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49. MEĐUNARODNI KONGRES I IZLOŽBA O KGH • 71
POBOLJŠANJE ENERGETSKIH PERFORMANSI OSNOVNE ŠKOLE ĆELE KULA
U NIŠU PRIMENOM SISTEMA PASIVNOG SOLARNOG DIZAJNA
IMPROVEMENT OF THE ENERGY PERFORMANCE OF ELEMENTARY SCHOOL
ĆELE KULA IN NIŠ BY APPLYING PASSIVE SOLAR DESIGN SYSTEMS
Dušan J. RANĐELOVIĆ
*
1, Miomir S. VASOV1, Marko G. IGNJATOVIĆ2,
Mirko M. STOJILJKOVIĆ2, Milena B. BLAGOJEVIĆ3
1 University of Niš, Faculty of Civil Engineering and Architecture, Serbia
2 University of Niš, Faculty of Mechanical Engineering in Niš, Serbia
3 University of Florence, Department of Architecture, Italy
Potreba za primarnom energijom se u poslednjih 30 godina gotovo utrostručila [1]. Zgradarstvo je energetski
najzahtevniji sektor u Srbiji, pa u skladu sa time energetska efikasnost predstavlja najveći energetski resurs, posebno
kroz potencijalnu mogućnost renoviranja postojećih objekata. Sektor školskih objekata je visoko na listi prioriteta veza-
nih za uštedu energije i predstavlja značajan resor koji je neophodno sanirati i poboljšati njegove performanse. Tre-
nutno stanje energetske efikasnosti u Srbiji u oblasti javnih zgrada je zabrinjavanjuće, što daje puno mogućnosti za
unapređenje i uštedu energije. Cilj ove studije predstavlja razvoj i određivanje optimalnog modela energetske sanacije
u procesu sveobuhvatne revitalizacije postojeće osnovne škole Ćele kula u Nišu implementacijom pasivnih solarnih
strategija, kao i njihova primena u lokalnim klimatskim uslovima. Pored pregleda literature, u ovom istraživanju je
korišćen metod modelovanja zasnovan na kompjuterskoj simulaciji reprezentativnog postojećeg objekta osnovne škole.
Primenom konkretnih intervencija pasivnog projektovanja, razvijeni su novi modeli energetske sanacije i smanjenje
godišnjih potreba za energijom za grejanje i hlađenje. Rezultati potrošnje energije osnovne škole Ćeleu kula u Nišu pre
i nakon implementacije strategija pasivnog solarnog dizajna su dobijeni na osnovu simulacija sprovedenih u softver-
skim paketima SketchUp i EnergyPlus.
Ključne reči: ušteda energije; pasivni solarni dizajn; EnergyPlus
The need for primary energy has almost tripled in the past 30 years [1]. Energy-efficiency of buildings is the
most demanding sector in Serbia, but also the largest energy resource, especially through the potential possibility of
renovating existing buildings. The school buildings sector is high on the priority list related to energy savings and rep-
resents an important sector that needs to be rehabilitated and improved. The current state of energy efficiency in Serbia
in the field of public buildings is worrying, giving a lot of opportunities for improving and saving energy. The aim of
this study is to develop and determine an optimal model of energy rehabilitation in the process of comprehensive revi-
talization of existing primary school building Ćele Kula in Niš by implementing passive solar design strategies, as well
as their application in local climatic conditions. In addition to the literature review, this research used a modeling
method based on a computer simulation of a representative existing elementary school building. With the implementa-
tion of concrete interventions of passive solar design principles, new models of energy rehabilitation and reduction of
annual needs for cooling and heating energy have been developed. The results of energy consumption of the primary
school Ćele kula in Niš before and after implementation of the passive design strategies were obtained by simulations
using SketchUp and EnergyPlus software packages.
Key words: energy savings; passive solar design; EnergyPlus
1
Introduction
The need for primary energy has almost tripled in the past 30 years [1]. The largest amount of primary energy is
used to generate electricity. The existing building fund must be treated as a resource of sustainable development, and
the adaptation of buildings is necessary for its rational exploitation. The modern approach to building adaptation pro-
vides numerous and varied opportunities for improvement that can be achieved in this way. Regarding the context in
which construction practice works in developed countries, it is possible to make the necessary adaptations of the exist-
ing construction fund because of the great potential for energy savings [2]. The current state of energy efficiency in
Serbia in the field of public buildings gives a lot of opportunities for improvement and energy savings. The current
situation is further complicated by the lack of reliable information related to the technical characteristics of the build-
ings themselves, the way they are used, poor information and motivation of their users for improving energy efficiency.
Investing in the construction of new primary schools and in modern sustainable technologies is extremely low. All this
is the result of an economic crisis that lasts for a long time in this region. Existing objects are on average over 50 years
old, and only some have done any intervention in order to improve energy performance and comfort. Reconstruction
and rehabilitation of existing elementary schools using the passive solar design system will improve the energy picture
‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒‒
*
Corresponding author’s e-mail: dusan.randjelovic@gaf.ni.ac.rs
72 • 49th INTERNATIONAL HVAC&R CONGRESS AND EXHIBITION
of this sector, and at the same time contribute to better promotion of green construction and preservation of the natural
environment.
The aim of this study is to develop and determine the optimal model of energy rehabilitation in the process of
comprehensive revitalization of existing facilities of primary schools by implementing passive solar design strategies, as
well as their application in local climatic conditions. It also provided a proposal for measures that could be implemented
to reduce energy consumption, and hence the negative impact on the environment and carbon dioxide emissions. The
assessment of buildings based on energy indicators, while defining the energy balance of buildings, is one of the most
important methods for determining their energy efficiency. Energy efficiency indicators are used to define energy sav-
ing potentials and determine the potential effects of applying energy efficiency measures. They provide an opportunity
to compare with standard values or with the same types of buildings in the developed countries.
In addition to the literature review, in this study modeling method based on computer simulation representative
of the existing building of primary school has been used. On the basis of the results of computer simulation of the appli-
cation of passive solar systems on the selected primary school in Nis, the guidelines for implementation of such systems
are given. These guidelines include the definition of interventions for the application of passive systems that would
contribute to energy savings, and an assessment of the effectiveness of the implementation of these strategies. By apply-
ing concrete interventions of passive solar design, new models of energy remediation have been developed and the
annual needs for energy for heating and cooling have been reduced.
