Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 121 (2015) 1274 – 1280
9th International Symposium on Heating, Ventilation and Air Conditioning (ISHVAC) and the 3rd International Conference on Building Energy and Environment (COBEE)
Experimental Research on the Insulation Effect of Water-retaining Roof Bricks in Transition season Yutao Qinga,*, Ling Wanga, Zhimao Xuc, Rubing Hana,b, Enshen Longb a
Southwest University of Science and Technology, Mianyang, 621010, China b Sichuan University, Chengdu, 610044, China c Sichuan Electric Power Design ˂ Consulting CO., ltd. Chengdu, 610016, China
Abstract The roof, as an important part of the building external envelope, has a significant impact on top floor rooms' indoor energy consumption and thermal comfort. A roof pond is a special system that can be used for passive cooling buildings during summer. Three experiments were carried out to test the insulation effect of water-retaining roof bricks under natural climatic conditions. It was concluded that water-retaining roof bricks have heat insulation effect. A brick model 300mm x 300mm x 240 mm was found to be most effective. The bricks were beneficial for indoor heat dissipation. This research helps to understand the heat and mass transfer mechanism and energy consumption characteristics of different roof structures. It can contribute to the design of a better thermal-performance water roof and energy efficient buildings in sub-tropical climatic conditions. © Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2015 2015The The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ISHVACCOBEE 2015. of ISHVAC-COBEE 2015 Peer-review under responsibility of the organizing committee
Keywords: roof pond; thermal performance; evaporation cooling; energy saving
1. Introduction Designing energy efficient buildings is one of the country's major strategic issues. In particular there is a need for energy conservation and transformation through retrofitting existing buildings' roofs. Large numbers of buildings are involved so this will play an important part in energy conservation. Because the roof receives the strongest direct radiation from the sun for the longest time compared with other parts of a building around 40% of the heat gain in * Corresponding author. Tel.: +86-158-8287-8159; fax: +86-158-8287-8159. E-mail address:
[email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ISHVAC-COBEE 2015
doi:10.1016/j.proeng.2015.09.002
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top floor rooms comes through the roof [4]. While in a hot climate the top rooms' thermal environment is very poor. Improving the thermal insulation of the roof to improve the indoor thermal environment can reduce energy consumption of air conditioning. Using the principles of evaporation cooling, heat preservation and heat insulation in roof renovations will not only improve the quality of the indoor environment, but also effectively alleviate urban heat island effect and dry island effect. In the context of seeking to achieve energy savings through buildings’ renovation the research team has tested a newly-built residential building on site (Fig.1).
Fig. 1. Test results.
In summer the top floor room’s temperature is 2 ~ 3 ćhigher than that of the middle-floor, and the ceiling temperature is 2 ~ 4 ć higher than the middle-floor's. The high temperature makes the habitability of these rooms quite poor. Professor Fanger has proved that the heat radiating surface of the hot roof will make people feel uncomfortable. Thermal insulation of roofs needs to be improved. Onmura, Matsumoto, and Hokoi studied the thermal insulation performance of a light vegetation roof and indicated that the roof’s outer surface temperature could be reduced by10̢30 ć [6]. Zhao, Tan and Tang and Zhao and Xue studied the thermal performance of a sedum lineare planted roof. Based on Shanghai’s weather condition, they indicated that the roof’s outer surface temperature was reduced by around 16.0ć compared to the original roof, while its inner surface temperature was reduced by about 3.3ć [14]. The research team has developed a series of energy-saving water-retaining roof bricks which are suitable for regions where no central heating system is provided. The structure of the brick is shown in Fig. 2. Domestic and foreign researches have focused on the improvement of heat transfer theory and water-retaining roof structures such as suspended particulate matter shade, water evaporation system performance, and flowing water evaporation cooling etc. No research on the structure of the water-retaining roof brick has been conducted. This article will study on the thermal insulation of water-retaining roof bricks.
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ceramsite
air
ceramsite
air
water
water
air
air
300
2-2 profile
1-1 profile
front view
topview
;;
;;
A
B Fig. 2. Schematics of the test structures (A and B are except lid).
