , since the outdoor
air temperature is less than supply air temperature, no need
for mechanical cooling and the free cooling capacity of outdoor
air is utilized directly. In free cooling mode, the outdoor air

Cumulative distribution of yearly total bin data for _ Istanbul.

ENERGY

ENERGY

COOLİNG

Fig. 8. Variation of monthly free cooling, night cooling and mechanical cooling
potential for _ Istanbul throughout year.

Variation of monthly free cooling potential at different supply air temperatures.

Fig 9: Variation of monthly free cooling potential at different supply air
temperatures.

chart of a fixed dry-bulb temperature economizer is depicted
in Fig. 4.
3. Analysis of outdoor air temperature
Detailed analysis of outdoor climatic conditions is required in

order to assess free cooling potential. In this study, the cooling sea-
son for _

Istanbul was determined by using long term daily mean

dry-bulb temperature. Fig. 5 shows the variation of daily mini-
mum, maximum and mean dry-bulb temperature data throughout

the year for _
Istanbul. These dry-bulb temperature data is obtained
from the means of 16 years (1981–1996) the long-term measured
data. As seen from Fig. 5, by choosing daily mean temperature
20 C (mean of maximum temperatures is about 24 C) as base
temperature, cooling season starts at the month of June (152nd
day), and ends at the month of September (273rd day). If the daily

mean temperatures are between 13 C and 20 C, this period is ac-
cepted as transition period. Thus, April, May and October can be

handled as the transition months for _
Istanbul.

In this study, the bin data for the dry-bulb temperature is used
to determine the potential of free cooling. The bin method requires
the bin weather data. Bin method is based on the concept that all

the hours of a month, season or year, when a particular tempera-
ture interval (bin) occurs, can be grouped together and the energy

calculations can be performed for those hours with the equipment

operating under those particular conditions. The instantaneous en-
ergy requirements are calculated at different values of the outdoor

dry-bulb temperature (To,i) and then are multiplied by the corre-
sponding number of hours (Nbin,i) in the temperature interval

(bin) centered on that temperature [20,21]. The result is the energy
consumption Qbin,i at the corresponding bin:


where Ktot and g are, respectively, total heat loss coefficient of the
building, and the efficiency of the HVAC system. The balance point
temperature Tb is the value of outdoor temperature below or above
which heating or cooling is needed. The bin method can be used for
estimating both heating and cooling energy requirements. The plus
subscript on the parenthesis of Eq. (1) is for heating and indicates

that only positive values are to be counted. For cooling, only nega-
tive values should be considered. Eq. (1) gives only the sensible en-
ergy requirements. The latent energy requirements can also be

calculated if the mean coincident wet-bulb temperature at each

temperature bin is known. Qbin,i values, which are calculated sepa-
rately for each temperature interval (bin), are summed to obtain to-
tal energy consumption:

where m is the total number of the temperature intervals (bins).
This procedure can be performed either with monthly or with
yearly data. It can account for the part-load performance of heating,

ventilating and air-conditioning equipment as well as for the vary-
ing performance of heat pump systems and primary HVAC equip-
ment. Since the ambient temperature effect on equipment

efficiency is taken into account, the accuracy of the energy calcula-
tions is significantly improved as compared to that of the degree-
day methods. The bin method can account for the part-load perfor-
mance of HVAC equipment and has been specially used for analysis

of heat pump systems. Additionally, by performing separate calcu-
lations for different time periods, variations of indoor loads with

time, occupancy patterns and operating schedules of HVAC systems
can be considered [20].
In this study, the bin data for dry-bulb temperature from 9 C
to 39 C with 3 C increments were calculated in six daily 4-h shifts
(1–4, 5–8, 9–12, 13–16, 17–20 and 21–24 h) for _

Istanbul. Bin data
were determined using hourly dry-bulb temperatures measured by

The State Meteorological Affairs General Directorate (Turkish ini-
tials DM_

I) in _

Istanbul (Göztepe) during a period of 16 years (be-
tween 1983 and 1998). Monthly bin data for cooling and

transition period are given in Table 1 in 4-h periods. The smallest
temperature bin observed in _

Istanbul is 4.5 C (6 C/3 C) with
11 h in February, whilst the maximum bin observed is 34.5 C
(33 C/36 C) with 3 h in July. Annual Nbin values for six separate
time periods of the day are presented in Table 2. As can be seen
from the table, the maximum yearly total Nbin value is 1144 h in
7.5 C (6 C/9 C) temperature interval.
Fig. 6 shows distribution of monthly total Nbin values for _

Istan-
bul. Heating and cooling periods can also be estimated from Fig. 6.

