This article appeared in a journal published by Elsevier. The attached
copy is furnished to the author for internal non-commercial research
and education use, including for instruction at the authors institution
and sharing with colleagues.
Other uses, including reproduction and distribution, or selling or
licensing copies, or posting to personal, institutional or third party
websites are prohibited.
In most cases authors are permitted to post their version of the
article (e.g. in Word or Tex form) to their personal website or
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.
Received 11 May 2010
Received in revised form 26 August 2010
Accepted 31 August 2010
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
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) . 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 . 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 .
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 .
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
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 .
The potential of free cooling represents a measure of the
capability of ventilation to ensure indoor comfort without using
mechanical cooling systems . 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.  showed that low-energy cooling systems
that maximize free cooling from outside air have the best energy
performance under mild UK climate conditions. Budaiwi 
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
 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.  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
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 . The
effectiveness of night ventilation technique for residential build-
ings in hot-humid climate of Malaysia was investigated by Kubato
et al. .
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 .
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 . 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  also investigated the free cooling potential
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
The main purpose of this study is to determine and analyse the
free cooling potential in _
Istanbul (latitude: 40580
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.
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 . 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 . 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.
Many control methods and applications which determine set
points and the control strategy for an economizer are recom-
mended to maximize energy saving . The enthalpy econo-
mizer control yields lower overall energy consumption.
Simulation results by Wacker  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 . 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
. 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
Fig. 8. Variation of monthly free cooling, night cooling and mechanical cooling
potential for _ Istanbul throughout year.
Fig 9: Variation of monthly free cooling potential at different supply air
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 _
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 .
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-
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 _
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 _
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
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 _
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
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
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
 Lam TNT, Wan KKW, Wong SL, Lam JC. Impact of climate change on
commercial sector air conditioning energy consumption in subtropical Hong
Kong. Appl Energy 2010;87(7):2321–7.
 Lygouras JN, Kodogiannis VS, Pachidis Th, Tarchanidis KN, Koukourlis CS.
Variable structure TITO fuzzy-logic controller implementation for a solar air-
conditioning system. Appl Energy 2008;85(4):190–203.
 Aktacir MA, Buyukalaca O, Yilmaz T. A case study for influence of building
thermal insulation on cooling load and air-conditioning system in the hot and
humid regions. Appl Energy 2010;87(2):599–607.
 Florides GA, Tassou SA, Kalogirou SA, Wrobel LC. Measures used to lower
building energy consumption and their cost effectiveness. Appl Energy
 Karunakaran R, Iniyan S, Goic R. Energy efficient fuzzy based combined
variable refrigerant volume and variable air volume air conditioning system
for buildings. Appl Energy 2010;87(4):1158–75.
 Wang SK. Handbook of air conditioning and refrigeration. 2nd ed. New
York: McGraw-Hill Companies, Inc.; 2001.
 Budaiwi IM. Energy performance of the economizer cycle under three climatic
conditions in Saudi Arabia. Int J Ambient Energy 2001;22(2):83–94.
 ASHRAE, ASHRAE-fundamental-handbook, American society of heating.
Refrigerating and air-conditioning engineers. Atlanta; 2001.
 Fanger PO. How to make indoor air quality one hundred times better while
saving energy. In: Proceedings of VI international HVAC+R technology
Istanbul, Turkey; 2004. p. 1–10.
 Ghiaus C, Allard F. Potential for free-cooling by ventilation. Sol Energy
 Olsen EL, Qinyan YC. Energy consumption and comfort analysis for different
low-energy cooling systems in a mild climate. Energy Build
 Wacker PC. Economizer savings study. ASHRAE Trans 1989;95:47–51.
 Kolokotroni M, Aronis A. Cooling-energy reduction in air-conditioned offices
by using night ventilation. Appl Energy 1999;63:241–53.
 Kubota T, Chyee DTH, Ahmad S. The effects of night ventilation technique on
indoor thermal environment for residential buildings in hot-humid climate of
Malaysia. Energy Build 2009;41(8):829–39.
 Aktacir MA, Bulut H. Investigation of free cooling potential of Kayseri province.
In: Proceedings of 16th national thermal science and technique congress, vol.
2, Kayseri, Turkey; 2007. p. 860–6.
 Aktacir MA, Bulut H. Temperature controlled free cooling and energy analysis
in all air conditioning systems. In: Proceedings of second national air
conditioning congress, Antalya, Turkey; 2007. p. 151–61.
 Aktacir MA, Bulut H. Investigation of free cooling potential of _
In: Proceedings of 8th national HVAC congress. _
Izmir, Turkey; 2007. p. 685–97.
IK, Turkish Statistical Institute. <www.tuik.gov.tr> [last visited May 2010].
 Feng J, Liu M, Pang X. Enthalpy economizer control using mixed air enthalpy.
In: The International Conference for Enhanced Building Operations-ICEBO-
 Bulut H, Büyükalaca O, Yılmaz T. Bin weather data for Turkey. Appl Energy
 Papakostas K, Tsilingiridis G, Kyriakis N. Bin weather data for 38 Greek cities.
Appl Energy 2008;85:1015–25.