2. Methodology 2.1 Measuring points of water-retaining roof brick Three comparison groups were established to study the insulation effect of water-retaining roof bricks. Comparative experiments were as follows: 1. Comparing the temperature of the roof laid with brick and the temperature of a normal roof. 2. Comparing the insulation effect of different height bricks. 3. Comparing water temperature, brick bottom temperature and the roof temperature under the brick. Experimental bricks' measuring points are: air, water, brick bottom, roof with brick, and roof. Measuring points and layout are shown in Fig.3. AT
WT
RT
Roof BT Layout
RTWB
Measure point
Fig.3.Measuring points and layout, Note: RT=roof temperature, AT=air temperature, BT=bottom temperature, RTWB=roof temperature with brick.
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2.2. Test equipment Roof temperature was tested by infrared thermometer Testo 830-S1, measurement accuracy: f1.5%. The water temperature was tested by Withfirst-TL8015 electronic thermometers, measurement accuracy: f1ć. Outdoor air temperature and humidity were tested by Virtues weather meter, temperature error: İ f0.9 ć humidity error: İ f4.9% RH. 2.3. Test data selection The experiment started on August 30th and finished at October 17th 2014. In fact the data collected on September 28th 2014 was benefit for analyze. Outdoor air temperature ranged from 28.1 ć to 34.7 ć. 3. Results 3.1. Analysis of the data from the water-retaining roof brick The roof brick has an obviously insulation effect. We put the brick full of water, when the roof temperature reached 40.8 ć, the roof temperature under the water-retaining brick was only 26.2 ć during daytime. For the roof temperature curves see Fig.4.
42 40
AT RT RTWB
38
7HPSHUDWXUH ć
36 34 32 30 28 26 24 22 20 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 Time (h)
Fig.4. Temperature variations, RT=roof temperature, AT=air temperature, RTWB=roof temperature with brick.
The following conclusion such as: (1)The roof temperature with brick changes are very small basically giving a smooth curve. On the contrary, the roof temperature shows a similar trend with the change of air temperature giving an upward convex parabola. (2)In the test period, the roof temperature was always higher than that of the air temperature, whereas the roof temperature with brick was below air temperature. At 13:00, the roof temperature was found at peak 40.8 ć, however, the roof temperature with brick was 26.2 ć, the difference is 14.6 ć. Therefore roof bricks can play a significant role in effectively reducing the roof surfaces temperature. 3.2. Effect of different height
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Our group has designed two different height roof brick for studying the theories of evaporative cooling and water retaining heat insulation. And results for the bearing load on roof are listed in Table. 1. Table 1. Bear load and mass of roof brick. Type
A
B
Mass(kg)
13.58
11.89
And another entry
150.89
131.44
We tested the roof temperature under the two different height brick to compare the heat insulation effect. The results are shown in Fig. 5. Note: RT=roof temperature. 30
RT(A) RT(B)
7HPSHUDWXUH ć
28
26
24
22
20 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 time (h)
Fig. 5. Roof temperature variations.
In this study, roof temperature with different height brick has almost the same change trend. The data reported here suggest that the temperature difference is not big, and the model B being slightly higher than that of model A. From the point of unit area bearing load and heat insulation effect, there is no advantage in increasing the roof bearing load for such a minimum heat insulation enhancement, so it is concluded that model B is the optimal choice. 3.3. Heat transfer characteristics research To study the water-retaining brick heat insulation and transfer properties, we tested the temperature of water, brick bottom and roof with brick by using model B (300mm x 300mm x 240 mm) full of water. A look at how this is distributed throughout the course of the day in Fig6.
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AT WT BT RTWB
36 34
7HPSHUDWXUHć
32 30 28 26 24 22 20 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 Time (h)
Fig. 6. Temperature variations. Note: AT=air temperature, WT=water temperature, BT=brick bottom temperature, RTWB=roof temperature with brick.
