Optimal combination of thermal resistance of insulation materials and primary fuel sources for six climate zones of Japan

Energy and Buildings 153 (2017) 403–411

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Replication Studies paper

Optimal combination of thermal resistance of insulation materials and primary fuel sources for six climate zones of Japan Jihui Yuan ∗ , Craig Farnham, Kazuo Emura Department of Housing and Environmental Design, Graduate School of Human Life Science, Osaka City University, Osaka 5588585, Japan

a r t i c l e

i n f o

Article history: Received 9 May 2017 Received in revised form 21 July 2017 Accepted 16 August 2017 Keywords: Japanese climate zones Optimal combination Thermal resistance Fuel sources Cost analysis Degree-day method

a b s t r a c t Since the East Japan great earthquake disaster in March 2011, liquefied natural gas (LNG) to replace nuclear energy has become the most-used fuel source for energy generation in Japan. Japan is implementing energy conservation policy. Building energy consumption accounts for a large proportion of the total energy consumption. To better combine the thermal insulation of external walls and fuel source utilization will play an important role in building energy conservation. This paper aims at finding the optimal combination from four different insulation materials and four different fuel sources for residences using electricity for heating and cooling in the six climate zones of Japan. The optimal thermal resistance (OTR) of insulation materials, energy cost saving per unit area of external walls and payback periods if the OTR is adopted for six climate zones are estimated via a cost analysis and degree-day (DD) method. According to the results, the optimal combination for all climate zones has been obtained by using rock wool as the insulation material and LNG as the fuel source. The energy cost saving and payback periods are 20.4 $/m2 -yr and 0.4 yrs respectively, while the OTR is 2.5 m2 K/W for Sapporo (in climate zone I), 14.1 $/m2 -yr and 0.5 yrs respectively, while the OTR is 2.1 m2 K/W for Akita (in climate zone II),11.2 $/m2 -yr and 0.6 yrs respectively, while the OTR is 1.8 m2 K/W for Fukushima (in climate zone III), 5.2 $/m2 -yr and 0.8 yrs respectively, while the OTR is 1.3 m2 K/W for Osaka (in climate zone IV), 2.5 $/m2 -yr and 1.2 yrs respectively, while the OTR is 0.9 m2 K/W for Kagoshima (in climate zone V), and there is no need to adopt thermal insulation for Naha (in climate zone VI). © 2017 Elsevier B.V. All rights reserved.

1. Introduction With the continuing increase of world’s population and standard of living, energy consumption becomes even more of an important issue. A continuous and cheap supply of energy is desired for economic and social development [1]. The use of non-renewable energy use can cause environmental problems, global warming and reduced quality of life. One of the most effective ways of reducing this energy use is to utilize fossil fuel sources efficiently [2,3]. A study showed that the major energy end use sectors are commercial, industrial, transportation and residential, with residential the largest consumer sector in many countries [4]. The energy demands of residential buildings are high due in part to the indoor thermal comfort requirements of buildings. As an effective measure of reducing building energy use, thermal insulation technologies of building external walls are often adopted [5,6]. The thermal insu-

∗ Corresponding author. E-mail address: [email protected] (J. Yuan). http://dx.doi.org/10.1016/j.enbuild.2017.08.039 0378-7788/© 2017 Elsevier B.V. All rights reserved.

lation of building external walls can decrease the heat loss or gain through the building envelopes, but at increasing cost [7]. The influence of insulation configuration on thermal loads of buildings had been detailed by many scholars. The thermal insulation of building external walls can significantly reduce the heating and cooling needs of the zone [8]. A study on performance of the heat and multilayer reflection insulators used for buildings was implemented in South Korea [9]. The result showed that the multilayer reflection insulator keeps the indoor wall surface temperature high during winter or low in summer, enhances the comfort of the building occupants and reduces thermal loads. The influence of insulation configuration on thermal loads of buildings was evaluated and a whole-building energy modeling was performed using DOE-2.1E to predict annual heating and cooling energy demand for a onestory residential building [10]. Results showed that the material configuration of the external wall can significantly affect the annual thermal performance of the whole building under different types of climate. Design and construction with optimal insulation thickness (OIT) should be considered as a prerequisite and a top priority for energy

