Thermoeconomic method for determination of optimum insulation thickness of external walls for the houses: Case study for Turkey

Sustainable Energy Technologies and Assessments 22 (2017) 1–8

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Original article

Thermoeconomic method for determination of optimum insulation thickness of external walls for the houses: Case study for Turkey Ö. Altan Dombayci a, Öner Atalay b,⇑, Sß engül Güven Acar b, Eylem Yilmaz Ulu b, Harun Kemal Ozturk c a

Faculty of Technology, Pamukkale University, Denizli, Turkey Sarayköy Vocational High School, Pamukkale University, Denizli, Turkey c Department of Mechanical Engineering, Faculty of Engineering, Pamukkale University, Denizli, Turkey b

a r t i c l e

i n f o

Article history: Received 23 January 2017 Revised 22 May 2017 Accepted 23 May 2017

Keywords: Life Cycle Cost Analysis (LCA) Thermoeconomic method Exergy Residential Insulation materials Optimum insulation thickness

a b s t r a c t The increment of the population, globalization of the world, improvement in technology and the increment of the welfare level causes to increase of energy use of goods and services. Energy consumption is commonly observed to occur in four main sectors: industry, residential, transport and agriculture. Residential sector in many countries is one of the largest energy consumers. The aim of this study to determine the optimum insulation thickness of the external wall of the housing for the selected province in four different climate regions in Turkey. As the insulation material, the polystyrene and polyurethane were used in the study. The optimum insulation thickness of the external wall was calculated using thermoeconomic method considering the effect of the inflation and interest rate, which is also called as Life Cycle Cost Analysis (LCA). For two different insulation materials, the minimum thickness was calculated for warm temperate climate region and the maximum thickness was calculated in the cold climate region as accepted. Also, the maximum savings was calculated for the cold climate region and the minimum savings was calculated for the warm temperate climate region. Ó 2017 Elsevier Ltd. All rights reserved.

Introduction Population growth, globalization, rapid advances in technology, the rise of income and welfare causes to increase in energy consumption in the world. The fossil fuel has been seen as the easiest way to meet this growing demand for many years. However, due to the limited amount of fossil fuels, the increase of the energy price, environmental problems and global warming, it is important to use energy efficiently at every stage from production to consumption [1–3]. The area of Turkey is 783,502 km2. Turkey is located at the meeting point of three continents - Asia, Europe and Africa. Turkey can be considered a natural bridge between West and East or Europe and Asia. Although Turkey is a rapidly growing, developing and industrialized country in the last two decades [4], developing and industrialized, according to the data of Ministry of Energy and Natural Resources (MENR), total energy consumption in Turkey in 2013 was 120.3 Million Tons of Oil Equivalent (MTOE). This amount corresponds to an increase of 127% compared to the consumption in 1990. 75.5% of the primary energy demand in 2013 could be met by import. The bulk of the demand (31.3%) was ⇑ Corresponding author. E-mail address: [email protected] (Ö. Atalay). http://dx.doi.org/10.1016/j.seta.2017.05.005 2213-1388/Ó 2017 Elsevier Ltd. All rights reserved.

met by natural gas which is mostly imported (98%). 28.2% of the energy demand is met by oil; 14.7% by coal; and 11% by lignite. The share of imports in energy sources is increasing rapidly [5]. Energy consumption is commonly observed to occur in four main sectors: industry, residential, transport and agriculture. Residential sector in many countries is one of the largest energy consumers. In the residential sector, the energy consumption for heating is two times more than the energy consumed for the functions such as water heating, cooking, cooling and freezing foods. However, energy consumption for heating can substantially be reduced by applying insulation in the residential sector [6]. Thermal insulation is the most important part of the energy efficiency in the world. TS 825 Regulations on Thermal Insulation Requirements for Buildings aims to provide energy savings by reducing the energy consumption of residential heating, that has the largest share in energy consumption [7]. Yuan et al. [8] set the optimum combination of external wall insulation and reflectivity for annual thermal load in Japan. Liu et al. [9], calculated the optimal insulation thickness of external walls in the housing by taking into account moisture transfer in China’s cold and hot climate zones. Ozel [10], has determined the optimum thickness of the external wall for the province of Antalya in Turkey by depending on the cooling load in a warm climate. Yuan et al. [11] calculated optimal insulation thickness of housing