2
Passive solar improvement measures
Passive solar design is based on spontaneous natural processes that do not use expensive technological solutions
and mechanical systems for their functioning. This technology is completely ecological and dates from the first human
settlements to the present. At the end of the last century, significant efforts were made in the use of solar energy, which
did not lead to its wider application, as conventional energy was abundant. Solar movement in the world today has
reached great proportions. Almost all developed countries have more or less extensive solar energy development pro-
grams. Some even have solar energy commissions, the highest ranks, equal to the commissions for nuclear energy. In
favor of the use of solar energy is the fact that it is available in unlimited quantities, and above all it is free and has no
harmful effects. In this way, heat and light gain is enabled without unnecessary energy consumption. Passive solar de-
sign is extremely important and is defined as: "Systems that collect, store and redistribute solar energy without the use
of mechanical systems". The benefit is reflected in the fact that lower initial costs are compared to active solar systems
because they represent a part of the building itself, not separate systems. The first passive building was built in 1991 in
Darmstadt-Kranichstein, Germany [3]. Passive architecture is constantly improving, so in time it has become a common
instrument in design. It is based on the concept of high-quality insulation, where the layers of outer shell of the building
are extremely important. This type of facility has no thermal bridges, is completely closed and designed to receive a
large amount of sunlight. The heat of solar radiation is directly transferred to the indoor air or is kept in order to control
its use for heating and cooling. Although it has started from a scientific experiment, it has been proven that the passive
way of heating achieves a saving of as much as 80% in relation to the conventional heating method. All this supports
the application of passive principles of design and investment in the development of knowledge about these systems.
Awareness of the importance of improving energy performance is known to the general public, and the main
question is how much isolation is sufficient and whether there are some systems that would provide internal comfort
without additional energy consumption. Passive use of solar energy is the basic and cheapest way to use energy from
the environment. Passive systems do not use expensive technology, but adapt their architecture to the maximum use of
solar energy. These systems represent a synergy of traditional values and modern knowledge, all of which need to be
carefully integrated into new and existing buildings. Architectural and urban planning of solar buildings, besides saving
energy, biological and chemically pure living is achieved. Passive design principles include a large number of interven-
tions and a much higher degree of project consideration than classical design. The concept of passive solar systems is
based on the knowledge and application of the laws of physics in order to improve heating, cooling, air circulation and
thermal insulation. In the passive design, the most solar energy is obtained by radiation and accumulation in certain
parts of the structure (walls, floors). Solar energy can be used indirectly and directly.
The most important parameters for design in accordance with the principles of passive-solar architecture are: the
surface of the walls, the orientation of the building, the windows (size and position) and the proper shading that controls
the amount of incident solar radiation. [4]. Stevanović [5] considers strategies for optimizing passive solar design in
buildings. It is obvious that energy building simulations must interact with optimization methods in order to achieve a
high level of energy performance, especially in the conceptual design phase when making decisions that have the great-
est impact on energy consumption in buildings. Energy-efficient design is primarily based on passive solar architecture,
the use of renewable energy systems and the use of construction materials with a small embodied energy. Designing
primarily depends on the climatic conditions of the site, the availability of local materials and technology. The main
goal of energy efficient design of buildings should be the achievement of the lowest possible energy consumption, the
maximal reduction of CO2 emissions with the least possible costs [6]. Gaitani, Mihalakakou & Santamouris [7] de-
scribed the procedure for designing and implementing several techniques based on the criteria of bioclimatic architec-
ture and passive principles of cooling and energy saving in order to improve the thermal comfort (microclimate) in the
open area in the Great Athens area. The total performance of passive solar houses is quite favorable from the point of
protecting the environment and reducing energy consumption [8].
49. MEĐUNARODNI KONGRES I IZLOŽBA O KGH • 73
Designing according to climatic conditions is one of the best ways to reduce energy consumption in buildings.
Proper design is the first step in defending against climate constraints. Buildings should be designed according to the
climatic characteristics of the site, which reduces the need for mechanical heating or cooling. Thus, natural energy can
be maximized to create a pleasant environment within a built object [9]. Precise knowledge of local climate factors and
characteristics of building materials from which the building was built is of crucial importance for every architect. They
represent a source of inspiration in the creative process [10]. Bekkouche and others in their work [11] suggest several
basic guidelines for designing passive houses in dry climatic conditions. However, the typical climatic conditions of the
20th century inadequately describe the potential extreme conditions that will arise during the lifetime of the constructed
facilities. Kalvelage, Passe, Rabideau, & Takle [12] develop future typical meteorological annual databases that de-
scribe the environmental conditions used in designing and modifying buildings in order to maintain thermal comfort in
buildings. The use of several different climate models creates an uncertain future for the future demand for energy.
Compared to the previous studies, these results show that with the existing buildings in the United States, the future
demand for energy for heating will decline, but that the need for cooling energy will increase. Increased air temperature
represents a new challenge of increasing humidity that will cause unpleasant conditions for staying in existing build-
ings. In addition, the authors of this paper identify the need to maintain thermal comfort, including the expected future
climate change in the United States.
General strategies for reducing energy consumption for heating and cooling using passive systems include the
avoidance of use-mechanical systems, their efficiency, the insulation of the building envelope, the application of solar
strategies (passive heating), and alternative cooling strategies. The "greenhouse effect" allows the penetration of solar
radiation, but it also needs to be protected against unwanted sunshine in certain periods. A potential reduction in con-
ductivity can be achieved by adding a thermal mass. Namely, the "internal thermal mass" can contribute to reducing the
need for air conditioning, while the "external thermal mass" contributes to the delay of the influence of the external
temperature oscillation. The sun's inclined angle varies throughout the year (which can be advantageously used in de-
sign). The layout and size of windows on the façade affect the energy efficiency of the building. Windows plays a key
role for solar gain inside the building, so it is necessary to properly control the shade and regulate sunlight penetration
depending on the climatic characteristics. External static shading is the most efficient system because it limits the pene-
tration of the sun's rays before interaction with building components (walls, windows) [4]. Windows and walls play an
important role in the final energy consumption of buildings. When designing, it is important to note that by applying a
window with a smaller U coefficient, the environment is less disturbed than by controlling the window to wall ratio,
[13, 14]. The position of fenestration is generally decisive in design (It is necessary to increase the southern exposition
and reduce the eastern and western exposures). Increasing glass surfaces can also have a negative effect on the function-
ing of the facility (it can cause large heat losses through glass surfaces, but also overheating in the summer period dur-
ing the day). Potential for overheating during the day and cooling overnight (The internal thermal mass contributes to
the reduction of this effect. It is important to take into account the position of the thermal mass, its color, but also possi-
ble obstructions from the surrounding furniture, carpets ...)
