J. Cent. South Univ. (2012) 19: 1663-1667
DOI: 10.1007/s11771-012-1190-3
An active pipe-embedded building envelope for utilizing low-grade energy sources
XIE Jun-long(谢军龙)1, ZHU Qiu-yuan(朱求源)2, XU Xin-hua(徐新华)2
1. Department of Refrigeration and Cryogenic Engineering, Huazhong University of Science and Technology,Wuhan 430074, China;
2. Department of Building Environment and Services Engineering, Huazhong University of Science and Technology,Wuhan 430074, China
? Central South University Press and Springer-Verlag Berlin Heidelberg 2012
Abstract: An active pipe-embedded building envelope, which is an external wall or roof with pipes embedded inside, was presented. This structure may utilize the circulating water in the pipe to transfer heat or coolth inside directly. This kind of structure is named “active pipe-embedded building envelope” due to dealing with the thermal energy actively inside the structure mass by circulating water. This structure not only deals with thermal energy before the external disturbance becomes cooling/heating load by using the circulating water, but also may use low-grade energy sources such as evaporative cooling, solar energy, and geothermal energy. In the meantime, this structure can also improve the indoor thermal comfort by tempering the internal wall surface temperature variation due to the thermal removal in the mass. This work further presents the thermal performance of this structure under a typical hot summer weather condition by comparing it with that of the conventional external wall/roof with numerical simulation. The results show that this pipe-embedded structure may reduce the external heat transfer significantly and reduce the internal wall surface temperature for improving thermal comfort. This work also presents the effects of the water temperature and the pipe spacing on the heat transfer of this structure. The internal surface heat transfer may reduce by about 2.6 W/m2 when the water temperature reduces by 1 ℃ as far as a brick wall with pipes embedded inside is concerned. When the pipe spacing reduces by 50 mm, the internal wall surface heat flux can also reduce by about 2.3 W/m2.
Key words: active pipe-embedded building envelope; thermal performance; thermal comfort; simulation analysis; low-grade energy source; pipe spacing
1 Introduction
Buildings consume about 30% of the energy use in China, and building operating energy consumption is nearly 20% of the total energy use. China has enacted a series of building energy efficiency standards for reducing energy consumption of buildings to cope with the increasing energy consumption. The heat transfer through the building envelope becomes cooling/heating load and results in energy consumption finally. Therefore, it is an important means to decrease the building energy consumption by reducing the heat transfer through building envelopes. The conventional means are to change the construction material, the thickness of the external wall or directly install insulation on the opaque building envelope, etc.
Usually, pipes can be embedded in floor concrete or plaster layer with cooled/hot water as the medium circulating in pipes for providing space air conditioning through cooling/heating radiation. It is a conventional engineering application, and usually is called floor radiation system [1-5]. This system can use low-grade energy sources such as cool air and geothermal energy (ground water or coupled with ground heat exchangers) etc [6-9] and enhance the efficiency of heat transfer. This work proposes a new building envelope structure, which is an external wall or roof with pipes embedded inside and utilizes the circulating water in the pipe to transfer heat/coolth inside the structure directly. This structure may significantly enlarge heat transfer surface between the structure mass and the water in the pipe for allowing substantial heat flows even for relatively small temperature differences between the mass and water. It may sufficiently utilize the low-grade energy source (such as groundwater, the water produced by cooling towers, and geothermal energy produced by the ground coupled heat exchanger system) to weaken the influence of outdoor climate on the indoor environment, reduce the external heat transfer and improve indoor thermal comfort. This is called “active pipe-embedded building envelope” due to dealing with the thermal energy actively inside the structure mass by circulating water.
As the indoor and outdoor air temperature and humidity are always changing, the heat transfer through the building envelope is unsteady. For the conventional and regular building envelopes without internal heat source, response factor method [10] or transfer function method [11] can be used to calculate the unsteady heat transfer through these building envelopes. However, these calculation methods are inappropriate for analyzing the thermal performance of the active pipe-embedded building envelope since the pipe-embedded building envelope (i.e., roof or external wall) is a complicated structure, and the circulating water in the pipe (inside the structure mass) can be considered as an internal heat source. For the complicated and irregular structure or the structure having internal heat sources, numerical analysis method is usually used for heat transfer analysis. Some researchers [12-15] used CFD technology to establish numerical simulation model for analyzing the unsteady heat transfer of the ground-coupled heat exchanger of a heat pump system. LI et al [16] analyzed the heat/coolth storage capacity of the concrete slab with embedded- pipes by using CFD technology. In this work, CFD technology is also used to simulate the heat transfer characteristics and the energy saving potential of the embedded-pipe building envelope. In the meantime, the effects of the water temperature and the pipe spacing on the heat transfer of this structure are also presented.
