Thermal performance of double-layer black tile roof in winter

ScienceDirect Energy Procedia 00 (2017) 000–000

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CISBAT 2017 International Conference – Future Buildings & Districts – Energy Efficiency from Nano to Urban Scale, CISBAT 2017 6-8 September 2017, Lausanne, Switzerland 15th International on District Heating Cooling ThermalThe performance ofSymposium double-layer black tileand roof in winter

Assessing the afeasibility using the demand-outdoor Lili Shen , Qun Zhaob, *,ofZhengrong Liaheat , Jingpeng Zhaoa HVAC andfor Gas Institute,Tongji University,Shanghai 200092,China temperature function a long-term district heat demand forecast a

b College

College of Architecture and Urban Planning,Tongji University,Shanghai 200092,China

a,b,c

I. Andrić

*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc

a

IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal Abstract b c

Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France

Black tile roofs (BTR) are a traditional roof construction in Southern China. As heat insulation and waterproofing are insufficient in single-layer roofs, more climate-adapted double-layer tile roofs are used to replace them. Although providing remarkable heat insulation in summer, winter thermal performances of the double-layer BTR are unclear. Experimental comparisons of thermal performances Abstract of single-layer and double-layer BTRs showed that the indoor temperature of the single-layer BTR was 5°C higher than that of the double-layer BTR in the daytime, but 2℃ lower in the night-time. Compared with the single-layer BTR, the heat gain of theheating double-layer BTRare was reduced by 85%, andina the 55%literature heat loss as reduction achieved. The overall thermal resistance of District networks commonly addressed one of was the most effective solutions for decreasing the the double-layer BTR was ten times higher. When considering the effects of ventilation in the air channel, the unventilated greenhouse gas emissions from the building sector. These systems require high investments which are returned throughdoublethe heat layer loss during theconditions night-time,and but building there wasrenovation no obvious policies, differenceheat in heat gain. in the future could decrease, sales.BTR Duereduced to the heat changed climate demand ©prolonging 2017 The Authors. Published byperiod. Elsevier Ltd. the investment return © 2017 The Authors. Published by Elsevier Ltd. Peer-review underofresponsibility scientific committee of thetemperature CISBAT 2017 International committee of the scientific The main scope this paper is of to the assess the feasibility of using heat demand – outdoor heat demand Peer-review under responsibility of the scientific committee of thethe CISBAT 2017 International Conferencefunction – Futurefor Buildings & Conference – Future Buildings & Districts – Energy Efficiency from to Urban Scale. Nano forecast.– The district of Alvalade, located in Lisbon Districts Energy Efficiency from Nano to Urban Scale (Portugal), was used as a case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Keywords:Double construction; tile roof; Ventilation; Thermal performance; renovation scenarios were Black developed (shallow, intermediate, deep). ToWinter estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications 1.(the Introduction error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). Forvalue conventional constructions in regions with high levels of solar radiation, can easily reach The of slope coefficient increased on average within the range of 3.8% up tosurface 8% pertemperatures decade, that corresponds to the decrease in the number heating hours oftilt 22-139h during the heating season constructions (depending on can the combination weatherheat and 75~80℃, depending onofthe orientation, and time of year [1]. Double significantlyofreduce renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. by Elsevier Ltd. * Corresponding author.Published Tel.: +86-021-69583962; fax: +86-021-69583962. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and E-mail address: [email protected] Cooling. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Keywords: under Heat demand; Forecast; Climate change Peer-review responsibility of the scientific committee of the CISBAT 2017 International Conference – Future Buildings & Districts – Energy Efficiency from Nano to Urban Scale.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the CISBAT 2017 International Conference – Future Buildings & Districts – Energy Efficiency from Nano to Urban Scale 10.1016/j.egypro.2017.07.463

Lili Shen et al. / Energy Procedia 122 (2017) 247–252 Lili Shen et al. / Energy Procedia 00 (2017) 000–000