2.1
Double skin facades
The behavior of the double skin façade (DSF) is directly related to orientation and climatic conditions, and in
general, it can be said that the greater the sun's radiation and the surrounding air temperature, the greater the final ener-
gy savings in the open ventilated facades [15]. The application of the double skin facade system proved to be justified
for the climatic conditions of Serbia. It is best to design the double skin facade-designed so that the space between the
two layers was closed to the circulation of air in the winter months. During the summer months, the façade should be
ventilated and thus provide comfort in the summer and winter periods of the year. In this way, it has been confirmed
that designing this type of construction can significantly save energy in the building while ensuring comfortable stay.
Although costly, properly designed and constructed double skin facades can provide the desired comfort within
the facility. Closing of the air intake in the winter period enables reduction of thermal losses of double facades in rela-
tion to single facades. As for the summer period, it is necessary to ventilate the double-facade of the enclosure to allow
the temperature in the space between the outside temperature to be lower [16]. Taking only thermal characteristics into
account, it has been proven that double skin facades reduce thermal losses as well as heat gains throughout the year,
while making a significant contribution to saving and reducing energy consumption in buildings [16]. If there is any
doubt whether is more convenient the use single or double skin facade in the winter, the answer would be on the side of
the double skin facades. Adding a double skin façade can lead to a reduction of the delivered heating energy by over
55%, while the amount of energy needed for cooling would increase if no adequate type of glass was selected [17]. The
best results are achieved using triple glazing from the inside of the double skin facade.
2.2
Trombe wall
Trombe wall (TW) is proven to be an appropriate solution for passive use of energy for the current ecological
and energy crisis [18]. Solid concrete Trombe walls with a thickness of 30-40cm work well in many geographical loca-
tions. Dark colors are recommended because they absorb much more energy. Strictly, correct insulation from the inside
of the wall is recommended in order to avoid the reverse heat transfer. Insulation not only increases the efficiency of the
solar system by up to 56%. but also reduces the size of mass walls, which reduces the weight of the entire building [18].
Any materials that have a high thermal capacity can be used in the Trombe wall. The high degree of heat accu-
mulation has concrete, stone, earth, but also water, so they are most suitable for making such heating elements. Con-
74 • 49th INTERNATIONAL HVAC&R CONGRESS AND EXHIBITION
crete, stone or brick walls, turning to the side most exposed to the sun during the year, serve to accumulate heat during
the day. Accumulated heat is emitted into rooms during cooler nights. This transmission does not have to be direct, but
the natural character of the air can be used, due to which the warmer air rises, and the cooler descends. Windows in
front of these elements are designed to prevent heat loss, while helping the heat accumulation.
The use of double instead of single glazing for the Trombe wall system not only reduces thermal losses in winter
but also improves passive cooling in summer. [19]. In order to increase the ventilation rate, the inner surface of the
Thromb wall should be isolated for summer cooling. This way also prevents the undesirable overheating of the internal
air due to heat transfer from the wall through convection and radiation [19]. Liu and others [20] state that the optimum
time to open the ventilation ducts on the Trombe wall is 2-3 hours after sunrise, while the optimal closing time is 1h
before sunset. The Trombe wall reaches the maximum value of the capacity of the heat accumulation at 16h, while the
minimum value is recorded at about 7-8 o'clock in the morning. The results of the survey provide a reference basis for
optimization and operational management in passive solar houses with Trombe wall.
Bojić, Johannes, and Kuznik [21] provide a comparison of energy consumption and environmental impacts for
buildings with and without a Trombe wall. The indicator for measuring the environmental impact is the sum of primary
operational energy for winter heating and energy consumption necessary for the functioning of the Trombe Wall on an
annual basis. The application of the Trombe wall represents a good choice in order to passively reduce the consumption
of energy. A number of constructive elements of a structure as well as behavior of its users, can significantly affect the
efficiency of this system. The benefits of using this system are primarily reflected in saving energy needed for heating
and cooling the space, which is mostly contributed to heat accumulation during the day and distribution of this heat
during the night [22]. The only better strategy for passive cooling by using solar energy is the solar chimney [19]. On
the other hand Quesada and others [23] note that although it is similar to Trombe wall, solar chimney is less advanced
technology. Some energy efficiency gains are expected, however, there is still significant potential for optimizing this
system. In order to prevent the penetration of solar radiation into the building, it is recommended to use blinds as well
as isolating curtains between glass and wall layers to avoid heat transfer to the building during summer. [24].
2.3
Green roof
The higher the ratio of the roof surface to the surface of the entire building envelope, the greater the potential en-
ergy losses. Griggs, Sharp & MacDonald [25] state that the surface of the roof of the one floor buildings can be as much
as 50-70% of the total surface of the envelope, which indicates that considerable attention should be given to this ele-
ment during the design process. Linda McIntyre and Edmund Snodgrass in their book comprehensively and thoroughly
examine the efficiency of green roof (GR) technology and provide answers to the questions how these systems would
function and be effective, as is the case in Germany, Switzerland and other European countries. as well as a number of
useful tips to architects, landscape architects and engineers of different profiles [26]. The efficiency of green roofs to a
large extent depends on the climatic characteristics of a particular climate [27, 28]. Reflecting roofs provide significant
advantages in sunny climates, while green roofs provide greater benefits in moderate and cold climatic conditions,
which only confirms the fact that the climate plays a very important role in the potential behavior of cold and green
roofs [29].
Green roofs have different thermal performance in different climatic zones. It should be more important to con-
sider the heat characteristics and selection of plants in order to improve the thermal characteristics of green roofs.
