Elsevier

Applied Energy

Volume 242, 15 May 2019, Pages 1010-1021
Applied Energy

Experimental study of the thermal performance of an extensive green roof on sunny summer days

https://doi.org/10.1016/j.apenergy.2019.03.153Get rights and content

Highlights

  • Equivalent thermal performance indices of a green roof are determined.

  • Correlations between climatic parameters and thermal performance are analyzed.

  • The temperature profile inside green roof is S-sharp at non-air-conditioned state.

  • The bottom layer of the soil of the green roof act like a “cooling source”.

  • Influence of the indoor critical air temperature on thermal resistance is discussed.

Abstract

Green roofs have shown a remarkable effect on the reduction of the building cooling load in summer. The assessment of the thermal performance of green roofs is essential for their design and evaluation. This study investigated the thermal characteristics of an extensive green roof under air-conditioned and non-air-conditioned states by using experimental data obtained on successive sunny summer days. Two indices of the green roof, namely the equivalent thermal resistance and the equivalent decrement factor, were estimated through comparison with a common bare roof. Under the non-air-conditioned state, the distribution of the average temperature on the green roof profile was S-sharp. The lowest average temperature of the green roof was observed on the interface between the planting soil and roof structure, which were 1.8 and 0.9 °C below the outdoor and indoor air temperature, respectively. This finding indicated that the bottom of the soil layer functioned as a “cooling source” that absorbed heat from the upside and downside. The indoor critical air temperature that maintains the heat flux equal to zero was investigated on the basis of the experimental data. The influence of indoor critical air temperature on the equivalent thermal resistance was discussed; for the same green roof under the same outdoor climatic conditions, an indoor air temperature that is closer to the indoor critical air temperature yields a higher equivalent thermal resistance. Therefore, the equivalent thermal resistance of the green roof obtained under low indoor air temperature is recommended for practical use.

Introduction

Green roofs have received considerable attention in recent years, because many countries have been dealing with various challenges, such as climate change, environmental deterioration, and energy shortages. These problems can be partially mitigated by installing green roofs. Many governments worldwide have promoted the implementation of green roof technology [1], [2], [3]. As an ecological strategy, green roofs can relieve the urban heat island effect [4], [5], [6], decrease water runoff [7], and reduce carbon emissions [8], [9]. Moreover, the green roof is an essential energy-saving technology that is suitable for buildings in hot-humid areas, where abundant rainfall provides the possibility for its implementation.

Previous research has demonstrated that green roofs significantly reduce the energy consumption of buildings through evapotranspiration, photosynthesis, solar shading, and thermal insulation of substrates [10], [11]. Mathematical models [12], [13], [14], [15] have been developed to quantify the thermal effects of green roofs, and a few of these models have been integrated into building energy simulation programs [16]. However, accurate, comprehensive, and quantitative assessment remains challenging because of the complexity and uncertainty of the parameters collected through experiments or from databases [17], [18]. When considering the numerous design criteria and parameters involved, incorporating the thermal performance of green roofs into building energy simulation remains difficult.

On the practical side, the thermal design and evaluation of building envelopes mostly use the properties of common materials. The components of green roofs are generally common materials, except for the water content and the plant. Early studies have simplified green roofs into layers of substrate and other common materials without plants by estimating the thermal resistance (R-value) [11] or thermal transmittance coefficient (U-value) [19], [20], [21] using the properties of each layer. Recently, researchers have assessed the thermal performance of green roofs through comparison with common roofs for pratical use. Moody et al. [22] proposed the dynamic benefit of green roof index using the ratio of the annual heating, ventilation, and air conditioning (HVAC) energy use for a building with a common roof to that of a green roof. Chan et al. [23] proposed a set of correction factors for the overall thermal transfer value evaluation of a green-roof-integrated building compared with that of a conventional roof. These studies quantified the effects of green roofs for an entire year by comparing them with common roofs. Nevertheless, the thermal performance of green roofs varies throughout the year. They significantly reduce the building cooling load in the summer but have minimal influence on the heating load in the winter [11], [12], [24], [25]. In general, a building’s thermal design is based on the maximum cooling/heating load. Thus, the performance of green roofs in summer is important.

Green roofs act as envelope insulation for air-conditioned buildings in summer. Although the mechanism of green roofs is complicated and difficult to model accurately, the comprehensive effect is to reduce the heat flux into the interior space. In this respective, it is the same as a common insulated roof. Therefore, experimental methods have been widely used to determine this comprehensive effect using the R-value or U-value. Meng [26] conducted a wind tunnel experiment on an extensive green roof. The measured results showed that the equivalent thermal resistance was 0.41–0.61 m2·K·W−1 under a wind speed of 1.5–1.8 m/s. Wong et al. [27] performed an on-site measurement on an intensive green roof and estimated the thermal resistances of three types of plants, turfing, shrubs and trees, as 0.36, 1.61, and 0.57 m2·K·W−1 respectively. Olivieri [28] conducted an experimental study on an extensive green roof situated in the Mediterranean region, in which an equivalent thermal resistance of 6.53 m2·K·W−1 was obtained. Moreover, many field measurements have revealed that green roofs effectively attenuate the daily periodic temperature acting on the surface of the building [29], [30], [31], [32], [33], [34], [35], which is important for the reduction of building peak cooling load. Evaluation indices such as the decrement factor [35], [36] were measured to quantify the resistance of green roofs against sol-air temperature fluctuations. However, the preceding experimental investigations mentioned above employed a variety of measurement methods and calculation approaches. For example, in the calculation of evaluation indices, the external temperature was taken differently in different studies—by using the soil surface temperature [27], [29], the sol-air temperature [28], or the surface temperature of a bare roof [37]—thereby leading to incomparable results.