248 2

gain in the daytime, reducing the impact of solar radiation on the indoor environment. For exposure to high intensity and long hours of solar radiation, the roof is a weak part in building envelopes. When double constructions are applied to roofs, they are ventilated. A great number of literatures, reporting both experiments [1,2,3] and simulations [4,5,6], have proved that ventilated roofs offer great heat insulation in summer. However, the majority of researchers focus on the thermal performance of ventilated roofs in summer. In fact, thermal performances throughout the year should be analyzed to estimate the applicability of ventilated roofs, while little attention is paid to investigate them in winter. In China, there are numerous examples in the design and construction of traditional buildings for adaptation to the climate and nature. The application of the double-layer tile roof is an example in the south of China. Compared with a single-layer tile roof, it has remarkable thermal benefits in summer and remains waterproof in the rainfall season on account of an additional tile layer and air ventilation created by buoyancy and wind effect. But as mentioned above, thermal performances of double-layer tile roofs in winter are unclear for lack of studies, which has a great impact on the application of the double-layer tile roof in the design and renovation of buildings. As one of Chinese traditional roofs, The BTR (an acronym of the black tile roof) is studied in this paper. Based on the experimental set-up, surface temperature distributions and indoor temperatures of single-layer and double-layer BTRs are investigated. Compared with the single-layer BTR, thermal performances of the double-layer BTR in winter are analyzed. Besides, when ventilation in the air channel is reduced, effects on thermal performances of the doublelayer BTR are considered as well. 2. Experimental arrangement The experimental set-up was located in Shanghai, China. It consists of two cubical boxes with mono-pitch roofs. The dimension is shown in Fig.1 (a). To simplify constructions of external envelops, the wall consists of 6mm glass and 50mm extruding polystyrene plate as the thermal insulation, and the floor consists of 50mm plastic plate and 50mm extruding polystyrene plate. The creamy-white wallpaper is attached on the outside of the wall. The indoor environment is without air-conditioning, similar with traditional buildings accommodated by natural ventilation.

Fig1. (a) The dimension of the cubical box; (b) The photograph of the experimental set-up

The dimension of the black tile is presented in Fig.2 (a). For the tile assembly, the upper tile covers half of the lower one, and the presentation in detail is given in Fig.2 (b). In this way, a layer of tiles is finished. The construction of the single-layer BTR is made by a layer of tiles, and the tile layer is directly laid on rafters without decks. The doublelayer BTR consists of two tile layers supported by rafters and an air channel between them. The clear height of the air channel is 10cm. Constructions of single-layer and double-layer BTRs are seen in Fig.1 (b).

Fig.2 (a) The dimension of the black tile; (b) The photograph of the tile assembly



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Both indoor and outdoor environmental parameters are monitored, and outdoor environmental parameters are from the nearby weather station. The details of sensors applied in experiments are presented in Table.1. The time interval of data recording is 5 minutes. The positions of different sensors on BTRs are shown in Fig.3 (a) (b) in detail. Table.1 Information of sensors used in the experiment Objective

Sensor

Range

Accuracy

Surface temperature of tile layer

T-type thermocouple

/

± 0.3℃

Indoor air temperature and relative humidity

HOBO temperature/relative humidity data logger

-20~70℃

± 0.21℃

1~95%

± 3.5%

Air temperature in air channel

HOBO temperature/relative humidity data logger

-20~70℃

± 0.21℃

Heat flux

KEM KR6 heat flux sensor

11.6~3480W/m2

±5%

Fig.3 (a) Sensors’ positions of the single-layer BTR; (b) Sensors’ positions of the double-layer BTR