Adapting the application of green roof technology to the most diverse climatic conditions and the proper selection of
plants are key elements of success [30, 31]. Green roofs can reduce outside temperature growth by about 42% and in-
crease the internal temperature by 8% during the day. During the night, they can maintain 17% of the outdoor tempera-
ture, thus providing thermal stability inside the building. The thermal characteristics of the extensive green roof are
closely related to the climatic characteristics. They vary depending on the climate, but also from the different times of
the day [32]. Extensive and moderately isolated green roofs are the most efficient systems that contribute to the reduc-
tion of energy needs for cooling, as well as the reduction of urban heating islands, primarily in areas with mild Mediter-
ranean climate [33]. A green roof in moderate climatic conditions, where energy consumption for cooling is low, can
have a negative effect from the energy consumption point of view due to unwanted evaporative cooling during the post-
season. However, it is important to note that the energy characteristics of the building are just few of the many potential
benefits that must be taken into account when assessing the relative benefits of green roofs [34]. The application of
green roofs in a very hot dry, hot-dry, and mixed-dry climate causes a decrease in energy consumption by about 8.5%,
9.2% and 6.6%, respectively [35]. If the reduction of energy consumption would be considered as the only benefit of
applying green roofs, the return period of the investments would be 25-57 years (depending on the climatic conditions).
Due to the large number of variables related to heat transfer mechanisms and due to the general lack of infor-
mation related to input data, the modeling of green roofs is very difficult. It is also difficult to correctly calculate their
performance using the simulation. In a study conducted by Zinzi and Agnoli [36] pointed out very important facts: The
behavior of the green roof system is significantly dependent on the amount of water in the adopted model. Wet green
roofs are favorable from the point of cooling the building. Unfortunately, this would involve reliance on precipitation,
so the energy performance of the roof and efficiency during the warm dry season, especially in the central and south-
eastern parts of the Mediterranean, would be an obvious problem. Compared to conventional roofs (CR), green roofs
improve building performance and reduce energy consumption for heating the building. A limited amount of water
reduces the amount of moisture in the soil layers, which actively increases the thermal resistance of the building. The
49. MEĐUNARODNI KONGRES I IZLOŽBA O KGH • 75
lower the amount of moisture on the roof, the less the need for heating. Water control should be calibrated according to
climatic conditions and the way of using energy.
Sailor developed the physical model of the energy balance of the green roof and integrated it into the energy
building simulation program - EnergyPlus [37]. This model of green roof enables the use of energy modeling to study
the design options for green roofs, including the thermal properties and depth of the substrate, as well as the vegetation
characteristic (types of plants, soil height and leaf area index - LAI). It is obvious that this method of simulating green
roofs can significantly contribute in further information and decision-making related to the design of green roofs.
3
Case study
analysis and implementation of passive solar design strategies
Sector of school buildings is high on the list of priorities in energy savings and represents an important sector
that needs to be rehabilitated and improved. One of the best examples of sustainability is the reuse of existing buildings
with the improvement of their performance. The importance of these measures is even greater if we take into account
the special sensitivity of the population involved, because the normal and complete development of young people re-
quires thermally neutral spaces that do not cause discomfort. The widely accepted definition is: "hermal comfort is the
condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation"
[38]. The largest number of existing educational buildings on the territory of southeastern Serbia is obsolete in therms
of energy efficiency. There is enormous energy consumption, and almost no facility that has been built in accordance
with the current energy consumption standards. The category of educational institutions, owned by the city of Niš, has
128 facilities, with the total area of 191,074.00 [m2]. In this subsector, in 2010, 5,795,186.30 [kWh] of electricity was
used, which represents a specific consumption of 30,33 [kWh/m2]. The property of the city of Nis has a total of 86
buildings of Primary schools, with total area of 121.301,00 [m2]. Total CO2 emissions in the city of Nis was 1065940.60
[tCO2] in 2010. The largest source of emissions, as well as energy consumption, was in the building sector, with total
emissions of 850,963.91 [tCO2]. Observed in relation to the number of inhabitants, emissions in the city of Niš amount-
ed to 4.13 [tCO2 / inhabitants][39].
Based on the case study of the representative Primary school in the city of Nis, the following steps were under-
taken: the identification of a representative example of the elementary school building in the city of Niš and a brief
presentation of its technical characteristics, defining the possible levels of energy efficiency improvement by applying
passive design principles. Energy efficient and conscientious design is a very responsible and complex task, and a bad
choice of one component can have a greater negative impact than all the positive effects of the best components [40].
Rehabilitation / renovation / restoration / reconstruction of the building envelope stabilize the internal microclimate. In
particular, as the thickness of the thermal insulation layer increases, the internal temperature fluctuations decrease. This
increases the thermal resistance and inertia of the building envelope [41]. The insulation of the building envelope aims
to reduce the negative impacts of the environment on the building. Depending on the particular climatic conditions, the
project should be adapted in order to achieve the best possible results. The ratio of the surface of the walls, windows
and roof, as well as the shape of the building, affects the heat flow through the structure. Adding thermal insulation
reduces the heat flow through the envelope of the building. The amount of energy needed for heating makes the largest
share of total energy consumption in buildings. In addition, most of the heat energy is lost through the building enve-
lope (walls, windows, ceilings and roofs) [42]. Heating of the space would be reduced by about 50%, when the econom-
ically viable measures of insulation of the building envelope, ie the ceilings and walls, would be applied. This leads to a
reduction in the consumption of appropriate fossil fuels and reduced emissions of harmful gases [43]. Proper design of
the building envelope significantly contributes to reduce the needs for heating and cooling. This improves the energy
performance of the building. The right choice of thermal insulation, glazing and shade types can contribute to reducing
the heat flow through the building envelope [44]. Generally, factors that affect total energy consumption in buildings
can be divided into seven categories [45]:
1) Climate (e.g. outdoor temperature, solar radiation, wind speed, etc.),
2) Characteristics of buildings (e.g. types, surfaces, orientations, etc.)
3) Characteristics of building users, other than social and economic factors (e.g. the presence of occupants, etc.)
4) Built-in mechanical systems and their operation (e.g. cooling/space heating, hot water supply, etc.)