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Nomenclature (CT )nins The CT for non-insulated walls ($/m2 -yr) (CT )otr The CT for OTR ($/m2 -yr) CDD28 The cooling degree-days at 28 ◦ C base dry-bulb temperature (◦ C-day) The unit cost of furels ($/kg) or ($/m3 ) Cf COPC The efficiency of air conditioning during cooling period [-] The efficiency of air conditioning during heating COPH period [-] CT The annual total cost per unit area of building external walls ($/m2 -yr) Cins The unit insulation cost ($/m3 ) The energy cost saving if the OTR is adopted ($/m2 Ecs yr) HDD18 The heating degree-days at 18 ◦ C base dry-bulb temperature (◦ C-day) Hu The lower calorific value of fuel sources (kJ/kg) or (kJ/m3 ) N The period of the cost analysis (yr) OIT The optimal insulation thickness of external walls (m) The optimal thermal resistance of insulation mateOTR rials (m2 K/W) PP The payback period if the OTR is adopted (yr) PWF The present worth factor (−) The thermal resistance of internal air film (m2 K/W) Ri Rins The thermal resistance of insulation material (m2 K/W) Ro The thermal resistance of external air film (m2 K/W) The total thermal resistance of the wall layers withRw out insulation (m2 K/W) The sum of Ri , RO and RW (m2 K/W) Rwt U The coefficient of heat transmission of external wall (W/m2 K) g The inflation rate (−) The interest rate (−) i k The thermal conductivity of insulation materials (W/mK) mC The annual fuel consumption for cooling period (kg/m2 -yr) mH The annual fuel consumption for heating period (kg/m2 -yr) n The day of heating or cooling periods (day) The heat loss from unit external wall surface q (W/m2 )The heat loss from unit external wall surface (W/m2 ) x The insulation thickness (m) The temperature difference between indoor and T outdoor sides (◦ C or K)  The electric generation efficiency for fuel sources (−) The design indoor dry-bulb temperature for cooling  ic period (◦ C) The design indoor dry-bulb temperature for heating  ih period (◦ C)  om(d) The mean daily outdoor dry-bulb temperature of the d-th day (◦ C)

savings in buildings [11]. The definition of OIT has been detailed by many researchers worldwide [12,13]. A numerical model was used to determine the annual thermal transmission loads, then the calculated thermal transmission loads were inputted to an eco-

