ASHRAE LO-09-076-2009 The Role of Plants in the Reduction of Heat Flux through Green Roofs Laboratory Experiments《通过绿化屋顶栽种植物来减少热流 实验室实验》.pdf
《ASHRAE LO-09-076-2009 The Role of Plants in the Reduction of Heat Flux through Green Roofs Laboratory Experiments《通过绿化屋顶栽种植物来减少热流 实验室实验》.pdf》由会员分享,可在线阅读,更多相关《ASHRAE LO-09-076-2009 The Role of Plants in the Reduction of Heat Flux through Green Roofs Laboratory Experiments《通过绿化屋顶栽种植物来减少热流 实验室实验》.pdf(10页珍藏版)》请在麦多课文档分享上搜索。
1、2009 ASHRAE 793ABSTRACT An interesting approach to reduce building energy consumption is to use green roofs as a part of building enve-lope. However, many building designers ignore this opportu-nity as it is quite difficult to estimate the resulting energy saving. This paper provides results from an
2、 ongoing experi-mental research project that focuses on the thermal perfor-mance of extensive green roofs when buildings are in the cooling mode. The paper discusses the importance of green roofs and reviews previous research studies. In particular, this paper focuses on the role of plants for the h
3、eat flux reduction through the roof structure. The performance of the plant mate-rial was assessed in an environmental chamber by experiments with two samples, one with the plant material, and another one without the plant material. Overall, plants reduced the measured heat flux through the green ro
4、of sample by 40-50% compared to the roof sample without plants. In conclusion, plants have an important role in reducing the heat flux by regu-lating: (1) latent heat flux through better water management and additional water storage in the plant leaves/roots, and (2) sensible heat flux through addit
5、ional shading provided by the plant leaves. Based on these results, future research will focus on thermal modeling of green roof including the role of plants.INTRODUCTIONGreen roofs are an emerging sustainable technology that is becoming more popular in North America (Miller et al., 2005). As a defi
6、nition, green roofs are “specialized roofing systems that support plant growth on rooftops” (Liu et al., 2004). From top to bottom, a typical green roof consists of several layers: (1) vegetation, (2) substrate, (3) filter membrane, and (4) drainage layer. Plants used for extensive green roofs are t
7、ypically drought tolerant, and selected from the group of native or Sedum plants. Substrate is the soil-like layer where plants grow, and it has to be porous, retain mois-ture and nutrients, and support plant growth (Snodgrass et al., 2006). The filter membrane prevents drainage clogging by containi
8、ng the substrate and roots. The drainage layer trans-ports the rainfall water runoff to the roof drainage (Peck, 2002; Snodgrass et al., 2006).There are basically two types of green roofs: extensive and intensive green roofs. Extensive green roofs have lower weight, lower capital cost, minimal maint
9、enance, and a substrate depth between 2 and 6 inches (5 and 15 cm). Inten-sive green roofs have greater weight, higher capital costs, wider planting selection, higher maintenance requirements, and a substrate depth between 8 and 24 inches (20 and 60 cm). However, intensive green roofs are less cost-
10、effective than extensive and required more structural support (Peck et al., 1999; Tanner, 2004). Moreover, extensive green roofs repre-sent about 2/3 of the total green roof square footage installed in North America (Johnston, 2007). Therefore, this research project focuses on summer thermal perform
11、ance of extensive green roofs as a more economically viable solution to be adopted in the building industry. The popularity of green roofs is increasing due to their potential benefits. In general, green roofs have a potential to (Liu et al., 2004):reduce energy demand on space conditioningreduce st
12、orm water runoffimprove air quality, and reduce the urban heat island effect in cities.The Role of Plants in the Reductionof Heat Flux through Green Roofs: Laboratory ExperimentsPaulo Cesar Tabares-Velasco Jelena Srebric, PhDStudent Member ASHRAE Member ASHRAEPaulo Cesar Tabares-Velasco is a graduat
13、e student and Jelena Srebric is an associate professor at the Department of Architectural Engineering, The Pennsylvania State University, University Park, PA.LO-09-076 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transact
14、ions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.794 ASHRAE TransactionsTherefore, green roofs can help address three of the four top problems facing th
15、e society in the next 50 years: energy, water, and environment (Smalley, 2005).GREEN ROOF ENERGY BALANCEThe main challenge of accurately modeling and measur-ing the thermal performance of green roofs is due to the complex heat and mass transfer processes through the roof by the means of (1) shading,
16、 (2) insulation, (3) evapotranspira-tion, and (4) thermal mass (Liu, 2004). Evapotranspiration represents a combined process of water lost from the soil (evaporation) and plants (transpiration). Transpiration occurs when water from the plant leaf surface goes into the air by diffusion or convection.
17、 Most of the water is lost by transpi-ration through plant stomata, which are adjustable small pores in the leaf that allow the entry of gases needed for photosyn-thesis such as CO2, and the release ofO2and water vapor. Thus, plants can control their transpiration rate by opening and closing their s
18、tomata (Nobel, 1983; Allen et al., 1998; Hillel, 1998).All of the heat transfer processes taking place on a green roof are combined in an energy balance equation as following (Hillel, 1998; Jones, 1992):(1)whereRn= net radiation, equal to solar gain minus infrared heat losses, Btu/hft2(W/m2)ET = eva
19、potranspiration, or latent heat flux, Btu/hft2(W/m2)Qsensible= convective or sensible heat flux, Btu/hft2(W/m2)Qcondution= heat flux trough roof, Btu/hft2(W/m2)Sthermal= thermal storage for substrate, plants, Btu/hft2(W/m2)M = metabolic storage (photosynthesis and respiration), Btu/hft2(W/m2)In Equa
20、tion 1, the metabolic storage is often neglected as its contribution to the total energy budget is around 1% to 2% of the net radiation (Jones, 1992; Gates, 1980). All fluxes are dependent greatly on the capacity of the plants/substrate to evaporate water as latent heat flux uses energy from the env
21、i-ronment to evaporate water, thus cooling down the plants surface and roof temperature. Consequently, the latent heat flux partially controls the heat flux going trough the substrate/roof that eventually converts to building cooling loads. Evapo-transpiration is also a phenomenon that enables green
22、 roofs to decrease the urban heat island effect by lowering the temper-ature of the roofs by evaporation of the rain water.As an example of energy balance, Figure 1 shows the percentages of each heat transfer component divided by the incoming shortwave radiation for a particular experimental setup w
23、ith a green roof. Experiments are described in more details later in the paper, while these results are presented here for the illustration of typical heat fluxes and heat balance on a green roof. Table 1 provides a summary of the total heat flux components for the two different sets of experiments,
24、 one with the plants and another without plants. Because all fluxes were divided by the incoming shortwave radiation having the same units, the results are dimensionless. The sum of all heat fluxes shown in Table 1 was very close to 100% for the experiments without plants. In contrast, the sum of he
- 1.请仔细阅读文档,确保文档完整性,对于不预览、不比对内容而直接下载带来的问题本站不予受理。
- 2.下载的文档,不会出现我们的网址水印。
- 3、该文档所得收入(下载+内容+预览)归上传者、原创作者;如果您是本文档原作者,请点此认领!既往收益都归您。
本资源只提供5页预览,全部文档请下载后查看!喜欢就下载吧,查找使用更方便
10000 积分 0人已下载
下载 | 加入VIP,交流精品资源 |
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- ASHRAELO090762009THEROLEOFPLANTSINTHEREDUCTIONOFHEATFLUXTHROUGHGREENROOFSLABORATORYEXPERIMENTS 通过 绿化

链接地址:http://www.mydoc123.com/p-455357.html