A Simple Method to Evaluate Adaptation Measures for Urban Heat Island

Abstract

In recent years, adaptation measures such as awnings, louvers, directional reflective materials, mist sprays, and evaporative materials, have been developed with the expectation that they will serve as effective solutions to outdoor human thermal environments that are under the influence of urban heat island. A simple method to evaluate the aforementioned adaptation measures is examined in this study, focusing on their appropriate introduction on urban space. The influence of the solar transmittance of adaptation measures such as shading, on mean radiant temperature (MRT) is approximately 1.5 °C per 0.10. If a shielding device that reflects a large amount of solar radiation and facilitates high levels of evaporation is developed, MRT and standard new effective temperature (SET*) will both decrease.

1. Introduction

Mitigation measures such as green roof, cool roof (with a high reflectance material), and water-retentive materials, have been developed with the expectation that they will serve as countermeasures to the urban heat island [1,2,3,4]. In recent years, in order to serve as effective solutions to outdoor human thermal environments under the influence of urban heat islands, adaptation measures such as awnings, louvers, directional reflective materials, mist sprays, and evaporative materials have been developed. A simple method to evaluate these adaptation measures focusing on their appropriate introduction into urban space has been here investigated.

The Japanese Ministry of the Environment developed the ‘Heat countermeasure guideline in the city’ [5], which includes basic, specific adaptation measures, and technical sections. The guideline states that ‘by understanding the factors that make it hot and implementing appropriate adaptation measures for places we have to wait for or places we want to spend comfortably such as bus stops and plazas, we can promote a healthy and comfortable environment in the urban area’ (p. 11 of [5]). In the basic section, the adaptation measures against heat are explained in an accessible manner for the Japanese administration and the general public. In the specific adaptation measures section, the type and effect of adaptation measure technologies and precautions to be considered upon introducing them are explained for the general public and practitioners involved in town development. In the technical section, technical information on adaptation measure technologies is explained for building and external construction design practitioners.

Several studies focused on effective measures against heat waves have been implemented in various countries [6,7]. Evaporative cooling effects such as irrigation [6,7], vegetation and pavement watering [7] have been studied by the numerical simulation. Some of those scenarios assumed the future climate affected by climate change [7,8]. Discussions including the improvement of thermal environments in the street canyon or in the plaza were not sufficiently conducted based on the evaluation of the human thermal comfort in previous examinations [9,10,11].

In Germany, several cities are considering adaptation measures. According to a report from Karlsruhe City [12], it is recommended that appropriate adaptation measures be introduced in ‘hot spots’ where temperatures are high. Several typical urban districts in cities that may undergo adaptation in the future are also discussed. Within the Osaka Heat Island Countermeasure Technology Consortium [13], adaptation technologies developed by various companies were presented and evaluation methods were discussed so that they may be properly implemented in society. In this study, a specific method to evaluate adaptation measures is discussed, considering these efforts in Japan.

2. Adaptation Measures

The adaptation measures for urban heat islands listed in the heat countermeasure guidelines established by the Japanese Ministry of Environment [5], the report by the Japanese Ministry of Environment [14], and the town planning idea competition considering the urban heat island presented at the Osaka Heat Island Countermeasure Technology Consortium [15] are shown in

Table 1. Adaptation measures for urban heat islands and their effects and associated evaluation indices.

