44

EXPRES 2017 ISBN 978-86-919769-1-0

Composite materials for wind power turbine blades R. SZABÓ a, L. SZABÓ b

a Ingtex Bt - Rejtő Sándor Foundation H-1056 Budapest Nyáry Pál u. 5, Hungary

E-mail: ingtex@t-online.hu b

Institute of Environmental Protection Engineering, Rejtő Sándor Faculty of Light Industry and Environmental Protection

Engineering, Óbuda University, H-1034 Budapest, Doberdó u. 6, Hungary

E-mail: szabo.lorant@rkk.uni-obuda.hu

Energy is an essential ingredient of socio-economic development and economic growth. Renewable energy sources like wind

energy is domestic and can help in reducing the dependency on fossil fuels. Renewable energy resources, of which wind

energy is prominent, are part of the solution to the global energy problem and reduce the CO₂ emission. Wind turbine and the

rotorblade concepts are reviewed, and loadings by wind and gravity as important factors for the fatigue performance of the

materials are considered.Composites are discussed as candidates for rotor blades. The fibers and matrices for composites are

described, and their high stiffness, low density, and good fatigue performance are emphasized. This presentation will explore

properties of carbon fiber, the advantages/ disadvantage between CFRP and competitive materials as GFRP.

Keywords: airfoil, wind turbine blade, carbon fiber, composites, CFRP, power

1. Introduction

Windmills have been assisting mankind to

convert the energy contained in wind to many

other useful forms for the last three thousand

years. In the early 70s, the serial production of

GRP (glass-fibre reinforced plastics) rotor blades

a length of 17 metres was developed on small

systems, starting in Denmark. In the initial

phases, the achieved performance was

approximately 10 kilowatts bu now we are able

to attain magnitudes of 8 to 10 Megawatts.

Today’s wind turbines are capable of converting

a great amount of energy in the wind into

electricity. This is due to blades which are

developed using state-of-the-art aerodynamic

analysis and other performance-enhancing

equipment. The wind turbine performance

improvement is the key to increase the length of

the blade. The wind blades to make are needed

extremely high loads, high stiffness and high-

strength structural materials. The environmental

load of electricity produced from wind power

generation is the most favourable, so you can

expect a bright future.

2. The wind blade functions

Main components of wind turbines:

Rotor – generates aerodynamic torque from the

wind

Nacelle – converters the torque into electrical

power

Tower – holds nacelle and rotor blades up in the

wind and provides access to the nacelle

Foundation – ensures that the turbine stays upright

The presentation mainly focused on the turbine

blade.

If the blowing wind can turn the blade, we will

receive electricity from the generator that is

attached to it. A lift force is produced when air

moves over an airfoil (Bernoulli-equation) (Fig. 1).

Fig. 1.

The pressure difference on the airfoil makes the

wind turbine blade turn. This way the wind turbine

achieves the basic rotation. Wind turbine blades

turn at a very low rate of rpm. So before the low

speed shaft connecting to the generator the speed

is increased in a gearbox (planetary gear set,

speed ratio ~ 1:90) to achieve the high speed

ratio. The moving wind turbine blade also

experiences the relative wind velocity is as

shown in Fig. 2. [12].

EXPRES 2017 ISBN 978-86-919769-1-0

Fig. 2.

As the blade velocity increases to the tip the

relative wind speed become more inclined

towards the trip. The blade has a lot of air foil

cross-section consisting of different sizes and

shapes from the root to tip. This means by the

manufacturing that a continuous twist is given to

the blade from the root to tip. Therefore the wind

turbine is positioned in a pitched manner in order

to align with the relative wind speed.

Wind turbine always be aligned with the wind

direction. Wind velocity sensor on top of the

nacelle measures the wind speed and direction.

The deviation in the wind’s direction yawing

mechanism to correct the error (picture).

According to the wind speed the relative velocity

angle of the wind also changes. A blade pitching

mechanism turns the blades and guarantees a

proper alignment of the blade with the relative

velocity. Thus the blades are always at the

optimum angle of attack with the relative wind

flow. Pitch control mechanism keep the rotation

speed of a turbine constant for changing wind

speed. This is necessary to keep the rpm of the

generator constant.