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Nomenclature C Cf Ct Ex HDD PWF h s k qA U T Ru P M (m)

cost ($) unit cost of fuel ($/kg) total heating cost at present value ($) exergy (kJ/m2) heating degree-days present worth factor enthalpy (kJ/kmol, kJ/kg) entropy (kJ/kmol K, kJ/kg K) thermal conductivity coefficient (W/m K) annual heat loss (W/m2) heat transfer coefficient (W/m2 K) temperature (K) universal gas constant (kJ/kmol K) pressure (Pa) mass rate (kg/m2)

ceiling in China’s hot and cold regions with the method of sol-airdegree -hours. Daouas [12] investigated the energy saving and optimal insulation thickness connected to the heating and cooling loads in housing in Tunisia. Ucar and Balo [13] determined the optimum insulation thickness of the external walls of the housing in Elazıg˘, Sßanlıurfa, Mersin and Bitlis in Turkey. Kaynakli [14] investigated optimal insulation thickness and heating energy demand in housing for 4 different types of fuel. Arslan and Kose [15] calculated the optimal insulation thickness for external walls in houses in Kutahya in Turkey, with the thermoeconomic method that takes into account the condensation. Ucar [16], calculated the optimum insulation thickness for the outer wall by using thermoeconomic optimization method in Adana, Istanbul, Elazıg˘ and Erzurum in Turkey. In this study, the optimum insulation thickness of the external wall of the housing in the provinces -Ankara, Izmir, Kars and Trabzon - located in four different climate regions of Turkey was calculated for the polystyrene and polyurethane insulation materials. The geographical locations of these 4 provinces are given in Fig. 1. In the study, natural gas was selected as a fuel. Since the thermoeconomic method is the combination of exergy and economy, it has been used for the calculation. In the literature, there are numerous studies about this method also called exergy cost analysis [17–30]. However, there are a few studies conducted by a combination of exergy analysis and lifecycle cost analysis. The purpose of this study is to determine the optimum insulation thickness of the external wall using thermoeconomic method

n EA R x

g

y

mole (kmol) annual energy demand (W/m2-year) thermal resistance (m2 K/W) insulation thickness (m) efficiency of the heating system mole fraction

Subscripts Q,loss losses due to heat transfer F(f) fuel a air ins insulation opt optimum s stack gas t total

considering the effect of the inflation and interest rate, called as Life Cycle Cost Analysis (LCA), which are not examined in previous studies. It is believed that this gives much more accurate results.

Method The amount of heat lost and consumption of the natural gas from the unit surface of the external wall has been calculated to find the annual energy needs using the degree day method for the selected degree day for the selected provinces [31]. The fuel consumption to meet the annual energy demands for the unit surface was calculated assuming the steady state exergy balance, and by using equations obtained by calculating entropy production and exergy destruction. Finally, the optimal insulation thickness of the external walls and exergy cost savings were calculated with the thermoeconomic providing exergy costs and lifecycle cost analysis for the selected provinces in Turkey given in Fig. 1.

The model of external wall used in calculations The external wall structure used in the calculations is shown in Fig. 2. As can be seen in the Figure, the external wall, which is also called as the ‘‘sandwich wall”, used in the calculations, consists of 2 cm inner plaster, two 8.5 cm horizontal hollow bricks and 3 cm exterior plaster. Expanded polystyrene and polyurethane was

Fig. 1. Geographic locations of selected cities in Turkey.