Menu	Evaluation Index	Main Effect Mechanism
From the heat countermeasure guidelines by the Japanese Ministry of Environment [5]
Green shade [16]	Solar transmittance, Evaporative efficiency	Sun shade, Evaporative cooling
Solar radiation shade [17]	Solar transmittance, Convection heat transfer coefficient	Sun shade, Convection heat transfer
Retroreflective surface [18,19]	Downward solar reflectance	Solar reflection
Water retentive pavement [20,21]	Evaporative efficiency	Evaporative cooling
Cool pavement [21]	Solar reflectance	Solar reflection
Green pavement [22]	Evaporative efficiency	Evaporative cooling
Green wall [23]	Evaporative efficiency	Evaporative cooling
Water-retentive wall [24]	Evaporative efficiency	Evaporative cooling
Fine mist spray [25,26]	Evaporation rate	Evaporative cooling
from the report by the Japanese Ministry of Environment [14]
Awning [27]	Solar transmittance	Sun shade
Fractal-shaped sunshade [17]	Solar transmittance, Convection heat transfer coefficient	Sun shade, Convection heat transfer
Mesh shade and water supply [14]	Solar transmittance, Evaporative efficiency	Sun shade, Evaporative cooling
Evaporative cooling louver [24]	Evaporative efficiency	Evaporative cooling
Greening cooling louver [14]	Evaporative efficiency	Evaporative cooling
Tree pot [14]	Solar transmittance, Evaporative efficiency	Sun shade, Evaporative cooling
Water-retentive block [20]	Evaporative efficiency	Evaporative cooling
Water surface [28]	Evaporative efficiency	Evaporative cooling
Fine mist spray with blower [25,26]	Evaporation rate	Evaporative cooling
Ceiling cooling system [14]	Surface temperature	Artificial cooling
Water cooling bench [14]	Surface temperature	Artificial cooling
from town planning idea competition by Osaka Heat Island Countermeasure Technology Consortium [15]
Water surface [28]	Evaporative efficiency	Evaporative cooling
Watering [28]	Evaporative efficiency	Evaporative cooling
Fine mist spray [25,26]	Evaporation rate	Evaporative cooling
Shading [27]	Solar transmittance	Sun shade
Tree planting	Solar transmittance, Evaporative efficiency	Sun shade, Evaporative cooling
Roof and ground greening [22]	Evaporative efficiency	Evaporative cooling
Wind use	Convection heat transfer coefficient	Convection heat transfer
Traffic mode control	Anthropogenic heat release	Reduction of anthropogenic heat release
Unused energy use, natural energy use	Anthropogenic heat release	Reduction of anthropogenic heat release
ICT use	Human body physiological amount	Reduction of human thermal load

. The mechanisms by which these methods work and the evaluation indices governing their effects are also presented. Heat is mainly mitigated by solar shading, solar reflection, and evaporation. Therefore, solar transmittance, solar reflectance, and evaporative efficiency (evaporative rate) are the primary evaluation indices. The increase in the convection heat transfer coefficient is the cause of cooling by fractal-shaped sunshades, and the artificial cooling is the cause of cooling by ceiling cooling systems and water cooling benches. Examples of adaptation measures developed by Japanese companies are shown in Figure 1, Figure 2 and Figure 3 [14]. Experiments demonstrating these measures are currently proceeding throughout Japan [5,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28].

3. Simple Evaluation Method of Adaptation Measures

3.1. Methods

The effect of the studied adaptation measures is evaluated by outdoor human thermal comfort, which is strongly correlated to the outdoor thermal environment. As Nouri et al. [29] pointed out, the selection of the index for the assessment of outdoor thermal comfort conditions is still a debated matter [30]. They stated that, “So far, within the international community various indices have been developed and disseminated, including the (i) Standard Effective Temperature (SET*) [31]; (ii) Outdoor Standard Effective Temperature (OUT_SET*) [32,33]; (iii) Perceived Temperature (PT) [34]; (iv) Predicted Mean Vote (PMV) [35,36]; (v) Index of Thermal Stress (ITS) [37]; (vi) Predicted Percentage of Dissatisfied (PPD) [35]; (vii) COMFA outdoor thermal comfort model [38]; (viii) Universal Thermal Climate Index (UTCI) [39,40,41]; (ix) Wet Bulb Globe Temperature (WBGT) [42,43]; and (x) Predicted Heat Strain (PHS) [44,45,46].” They also demonstrated the necessity of standardizing a thermal comfort index for specific regions. Currently, different indices are widely used by each academic community. Physiologically Equivalent Temperature (PET) is widely used in Europe; it has been defined as the air temperature at which, in a typical indoor setting, the human energy budget is maintained by the skin temperature, core temperature, and perspiration rate, which are equivalent to those under the conditions to be assessed [47,48]. In Japan, SET* and WBGT are mainly used. WBGT, which is a stress index worldwide accepted as a preliminary tool for the assessment of hot thermal environments [49,50,51], is often used under more severe conditions to warn of the risk of heat stroke. SET* is defined as the equivalent dry bulb temperature of an isothermal environment at 50% RH in which a subject, while wearing clothing standardized for the activity concerned, would have the same heat stress and thermo-regulatory strain as in the actual test environment [31], is used to evaluate the thermal environment [5]. The relationship between SET* and thermal comfort, which is based on the results of a declaration test for the outdoor comfort of Japanese people, is shown in