nomic model to determine the OIT for a south-facing wall in the climatic conditions of Elazı˘g, Turkey [14]. The degree-day (DD) method is commonly used to calculate the energy needs of buildings, and methods of calculating the OIT are proposed based on the DD method and life-cycle cost analysis (LCCA) which is widely used in different fields [15–17]. Dombaycı et al. [18] have calculated the OIT of external wall for five different energy sources (coal, natural gas, LPG, fuel oil and electricity) and two different insulation materials (expanded polystyrene (EPS) and rock wool) for Denizli, Turkey. It shows that the OIT is obtained by using the coal as the primary energy source and EPS as the insulation material. In addition, a study on the determination of OIT of indoor pipelines of split air conditioning systems with consideration of four different insulation materials (EPS, rock wool, foam board and extruded polystyrene (XPS)) was also carried out using the LCCA [19]. It shows that EPS is a better choice when the OIT is an important consideration. Japan has high electricity costs compared with other major economies, in 2013 nearly 50% higher than the OECD average unit cost. The portion of consumer spending on energy reached a record high of 8.6% as of 2014, chiefly due to electricity price increases [20]. Part of Japan’s Energy Plan for FY2030 includes a focus on residential energy conservation through building and renovating houses, including improved thermal insulation and high-efficiency air-conditioners with the goal of certification as low-carbon buildings [21]. The Government of Japan aims by 2030 for all newly constructed houses to be zero energy buildings, though electricity generation will still largely depend on increasingly-efficient fossil fuel use [22]. In addition, Japan is reforming its electricity market, which had been dominated by the 10 regional Electricity Production Company (EPCOs) as near monopolies. One of the 3 stated goals of the 2013 Cabinet plan is “expanding choices for consumers and business opportunities” [23]. Retail competition for electricity distribution was expanded to the residential sector in 2016. Consumers and Energy Services Company (ESCOs) have a much greater range of options. This paper has the goal of finding the optimum thermal resistance (OTR) of insulation materials based on the insulation cost in a residence using electricity for heating and cooling combined with the cost of the fuel source chosen by the electric utilities. Thus the OTR here is not from the viewpoint of the individual homeowner, but rather for regional or national economization goals, or for an ESCO or the new electricity retailers, trying to maximize efficiency, or minimize costs, by making recommendations to builders or remodelers concerning insulation, while also considering the primary fuel sources to aid in minimizing costs to the electric producers, which should in turn help reduce electric prices for the individual consumer and the ESCO or retailer. The analysis is done for the six climate zones of Japan. According to the standards of energy conservation for residential and commercial building owners of Japan in 2013 [24], a total of eight climate zones were classified by considering the average heat transmission of external walls and the average rate of solar radiation gain during the cooling period as classification index. However, the eight climate zones also can be assembled into six climate zones, which includes zone I (Ia , Ib ), zone II, zone III, zone IV (IVa , IVb ), zone V, and zone VI. For the six climate zones of Japan, the OTR of insulation materials, total energy cost saving per unit area of external walls and payback period (PP) if the OTR is adopted by different combination of four different insulation materials (EPS, rock wool, foam board and XPS) and four different fuel sources for electric power generation (coal, natural gas, LNG and fuel oil) are calculated here using a cost analysis and DD method.

J. Yuan et al. / Energy and Buildings 153 (2017) 403–411

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Fig. 1. Six typical cites respectively in six climate zones of Japan.

2. Material and methodology

2.2. Heating and cooling degree-days

2.1. Six climate zones of Japan

For determination of the heating degree-day (HDD) and cooling degree-day (CDD) in this study, we obtained the mean daily outdoor dry-bulb temperature of 35 years (1981–2015) of six typical cities representing six climate zones from the Expanded AMeDAS (EA) weather data of Japan [25]. Under the current premise of energy conservation, the HDD was calculated at base dry-bulb temperature of 18.0 ◦ C, and the CDD was calculated at base dry-bulb temperature

Six typical cities (Sapporo, Akita, Fukushima, Osaka, Kagoshima and Naha) respectively in six climate zones of Japan which is shown in Fig. 1 are chosen to calculate the OTR of insulation materials, total energy cost saving per unit area of external walls and PP if the OTR is adopted in this study. The geographic locations of the six chosen cities are detailed in Table 1.

Table 1 Geographic locations, HDD18 and CDD28 of six typical cities respectively in six climate zones of Japan. Capital Cities Sapporo Akita Fukushima Osaka Kagoshima Naha

Climate zones I II III IV V VI

Location (Lat., Long.) ◦

◦

43.1 N, 141.3 E 39.2◦ N, 140.1◦ E 37.8◦ N, 140.5◦ E 34.7◦ N, 135.5◦ E 31.6◦ N, 130.6◦ E 26.2◦ N, 127.7◦ E

HDD18 (◦ C-day)

CDD28 (◦ C-day)

3530.1 2746.3 2362.2 1485.5 1024.3 60.0

0.0 0.0 0.0 28.6 25.4 52.8

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J. Yuan et al. / Energy and Buildings 153 (2017) 403–411

of 28.0 ◦ C in this study. The HDD and CDD are determined using the following Eqs. (1) and (2), respectively,

 n

HDD18 =

(ih − om(d) )

PWF =

CDD28 =

(2)