The swept area increases by approximately the

square of blade length. Spinning blades cause

wind behind turbine to rotate. Faster moving

blades tip cause less rotation and have less wake

rotation losses. The longer the turbine blades to

achieve the goal.

3. Wind turbine power

The wind power depends on the wind speed.

During excessively windy (cut off speed ~ 80

km/h) the brake is arrest wind blade rotation (Fig.

3).

Fig. 3.

The power of the wind turbine

P = 1/2 η ρ A v v² = 1/2 η ρ π r² v³

where:

P – Performance of wind turbine,

η – Efficiency of wind turbine,

ρ – Air density (1.2 kg/m³),

A – Swept area of wind turbine (A = L²π),

L – Length of blade,

v – wind velocity.

To gain a good insight into wind turbine

efficiency assume that you are measuring wind

speed at upstream and downstream over wind

turbine. The wind speed at the downstream is

much smaller than the upstream because the

blades absorb some kinetic energy from the

wind.

The efficiency of wind turbine (η): Energy out/

Energy in.

The concept would also allow the blades to

spread out when the wind is blowing lightly to

capture as much power as possible.

A portion of the kinetic energy of wind the

turbine blades converts the torque work (M= F x

r). The wind turbines theoretically of the wind

kinetic energy is 59.26% (Betz's law) exploits

(Fig. 4).

Fig. 4

Increasing wind turbine blade lengths lead to

higher energy production (Fig. 5).

45

EXPRES 2017 ISBN 978-86-919769-1-0

Composite materials for wind power turbine blades R. SZABÓ a, L. SZABÓ b

a Ingtex Bt - Rejtő Sándor Foundation H-1056 Budapest Nyáry Pál u. 5, Hungary

E-mail: ingtex@t-online.hu b

Institute of Environmental Protection Engineering, Rejtő Sándor Faculty of Light Industry and Environmental Protection

Engineering, Óbuda University, H-1034 Budapest, Doberdó u. 6, Hungary

E-mail: szabo.lorant@rkk.uni-obuda.hu

Energy is an essential ingredient of socio-economic development and economic growth. Renewable energy sources like wind

energy is domestic and can help in reducing the dependency on fossil fuels. Renewable energy resources, of which wind

energy is prominent, are part of the solution to the global energy problem and reduce the CO₂ emission. Wind turbine and the

rotorblade concepts are reviewed, and loadings by wind and gravity as important factors for the fatigue performance of the

materials are considered.Composites are discussed as candidates for rotor blades. The fibers and matrices for composites are

described, and their high stiffness, low density, and good fatigue performance are emphasized. This presentation will explore

properties of carbon fiber, the advantages/ disadvantage between CFRP and competitive materials as GFRP.

Keywords: airfoil, wind turbine blade, carbon fiber, composites, CFRP, power

1. Introduction

Windmills have been assisting mankind to

convert the energy contained in wind to many

other useful forms for the last three thousand

years. In the early 70s, the serial production of

GRP (glass-fibre reinforced plastics) rotor blades

a length of 17 metres was developed on small

systems, starting in Denmark. In the initial

phases, the achieved performance was

approximately 10 kilowatts bu now we are able

to attain magnitudes of 8 to 10 Megawatts.

Today’s wind turbines are capable of converting

a great amount of energy in the wind into

electricity. This is due to blades which are

developed using state-of-the-art aerodynamic

analysis and other performance-enhancing

equipment. The wind turbine performance

improvement is the key to increase the length of

the blade. The wind blades to make are needed

extremely high loads, high stiffness and high-

strength structural materials. The environmental

load of electricity produced from wind power

generation is the most favourable, so you can

expect a bright future.

2. The wind blade functions

Main components of wind turbines:

Rotor – generates aerodynamic torque from the

wind

Nacelle – converters the torque into electrical

power

Tower – holds nacelle and rotor blades up in the

wind and provides access to the nacelle

Foundation – ensures that the turbine stays upright

The presentation mainly focused on the turbine

blade.