5) Behavior and activities of building users,
6) Socio-economic factors (e.g. education level, energy price, etc.),
7) Air quality inside the building
Among these seven factors, social and economic factors will partially determine the user's relation to energy
consumption. These factors may have an impact on their everyday activities and behavior, which affects the consump-
tion of energy in buildings [45]. In addition to environmental and economic benefits, the application of these energy
performance improvement measures would also affect the improvement of the interior comfort. The benefit is also re-
flected in the fact that all this positively reflects on the effect of learning, motivation, but also the mood [46]. The re-
construction of the facade contributes to the architectural and visual identity of the contemporary architectural expres-
sion, all with the aim of achieving the feeling of comfort and the need of students to more easily accept the environment
in which they spend a significant time during the day.
76 • 49th INTERNATIONAL HVAC&R CONGRESS AND EXHIBITION
3.1
Structure of the existing thermal envelope - prior to energy performance improvement
The existing structure of the thermal envelope of Elementary School Ćele Kula in Niš was designed in the period
when there were no clearly defined regulations related to energy savings. However, in 2011 The Government of the
Republic of Serbia adopted a new regulation on the energy efficiency of the buildings which is enacted on 30th Septem-
ber 2012 [47]. This regulation defines the maximum permissible value of heat conduction coefficient of the thermal
envelope of the building.
Elements of the envelope structure of existing primary school Ćele Kula do not meet the regulations [22] in
terms of maximum permissible values of thermal transmittance coefficients (for walls and bottom floor
Umax=0.3Wm2/K, for a flat roof Umax=0.15Wm2/K, for the glazed part of the façade Umax=1.5Wm2/K). The structure of
the building is made of the following materials (inside to outside):
Default Exterior walls: mortar 1,5cm thick, brick 25cm thick, expanded polystyrene 5cm thick and Finishing
Mortar 2.5cm thick. Thermal transmittance coefficient U=1.072 Wm2/K.
Default Flat roof: mortar 2cm thick, hollow clay block for interfloor and attic construction – MONTA 16cm
thick, reinforced concrete 4cm thick, layer for inclination 5cm thick, vapor dam 0,5cm thick, expanded polystyrene
10cm thick, PVC soft foil 0.01 cm thick, bitumen roofing paper 1.3cm thick, dry sand 4cm thick, dry gravel 2.5cm
thick. Thermal transmittance coefficient U=0.306Wm2/K.
Default Floor: Granite ceramics 1.5cm thick, elasticated polystyrene 8cm thick, reinforced concrete 10cm thick.
Thermal transmittance coefficient U=0.380Wm2/K.
Default Windows: The windows on the building are made of PVC, with double glazing and air between the glass
panes (4-12-4), with no screens or rolling shutters. U-value 1.9Wm2/K, Solar Heat Gain Coefficient of 0.63.
4
Simulation
In Figure 1 3D models of Primary school as well as modified model with implemented Trombe Wall on parapet
walls; and Model with implemented Double skin facade are shown. Models were created in Google Sketchup with
OpenStudio plugin, and then exported to EnergyPlus.
Figure 1 - Appearance of 3D models of Primary school: a) Basic model; b) Model with implemented
Trombe Wall on parapet walls; c) Model with implemented Double skin facade
Thermal zoning of the building includes the grouping of individual parts of the building in accordance with their
needs for the maintenance of certain thermal conditions. According to the Regulations [47], a building with several
energy zones is a building that has several special parts:
1) consisting of technical-technological and functional units, which have different purposes and, accordingly,
have the possibility of separate heating and cooling systems or differ by internal project temperature by more than 4°C,
2) where more than 10% of the net surface area of the building with controlled temperature is maintained for
other purposes,
3) where the parts of the building, which are technical-technological and functional entities, have different
HAVC systems and / or substantially different regimes of the use of HVAC systems;
In Figure 2 modeled floor plans and thermal zones of primary school Ćele Kula are shown. As we can see on
this figure, ground floor and first floor plans are divided into different thermal zones based on space types. For the loca-
tion where the simulation of the working model was performed, the meteorological data of the city of Niš [48] were
used - in EPW (EnergyPlusWeather) format.
Simulation of the working model implies a calculation for a time step of 5 minutes. Working hours are from
08:00 to 19:00, five days a week. However, due to the possible early arrival at work, as well as leaving after working
hours to complete the daily obligations, half of the employees are expected to be present from 07:30 to 08:00, and from
19:00 to 20:00. During working hours in two shifts during the school year (in the simulation the months of July and
August were not observed), the heating is switched on when the temperature falls below 20°C, and the cooling starts
when the temperature rises above 26°C.
49. MEĐUNARODNI KONGRES I IZLOŽBA O KGH • 77
Figure 2 - Floor plans and thermal zones of primary school Ćele Kula
The largest heat losses occur through the building envelope. In the performed simulations, we start from the
assumption that constant internal temperature is maintained in each zone (in the winter mode 20°C, in the summer mode
26°C). An ideal HVAC system (the IdealLoadsAirSystem) has been adopted to provide thermal comfort in the modified
thermal zones, enabling each zone to deliver the energy required in a given moment. This type of modeling is primarily
used in the conceptual phase of architectural design, when it is necessary to check different architectural solutions, as is
the case with the application of passive solar design systems. Out of working hours when employees and students are
not present in the building, heating is switched on when the temperature falls below 12°C, and cooling is switched on
when the temperature rises above 30°C in order to prevent excessive temperature oscillations.
The internal gains are determined by the number of students and teachers. For various space types different
number of people are defined, as well as adequate degree of their activity and their presence during the day.
4.1
Parametric simulation and optimization of the working model
According to the Regulations on energy efficiency of buildings [47], energy rehabilitation of buildings means
construction and other works on the existing building, as well as the repair or replacement of equipment, plant, equip-
ment and installations of the same or lower capacity. It does not affect the stability and security of the facility, not
change structural elements or affecting the safety of the surrounding facilities, does not affect the protection against fire
and protection of the environment, or which can change the external appearance with the necessary approvals, in order
to increase energy efficiency of the building. The planned renovation of the building includes energy rehabilitation of
transparent and non-transparent construction elements of the building envelope without intervention on the technical
systems of the building.
Energy rehabilitation was carried out by intervention on transparent and non-transparent surfaces of the thermal
envelope. In general, the intervention for the purpose of energy performance improvement is based on correcting the
existing values of the coefficient of heat transfer U (W/m2K) of the analyzed elements of the thermal building shell on
the prescribed rules. Bearing in mind that the number of existing school buildings on the territory of the Republic of
Serbia is on a respectable level, the presented case study analysis aims to provide basic guidelines for the principle and
scope of this type of intervention. In order to determine the energy consumption, with providing the necessary comfort,
11,745 separate simulations were carried out. The simulations were carried out on the model of the existing Primary
school after energy rehabilitation, as well as after implementation and combination of passive solar designs strategies.