1

where “HDD18 ” is the heating degree-days at base dry-bulb temperature of 18.0 ◦ C, [◦ C-day]; “CDD28 ” is the heating degree-days at base dry-bulb temperature of 28.0 ◦ C, [◦ C-day]; “ ih ” is the design indoor dry-bulb temperature of 18.0 ◦ C for heating period; “ ic ” is the design indoor dry-bulb temperature of 28.0 ◦ C for cooling period; “ om(d) ” is the mean daily outdoor dry-bulb temperature of the d-th day, [◦ C]; and “n” is the days of heating or cooling periods, [day]. The annual HDD18 and CDD28 for six typical cities in six different climate zones of Japan are calculated and shown in Table 1. The results of DD calculations show that HDD18 is relatively larger than CDD28 for all six climate zones. 2.3. Determination of OTR of insulation materials Heat loss or gain from unit external wall surface can be calculated using the following Eqs. (3) and (4), q = U · T U=

1 1 = Ri + Rw + Rins + Ro Rwt + Rins

(3) (4)

where “q” is heat loss or gain from unit external wall surface, [W/m2 ], “U” is the coefficient of heat transmission of external wall, [W/m2 K]; “T” is the temperature difference between indoor and outdoor sides, [◦ C] or [K]; “Ri ” is the thermal resistance of internal air film, [m2 K/W]; “Ro ” is the thermal resistance of external air film, [m2 K/W]; “Rw ” is the total thermal resistance of the wall layers without insulation materials, [m2 K/W], “Rwt ” is the sum of Ri , Ro and Rw , [m2 K/W]; and “Rins ” is the thermal resistance of insulation material, [m2 K/W]. The thermal resistance of insulation material, Rins , can be derived through the following Eq. (5), Rins =

x k

(5)

where “x” is the insulation thickness, [m]; and “k” is the thermal conductivity of insulation material, [W/mK]. This study focuses on Japanese houses which solely use electricity for heating and cooling. Hence, the fuel consumption by the electricity producers per unit area of building external walls for heating and cooling periods can be derived using the following Eqs. (6) and (7), respectively, 86.4HDD18 mH = (Rwt + Rins )Hu COPH

(6)

86.4CDD28 (Rwt + Rins )Hu COPC

(7)

mC =

r(1 + r)N

,

1+g

(9)

⎪ ⎩ i < g, r = g − i

1+i

PWF = (om(d) − ic )

(1 + r) − 1

(1)

1 n 

⎧ i−g ⎪ ⎨ i > g, r =

N

1 ,i = g 1+i

(10)

where “CT ” is the total cost per unit area, [$/m2 -yr]; “PWF” is the present worth factor, [-]; “Cf ” is the unit cost of fuels, [$/kg] or [$/m3 ]; “Cins ” is the unit insulation cost, [$/m3 ]; “i” is the interest rate, [-]; “g” is the inflation rate, [-]; and “N” is the period of the cost analysis, [yr]. Here we choose 10 years, as a possible ESCO contract length as far as realizing net cost savings to yield a return within a reasonable period. The optimal thermal resistance of insulation materials can be also derived using the following Eq. (11),

 OTR =

86.4·HDD18 ·Cf ·PWF·k Hu ·Cins ··COPH

+

86.4·CDD28 ·Cf ·PWF·k Hu ·Cins ··COPC

− Rwt · k

k

where “OTR” is the optimal thermal resistance of insulation materials, [m2 K/W]. 2.4. Determination of total energy cost saving The total potential energy cost saving per unit area of building external walls if the OTR is adopted can be derived by subtracting the total cost per unit area of building external walls for noninsulated walls and the total cost per unit area of building external walls for external walls with OTR. The relationship is shown as the following Eq. (12), Ecs = (CT )nins − (CT )otr

(8)

(12)

where “Ecs ” is the total potential energy cost saving per unit area of building external walls if the OTR is adopted, [$/m2 -yr]; “(CT )nins” is the total cost per unit area of building external walls for noninsulated walls, [$/m2 -yr]; and “(CT )otr” is the total cost per unit area of building external walls for external walls with OTR, [$/m2 yr]. 2.5. Determination of payback periods The payback period refers to the period of time required to recoup the funds expanded in an investment, or to reach the breakeven point. It can be obtained by the following Eq. (13), pp =

Cins · x Ecs

where “PP” is the payback period if the OTR is adopted, [yr].

where “mH ” and “mC ” are the fuel consumption per unit area of building external walls for heating and cooling periods, respectively, [kg/m2 -yr]; “” is the electric generation efficiency for different fuel sources, [-]; “Hu ” is the lower calorific value of the fuel, [kJ/kg], “COPH ” and “COPC ” are the efficiency of air conditioning during heating and cooling periods, respectively, [-]. The total cost per unit area of building external walls can be derived by the following Eqs. (8)–(10), CT = PWF · − − −Cf · (mH + mC ) + Cins · x

(11)

Fig. 2. Cross-sectional view of the external wall structure.