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EA ¼

86:4HDD ðRwt þ kxÞg

ð7Þ

As a result, exergy loss realized by heat transfer from the unit outer wall surface,

ExQ ;loss ¼

86:4HDDU

g

ð1 

T0 Þ Tb

ð8Þ

where, T 0 environmental reference temperature (boiler room temperature) and T b boundary wall temperature. Calculation of chemical exergy of fuel and exergy destruction due to the heat transfer

Fig. 2. The model of external wall.

applied as the insulation material. The parameters used for the external wall is given in Table 1. Exergy analysis of external wall The heat transfer from the indoor to the outdoor through the external walls causes the heat loss during the heating period. Heat loss from the houses occurs through external walls, roof, windows, air infiltration and ventilation. Since the effect of air infiltration and ventilation on heat transfer is limited, they were not taken into account [16] heat loss at the unit outer wall surface,

q ¼ U DT

ð1Þ

where, U is the heat transfer coefficient. Annual heat loss from the unit wall surface qA and the annual energy needs EA depending on this annual heat loss are calculated by using the number of heating degree days (HDD) [32].

qA ¼ 86:4HDDU EA ¼

ð2Þ

86:4HDDU

ð3Þ

g

where g is the boiler efficiency U for a typical external wall,

U¼

1 Ri þ Rw þ Rins þ Ro

ð4Þ

Natural gas contains more than 90% methane (CH4). Therefore, methane is selected in the equations of the combustion. For ease of calculation, it was assumed to be complete combustion [33]. If n mole fuel is burned for annual energy needs of a unit surface, the combustion reaction equation;

nCH4 þ 2nðO2 þ 3:76N2 Þ ! nCO2 þ 2nH2 O þ 7:52nN2

Typical housing system for exergy balance is shown in Fig. 3. Exergy balance is given below for the chemical exergy of fuel by accepting steady state for fuel, air and waste gases [34].


CH þ 2h
O þ 7:52h
N  h
CO  h
H O  7:52h
N  T 0 ð
sCH þ 2
sO n½h 4 2 2 2 2 2 4 2 þ 7:52
sN2 
sCO2  2
sH2 O  7:52
sN2 Þ  ExQ;loss  Ex;d ¼ 0 ð10Þ
and
s are the molar enthalpy and entropy of reactant and where h products respectively, Ex;d is the exergy destruction, occurring due to the heat lost from the unit wall surface. It has been considered that the fuel and air enter the combustion system in environment reference conditions ðT0 ¼ 25 C;P0 ¼ 1 atmÞ and waste gasses exit from the chimney at 150  C ðTstack Þ. Any system that is assumed in equilibrium with its surroundings has zero exergy and is said to be at the dead state. The temperature and pressure of the environment are known to be uniform at T0 and P0. It has been presented that there is the insignificant effect on the result of exergy and energy analysis small change of dead state. It has been presented that the effects on the results of energy and exergy analyses of variations in dead-state properties are insignificant [35].

where Ri and Ro are respectively the thermal resistance of internal and external air film. Rw is the thermal resistance of the uninsulated wall layers. The thermal resistance of the insulating material,

Rins ¼

x k

ð9Þ

Exstack gases (CO2,H2O, N2) (P0,Tstack)

ð5Þ

where x and k are respectively the thickness of the insulating material and thermal conductivity coefficient.

Rwt ¼ Ri þ Rw þ Ro

ð6Þ

According to this formula, annual energy demand for heating,

Table 1 Parameters used of energy demand calculations. Parameter

Value

_ HDD (Izmir) [31] HDD (Ankara) [31] HDD (Trabzon) [31] HDD (Kars) [31] Rwt [32] ɳ [32]

1583 3214 2223 5451 0.592 m2 K/W 0.93

Environmental state (P0,T0),which is boiler room conditions Exfuel

ExQ,loss Exd

Exair

Fig. 3. Systematic residential building for chemical exergy of fuel.

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Steady state - entropy balance system is systematically shown in Fig. 4. Exergy destruction can be determined by calculating the entropy production due to the second law analysis of the combustion equation [36].

