Surface heat budget on green roof and high reflection roof for mitigation of urban heat island

doi:10.1016/j.buildenv.2006.06.017

Building and Environment

Volume 42, Issue 8, August 2007, Pages 2971-2979

https://doi.org/10.1016/j.buildenv.2006.06.017 Get rights and content

Abstract

In this study, the surface temperature, net radiation, water content ratio, etc., of green roofs and high reflection roofs are observed. The heat and water budget are compared to each other. In the daytime, the temperature of the cement concrete surface, the surface with highly reflective gray paint, bare soil surface, green surface and the surface with highly reflective white paint are observed to be in descending order. On a surface with highly reflective white paint, the sensible heat flux is small because of the low net radiation due to high solar reflectance. On the green surface, the sensible heat flux is small because of the large latent heat flux by evaporation, although the net radiation is large. On the cement concrete surface and the surface with a highly reflective gray paint, the sensible heat fluxes have almost the same values because their solar reflectance is approximately equal. These tendencies of the sensible heat flux accord with the pitch relation of the surface temperature. Methods to estimate the quantity of evaporation, evaporative efficiency, heat conductivity, and thermal capacity are explained, and the observation data is applied to these methods.

Introduction

Improvement in the surface cover of buildings and constructions that have been covered with cement and asphalt concrete is examined as one of the measures to mitigate the urban heat island phenomenon. Green roofs, high reflection roofs, wall planting, etc., are suggested from the viewpoint of building planning, and green parks, roadside trees, etc., are suggested from the viewpoint of urban planning.

The use of highly reflective paint for cool roofs and road pavements is examined by Akbari et al. [1], [2], [3], [4] from the heat island group of the Lawrence Berkeley National Laboratory for reducing urban heat islands and energy consumption. According to their test calculation, the possibility of reduction or savings in the annual air conditioning costs was estimated to be 35 million dollars in Los Angeles, 16 million dollars in New York, and 10 million dollars in Chicago.

At the Lawrence Berkeley National Laboratory [5] and the Oak Ridge National Laboratory [6], the database of the characteristics of the roof materials (reflectivity and emissivity) available in the market is prepared and new products are developed.

The purpose of this study is as follows.

(1)
From the viewpoint of urban heat island mitigation, sensible heat flux reduction for the roof surface serves as an evaluation index. Therefore, the surface heat budgets on some roof covers are examined under the same weather condition. Consequently, sensible heat flux from the surface to air is compared from the heat island mitigation effect.
(2)
The estimation method of the sensible heat flux from each roof surface is investigated. The heat budget components are derived from the observation results. Finally, the sensible heat flux from each roof surface is estimated by the residue of the surface heat budget.

Section snippets

Outline of observation

The observation period is from July 2003, to the present. It is carried out on the experimental roof of the 8-story building of the Kobe University. In this study, the observation data for August and November 2004 is used for the detailed discussion. Unit numbers 1, 2, 9, 11, and 12 in Fig. 1 are used for the detailed discussion. The unit size for the bare soil surface (unit number 1) is 1.175 m×1.6 m; for the green (lawn) surface (unit number 2), 0.95 m×1.6 m; and for the paint and cement concrete

Observation result of the surface temperature

Weather condition in August and November 2004 are shown in Fig. 3, Fig. 4. Observation results of the surface temperature in August and November 2004 are shown in Fig. 5, Fig. 6. Although the leaf surfaces and the region around the roots in a lawn are expected to have different temperatures, only the result of the leaf surface temperature is shown, which is observed continually by a radiation thermometer.

In August, the surface temperature of the (cement) concrete slab and the highly reflective

Surface heat budget

The surface heat budget is expressed as follows: It is supposed that evaporation is only carried out at the surface. $Rn = A + V + lE,$ $A = {- λ \frac{\partial T}{\partial z} |}_{z - 0},$ $V = α (T_{s} - T_{a}),$ $lE = l β α_{w} (X_{s} - X_{a}) = l β α / C_{p} (X_{s} - X_{a}) .$

Here, Rn is the net radiation (W/m²), A the conduction heat flux (W/m²), V the sensible heat flux (W/m²), lE the latent heat flux (W/m²), λ the heat conductivity (W/mK), α the convective heat transfer coefficient (W/m²K), T_s the surface temperature (°C), T_a the air temperature (C), β the evaporative efficiency (−), C_p

Calculation of the water content ratio in the soil

Eqs. (8), (9) are calculated by the retreat difference method. The comparison between the calculation result and the observation result of the water content ratio in some layers and the quantity of evaporation for November 5–7, 2004 are shown in Fig. 15, Fig. 16. The quantity of evaporation is calculated by using the calculation result of the water content ratio. The observation values of the water content ratio in the 2- and 21-cm layer are used for boundary condition. The parameters for the

Summary

The surface temperature, net radiation, water content ratio, etc., are observed on the green roof and the high reflection roof. Then, the heat and water budget are compared to each other. Summaries of this study are as follows.

(1)
On the surface with highly reflective white paint, the sensible heat flux is small because of the low net radiation by high solar reflectance. On the green surface, the sensible heat flux is small because of the large latent heat flux by evaporation, although the net

Acknowledgment

The authors wish to thank Dr. Yasunobu Ashie at the Building Research Institute of Japan for his advice and assistance. This study was funded by the Asahi Glass Foundation, the Kajima Foundation, and the 21st Century COE Program “Design Strategy towards Safety and Symbiosis of Urban Space.” This study was performed with the cooperation of the Miki Coating Design Office and Toho-Leo Co.

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