If the blowing wind can turn the blade, we will

receive electricity from the generator that is

attached to it. A lift force is produced when air

moves over an airfoil (Bernoulli-equation) (Fig. 1).

Fig. 1.

The pressure difference on the airfoil makes the

wind turbine blade turn. This way the wind turbine

achieves the basic rotation. Wind turbine blades

turn at a very low rate of rpm. So before the low

speed shaft connecting to the generator the speed

is increased in a gearbox (planetary gear set,

speed ratio ~ 1:90) to achieve the high speed

ratio. The moving wind turbine blade also

experiences the relative wind velocity is as

shown in Fig. 2. [12].

EXPRES 2017 ISBN 978-86-919769-1-0

Fig. 2.

As the blade velocity increases to the tip the

relative wind speed become more inclined

towards the trip. The blade has a lot of air foil

cross-section consisting of different sizes and

shapes from the root to tip. This means by the

manufacturing that a continuous twist is given to

the blade from the root to tip. Therefore the wind

turbine is positioned in a pitched manner in order

to align with the relative wind speed.

Wind turbine always be aligned with the wind

direction. Wind velocity sensor on top of the

nacelle measures the wind speed and direction.

The deviation in the wind’s direction yawing

mechanism to correct the error (picture).

According to the wind speed the relative velocity

angle of the wind also changes. A blade pitching

mechanism turns the blades and guarantees a

proper alignment of the blade with the relative

velocity. Thus the blades are always at the

optimum angle of attack with the relative wind

flow. Pitch control mechanism keep the rotation

speed of a turbine constant for changing wind

speed. This is necessary to keep the rpm of the

generator constant.

The swept area increases by approximately the

square of blade length. Spinning blades cause

wind behind turbine to rotate. Faster moving

blades tip cause less rotation and have less wake

rotation losses. The longer the turbine blades to

achieve the goal.

3. Wind turbine power

The wind power depends on the wind speed.

During excessively windy (cut off speed ~ 80

km/h) the brake is arrest wind blade rotation (Fig.

3).

Fig. 3.

The power of the wind turbine

P = 1/2 η ρ A v v² = 1/2 η ρ π r² v³

where:

P – Performance of wind turbine,

η – Efficiency of wind turbine,

ρ – Air density (1.2 kg/m³),

A – Swept area of wind turbine (A = L²π),

L – Length of blade,

v – wind velocity.

To gain a good insight into wind turbine

efficiency assume that you are measuring wind

speed at upstream and downstream over wind

turbine. The wind speed at the downstream is

much smaller than the upstream because the

blades absorb some kinetic energy from the

wind.

The efficiency of wind turbine (η): Energy out/

Energy in.

The concept would also allow the blades to

spread out when the wind is blowing lightly to

capture as much power as possible.

A portion of the kinetic energy of wind the

turbine blades converts the torque work (M= F x

r). The wind turbines theoretically of the wind

kinetic energy is 59.26% (Betz's law) exploits

(Fig. 4).

Fig. 4

Increasing wind turbine blade lengths lead to

higher energy production (Fig. 5).

46

EXPRES 2017 ISBN 978-86-919769-1-0

Fig. 5.

The ability to „add a few meters” to a wind

turbine blade set is far from simple.

Theoretically, blade weights increase as a cubic

function of blade length when similar designs

and processes are used. The use of alternate

materials, most notably carbon fiber, can help

reduce the weight penalty associated with

increasing blade length.

4. The wind blades loads

The main load of blades is the aerodynamic load

in flap wise direction. This trust force produces

the torque, so a power. The turbine blade sheet is

parallel to the rotation sheet to blades. The high

loads due to the blades tip moves toward the

tower. The blades collision to the tower can be

eliminated with a large sheet directional rigidity

of blades (Fig. 6) [8].

Fig. 6

The plane of the turbine blades are parallel to the

rotating basis. The high load causes the blade tip

is moved towards the turret. The collision of a

large tower paddle board directional stiffness

eliminated. The other loads on the blades made

the gravity in edgewise direction (Fig. 7).