3. Data processing 3.1. Fundamental assumptions To counter the complexity of the black tile’s physical parameters and roof constructions, some fundamental assumptions are given to simplify the data processing: • The distribution of surface temperatures on BTRs is two-dimensional, neglecting differences along the roof width; • On account of sunny days during testing, physical parameters of black tiles are constant; • The double-layer BTR is composed of two tile layers with the same thermal performances. 3.2. Processing method The testing time was from February 20th to March 16th in 2017, and data of three days with the clear sky was selected in this paper, with solar irradiance exceeding 700 W/m2 and ambient air temperature ranging from 3°C to 18°C. As the heat transfer coefficients are not known, I is put forward to calculate heat gain and loss of BTRs. Based on assumptions, it represents the quantity of heat within 5 minutes when the temperature difference between upper and lower surfaces of the tile layer is 1℃.So heat gain or loss of single-layer and double-layer BTRs is: ′ ′ ′ ′ )𝐼𝐼𝐼𝐼 ′ ′ − 𝑇𝑇𝑇𝑇41 )𝐼𝐼𝐼𝐼 + (𝑇𝑇𝑇𝑇12 − 𝑇𝑇𝑇𝑇42 )𝐼𝐼𝐼𝐼 + (𝑇𝑇𝑇𝑇13 − 𝑇𝑇𝑇𝑇43 ]/3 = ∑𝑡𝑡𝑡𝑡(𝑇𝑇𝑇𝑇�1′ − 𝑇𝑇𝑇𝑇�4′ )𝐼𝐼𝐼𝐼 (1) 𝑄𝑄𝑄𝑄𝑠𝑠𝑠𝑠 = ∑𝑡𝑡𝑡𝑡[(𝑇𝑇𝑇𝑇11 ���3 − ��� 𝑄𝑄𝑄𝑄𝑑𝑑𝑑𝑑 = ∑𝑡𝑡𝑡𝑡[(𝑇𝑇𝑇𝑇31 − 𝑇𝑇𝑇𝑇41 )𝐼𝐼𝐼𝐼 + (𝑇𝑇𝑇𝑇32 − 𝑇𝑇𝑇𝑇42 )𝐼𝐼𝐼𝐼 + (𝑇𝑇𝑇𝑇33 − 𝑇𝑇𝑇𝑇43 )𝐼𝐼𝐼𝐼 ]/3 = ∑𝑡𝑡𝑡𝑡(𝑇𝑇𝑇𝑇 𝑇𝑇𝑇𝑇4 )𝐼𝐼𝐼𝐼 (2) A positive Q means heat gain. And temperatures mentioned in formulae correspond to surface temperatures of BTRs in Fig.3 (a) (b). In order to assess thermal performances of the double-layer BTR quantitatively, S is adopted to show the relative difference, represented by the ratio: (3) S = (𝑄𝑄𝑄𝑄𝑑𝑑𝑑𝑑 − 𝑄𝑄𝑄𝑄𝑠𝑠𝑠𝑠 )/|𝑄𝑄𝑄𝑄𝑠𝑠𝑠𝑠 | × 100% When the double-layer BTR presents less heat gain or more heat loss, S is negative. Thus, a positive S means that the double-layer BTR performs better in winter.

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4. Experimental results and discussions 4.1. Temperature distributions of BTRs Fig. 4(a) illustrates average temperatures of interior surfaces of single-layer and double-layer BTRs. The interior surface temperature impacts on the indoor mean radiant temperature, directly influencing the human thermal sensation. After 8 am, the surface temperature of a single-layer BTR exceeded that of the double-layer one, and it was higher by about 8°C around midday, but it dropped sharply without solar radiation after 6 pm. There was no remarkable difference between the surface temperature of the double-layer BTR and the outdoor air temperature during nighttime. But for the single-layer BTR, the temperature difference was about 3°C, because more longwave radiation leads to lower surface temperature. However, the first tile layer of the double-layer BTR blocks heat loss at night. But it also blocks heat gain in the daytime, which is unfavorable in winter but beneficial in summer. Differences between indoor temperatures with the single-layer BTR and that with the double-layer BTR can be seen in Fig. 4(b). The variation trend of indoor temperatures was similar with surface temperatures above. With both the singlelayer BTR and the double-layer BTR, there was a significant daily range. Compared with the single-layer BTR, the indoor temperature of the double-layer one was lower by 5°C in the daytime and higher by 2°C in the night-time.