(13)

J. Yuan et al. / Energy and Buildings 153 (2017) 403–411

3. Results and discussion In this study, the structure of external wall which is shown in Fig. 2 is used for calculation. The energy consumed in buildings for heating and cooling space is electricity (secondary energy) which is converted from four different fuel sources (coal, natural gas, LNG and fuel oil). All the parameters [26–30] of calculation are given in Table 2. 3.1. Optimal thermal resistance (OTR) The annual thermal loads tend to decrease with increasing thermal resistance of the building external walls. The cost of the insulation material increases linearly with the thermal resistance. The energy cost saving increases up to a certain value of the thermal resistance, and beyond this level, the energy cost saving is

407

decreased. The curves of energy cost saving versus the thermal resistance of insulation materials for example city of Osaka are shown in Fig. 3. The OTR of insulation materials for six typical cities in six climate zones of Japan by different combination of four insulation materials and four fuel sources are calculated and detailed in Table 3. Results show that the largest OTR of insulation materials is obtained by using rock wool as the insulation material and LNG as the fuel source for all climate zones of Japan, thus it is considered that the optimal combination is composed of rock wool and LNG for all climate zones. The largest OTR is about 2.5 m2 K/W for Sapporo (in climate zone I), about 2.1 m2 K/W for Akita (in climate zone II), about 1.8 m2 K/W for Fukushima (in climate zone III), about 1.3 m2 K/W for Osaka (in climate zone IV), about 0.9 m2 K/W for Kagoshima (in climate zone V), and there is no need to adopt thermal insulation for Naha (in climate zone VI). In addition, we

Fig. 3. Energy cost saving versus thermal resistance of external walls for Osaka.

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J. Yuan et al. / Energy and Buildings 153 (2017) 403–411

Table 2 Parameter value used in the calculations. Parameter

Unit

Value

Sapporo (Climate zone-I) [m2 K/W]

Cost of insulation material (Cins )[26] Expanded polyurethane (EPS) Foam board Rock wool Extruded polyurethane (XPS)

$/m $/m3 $/m3 $/m3

155 193 90 224

Cost of Fuels (Cf ) [27] Coal Natural gas Liquefied natural gas (LNG) Fuel oil

$/kg $/m3 $/kg $/kg

0.164 0.223 1.218 0.616

Lower calorific value of fuel (Hu ) Coal Natural gas Liquefied natural gas (LNG) Fuel oil

Table 3 Optimal thermal resistance (OTR) for six climate zones by different combination of four insulation materials and four fuel sources.

3

kJ/kg kJ/m3 kJ/kg kJ/kg

25,122 34,542 46,475 40,614

Coal Natural gas LNG Fuel oil

EPS

Foam board

Rock wool

XPS

0.82 0.67 2.12 1.80

0.65 0.52 1.84 1.55

1.03 0.86 2.47 2.12

0.26 0.16 1.18 0.96

EPS

Foam board

Rock wool

XPS

0.60 0.47 1.74 1.46

0.45 0.33 1.50 1.24

0.78 0.64 2.06 1.74

0.11 0.02 0.92 0.72

Akita (Climate zone-II) [m2 K/W]

Coal Natural gas LNG Fuel oil

Fukushima (Climate zone-III) [m2 K/W]

Conductivity of insulation materials (k) Expanded Polyurethane (EPS) Foam board Rock wool Extruded polyurethane (XPS)