2 Descriptions of embedded-pipe wall and heat transfer calculation model
A brick wall with two bricks of 120 mm in thickness in series is widely used in buildings as external wall in China. In this work, this wall is used for the prototype to construct a pipe-embedded wall. A concrete layer of 40 mm in thickness is added between these two bricks of 120 mm in thickness to embody the embedded pipes, as shown in Fig. 1. The pipe is in the middle of the concrete layer. The pipe material should be extraordinary and durable, but allow heat to pass through efficiently. In this work, polybutylene tube with the diameter of 20 mm is used in the structure to investigate the thermal performance of the active pipe-embedded wall (called pipe-embedded wall thereafter). In the meantime, the thermal performance of the pipe-embedded wall is compared with that of the brick wall with the same concrete layer, but without pipes inside (called conventional wall thereafter). Table 1 gives the physical properties of each layer of the walls of concern.
Fig. 1 Schematic diagram of cross section of embedded-pipe wall
Table 1 Physical properties of wall layers
Figure 1 shows the heat transfer model. It is assumed that the face between two adjacent embedded pipes is adiabatic due to the symmetry. The model is considered to be two-dimensional when the heat transfer along the length direction of the pipe is neglected. In practical applications, the embedded-pipe building envelope may have different pipe spacings such as 100, 150, 200, 250 and 300 mm. The supply water temperature may also be different such as 19, 20, 21, 22 and 23 ℃, according to the low-grade energy sources. In this model, the third boundary conditions are used (i.e., the heat transfer coefficients on the internal and external wall surfaces are specified, and the outside and inside air temperatures are specified). The coefficients of the internal and external surfaces are 18.3 W/(m2·K) and 8.3 W/(m2·K), respectively. The indoor and outdoor air temperatures (solar-air temperature) are ever-changing, as shown in Fig. 2, which is the typical temperature profile of the hot-summer region in Wuhan.
The heat transfer of this model was simulated by using CFD technology (Fluent was used in this work). The heat conduction within the solid (pipe wall, concrete and bricks) was computed without modeling the fluid itself. Conventional CFD software cannot deal with periodical boundary conditions automatically. In this work, a CFD interface program was developed to import the periodical temperature boundary conditions since periodical heat transfer of the building envelope model needs to be calculated. In this simulation, one cycle is 24 h, and the periodical conditions repeat on this model continuously until the heat transfer reaches a periodically steady state. The judge criterion is that the heat flow and temperature of the internal wall surface in the present day are the same as the previous day.
Fig. 2 Temperature of outdoor and indoor
3 Results and analysis
3.1 Effects of embedded pipes
For evaluating the effects of the embedded pipes on the heat transfer of the pipe-embedded wall, the heat transfers and the internal wall surface temperatures of the pipe-embedded wall and the conventional wall are compared, respectively. For the pipe-embedded wall, the pipe spacing is 200 mm, and the inlet water temperature is 21 ℃.
Figure 3 shows the internal wall surface heat flux profile of the pipe-embedded wall and the conventional wall. When the heat flux is positive, it indicates that the heat is transferred from the internal wall surface to indoor air, resulting building cooling load. This means that the internal wall surface temperature is higher than indoor air temperature. When the heat flux is negative, it indicates the internal wall surface absorbs heat from indoor air for reducing building cooling load, and the indoor air temperature is higher than the internal wall surface temperature. For the conventional wall, the heat is transferred from outside air to indoor air through the wall, resulting in building cooling load. The minimum heat transfer is 16 W/m2, and the maximum is 35 W/m2. The total transferred heat amount for 1 d (i.e., 24 h) is 2.1 MJ/m2. However, for the pipe-embedded wall, the heat flux is negative from 0:00 to 10:00, indicating that the heat is transferred from indoor air to the wall mass for reducing the building cooling load significantly. For other time, although the heat flux is positive, the heat flux is much less than that for the conventional wall in the coincident period, and the difference is about 24 W/m2. For 1 d, the pipe-embedded wall transfers 0.13 MJ/m2 to indoor air, absorbs 0.39 MJ/m2 heat flux from indoor air, and the equivalent heat transfer is -0.26 MJ/m2. This means that the wall absorbs heat from indoor air from the view point of 1 d. These results show that the energy consumption can be decreased significantly when the pipe-embedded building envelope is used.