SCH4 þ Sair  Sstack  SQ þ Sproduction ¼ 0

ð11Þ

and Sproduction ,

Sproduction ¼ Sstack  SQ  SCH4  Sair

ð12Þ

the entropy of fuel, air and waste gases,

Calculation of optimum insulation thickness Exergy cost of a system can be calculated using the thermoeconomic analysis method as given Eq. (24) [15,16,37]. OM C T ¼ C F þ Z CI tot þ Z tot

ð24Þ

where CT is the total cost; CF is fuel cost, Z CI tot is capital investment; Z OM tot is operating and maintenance cost. Since the maintenance and replacement of the insulation in the external walls of housing is not

SCO2 ¼ n½
sCO2 ðT stack ; P0 Þ  Ru lnyCO2 

ð13Þ

SH2 O ¼ 2n½
sH2 OðT stack ; P0 Þ  Ru lnyH2 O

ð14Þ

SCO2 ¼ 7:52n½
sN 2 ðT stack ; P0 Þ  Ru lnyN 2 

ð15Þ

SCH4 ¼ n½
sCH4 ðT 0 ; P 0 Þ  Ru lnyCH4 

ð16Þ

where cf is unit fuel cost ($/kg) and mf (kg) is the fuel mass. From Eq. (26) total fuel cast can be calculated [15,16],

SO2 ¼ 2n½
sO2 ðT 0 ; P0 Þ  Ru lnyO2 

ð17Þ

CF ¼

SN2 ¼ 7:52n½
sN 2 ðT 0 ; P0 Þ  Ru lnyN 2 

ð18Þ

WhereRu is the universal gas constant, y is the mole percent of the gas mixture. Entropy and exergy destruction due to heat loss can be calculated with Eqs. (19)–(21),

SQ ¼

86:4HDD ðRwt þ kxÞT b

ð19Þ

Ex;d ¼ T 0 Sproduction

ð20Þ

Steady state exergy balance is shown in Fig. 3,

Exf þ Exa  Exs  EQloss  Exd ¼ 0

ð21Þ

The number of fuel moles (n) to meet the annual energy needs caused by the heat loss from the unit external wall,

n¼

EQloss Exf þ Exa  Exs  Exd

ð22Þ

In Eq. (21), Exf, Exa, Exs are chemical exergy values of fuel, air and stack gas respectively. In Eq. (21), ExQloss is the exergy caused by heat loss from the unit external wall surface, and Exd is exergy destruction. The fuel mass can be calculated from the below equation for the annual energy demand for the molecular mass of methane, MCH4,

mf ¼ nMCH4

ð23Þ

possible, Z OM tot was taken as zero in this study. Fuel cost, CF ($) can be calculated from below equation

C F ¼ mf cf

ð25Þ

EQloss MCH4 cf Exf þ Exa  Exs  Exd

ð26Þ

Lifecycle cost analysis is used to evaluate the economic benefits by taking the life time of energy conversion systems [38,39]. Parameters used for calculation of optimum insulation thickness are given in Table 2. The total cost according to lifecycle cost method can be calculated using Eq. (27) [7,32],

C T ¼ PWFC F þ C ins x

ð27Þ

where, PWF is present worth factor, Cins is unit insulation cost ($ /m3) and x is the insulation thickness (m). The optimum insulation thickness for external walls can be calculated using the first taking into account total derivative based on the cost of the insulation thickness and then by equalizing it with zero. The total cost of the insulation thickness x calculated with synchronized zero [40]. As can be seen from the Eqs. (28) and (29), the optimum insulation thickness for external walls can be calculated taking first derivative of total cost according to the insulation thickness and equating the result to zero. [40].

dC T d ¼ ðPWFC F þ C ins xÞ dx dx

ð28Þ

and,

dC T ¼0 dx

ð29Þ

Exergy cost savings ($/m2) can be calculated from Eq. (30),

Excostsav ing ¼ ðC T Þnins  ðC T Þins

Sstacks

ð30Þ

where, ðC T Þnins and ðC T Þins are the total exergy cost for the uninsulated and insulated walls respectively. Sfuel

Table 2 Parameters used in calculations of optimum thickness.

SQ,loss Sair

Fig. 4. System of entropy calculations.