Fig. 7

Wind plate air foil cross section illustrated in 8.

Figure.

Fig. 8.

The critical situation caused by the flap wise load

direction. Aerodynamic wing thickness point of

view of reduction is preferred, and the long blade

large load requires high bending stiffness.

Estimate a number of load cycles:

5. CF and CFRP features

Lightness the key to the future. There is virtually

no alternative to lightweight constructions:

carbon fibres, the black wonder fibres that are

superior to steel and aluminium in almost all

respects when it comes to cutting weight. And, in

terms of stability and lightness, carbon-

reinforced plastic is simply unbeatable. The

aerospace, automobile and wind-power industries

have been aware of this and have been using

EXPRES 2017 ISBN 978-86-919769-1-0

carbon fiber reinforced polymer for many years.

Without it, much of what we nowadays regard as

given – from Formula 1 racing cars, via the

Airbus A350 to the innumerable large wind

turbines – would not be possible.

The quantity of CF (Carbon Fiber) to other

structural material is a little, but many specific

area key important (Fig. 9).

Fig. 9.

6. Carbon fiber

The excellent mechanical properties of

composites with a thin fibrous materials in the

array relative to a significantly higher strength

and, secondly, in accordance with the load

direction are arranged in a large number of

matrix imbedded fiber structure (anisotropic)

results (Fig. 10) [1].

Fig. 10

The reinforcing fibers (carbon fiber, glass fiber,

p-aramid fibers), and comparison of the steel and

the aluminium main mechanical properties are

shown in Table 1.

Table 1. Strength properties of structural materials

7. Carbon fiber production and

characteristics

Most of the carbon fiber is used reinforced

composite. The growth of demand and

production capacity of carbon fibers is shown in

Figure 11.

Fig. 11.

The structure of carbon fiber from the atomic

structure to the composites are shown in 12.

figure [6].

Fig. 12

The engineering practice previously used for

steel, the strength and stiffness values given in

pascals (N/m²). The density of composites more

lighter then steel, so advisable the specific

strength (free breaking length) and specific

stiffness to use (Fig. 12) [2].

47

EXPRES 2017 ISBN 978-86-919769-1-0

Fig. 5.

The ability to „add a few meters” to a wind

turbine blade set is far from simple.

Theoretically, blade weights increase as a cubic

function of blade length when similar designs

and processes are used. The use of alternate

materials, most notably carbon fiber, can help

reduce the weight penalty associated with

increasing blade length.

4. The wind blades loads

The main load of blades is the aerodynamic load

in flap wise direction. This trust force produces

the torque, so a power. The turbine blade sheet is

parallel to the rotation sheet to blades. The high

loads due to the blades tip moves toward the

tower. The blades collision to the tower can be

eliminated with a large sheet directional rigidity

of blades (Fig. 6) [8].

Fig. 6

The plane of the turbine blades are parallel to the

rotating basis. The high load causes the blade tip

is moved towards the turret. The collision of a

large tower paddle board directional stiffness

eliminated. The other loads on the blades made

the gravity in edgewise direction (Fig. 7).

Fig. 7

Wind plate air foil cross section illustrated in 8.

Figure.

Fig. 8.

The critical situation caused by the flap wise load

direction. Aerodynamic wing thickness point of

view of reduction is preferred, and the long blade

large load requires high bending stiffness.

Estimate a number of load cycles:

5. CF and CFRP features

Lightness the key to the future. There is virtually

no alternative to lightweight constructions:

carbon fibres, the black wonder fibres that are

superior to steel and aluminium in almost all

respects when it comes to cutting weight. And, in

terms of stability and lightness, carbon-

reinforced plastic is simply unbeatable. The

aerospace, automobile and wind-power industries

have been aware of this and have been using

EXPRES 2017 ISBN 978-86-919769-1-0

carbon fiber reinforced polymer for many years.

Without it, much of what we nowadays regard as

given – from Formula 1 racing cars, via the

Airbus A350 to the innumerable large wind

turbines – would not be possible.

The quantity of CF (Carbon Fiber) to other

structural material is a little, but many specific

area key important (Fig. 9).