Fig. 3 Internal wall surface heat flux profiles of conventional wall and pipe-embedded wall
Figure 4 shows the internal surface temperature profiles of the conventional wall and the pipe-embedded wall. For the conventional wall, the maximum temperature difference between the internal surface and the indoor air is 4.10 ℃, and the minimum is 1.88 ℃. The absolute average temperature difference is 2.93 ℃. However, for the pipe-embedded wall, the maximum temperature difference between the internal surface and the indoor air is only 0.81 ℃, 3.29 ℃ lower than that of the conventional wall. The absolute average temperature difference is 0.72 ℃, 2.21 ℃ lower than that of the conventional wall. The lower temperature difference between the internal surface and indoor air indicates that the heat radiation from the surface is less, and the indoor thermal performance may be improved significantly by using pipe-embedded building envelopes in real building system. Obviously, the lower surface temperature results in less energy consumption for air conditioning systems.
Fig. 4 Temperature profiles of wall surfaces and indoor air
3.2 Effects of water temperature
The pipe spacing of 200 mm in the pipe-embedded wall model is used for analyzing the effects of the water temperature on the wall heat transfer. Figure 5 shows the internal wall surface heat flux profiles at different water temperatures. The water temperatures are from 19 to 23 ℃ with the step of 1 ℃. Figure 5 demonstrates that the heat flux from the wall surface to indoor air reduces as the water temperature decreases. The heat flux reduces by about 2.6 W/m2 when the water temperature decreases by 1 ℃. Particularly, when the water temperature is bellow 20 ℃, the internal wall surface basically absorbs heat from indoor air in the most time of 1 d. The total heat transfer amounts (i.e., the summation of the heat transfer in one day) are -0.72, -0.49, -0.25 and -0.03 MJ/m2 for the water temperatures of 19, 20, 21 and 22 ℃, respectively. This means that the wall absorbs heat from indoor air at these water temperatures from the view point of 1 d. When the water temperature is 23 ℃, the total heat transfer amount is 0.19 MJ/m2 for 1 d, only 9.1% of the heat transfer of the conventional wall (2.09 MJ/m2). The maximum heat flux of the pipe- embedded wall is 11.98 W/m2 when the water temperature is 23 ℃. It is about one third of the maximum heat flux of the conventional wall (33.88 W/m2).
Fig. 5 Internal wall surface heat flux profiles of embedded-pipe wall at different water temperatures
3.3 Effects of pipe spacing
Figure 6 shows the internal wall surface heat flux at different pipe spacings with the water temperature keeping at 21 ℃. When the pipe spacing is 300 mm, the heat is transferred from outdoor air to indoor air, resulting in building cooling load in the most time of 1 d (from 9:00 to 23:00). At other time, the wall absorbs heat from indoor air for reducing building cooling load. With the decrease of the pipe spacing, the heat transfer from the internal surface to indoor air reduces, and the heat transfer time for this direction is also reduced. This means that the heat absorption duration from indoor air increases. When the pipe spacing is 100 mm, the pipe-embedded wall basically absorbs heat from indoor air. As the pipe spacing reduces by 50 mm, the heat flux of the internal wall surface reduces by about 2.3 W/m2. Therefore, reducing the pipe spacing may decrease the heat transfer from outside air to indoor air, and at the same time increase the heat absorption of the internal wall surface from indoor air for reducing the building cooling load finally.
Fig. 6 Internal wall surface heat flux profiles of pipe-embedded wall at different pipe spacing
4 Conclusions
1) For the conventional wall, the daily heat amount transferred from outside air to indoor air through the wall is 2.1 MJ/m2. For the pipe-embedded wall, the daily net heat transfer is -0.26 MJ/m2 with 0.13 MJ/m2 transferred to indoor air and 0.39 MJ/m2 absorbed from indoor air. These results show that the energy consumption can be decreased significantly when the pipe-embedded building envelope is used.