Parameter

Unit

Value

Cost of insulation materials Expanded polystyrene [7] Polyurethane [7] Cost of fuel (CH4) [33]

$/m3 $/m3 $/kg

120 260 0.53

W/mK W/mK

0.039 0.024 9.83

Conductivity of insulation material Expanded polystyrene [7] Polyurethane [7] PWF [7]

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In this study, the optimum insulation thickness of a typical external wall with two different insulation materials in houses was calculated using thermoeconomic optimization method for the selected provinces (Izmir, Ankara, Trabzon and Kars) in Turkey’s at four different climatic zones. Natural gas was used as fuel; and expanded polystyrene and polyurethane were used as insulation material for residential heating. The geographic locations of the selected provinces are presented in Fig. 1. The energy demand for heat loss from the external wall was calculated by the degreedays method. Parameters and the values used for the calculations are shown in Table 1. Then, system exergy analysis was performed, and annual fuel consumption for the unit wall surface is determined by calculating fuel chemical exergy, exergy loss due to the heat transfer through the outer wall surface, and the exergy destruction. The parameters used in the calculation of optimum insulation thickness for the external wall are given in Table 2. The annual fuel cost changes depending on the expanded polystyrene insulation material thickness for Izmir, Ankara, Trabzon and Kars are shown in Figs. 5–8. The effect of the polyurethane insulation material on the annual fuel cost is given in Figs. 9–12. The most economical insulation thickness is the thickness corresponding to the minimum point of the total cost, which is also known as the optimum insulation thickness. As can be seen from the Figures, the optimal insulation thicknesses for expanded

polystyrene insulation are 0.046 m for Izmir, 0.077 m for Ankara, 0.06 m for Trabzon, and 0.107 m for Kars. The optimal insulation thicknesses for polyurethane insulation are 0.023 m for Izmir, 0.039 m for Ankara, 0.0304 m for Trabzon and 0.055 m for Kars. For two different insulation materials, the minimum thickness is

50

Annual cost ($/m2 year)

Results and discussion

20

10

0,05

0,1

Insulation cost

15 10

0,15

0,2

0,25

Insulation thickness (m) Fig. 7. Annual cost versus insulation thickness of Trabzon for expanded polystyrene.

Annual cost ($/m2 year)

Annual cost ($/m2year)

30

90

Total cost Fuel cost

20

5 0 0

insulation cost

0 0

30 25

total cost fuel cost

40

total cost fuel cost

80

insulation cost

70 60 50 40 30 20 10

0,05

0,1

0,15

0,2

0 0

0,25

0,05

0,1

0,15

0,2

0,25

Insulation thickness (m)

Insulation thickness (m)

_ Fig. 5. Annual cost versus insulation thickness of Izmir for expanded polystyrene.

Fig. 8. Annual cost versus insulation thickness of Kars for expanded polystyrene.

50

50

Annual cost ($/m2year)

Annual cost ($/m2 year)

60 total cost fuel cost insulation cost

40 30 20 10 0 0

0,05

0,1

0,15

0,2

0,25

Insulation thickness (m) Fig. 6. Annual cost versus insulation thickness of Ankara for expanded polystyrene.

Total cost Fuel cost insulation cost

40

30

20

10

0 0

0,05

0,1

0,15

0,2

Insulation thickness (m) _ Fig. 9. Annual cost versus insulation thickness of Izmir for polyurethane.

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Ö. Altan Dombayci et al. / Sustainable Energy Technologies and Assessments 22 (2017) 1–8

12 total cost fuel cost insulation cost

50

Exergy cost saving ($/m2 year)

Annual cost ($/m2 year)

60

40 30 20 10 0 0

0,05

0,1

0,15

0,2

0,25

10 expanded polystyrene polyurethane

8 6 4 2 0 0

0,02

Insulation thickness (m) Fig. 10. Annual cost versus insulation thickness of Ankara for polyurethane.

40 30 20 10 0,1

0,15

0,2

0,25

Exergy cost saving ($/m2year)

Annual cost ($/m2 year)

fuel cost insulation cost

0,05

50 40 30 20 10 0,05

0,1

15 10 5 0,05

0,1

0,15

0,15

0,2

0,25

Fig. 14. Variation of exergy cost saving with insulation thickness for Ankara.