Fig. 9.

6. Carbon fiber

The excellent mechanical properties of

composites with a thin fibrous materials in the

array relative to a significantly higher strength

and, secondly, in accordance with the load

direction are arranged in a large number of

matrix imbedded fiber structure (anisotropic)

results (Fig. 10) [1].

Fig. 10

The reinforcing fibers (carbon fiber, glass fiber,

p-aramid fibers), and comparison of the steel and

the aluminium main mechanical properties are

shown in Table 1.

Table 1. Strength properties of structural materials

7. Carbon fiber production and

characteristics

Most of the carbon fiber is used reinforced

composite. The growth of demand and

production capacity of carbon fibers is shown in

Figure 11.

Fig. 11.

The structure of carbon fiber from the atomic

structure to the composites are shown in 12.

figure [6].

Fig. 12

The engineering practice previously used for

steel, the strength and stiffness values given in

pascals (N/m²). The density of composites more

lighter then steel, so advisable the specific

strength (free breaking length) and specific

stiffness to use (Fig. 12) [2].

48

EXPRES 2017 ISBN 978-86-919769-1-0

Fig 12

The properties and the price can be very different

in the different types of carbon fiber (Fig. 13).

Fig. 13.

8. Composites properties

Composites differ from traditional materials in

that composite parts comprise two distinctly

different components — fibers and a matrix

material (most often, a polymer resin) — that,

when combined, remain discrete but function

interactively to make a new material, the

properties of which cannot be predicted by

simply summing the properties of its

components. In fact, one of the major advantages

of the fiber/resin combination is its

complementary nature.

Thin high-tech fibers, for example, exhibit

relatively high tensile strength, but are

susceptible to damage. By contrast, most

polymer resins are weak in tensile strength but

are extremely tough and malleable. When

combined, however, the fiber and resin each

counteract the other’s weakness, producing a

material far more useful than either of its

individual components (Fig. 14).

Fig. 14

Overall, the properties of the composite are

determined by:

- The properties of the fiber

- The properties of the resin

- The ratio of fiber to resin in the

composite (Fiber Volume Fraction)

- The geometry and orientation of the

fibers in the composite.

9. Wind blade architecture, the spar

caps structure

In many areas, the rapidly growing use of CFRP

production of wind blades to dominate the field

(Fig. 15).

Fig. 15

The development of wind plate is one of the most

mechanical critical requirement for ensuring the

high flap wise stiffness of the blade. The spar

caps stucture of the length wind blades

particularly rigid CFRP (Carbon Fiber

EXPRES 2017 ISBN 978-86-919769-1-0

Reinforced Polimer) with UD (Uni Directionally)

fiber orientacio. Wind blade is some of the

world's biggest constructions (fig. 16).

Fig. 16

Global wind installations were 54.6 GW in 2016,

some 14% lower than the record 63.6 GW

installed in 2015. In 2016 to implement the ten

largest wind turbine capacity increase investment

in the country's pie chat illustrated.

10. Market characteristics of wind

turbines

Global wind installations were 54.6 GW in 2016,

some 14% lower than the record 63.6 GW

installed in 2015. In China, the most active wind

power market, annual wind installations fell from

30 GW in 2015 to 23 GW in 2016 (Fig. 16).

Fig. 16

The world wind power capacity and the increase

of the annual variation in the 17. Figure

illustrates [10].

Fig. 17

The wind turbines can be placed on land (onshore)

at sea (offshore fix and float). The greatest wind

blade (d=180 m) constructed offshore, the water

depth 10-50 m. By countries wind power capacity

in 2015 and has an installed capacity shown in Fig.

18.

Fig. 18

Europe wind investments rise 5% on offshore

surge. Europe's total installed wind capacity was

153.7 GW at the end of 2016. Wind power

provided 10% of Europe's power last year

(Fig.19).

Fig. 19

49

EXPRES 2017 ISBN 978-86-919769-1-0

Fig 12

The properties and the price can be very different

in the different types of carbon fiber (Fig. 13).

Fig. 13.