2) Water temperature and pipe spacing have significant effects on the heat transfer of the pipe-embedded wall. The results show that the internal surface heat transfer may reduce by about 2.6 W/m2 when the water temperature reduces by 1 ℃. When the pipe spacing reduces by 50 mm, the internal wall surface heat flux can also reduce by about 2.3 W/m2.
3) It is worthy to point out that the pipe-embedded building envelope can reduce the influence of the outdoor environment on the indoor environment by fully utilizing renewable energy or low-grade energy sources.
4) This work just makes a simple introduction and energy saving potential analysis of this technology. More fundamental works are still worthwhile for promoting the active structure to be used in low energy or green buildings wherever the low-grade energy sources are favorable.
References
[1] SNIJDERS A L. Aquifer seasonal cold storage for space conditioning: Some cost-effective applications [J]. ASHRAE Transactions, 1992, 98(1): 1015-022.
[2] MEIERHANS R A. Slab cooling and earth coupling [J]. ASHRAE Transactions, 1993, 99(2): 511-518.
[3] MEIERHANS R A. Room air conditioning by means of overnight cooling of the concrete ceiling [J]. ASHRAE Transactions, 1996, 102(1): 693-697.
[4] XU Xin-hua, ZHOU Li-jun, ZHU Qiu-yuan. Technologies and applications of pipe-embedded building structures for air- conditioning system [J]. Building Science, 2009, 125(10): 105-109. (in Chinese)
[5] XU Xin-hua, WANG Sheng-wei, WANG Jin-bo, XIAO Fu. Active pipe-embedded structures in buildings for utilizing low-grade energy sources: A review [J]. Energy and Buildings, 2010, 42((10): 1567-1581.
[6] LEHMANN B, DORER V, KOSCHENZ M. Application range of thermally activated building systems tabs [J]. Energy and Buildings, 2007, 39(5): 593-598.
[7] KENNETT S. Surface treatment [J]. Building Services Journal, 2006, 28(12): 39-41.
[8] FLORIDES G, KALOGIROU S. Ground heat exchangers-A review of systems, models and applications [J]. Renewable Energy, 2007, 32(15): 2461-2478.
[9] LIU X B, REES S J, SPITLER J D. Modeling snow melting on heated pavement surfaces: Part I. Model development [J]. Applied Thermal Engineering, 2007, 27(5/6): 1115-1124.
[10] STEPHENSON D G, MITALAS G P. Cooling load calculations by thermal response factors [J]. ASHRAE Transactions, 1967, 73: III1.1–III1.7.
[11] WANG Sheng-wei, CHEN Y M. A simple procedure for calculating thermal response factors and conduction transfer functions of multilayer walls [J]. Applied Thermal Engineering, 2002, 22: 333-338.
[12] TANG Zhi-wei, JIN Nan, YAN Gui-lan. Numerical study on unsteady heat transfer of underground heat exchanger of GCHP [J]. Renewable Energy, 2008, 26(1): 55-58. (in Chinese)
[13] LIU Jun, ZHANG Xu, GAO Jun, LI Kui-shan. Research of soil temperature restoration characteristics of ground-source heat pump systems [J]. Heating Ventilating & Air Conditioning, 2008, 38(11): 147-150. (in Chinese)
[14] OPPELT T, RIEHL I, GROSS U. Modelling of the borehole filling of double U-pipe heat exchangers [J]. Geothermics, 2010, 39(3): 270-276.
[15] WU Yu-peng, GAN Guo-hui, VERHOEF A. Experimental measurement and numerical simulation of horizontal-coupled slinky ground source heat exchangers [J]. Applied Thermal Engineering, 2010, 30(16): 2574-2583.
[16] LI Na, YUAN Dong-li, WANG Yong-hong, DI Yan-qiang. Simulation and analysis of cool storage capability of radiant concrete ceilings [J]. Heating Ventilating & Air Conditioning, 2009, 39(7): 143-145. (in Chinese)
(Edited by HE Yun-bin)
Foundation item: Project(51178201) supported by the National Natural Science Foundation of China; Project(2011CDB292) supported by the Natural Science Foundation of Hubei Province, China
Received date: 2011-07-26; Accepted date: 2011-11-14
Corresponding author: XU Xin-hua, Professor, PhD; Tel: +86-15972048683; E-mail: bexhxu@hust.edu.cn