Exergy cost saving ($/m2year)

Annual cost ($/m2 year)

insulation cost

60

0 0

20

20

total cost fuel cost

70

25

Insulation thickness (m)

Fig. 11. Annual cost versus insulation thickness of Trabzon for polyurethane.

80

0,1

expanded polystyrene polyurethane

30

0

Insulation thickness (m)

90

0,08

35

total cost

50

0 0

0,06

_ Fig. 13. Variation of exergy cost saving with insulation thickness for Izmir.

70 60

0,04

Insulation thickness (m)

0,2

0,25

Insulation thickness (m)

expanded polystyrene polyurethane

15

10

5

0 0

0,05

0,1

0,15

0,2

0,25

Insulation thickness (m)

Fig. 12. Annual cost versus insulation thickness of Kars for polyurethane.

Fig. 15. Variation of exergy cost saving with insulation thickness for Trabzon.

in Izmir, and the maximum thickness is in Kars. Because, Izmir is located in the warm temperate climate region, while Kars is located in the cold climate region. The exergy cost changes depending on the insulation thickness for two different materials in selected provinces are shown in Figs. 13–16. As can be seen from the Figures, the exergy cost savings increases up to the point corresponding to the optimum

thickness values, then gradually decreases after this point. The continuous increase of the insulation thickness after the optimum thickness also rises the total cost and reduces the economic benefits. Exergy cost savings corresponding to the optimum thickness of expanded polystyrene insulation material is about 11 $/m2 in _ Izmir, 30 $/m2 in Ankara, 18.5 $/m2 in Trabzon and 60 $/m2 in Kars. Exergy cost savings corresponding to the optimum thickness of

Ö. Altan Dombayci et al. / Sustainable Energy Technologies and Assessments 22 (2017) 1–8

Exergy cost saving ($/m2year)

70

expanded polystyrene polyurethane

60 50 40 30 20 10 0 0

0,1

0,2

0,3

0,4

0,5

0,6

Insulation thickness (m) Fig. 16. Variation of exergy cost saving with insulation thickness for Kars.

_ polyurethane insulation material is about 9 $/m2 in Izmir, 27 $/m2 2 2 in Ankara, 16 $/m in Trabzon and 53 $/m in Kars. The savings rates for expanded polystyrene in Izmir, Ankara, Trabzon and Kars are 27%, 54%, 51% and 56.6% respectively. On the other hand, the savings rates for polyurethane insulation material in Izmir, Ankara, Trabzon and Kars are 22%, 28%, 47.5% and 51% respectively. For two different insulation materials used in this study, the maximum savings was in Kars, the minimum savings was in Izmir. As it is apparent from the results, the optimum thickness calculation for external walls is especially very important in cold climates. Conclusions In this study, the optimum insulation thickness for the external wall surface of houses was calculated using the thermoeconomic analysis with the LCA method for the provinces of Izmir, Ankara, Trabzon and Kars in Turkey. Natural gas was used as fuel; and expanded polystyrene and polyurethane were used as the insulation materials for residential heating. In addition, exergy cost savings due to optimal insulation thickness was determined for each province. Primary implications of the study are listed below; 1) While insulation thickness increases, the annual cost decreases; but, it increases rapidly after a certain minimum point. 2) Optimum insulation thickness for the external wall in Izmir, Ankara, Trabzon and Kars were 0.046, 0.077, 0.06 and 0.107 m respectively for the expanded polystyrene materials, and 0.023, 0.039, 0.0304 and 0.055 m for the polyurethane insulation materials. 3) Exergy cost savings in optimal insulation thickness in the selected provinces were maximum. Exergy cost savings reduces after the optimum thickness. 4) The maximum savings for both of insulation materials was in Kars, while the lowest savings was in Izmir. Exergy cost savings are 56.6% in Kars 27% in Izmir for the expanded polystyrene materials, and 51% in Kars, 22% in Izmir for the polyurethane insulation materials.

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