8. Composites properties

Composites differ from traditional materials in

that composite parts comprise two distinctly

different components — fibers and a matrix

material (most often, a polymer resin) — that,

when combined, remain discrete but function

interactively to make a new material, the

properties of which cannot be predicted by

simply summing the properties of its

components. In fact, one of the major advantages

of the fiber/resin combination is its

complementary nature.

Thin high-tech fibers, for example, exhibit

relatively high tensile strength, but are

susceptible to damage. By contrast, most

polymer resins are weak in tensile strength but

are extremely tough and malleable. When

combined, however, the fiber and resin each

counteract the other’s weakness, producing a

material far more useful than either of its

individual components (Fig. 14).

Fig. 14

Overall, the properties of the composite are

determined by:

- The properties of the fiber

- The properties of the resin

- The ratio of fiber to resin in the

composite (Fiber Volume Fraction)

- The geometry and orientation of the

fibers in the composite.

9. Wind blade architecture, the spar

caps structure

In many areas, the rapidly growing use of CFRP

production of wind blades to dominate the field

(Fig. 15).

Fig. 15

The development of wind plate is one of the most

mechanical critical requirement for ensuring the

high flap wise stiffness of the blade. The spar

caps stucture of the length wind blades

particularly rigid CFRP (Carbon Fiber

EXPRES 2017 ISBN 978-86-919769-1-0

Reinforced Polimer) with UD (Uni Directionally)

fiber orientacio. Wind blade is some of the

world's biggest constructions (fig. 16).

Fig. 16

Global wind installations were 54.6 GW in 2016,

some 14% lower than the record 63.6 GW

installed in 2015. In 2016 to implement the ten

largest wind turbine capacity increase investment

in the country's pie chat illustrated.

10. Market characteristics of wind

turbines

Global wind installations were 54.6 GW in 2016,

some 14% lower than the record 63.6 GW

installed in 2015. In China, the most active wind

power market, annual wind installations fell from

30 GW in 2015 to 23 GW in 2016 (Fig. 16).

Fig. 16

The world wind power capacity and the increase

of the annual variation in the 17. Figure

illustrates [10].

Fig. 17

The wind turbines can be placed on land (onshore)

at sea (offshore fix and float). The greatest wind

blade (d=180 m) constructed offshore, the water

depth 10-50 m. By countries wind power capacity

in 2015 and has an installed capacity shown in Fig.

18.

Fig. 18

Europe wind investments rise 5% on offshore

surge. Europe's total installed wind capacity was

153.7 GW at the end of 2016. Wind power

provided 10% of Europe's power last year

(Fig.19).

Fig. 19

50

EXPRES 2017 ISBN 978-86-919769-1-0

11. Conclusion

To lower the cost of energy from wind turbines,

designers have focused on the power-generating

capacity of each turbine. Carbon fibre-reinforced

plastics make it possible to create even larger and

better optimised rotor blades for wind power

systems. They have lengthened blades to capture

more wind. The size of these gigantic blade

length has increased 10 per cent annually and

doubled approximately every seven years.

Automated manufacturing processes will be

further developed to reduce costs and improve

quality. Further, performance, aerodynamics and

aero acoustics will also be improved. And we

will have large, divided blades which will enable

the challenges of transportation and installation

to be solved more easily.

References

[1] Steinmann: Carbon fibers: an overview on manufacturing, research and market ITA/RWTH Aachen University Mitteilungen 2015.

[2] Szabó L. – Szabó R.: Kompozitok Magyar Textiltechnika 2014/4. p. 2-7.

[3] T. Gries, – B.: Veihelmann: Kombinierte Faserverbundstrukturen zum Aufprallschutz Institut für Textiltechnik der RWTH Aachen (ITA)

[4] Czvikovszky T. – Nagy P. – Gaál J.: A polimertechnika alapjai Műegyetemi

[5] K. Durst: Faserverbunde im Automobilbau: Warum „leicht“ schwer ist Material 13. 10. 2009, München

[6] Szabó R.: A könnyű a jövő XVII. ENELKO 2016. Kolozsvár, 2016. október 6-9. p. 146-151.