Fig.4 (a) Average temperatures of interior surfaces of single-layer and double-layer BTRs; (b) Indoor temperatures of single-layer and double-layer BTRs

Fig.5 Average temperature distributions of the air channel

Fig.5 presents average temperature distributions of the air channel. The lower surface temperature of the first tile layer was slightly higher than that of upper surface of the second tile layer in the daytime, by less than 1°C. The situation was reversed in the night-time. The air temperature in the channel was nearly equal to that of upper surface of the second tile layer after 6 pm. The difference in surface temperatures and air temperature was so small that the energy exchange driven by convective heat transfer was little, in the case of reduced air velocity in the air channel. 4.2. Analysis of heat gain and loss of BTRs Temperature differences in tile layers are presented in Fig.6, which are used to calculate heat gain and heat loss of BTRs. For the single-layer BTR, after 7:30 am, the temperature of upper surface was higher than that of lower surface, and the difference increased rapidly, ranging from 0 to 17℃. But after 4:00 pm, the situation was reversed and the maximum difference approximated -8°C. The surface temperature difference of the double-layer BTR was less sensitive to ambient variations, and it was less than ± 4°C all the time.



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Fig.6 Temperature differences of single-layer and double-layer BTRs used in the energy calculation

According to the calculation method mentioned in Section 3.2, the results are shown in Table.2. Remarkable differences were presented in heat gain and loss of two BTRs. In contrast to the single-layer BTR, heat gain of the double-layer BTR was reduced by about 85% due to the shading of the first tile layer and ventilation. And about 55% reduction in heat loss was achieved on account of less longwave radiation and convective heat transfer. Table.2 Heat gain and loss of single-layer and double-layer BTRs Day 1 Qs

Qd

S

Day 2 Qs

Qd

S

Day 3 Qs

Qd

S

Heat gain

1142.8I

140.4I

-88%

962.2I

120.7I

-87%

1017.9I

190.7I

-81%

Heat loss

-585.4I

-302.1I

48%

-738.6I

-201.8I

59%

-667.2I

-265.1I

60%

4.3. Comparisons between overall thermal resistances of BTRs Without air-conditioning equipment, the heat flux measured was much smaller, especially for the double-layer BTR. In most of the time, the value was less than or slightly more than the lower limit of heat flux sensors’ measurement range. To get thermal resistances of BTRs in the experimental condition, the heat flux of the single-layer BTR was adopted from 12 am to 2 pm. Based on quantitative analysis of heat gain above, the value of S was almost same for days. Therefore, S, as a scaling factor, was applied to calculate the heat flux of the double-layer BTR. The overall thermal resistances of the single-layer BTR (Rs) and the double-layer BTR (Rd) are: ′ ′ ��������� 𝑅𝑅𝑅𝑅𝑠𝑠𝑠𝑠 = (𝑇𝑇𝑇𝑇 �𝑠𝑠𝑠𝑠 (4) 1 − 𝑇𝑇𝑇𝑇4 )/𝑞𝑞𝑞𝑞 ��������� − 𝑇𝑇𝑇𝑇 )/(𝑞𝑞𝑞𝑞 � 𝑆𝑆𝑆𝑆 + 𝑞𝑞𝑞𝑞 � ) (5) 𝑅𝑅𝑅𝑅𝑑𝑑𝑑𝑑 = (𝑇𝑇𝑇𝑇 1 4 𝑠𝑠𝑠𝑠 𝑠𝑠𝑠𝑠 The calculations are shown in Table.3. The average thermal resistance of the single-layer BTR was 0.75m2·K/W, and that of the double-layer one was ten times higher, about 8.75 m2·K/W. For the shading and ventilation caused by additional tile layer, the heat flux entering into the room decreased dramatically, reflected by the value of S. So based on the definition of thermal resistances, the wide difference in two BTRs appeared. Obviously, the great heat insulation of double constructions is conducive to reduce cooling and heating load when designing air-conditioning systems. Table.3 Thermal resistances of single-layer and double-layer BTRs Single-layer BTR