W/mK W/mK W/mK W/mK

0.028 0.027 0.039 0.039

Thermal resistance of wall structures (R) External air film (Ro ) Interior air film (Ri ) Non-insulated wall layers (Rnins )

m2 K/W m2 K/W m2 K/W

0.043 0.11 0.9

Osaka (Climate zone-IV) [m2 K/W]

Electric generation efficiency for fuel sources () [28] Coal [–] [–] Natural gas [–] Liquefied natural gas (LNG) [–] Fuel oil

0.43 0.50 0.57 0.43

Coal Natural gas LNG Fuel oil

Energy efficiency of HVAC (COP) COPH during heating period COPC during cooling period

3.0 3.5

Present worth factor (PWF) Interest rate (i) [29] Inflation rate (g) [30] N

[–] [–] [–] [–] [yr]

0.73% 2.3% 10

Coal Natural gas LNG Fuel oil

EPS

Foam board

Rock wool

XPS

0.48 0.36 1.54 1.28

0.34 0.23 1.31 1.07

0.65 0.52 1.83 1.54

0.03 – 0.78 0.59

EPS

Foam board

Rock wool

XPS

0.17 0.07 1.02 0.81

0.06 – 0.84 0.65

0.31 0.20 1.25 1.02

– – 0.41 0.26

Kagoshima (Climate zone-V) [m2 K/W]

Coal Natural gas LNG Fuel oil

EPS

Foam board

Rock wool

XPS

– – 0.67 0.50

– – 0.52 0.36

0.08 – 0.87 0.67

– – 0.16 0.04

Naha (Climate zone-VI) [m2 K/W]

Coal Natural gas LNG Fuel oil

Fig. 4. Relationship between OTR and HDD by different combination of four insulation materials and four fuel sources.

can see that the OTR of insulation materials tends to decrease from climate zone I to VI (from low latitude to high latitude). The relationship between OTR and HDD by different combination of four insulation materials and four fuel sources is shown in Fig. 4. Here, the CDD is included, but the values of CDD and their influence is so

EPS

Foam board

Rock wool

XPS

– – – –

– – – –

– – – –

– – – –

low that they can be ignored, making the OTR a simpler function of HDD alone. Note, the trends in Fig. 4 are only valid for CDD «HDD, or CDD is relatively close to zero. We can see that the OTR increases as the HDD increases for all combinations of insulation materials and fuel sources. This OTR is from the standpoint of residences using electricity for cooling and heating, with the selection of primary energy fuel source and national or regional cost savings rather than cost of the individual residential consumer (though this optimization of fuel source may lead to lower electric rates for individuals any way). To check the effect of possible changes in electric generation efficiency, HVAC efficiency and present worth factors (interest rates, inflation rates), we repeated the OTR calculations; varying the electric power generation efficiency increasing by 5% or the COPH of HVAC from 3.0 to 4.0 or the COPC of HVAC from 3.5 to 4.5 or the interest and inflation rates to raise the PWF from 9.83 to 10.83, all yielded changes of under 3% in the OTR results. All these are greatly outweighed by small changes in HDD.

J. Yuan et al. / Energy and Buildings 153 (2017) 403–411 Table 4 Total energy cost saving (Ecs ) if the OTR is adopted for six climate zones by different combination of four insulation materials and four fuel sources. Sapporo (Climate zone-I) [$/m2 -yr]