[7] www.google.hu The wind resource - Sustainable Energy - TU Delft

[8] www.google.hu Aerodynamic theory - Sustainable Energy - TU Delft

[9] www.google.hu Future trends in wind energy - Sustainable Energy - TU Delft

[10] www.google.hu Global wind report annual market update 2015

[11] www.google.hu Global wind installs dip as China growth slows; Vestas

[12] www.google.hu How do Wind Turbines work ?

EXPRES 2017 ISBN 978-86-919769-1-0

Application of machine learning techniques in forecasting energy efficiency in buildings

O. GRLJEVIĆ a, L. SERES b, S. BOŠNJAK c

Department of Business Informatics and Quantitative Methods, University of Novi Sad, Faculty of Economics Subotica,

Segedinski put 9-11, Serbia a E-mail: oliverag@ef.uns.ac.rs b E-mail: laci@ef.uns.ac.rs c E-mail: bsale@ef.uns.ac.rs

Improvement of energy efficiency in buildings is crucial to achieving the goals of efficient energy consumption, to tackle climate

change and secure energy supply. This paper illustrates potential use of machine learning approaches, in particular classification,

for prediction of the heating load and the cooling load. These two aspects are crucial for energy efficient building design. We

study the effect of eight different variables on the heating and cooling loads and examine results of different classifiers. The

results prove suitability of machine learning approach in the field of energy efficiency in buildings, as well as the fact that rule-

based classifiers perform better on the given set of inputs.

Keywords: Building energy evaluation, Heating load, Cooling load, Machine learning, Classification

1. Introduction Efficient consumption of energy, as a general social

imperative, implies ensuring a stable financial system,

appropriate institutional and legislative framework

and activities to promote public awareness and change

in the habits regarding energy consumption.

The European Union (EU) has adopted Directive

[3] for efficient usage of energy among end-users and

energy services which include achieving significant

energy savings (20% of the greenhouse gases

emissions by 2020 and a 20% energy savings by

2020), as well as obligations to introduce measures

and incentives to promote energy efficiency at the

national level for all Member States [1]. Under the

Energy efficiency Directive EU countries must draw

up National Energy Efficiency Action Plans every

three years. These Action Plans include improvement

measures for energy performance of products,

buildings and services, the improvement of energy

production and distribution, reduction of the impact of

transport on energy consumption, facilitation of

financing and investments in the sector, and

encouragement of rational energy consumption.

The population can have little influence on

energy supply, due to the given natural resources.

However, they can influence the energy demand by

reducing energy consumption. In order to mitigate

climate change the general public has to change its

lifestyle to a less energy-intensive. According to the

report presented in [2] average global temperatures are

likely to rise by 2–3 °C within the next 50 years if a

current emission trends continue. Efficient

consumption of the end-users is also gaining an

increased importance within the European national

policy and the EU objectives related to climate and

energy. According to the European Commission,

residential buildings are responsible for 40% of

energy consumption and 36% of carbon dioxide

emissions in the EU. Potential energy savings in

residential areas and commercial buildings are

estimated at about 27% and 30%, respectively [1].

Similarly to the EU, residential buildings are the

largest consumers of energy in Serbia. The share of

renewable energy in total energy consumption of the

EU for the year 2014 was only 12.5% [3], and Serbia

is only at the beginning of use of renewable energy

sources.

Heating, ventilation, and air conditioning

(HVAC) play crucial role in regulation of indoor

temperatures and have the highest impact on energy

consumption in buildings. One of the ways to mitigate

increased demand for energy is to design and build

energy efficient buildings. EU Directive on energy

performance of buildings [1] obligates Member States

to apply minimum requirements to the energy

performance of new and existing buildings, to ensure

the certification of their energy performance and

require the regular inspection of boilers and air

conditioning systems in buildings. Similar to the EU

Directive, Serbia complies to the Regulations on

energy efficiency of buildings [4] which regulates

assessment of the energy efficiency of the building

based on the heating load, while it is planned to

include the cooling load in the future. Evaluation of

the overall energy efficiency of a building, according