Simulations were conducted using software packages SketchUp and EnergyPlus. Considering the set goal through the
aspect of adequate construction-architectural intervention, the conducted analysis is based on a parametric study of the
change in the characteristics of non-transparent and transparent elements of the thermal envelope of the building, as
well as the implementation of a passive solar design strategies such as a Trombe wall, Double skin facade and Green
roof. The analysis includes seven variant solutions for rehabilitation according to the principle, method and scope of
intervention:
Variant I: Basic model of the existing Primary school Ćele Kula in Niš.
Variant II: Energy performance improvement is done by increasing the thickness of the thermal insulation layer
of the building elements of the corresponding non-transparent part of the thermal envelope to satisfy the conditions ≤
Umax (W/m2K) for the existing building, while the transparent part of the envelope corrects the prescribed value of the
heat transfer coefficient Umax=1.5Wm2/K.
Variant III: Variant II, as well as the implementation of green instead of a conventional roof.
Variant IV: Variant II with the implementation of the Trombe wall in the parapet of the walls of predominantly
south-oriented classrooms
FLOOR PLANS THERMAL ZONES
GROUND FLOOR
FIRST FLOOR
Legends:
1. Corridors
2. Classrooms
3. Offices
4. Toilets
5. Social contents
6. Utility rooms
7. Wardrobes
8. Gym
9. Kitchen
10. Courtyard
78 • 49th INTERNATIONAL HVAC&R CONGRESS AND EXHIBITION
Variant V: Variant IV, as well as the implementation of green instead of a conventional roof.
Variant VI: Variant II with the implementation of Double skin facade.
Variant VII: Variant , as well as the implementation of green instead of a conventional roof.
The applied materials used for the building structure are primarily selected in accordance with the current Regu-
lations on energy efficiency of buildings [47] regarding the maximum allowed U-value. Reinforced concrete material is
imposed because it is one of the most flexible materials, and in addition it can satisfy all architectural constructive re-
quirements, such as burial in the ground, bridging large ranges, variety of form and actuality in contemporary design.
When applying these interventions, the characteristics of the green roof, the thickness and type of the thermal
mass in the Trombe wall, and the type of glazing and air cavity in the Double skin facade were also varied. As noted,
the working model has some variable parameters which are shown in Graph 1.
Graph 1 - Values of variable parameters of applied strategies
As for the energy model on which the simulations were performed, two components of the double skin façade
were varied. Namely, besides the type of glass, different sizes of air cavity between the facade and glazing are also
varied. The types of glass used, as well as type and thickness of the thermal mass are given in the diagram, while the
characteristics of these glasses are shown in the Table 1. Two components of the Trombe wall were varied. Namely,
besides the type of glass, different materialization and thickness of the thermal mass, were varied. The structure of the
Trombe wall, therefore, consists of: concrete material (thickness varies from 15cm to 25cm) or brick wall material
(thickness varies from 25cm to 38cm) with the solar absorber surface as the innermost layer of the wall. The absorber is
a selective surface material with very high absorbance and very low emissivity - copper with a special black surface
treatment. Modeled Trombe wall is sealed (unvented) with a different type of glazing. which covers all of the wall area
and has very high transmittance to allow the maximum amount of solar flux into the Trombe zone. Of all the compo-
nents that constitute the eco-roof model, the Height of Plants (HOP) {m}, Leaf Area Index (LAI) {dimensionless}, and
Soil Thickness (SOIL_THICK) {m} were varied. The values components that constitute the eco-roof model are shown
in the Graph 1.
Table 1 - Different types of glazing applied in simulations
G1
single, 6 mm
5,8
0,83
G3
double, transparent,4-12-4 mm
3,0
0,71
G6
triple, transparent, 6-12-6-12-6 mm
1,9
0,63
G8
double, low-e, 4-16-4 mm (air)
1,5
0,61
G10
double, low-e, 4-12-4 mm (Kr)
1,1
0,62
G12
triple, low-e, 4-8-4-8-4 mm (Kr)
0,7
0,48
G15
double, reflective, 6-12-4 mm (Ar)
1,4
0,27 - 0,44
index
GLAZING TYPE
Ug [W/m2K]
W/(m2xK)
g
49. MEĐUNARODNI KONGRES I IZLOŽBA O KGH • 79
5
Results and discussion
Implementation of these strategies of passive solar design can lead to significant savings. The simulations were
conducted for a run period of one school year (the months of July and August were not taken into account). Graph 2
shows the least specific heating needs for the basic model variant. It is also shown the possibility of implementing a
green roof, double skin facade or trombe wall, as well as their combination on the existing model without improving its
performance in any other way.
Graph 2 - Min specific energy required for heating for run period – BASELINE MODEL (kWh/m2)
Graph 3 shows the least specific cooling needs for the basic model variant. It is also shown the possibility of
implementing a green roof, double skin facade or trombe wall, as well as their combination on the existing model
without improving its performance in any other way.
Graph 3 - Min specific energy required for cooling for run period – BASELINE MODEL (kWh/m2)
On the basis of the obtained results it can be concluded that by applying the passive solar design system it is pos-
sible to achieve significant savings. For example, using an adequate combination of a trombe wall and a green roof, it is
possible to save as much as 17.85% of the energy for heating, while using an adequate combination of double skin fa-
çade and a green roof would save as much as 10.32% of the cooling energy. It is interesting that the application of these
interventions can lead to adverse effects, so it should be kept in mind that by adding a trombe wall, the demand for
cooling energy will increase by 11.32%, although the same intervention could lead to saving of heating energyup to
12,37%. It is important to note that the need for heating is tripled, so, nevertheless, the total energy consumption would
make significant savings.