Coal Natural gas LNG Fuel oil

Foam board

Rock wool

XPS

2.75 1.86 18.5 13.3

2.11 1.34 16.8 11.9

3.51 2.49 20.4 14.9

0.58 0.22 11.6 7.58

EPS

Foam board

Rock wool

XPS

1.46 0.90 12.5 8.79

1.01 0.56 11.1 7.62

2.03 1.36 14.1 10.1

0.10 – 7.00 4.29

EPS

Foam board

Rock wool

XPS

0.94 0.53 9.79 6.74

0.58 0.27 8.55 5.71

1.40 0.89 11.2 7.90

0.01 – 4.99 2.89

EPS

Foam board

Rock wool

XPS

0.12 0.02 4.30 2.71

0.02 – 3.49 2.08

0.31 0.13 5.24 3.47

– – 1.39 0.56

1.3 1.6 0.5 0.6

1.6 2.0 0.6 0.7

1.0 1.2 0.4 0.5

Coal Natural gas LNG Fuel oil

EPS

Foam board

Rock wool

XPS

1.8 2.3 0.6 0.7

2.3 3.1 0.7 0.9

1.4 1.7 0.5 0.6

9.6 – 1.2 1.5

Coal Natural gas LNG Fuel oil

EPS

Foam board

Rock wool

XPS

2.2 3.0 0.7 0.8

3.1 4.5 0.8 1.0

1.6 2.0 0.6 0.7

– – 1.4 1.8

Coal Natural gas LNG Fuel oil

EPS

Foam board

Rock wool

XPS

6.2 – 1.0 1.3

– – 1.3 1.6

3.4 5.2 0.8 1.0

– – 2.6 4.0

EPS

Foam board

Rock wool

XPS

– – 1.6 2.1

– – 2.0 2.9

– – 1.2 1.6

– – 6.4 –

Kagoshima (Climate zone-V) [yrs]

EPS

Foam board

Rock wool

XPS

– – 1.87 1.02

– – 1.35 0.65

0.02 – 2.50 1.51

– – 0.22 0.01

Naha (Climate zone-VI) [$/m2 -yr]

Coal Natural gas LNG Fuel oil

Rock wool

Osaka (Climate zone-IV) [yrs]

Kagoshima (Climate zone-V) [$/m2 -yr]

Coal Natural gas LNG Fuel oil

Foam board

Fukushima (Climate zone-III) [yrs]

Osaka (Climate zone-IV) [$/m2 -yr]

Coal Natural gas LNG Fuel oil

Coal Natural gas LNG Fuel oil

EPS

Akita (Climate zone-II) [yrs]

Fukushima (Climate zone-III) [$/m2 -yr]

Coal Natural gas LNG Fuel oil

Table 5 Payback period (PP) if the OTR is adopted for six climate zones by different combination of four insulation materials and four fuel sources. Sapporo (Climate zone-I) [yrs]

EPS

Akita (Climate zone-II) [$/m2 -yr]

Coal Natural gas LNG Fuel oil

409

Coal Natural gas LNG Fuel oil

Naha (Climate zone-VI) [yrs]

EPS

Foam board

Rock wool

XPS

– – – –

– – – –

– – – –

– – – –

Coal Natural gas LNG Fuel oil

EPS

Foam board

Rock wool

XPS

– – – –

– – – –

– – – –

– – – –

3.2. Total potential energy cost saving (Ecs ) 3.3. Payback period (PP) for insulated buildings The total potential energy cost saving per unit area of building external walls, Ecs , if the OTR is adopted was derived from the above Eq. (12). A three-dimensional graph which reflects the relationship of Ecs if the OTR is adopted, kinds of insulation materials and kinds of fuel sources for example city of Osaka is shown in Fig. 5. The Ecs for six typical cities in six climate zones of Japan by different combination of four insulation materials and four fuel sources are calculated and detailed in Table 4. The same as the result of 3.1, the largest Ecs is also obtained by using the optimal combination of rock wool and LNG for all climate zones of Japan. The largest Ecs is about 20.4 $/m2 -yr for Sapporo (in climate zone I), about 14.1 $/m2 -yr for Akita (in climate zone II), about 11.2 $/m2 -yr for Fukushima (in climate zone III), about 5.2 $/m2 -yr for Osaka (in climate zone IV), and about 2.5 $/m2 -yr for Kagoshima (in climate zone V). Additionally, we can also see that the Ecs tends to decrease from climate zone I to VI (from low latitude to high latitude).

The payback period for insulated buildings if the OTR is adopted, PP, is derived from the Eq. (13). A three-dimensional graph which reflects the relationship of PP, kinds of insulation materials and kinds of fuel sources for example city of Osaka is shown in Fig. 6. The PP for six typical cities in six climate zones of Japan by different combination of four insulation materials and four fuel sources are calculated and detailed in Table 5. The shortest PP is also obtained by using the optimal combination of rock wool as the insulation material and LNG as the fuel source for all climate zones. The shortest PP is about 0.4 yrs for Sapporo (in climate zone I), about 0.5 yrs for Akita (in climate zone II), about 0.6 yrs for Fukushima (in climate zone III), about 0.8 yrs for Osaka (in climate zone IV) and about 1.2 yrs for Kagoshima (in climate zone V). Additionally, we can also see that the PP tends to increase from climate zone I to VI (from low latitude to high latitude).