As seen from Fig. 6, while winter months lay the left side of the

graph, the summer months are at the right side of graphs. The tran

sitions season remains in the middle of the graph. Cumulative dis-
tribution of yearly bin data for _

Istanbul is shown in Fig. 7. From
Fig. 7, one can get approximately the number of hours for heating
season or cooling season by choosing a base temperature. For
example, it can be easily determined that the heating season is
6200 h for base temperature of 18 C and cooling season is 980 h
for 22 C base temperature.
4. Analysis of free cooling potential
4.1. Monthly potential of free cooling
Free cooling potential for an all-air HVAC system in _
Istanbul,

Turkey were determined for different outdoor air temperature val-
ues (15 C, 18 C, 21 C and 24 C). It is accepted that the supply air

temperature (Tsa) equals or less than the outside temperature and a
temperature raise due to fan dissipation is neglected. In Fig. 8,
monthly distribution of free cooling, night cooling and mechanical
cooling for _
Istanbul are shown when outdoor air temperature
equal or less than the supply air temperature, i.e. when Tsa 6 15 C,
Tsa 6 18 C, Tsa 6 21 C and Tsa 6 24 C. For night cooling, 17:00–

08:00 time period was taken out in the analysis. The impact of sup-
ply air temperature on free cooling potential is obvious as indi-
cated by Fig. 8. Variations of monthly free cooling at different

supply air temperatures are shown in Fig. 9. As shown in Figs. 8
and 9, free cooling potential is high in April and October but the
significant portion of this potential is appearing in night period.
From June to August, the high outdoor air temperature made the
system not beneficial for free cooling except at high supply air
temperature. The free cooling potential goes up with increase of
supply air temperature. The higher the supply air temperature,
the greater is the cooling potential of the outdoor air. The increase
of free cooling potential for Tsa = 24 C is very significant. Almost
more than half of the air-conditioned time from June to August
was favorable for free cooling to save coil energy consumption

Hourly free cooling potential at different supply air temperatures.

for Tsa = 24 C. The values of free cooling potential in the period of
April to October are relatively high and should not be neglected.
4.2. Daily potential of free cooling

Hourly variation of free cooling potential when outdoor tem-
perature equal or less than the supply air temperature of 15 C,

18 C, 21 C and 24 C is given in Table 3. As indicated by Table 3,
the free cooling potential is high in April, May, and October as

months of the transition period. As the supply air temperature in-
creases, this potential increases parallelly. The free cooling poten-
tial remains relatively low in June, July, August and September as

month of cooling season. The months of July and August which

are the hottest month in the year has the lowest free cooling po-
tential. As expected, while free cooling potential is low during day-
time period especially during midday hours when the outdoor air

temperature is high, the potential is high during nighttime period
which has relatively low outdoor air temperature. It is evident that
there is no significant potential during nighttime period of cooling
season (especially July and August). But, at high value of supply air
temperature (i.e. Tsa24 6 24 C), the potential is up to 100%.
As a case study, the operation hours of an all-air HVAC system
(indoor set temperature is 24 C) with economizer (free cooling
mode or partially free cooling mode) and without economizer
(mechanical cooling mode) is given in Table 4. As shown in Table 4,

for Tsa 6 15 C at period of 09:00–12:00 in April, 76 h of total run-
ning hours of 120 is free cooling mode and the remaining time is

mechanical cooling mode. For Tsa 6 24 C at the same period, there
are 76 h for free cooling, 43 h for partially free cooling, and only 1 h
for mechanical cooling.

The amount of the saving energy in a chiller system for yearly
operation of an all-air HVAC system with economizer is given in
Table 5. In this case, for Tsa 6 24 C, cooling unit installed in _

Istan-
bul will achieve 100% free cooling for 4671 h which represents 54%

of the year. While, cooling unit will achieve partial free cooling for
3108 h, which is 35% of the year. Therefore, total free cooling is

available for 89% of the year, which represents major potential en-
ergy and cost savings. If the installed compressor power is 500 kW

in the refrigeration unit, the average part-load ratio is 0.5 and the
average energy cost in _

Istanbul is 11 c/kWh, then the saving operating
cost for Tsa 6 24 C will be 342.375 $ per annum as shown in
Table 5.
5. Conclusion and recommendations
The determination and analysis of free cooling potential for an
all-air HVAC system is carried out under outdoor conditions of
_
Istanbul, Turkey. It is found that there is an energy saving potential
during a significant portion of the year especially in transition

months. The free cooling potential varies with supply air tempera-
ture and months. It is determined that the transitions months

(April, May, September and October) have the highest potential.
From June to August, the high outdoor air temperature made the
system not beneficial for free cooling except at the high supply
air temperature. As increase in supply air temperature, the greater
potential for energy savings by the economizer cycle can be
achieved.
The economizer cycle is a proven method for allowing many
hours of free cooling in many applications at lower operating cost.

Parallel to the economic growth of Turkey, package air-condition

ers are used more and more frequently for thermal comfort. There-
fore, the HVAC systems which have a free cooling option should be

preferred, if the climate is favorable. In order to find the exact ben-
efit of economizer cycle, an economic assessment including cost

analysis and detailed weather data analysis should be carried
out. The free cooling potential of the other locations should be also
determined.

The hours of operation of an all-air HVAC system with economizer and without economizer during transition months.

Yearly operation of an all-air HVAC system with economizer and the amount of the saving energy for chiller system.

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