When it comes to an energy revitalized model, it could be said that with the improvement of the glazing type and
addition of thermal insulation of the envelope of the school building can lead to savings of as much as 32.40% of the
energy for heating, while the cooling energy in this combination of interventions would be improved for 7.09%. Inter-
estingly, only the addition of a double skin facade contributes to significant savings, but in comparison with the classi-
cally energy-revitalized model, the savings for heating are reduced just a little bit. The variant of the energy revitalized
model with the use of a trombe wall and a green roof distinguished as the best combination, where heating energy sav-
ings in relation to the basic model were achieved by as much as 44.32% (Graph 4).
If we look at the cooling needs (Graph 5), the energy revitalized model (improvement of the glazing type and the
addition of thermal insulation on the building envelope) with the application of the trombe wall would, in relation to the
classical energy-revitalized model, record a smaller savings. The variant of the energy revitalized model with the use of
a double skin facade and a green roof distinguished as the best combination, where cooling energy savings in relation to
the basic model were achieved by as much as 24.41%. Independent addition of a green roof or double skin facade
would also lead to significant savings.
The greatest savings in thermal energy consumption was achieved in the simulation number 7395, when the spe-
cific heat consumption was reduced by as much as 44.32%, while the greatest saving in cooling energy consumption
was achieved in simulation number 4281, when the specific consumption of cooling energy reduced by 24.41%.
80 • 49th INTERNATIONAL HVAC&R CONGRESS AND EXHIBITION
Graph 4 – Min specific energy required for heating for run period – RETROFIT MODEL (kWh/m2)
Graph 5 – Min specific energy required for cooling for run period – RETROFIT MODEL (kWh/m2)
A comparative analysis of the implantation of these passive systems in the primary school model shows that the
application of some interventions can lead to undesirable consequences if we observe a period of one school year. For
example, some interventions provide savings in energy consumption for heating, while cooling energy consumption has
increased relative to the initial model. Of course, in some scenarios, a reverse situation is possible. Some of the inter-
ventions in which great savings in energy consumption for heating and cooling have been achieved are simulation num-
ber 399 (retrofit model with the implementation of green roof only), and, for example, simulation 4281 (retrofit model
with the implementation of double skin façade and green roof). The mentioned variants have achieved savings through-
out the entire school year.
In the simulation number 5266 (basic model with the implementation of only the trombe wall), energy savings
for heating of 12.37% was achieved, but the energy consumption for cooling significantly increased (as much as
12.98%) which is a negative example of implementation of passive solar design strategies.
In the end the highest and lowest recorded values of heating and cooling energy consumption for conducted sim-
ulations are shown in Table 2.
6
Conclusions
The goal of sustainability is to provide safe and sufficient energy supply while at the same time reducing the
negative impact on the environment. Therefore, it is clear that all segments of society related to energy consumption
must develop in a sustainable way. The most economically viable and crucial mechanism for reducing energy consump-
tion and achieving these goals is to improve energy efficiency. In accordance with this endeavor, the improvement of
energy efficiency is becoming a more current design challenge, especially in the domain of finding the optimal way of
energy rehabilitation of the existing architectural and construction fund. Reconstruction and re-use of buildings is an
important component in order to achieve the best possible results in improving the energy efficiency of the existing
construction fund. The process of critical review of the existing energy and architectural concepts of primary schools is
based on the need for their spatial and temporal framework as a whole to comply with modern regulations concerning
energy efficiency. One of the characteristics of a large number of primary schools in the analyzed area is the irrational
high consumption of all types of energy, primarily for heating, but also for cooling, due to increase in average tempera-
tures during summer.
The results of the performed simulations indicate a clear linear dependence of the heat loss effects across the
building envelope; however, the observable trend of reducing energy consumption shows an obvious non-linearity,
which indicates the justification of doing such research in order to find the most optimal strategies for achieving the best
results. All results of this study were obtained for ideal simulation conditions. Aggravating circumstances, which are
inevitable in the actual conditions of exploitation of the building (negligence of the building user, reduction in the per-
formance of applied materials over the years, etc.), are not taken into account. Additional studies are needed to compre-
hensively address these problems. On the basis of a comparative analysis of the obtained results , the most effective
49. MEĐUNARODNI KONGRES I IZLOŽBA O KGH • 81
measures can be used to save energy, thus providing both energy savings and preservation of the natural environment.
Regardless of the fact that the highest energy savings are achieved through the use of certain interventions, a compre-
hensive review makes it easy to conclude that only the change in the daily use regime or the inclusion of the economic
aspect can significantly change the decision on determining the best strategy for applying the mentioned measures.
The practical applicability of the obtained results is reflected in the establishment of a methodological and sys-
tematic framework for the management of energy performance improvement measures in the process of comprehensive
revitalization of primary schools. Implemented systems can gain more significance if they have the support of local
communities, the administration of school institutions, but also the relevant ministry in order to create optimal, healthy
conditions for the care of future generations. The contribution of this study is even greater if one considers that the ob-
tained results are useful for architects and all engineers when it comes to choosing an approach based on passive solar
design.