From graph can be concluded as follows: (1)The roof temperature with brick is higher than brick bottom, the difference being vary from 0.7 ć to 2.1 ć. This means the roof brick is insulating through two aspects. On one hand, it prevents the heat from the outside world being transferred indoors; on the other hand, there is a difference in temperature between the brick bottom temperature and the roof which speeds up the indoor heat dissipation to the outside world. (2) In terms of heat distribution in the brick itself, the water temperature is higher than the brick bottom temperature and the temperature decreased in a vertical direction downwards. 4. Discussion Considerable interest has been shown in greening to extend a building’s life span and offer high-quality green amenity spaces (Jim & Tsang, 2011)[12]. In this regard, numerous research works have been undertaken to investigate the thermal and energy performance of greening and its best practice. Wangshen Yang’s test results showed that rooms with green planting roofs had the lowest indoor air temperature (on average), and were around 0.95 ć, 0.8 ć and 0.5 ć lower than rooms with exposed, ceramic and clay laid evaporating roofs[11]. This paper give a similar conclusion, the roof surface temperature is lower by about 14.6 ć (maximum value) than the original roof surface. Also, Wangshen Yangÿs test results show us other points about temperature attenuation and delay characteristics. From the test results of this paper, the temperature of the roof with bricks also has a certain delay. The original roof temperature peak appeared at 13:00, while the brick paved roof peak appeared at 16:00. It clearly shows temperature attenuation and delay characteristics. All in all, indications are that the evaporating mechanisms can play an effective role in reducing a building’s air conditioning loads. 5. Conclusion This research described a dedicated experimental study on the performance of water-retaining roof bricks. And it is considered to be a first step in terms of applying this technology in a sub-tropical climatic region (Mianyang). The test results will help in understanding heat and mass transfer mechanism associated with water-retaining bricks and conventional roof, thus aiding in the design for a better thermal performance roof brick and energy efficient buildings under sub-tropical climatic conditions. Compared to the original roof, the roof with water-retaining bricks had significantly enhanced cooling and thermal insulation effects. The contrast experiments comparing different height bricks indicated that model B
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(300mm x 300mm x 240 mm) had the best insulation effect. The comparison of the temperatures of the water, brick bottom, roof with bricks, and roof showed that a water-retaining brick is advantageous to effect indoor heat dissipation. Acknowledgments This work was supported by National Natural Science Foundation of China (No.51408512). References [1] H. Gerson dos Santos, M. Nathan, Numerical analysis of passive cooling using a porous sandy roof, Appl. Therm. Eng. 51(2013) 25-31. [2] P. David, R. Siga, Performance analysis of a simple roof cooling system with irrigated soil and two shading alternatives, Energ. Buildings. 40 (2008) 855ˀ864. [3] J. Dilip, Modeling of solar passive techniques for roof cooling in arid regions, Build. Environ. 41 (2006)277ˀ287. [4] T. Runsheng, Y. Etzion, E. Erell, Experimental studies on a novel roof pond configuration for the cooling of buildings, Renew. Energ. 28(2003) 1513ˀ1522. [5] C.F. Liu, Y. Feng, Q.G. Chen. Study on Heat Insulation and Use of Passive Evaporated Roof, Journal of Chongqing University (Natural Science Edition), 2001(03)41-44. [6] S. Onmura, M. Matsumoto, S. Hokoi, Study on evaporative cooling effect of roof lawn gardens, Energ. Buildings. 33(2001) 653-666. [7] Y.X. He. Building energy consumption ranking first of all types in China, People's Daily, 2005. [8] R.B. Han, E.S. Long, Z.Y. Wang. One kind of water double insulation brick, China, 2013. ZL2010105871072. [9] Q.L. Meng. Study on the attenuation of temperature wave of evaporation layer on a passive cooling roof, Acta energiae solaris sinica, 23 (2002) 667-669. [10] R.S. Tang, Y. Etzion. Cooling performance of roof ponds with gunny bags floating on water surface as compared with a movable insulation, Renew. Energ. 30 (2005) 1373ˀ1385. [11] W.S. Yang, Z.Y. Wang, Comparative study of the thermal performance of the novel green (planting) roofs against other existing roofs, Sustainable Cities and Society, 16 (2015) 1ˀ12. [12] C. Y. Jim, S. W. Tsang, Modeling the heat diffusion process in the abiotic layers of green roofs, Energ. Buildings. 43 (2011) 1341ˀ1350. [13] D. Zhao, W. Xue, The cooling effect of light roof greening, Acta Agriculturae Shanghai, 22 (2006) 53ˀ55. [14] D. Zhao, Y. Tan, M. Tang, Quantitative analysis of energy saving green roofs, Chinese Horticultural Abstract, 12 (2009) 27.