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J. Yuan et al. / Energy and Buildings 153 (2017) 403–411

Fig. 5. Three-dimensional graph which reflects the relationship of total energy cost saving (Ecs ) if the OTR is adopted, kinds of insulation materials and kinds of fuel sources for Osaka.

Fig. 6. Three-dimensional graph which reflects the relationship of payback periods (PP) if the OTR is adopted, kinds of insulation materials and kinds of fuel sources for Osaka.

J. Yuan et al. / Energy and Buildings 153 (2017) 403–411

4. Conclusions and future work In order to contribute to the thermal insulation design of building external walls and effective utilization of energy sources for different climate zones of Japan as the national plan toward conservation and net zero energy housing progresses, this study aimed at finding the optimal combination of thermal resistance of insulation materials and primary energy fuel sources for electric power by combining four different insulation materials and four different fuel sources. The OTR of insulation materials, Ecs and PP if the OTR is adopted by different combination of insulation materials and fuels sources are calculated using the DD method and cost analysis. It is shown that the largest OTR, largest Ecs and shortest PP are all obtained from the combination of rock wool as the insulation material and LNG as the fuel source for all six climate zones of Japan. Hence, it is considered that the optimal combination is composed of rock wool and LNG in this study, targeting residences using electricity for cooling and heating. The OTR and Ecs tend to decrease from climate zone I to VI (from low latitude to high latitude). However, the PP for insulated buildings has the opposite trend. Future research will be focused on the impact of the other parameters, i.e., solar gains of external walls on the OTR design for external walls, total energy cost, and CO2 emissions. Furthermore, the potential for the possibility of introducing solar energy as a renewable energy to substitute the non-renewable energy in different climate zones of Japan will be considered. References [1] Dombaycı ÖA, Investigation of the Effect of Thermal Insulation for a Model House in Cold Regions: A Case Study of Turkey, Environ. Prog. Sustainable Energy 33 (2013) 527–537. [2] M. Mardookhy, R. Sawhney, S. Ji, X. Zhu, W. Zhou, A Study of Energy Efficiency in Residential Buildings in Knoxville, Tennessee, J. Cleaner Produc. 85 (2014) 241–249. [3] A. Baniassadi, B. Sajadi, M. Amidpour, N. Noori, Economic optimization of PCM and insulation layer thickness in residential buildings, Sustainable Energy Technol. Assess. 14 (2016) 92–99. [4] Dombayci ÖA, H.K. Ozturk, Atalay Ö, Acar S¸G, E.Y. Ulu, The impact of optimum insulation thickness of external walls to energy saving and emissions of CO2 and SO2 for Turkey different climate regions, Energy Power Eng. 8 (2016) 327–348. [5] K. C¸omaklı, B. Yüksel, Optimum insulation thickness of external walls for energy saving, Appl. Thermal Eng. 23 (4) (2003) 473–479. [6] M. Ozel, Effect of wall orientation on the optimum insulation thickness by using a dynamic method, Appl. Energy 88 (7) (2011) 2429–2435. [7] J. Yuan, C. Farnham, K. Emura, M.A. Alam, Proposal for optimum combination of reflectivity and insulation thickness of building external walls for annual thermal load in Japan, Build. Environ. 103 (2016) 228–237. [8] E. Wati, P. Meukam, M.K. Nematchoua, Influence of external shading on optimum insulation thickness of building walls in a tropical region, Appl. Therm. Eng. 90 (2015) 754–762. [9] M.J. Lee, K.G. Lee, W.D. Seo, Analyses on performances of heat and multilayer reflection insulators, J. Cent. South Univ. 19 (2012) 1645–1656.

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