Double-layer BTR

Heat flux

△T

Rs

(W/m )

(K)

Day 1

22.1

Day 2

16.1

Day 3

15.2

2

Heat flux

△T

Rd

(m ·K/W)

Scaling factor

(W/m )

(K)

(m2·K/W)

15.1

0.68

-91%

2.0

18.7

9.35

12.1

0.75

-89%

1.8

16.2

9.00

12.7

0.83

-85%

2.3

18.2

7.91

2

2

4.4. Effects of ventilation on thermal performances of the double-layer BTR As shown in Fig.7 (a), the case of the unventilated double-layer BTR refers to minimized wind effect and reduced buoyancy effect, achieved by covering over 90% of the flow area at the inlet and outlet of the air channel. Temperature differences used in the energy calculation can be seen in Fig.7 (b). According to results presented in Table.4, compared with the single-layer BTR, 88% of heat gain and 70% of heat loss were reduced. When ventilation was reduced, there

Lili Shen et al. / Energy Procedia 122 (2017) 247–252 Lili Shen et al. / Energy Procedia 00 (2017) 000–000

252 6

was a great improvement in heat loss decrease, but no remarkable change in heat gain. The phenomenon suggests that radiation heat transfer between sides of the air channel is much stronger than convective heat transfer in the daytime in winter. At night, the unventilated channel greatly blocks heat loss by convective heat transfer.

Fig.7 (a) The photograph of the unventilated double-layer BTR; (b) Temperature differences of single-layer and unventilated double-layer BTRs used in the energy calculation Table.4 Heat gain and loss of single-layer and unventilated double-layer BTRs Day 1

Day 2

Day 3

Qs

Qd

S

Qs

Qd

S

Qs

Qd

S

Heat gain

1092.7I

129.7I

-88%

1128.8I

108.7I

-90%

1089.6I

126.9I

-88%

Heat loss

-772.3I

-232.3I

70%

-764.0I

-210.8I

72%

-779.9I

-225.3I

71%

5. Conclusions Based on the experimental set-up and results, comparisons between single-layer and double-layer BTRs were made in this paper. And conclusions are presented as followed: • As for the indoor thermal environment, the single-layer BTR performs much better in the daytime in winter, and the indoor temperature was higher by 5°C. While the double-layer BTR is superior in the night-time, higher by 2°C. • Compared with the single-layer BTR, due to the shading of the first tile layer and effects of ventilation, heat gain of the double-layer BTR was reduced by 85%, and 55% of heat loss was cut down in winter. • The overall thermal resistance of the double-layer BTR is ten times higher than that of the single-layer BTR. • For the double-layer BTR, reduced ventilation in the air channel is considered to be beneficial in reducing heat loss in the night-time, but it has no obvious impact on heat gain. References [1] A Dimoudia, A Androutsopoulos, S Lykoudis. Summer performance of a ventilated roof component. Energy and Building. 2006;38:610-617 [2] Sunwoo Lee, Sang Hoon Park, Myong Souk, Yeo. An experimental study on airflow in the cavity of a ventilated roof. Building and Environment.2009;44:1431-1439 [3] L Susanti, H Homma, H Matsumoto. A laboratory experiment on natural ventilation through a roof cavity for reduction of solar heat gain. Energy and Building.2008;40:2196-2206 [4] P H Biwole, M Woloszyn, C Pompeo, Heat transfers in double-skin ventilated by natural convection in summer time, Energy and Buildings. 2008;40:1487-1497 [5] A Gagliano, F Patania, F Nocera et al. Thermal performance of ventilated roofs during summer period.2012;49:611-618 [6] Shanshan Tong, Hua Li. An efficient model development and experimental study for the heat transfer in naturally ventilated inclined roofs, Building and Environment.2014;81:296-308