Previous research has revealed the divergence of the heat flux direction through green roofs under varying indoor air temperatures. Takakura et al. [38] performed an experimental study under a non-air-conditioned state and showed that a concrete roof with green cover had heat flux transfer from the interior to the outside (i.e., negative heat flux). The negative heat flux through the green roof in a non-air-conditioned room was also found experimentally by Yang et al. [39], Jiang and Tang [40], and Tian et al. [41]. In contrast, when the indoor air temperature was relatively low under the air-conditioned state, the heat flux through the green roof was constantly positive [13], [42]. These studies demonstrate the effect of the indoor air temperature on the direction of heat flux through the green roof and indicate the existence of an indoor critical air temperature that leads to zero heat flux [43]. This thermal characteristic of green roofs is fundamentally different from common roofs. In this regard, knowledge of the heat flux and temperature distribution inside a green roof under different indoor conditions is essential for a complete understanding of this unusual thermal behavior of green roofs.

One of the major purposes of research on green roofs is to popularize their application as a building energy-saving product. To be such a product, it is necessary to quantify their thermal performance in terms of indices that could be used for standardized building thermal design procedure. The preceding up-to-date literature review indicates the lack of consideration in two main aspects. First, in the majority of the experimental studies, the evaluation indices of green roofs were incomparable to common insulated roofs. The climatic influences, which were determined to be not negligible [44], were not considered. Second, the influence of the indoor critical air temperature on the equivalent thermal resistance of green roofs has not been experimentally evaluated.

The current research explores the thermal behavior of an extensive green roof on the basis of experimental data obtained under non-air-conditioned and air-conditioned states on sunny summer days. The equivalent thermal resistance and equivalent decrement factor of the green roof are estimated. Temperature profiles and heat flux inside the green roof are analyzed. The indoor critical air temperature is determined, and the impact of the indoor critical air temperature on the equivalent thermal resistance of the green roof is discussed. The remainder of this paper is organized as follows: Section 2 describes the steps of the experiment and the methodology used to determine the equivalent indices for the green roof. Section 3 presents an analysis of the experimental results. Section 4 provides the discussion. The main conclusions are presented in Section 5.

Section snippets

Experimental site

The experiment was conducted in Xinzhuang, Shanghai (31°07′N latitude, 121°21′E longitude, and 6 m altitude). The site was located in a typical urban area in the hot-summer/cold-winter zone of China. To investigate the characteristic of an extensive green roof, half of the roof of a four-room, single-storey building was covered with sedum lineare planting modules (Fig. 1). The two adjacent rooms in the middle were selected for experimental use, as illustrated in Fig. 2. Each room had a

Results

The results presented in this paper were obtained from 7 to 30 August, and this period was preceded by rain on 3 August and 6 August. The rain before the monitored days provided sufficient water content for the substrates. Measurement of the solar radiation demonstrated that the majority of the 24 days were sunny days with average and maximum air temperatures of approximately 30 and 39 °C, respectively. The climatic conditions of the two observation periods were similar. Therefore, the measured

Discussion and limitations

A traditional approach to determine the equivalent thermal resistance of a green roof (Rg) is to measure the heat flux, external roof surface temperature, and inner roof surface temperature, and then calculate Rg thereafter. Given that the measured external temperature of a green roof (e.g., soil surface temperature or the surface temperature of the substrate layer) is substantially lower than that of a common insulated roof because of evapotranspiration by vegetation and soil, the calculated

Conclusions

This article analyzed the heat transfer characteristics of the green roof under different indoor air temperatures during summer. The main conclusions are listed below:

  • 1.

    The green cover reduced the cooling energy consumption by 14.7% and the heat flux by 76.1%. The reduced heat flux by the extensive green roof (qi.b − qi.g) was strongly associated with solar radiation and the outdoor air temperature, with correlation coefficients of 0.84 and 0.82, respectively.

  • 2.

    Compared with a common bare roof, the

Acknowledgment

This project was funded by the National Natural Science Foundation of the Peoples Republic of China (Grant No. 51478059). The authors would like to express sincere thanks to Dingguo Zhao, Weicheng Xue, Zhenjing Yang and Shukui Zheng for their assistance in conducting the experiments.

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