SIMULATION NUMBER
COMMENT
WALL_THICK [m]
WALL_INS_THICK [m]
ROOF_INS_THICK [m]
GLAZ_TYPE
TW_GLAZ_TYPE
TW_TYPE
TW_CONCRETE_THICK
DSF_GLAZ_TYPE
DSF_CAVITY [m]
ROOF_TYPE
HOP [m]
LAI
SOIL_THICK [m]
SPEC. HEAT. ENERGY
[kWh/m2]
SPEC. COOL. ENERGY
[kWh/m2]
SAVINGS - HEATING[%]
SAVINGS - COOLING[%]
1 BASIC MODEL 0.25 0.05 0.1 G6g - - - - - CR - - - 66.04 20.66 0.00% 0.00%
10 RETROFIT MODEL 0.25 0.05 0.15 G15g - - - - - CR - - - 56.21 18.13 14.88% 12.26%
44 RETROFIT MODEL 0.25 0.15 0.25 G12g - - - - - CR - - - 45.90 23.52 30.50% -13.81%
45 RETROFIT MODEL 0.25 0.15 0.25 G15g - - - - - CR - - - 44.65 19.20 32.40% 7.09%
46 BASIC + GR 0.25 0.05 0.1 G6g - - - - - GR 0.05 1 0.1 62.97 20.81 4.66% -0.70%
47 BASIC + GR 0.25 0.05 0.1 G6g - - - - - GR 0.05 1 0.15 62.62 20.73 5.18% -0.32%
52 BASIC + GR 0.25 0.05 0.1 G6g - - - - - GR 0.15 3 0.1 63.68 19.11 3.57% 7.51%
53 BASIC + GR 0.25 0.05 0.1 G6g - - - - - GR 0.15 3 0.15 63.37 19.09 4.04% 7.60%
85 RETROFIT + GR 0.25 0.05 0.1 G15g - - - - - GR 0.15 3 0.15 56.45 16.51 14.53% 20.07%
390 RETROFIT + GR 0.25 0.15 0.25 G12g - - - - - GR 0.05 1 0.1 44.64 23.63 32.41% -14.36%
399 RETROFIT + GR 0.25 0.15 0.25 G15g - - - - - GR 0.05 1 0.15 43.33 19.24 34.40% 6.89%
406 BASIC + DSF 0.25 0.05 0.1 G6g - - - G1g 0.3 CR - - - 63.38 22.11 4.04% -6.99%
411 BASIC + DSF 0.25 0.05 0.1 G6g - - - G3g 0.5 CR - - - 64.11 20.47 2.93% 0.94%
414 BASIC + DSF 0.25 0.05 0.1 G6g - - - G6g 0.5 CR - - - 64.07 19.91 2.98% 3.65%
415 BASIC + DSF 0.25 0.05 0.1 G6g - - - G8g 0.3 CR - - - 62.92 20.62 4.73% 0.19%
922 RETROFIT + DSF 0.25 0.15 0.25 G12g - - - G1g 0.3 CR - - - 45.98 23.03 30.39% -11.48%
934 RETROFIT + DSF 0.25 0.15 0.25 G15g - - - G1g 0.3 CR - - - 44.98 19.19 31.89% 7.14%
942 RETROFIT + DSF 0.25 0.15 0.25 G15g - - - G6g 0.5 CR - - - 46.44 17.00 29.68% 17.72%
946 BASIC + DSF + GR 0.25 0.05 0.1 G6g - - - G1g 0.3 GR 0.05 1 0.1 59.91 22.65 9.28% -9.61%
992 BASIC + DSF + GR 0.25 0.05 0.1 G6g - - - G3g 0.5 GR 0.15 3 0.1 61.46 19.10 6.93% 7.56%
1017 BASIC + DSF + GR 0.25 0.05 0.1 G6g - - - G6g 0.5 GR 0.15 3 0.15 61.13 18.53 7.44% 10.32%
1019 BASIC + DSF + GR 0.25 0.05 0.1 G6g - - - G8g 0.3 GR 0.05 1 0.15 59.11 21.05 10.50% -1.88%
4114 RETROFIT + DSF + GR 0.25 0.15 0.1 G12g - - - G1g 0.3 GR 0.05 1 0.1 48.24 23.49 26.96% -13.71%
4281 RETROFIT + DSF + GR 0.25 0.15 0.1 G15g - - - G6g 0.5 GR 0.15 3 0.15 49.24 15.62 25.45% 24.41%
5171 RETROFIT + DSF + GR 0.25 0.15 0.25 G15g - - - G1g 0.3 GR 0.05 1 0.15 43.47 19.41 34.18% 6.06%
5266 BASIC + TW 0.25 0.05 0.1 G6g G1g TWbrickIN - - - CR - - - 57.87 23.34 12.37% -12.98%
5438 RETROFIT + TW 0.25 0.15 0.25 G12g G1g TW brickIN - - - CR - - - 39.65 26.53 39.96% -28.41%
5442 RETROFIT + TW 0.25 0.15 0.25 G15g G1g TW brickIN - - - CR - - - 38.15 21.91 42.24% -6.03%
5454 BASIC + TW 0.25 0.05 0.1 G6g G6g TWconcret eIN 0.25 - - CR - - - 59.80 23.00 9.46% -11.32%
5455 BASIC + TW 0.25 0.05 0.1 G6g G8g TWconcret eIN 0.15 - - CR - - - 59.89 23.10 9.32% -11.80%
5502 RETROFIT + TW 0.25 0.05 0.1 G15g G6g TWcon creteIN 0.25 - - CR - - - 52.70 20.35 20.21% 1.50%
5986 BASIC + TW + GR 0.25 0.05 0.1 G6g G1g TWbrickIN - - - GR 0.05 1 0.1 54.57 23.90 17.37% -15.69%
5987 BASIC + TW + GR 0.25 0.05 0.1 G6g G1g TWbrickIN - - - GR 0.05 1 0.15 54.25 23.80 17.85% -15.20%
7042 RETROFIT + TW + GR 0.25 0.15 0.1 G12g G1g TWb rickIN - - - GR 0.05 1 0.1 41.74 26.94 36.80% -30.38%
7395 RETROFIT + TW + GR 0.25 0.15 0.25 G15g G1g TWb rickIN - - - GR 0.05 1 0.15 36.77 22.17 44.32% -7.31%
7497 BASIC + TW + GR 0.25 0.05 0.1 G6g G6g TWcon creteIN 0.25 - - GR 0.15 3 0.15 56.90 21.64 13.85% -4.74%
7504 BASIC + TW + GR 0.25 0.05 0.1 G6g G8g TWcon creteIN 0.15 - - GR 0.15 3 0.1 57.27 21.75 13.28% -5.26%
7881 RETROFIT + TW + GR 0.25 0.05 0.1 G15g G6g TWcon creteIN 0.25 - - GR 0.15 3 0.15 49.76 18.94 24.66% 8.32%
Table 2 - The highest and lowest recorded values of heating and cooling energy consumption for conducted simulations
Therefore, the investment in improving energy efficiency in schools is an area with high potential. The estab-
lishment of a mechanism that would allow for a permanent reduction of energy consumption in schools (new design
approaches and application of passive solar design systems) and proper renovation of existing buildings is the main goal
of energy efficiency in schools. Increasing the energy efficiency of buildings does not always have to be based on mon-
etary investments, but also by conscientious and responsible behavior, significant savings can also be achieved which
would also affect the energy properties of buildings.
7
Acknowledgements
The paper is a part of the research done within the project TR36037. The authors would like to thank to everyone who
in any way participated in the preparation of this work and who through their advice and experience contributed to writ-
ing and publishing this work.
82 • 49th INTERNATIONAL HVAC&R CONGRESS AND EXHIBITION
8
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