Table 2. Relationship between Standard Effective Temperature (SET*) and thermal comfort.

SET* (°C)	Thermal Comfort
33.3 32.1 30.8 28.4 27.0	extremely uncomfortable
	Uncomfortable
	slightly uncomfortable
	Neither
	slightly comfortable
	Comfortable

[52]. SET* is desirable as an index from the viewpoint of appropriately introducing adaptation measures in urban areas and developing a more comfortable outdoor space as it exhibits a good relationship with outdoor thermal comfort [53].

3.1.1. Sensitivity Analysis

Assuming a typical summer day as a standard condition; under which the air temperature is 34 °C, relative humidity is 50%, wind speed is 1 m/s, mean radiant temperature (MRT) in a sunny place is 50 °C or 37 °C in a shaded place, clothing insulation is 0.6 clo, and metabolic rate is 2 Met; a SET* sensitivity analysis was conducted with a variation range of 20 °C to 40 °C for air temperature, 30% to 80% for relative humidity, 0.5 m/s to 3 m/s for wind speed, and 20 °C to 60 °C for MRT [54,55].

3.1.2. MRT and Surface Temperature Reduction Evaluation

The decrease in MRT caused by solar radiation shielding was dominant over the improvement in SET*. Assuming the implementation of adaptation measures such as shading, MRT was evaluated using the following indices: solar transmittance τ, evaporation rate E, solar absorptance a, and convective heat transfer coefficient h. Assuming that the human body is spherical with a solar absorptance a_h which is assumed to be 0.5, MRT can be calculated from Equation (1) [55,56,57]:

$$MRT={\left(a_{h}Q/\sigma +{\displaystyle \sum }_{i=1}{\Phi }_i{T}_i^4\right)}^{\frac{1}{4}}$$

(1)

With reference to previous studies in Japan [54,55], the weather conditions during a typical summer day were assumed as follows; solar radiation J was 1000 W/m² (direct solar radiation was 900 W/m² and diffuse solar radiation was 100 W/m²), each surface temperature T_i was the same as the air temperature T_a (T_i = T_a = 34 °C), the MRT under clear sky conditions was 56.2 °C. While the relationship between the human body and the surrounding objects is varied actually, in order to simplify the discussion, it is supposed to be a human body on a green area that has been thoroughly irrigated. The incident solar radiation on the human body was calculated by Q = 900/4 + 100 W/m², as the human body was assumed to be a sphere. σ is the Stefan–Boltzmann constant (=5.67 × 10⁻⁸ W/(m²K⁴)), and Φ_i is the shape factor between the human body and each surface.

Surface temperature T_s of the adaptation measures is calculated from Equation (2):

$$T_s=\frac{1}{h}\left(aJ+\epsilon q-lE\right)+T_a$$

(2)

where, ε is emissivity, q is net infrared radiation and l is the latent heat of vaporization of water (=2500 kJ/kg).

3.2. Results

3.2.1. Sensitivity Analysis

Sensitivity analysis results are shown in Figure 4. The sensitivities by air temperature, relative humidity, wind speed, and MRT were 0.63 °C/°C, 0.13 °C/%, 1.4 °C/(m/s), and 0.21 °C/°C, respectively. The sensitivities by MRT and wind speed were larger than those by air temperature and relative humidity, however, they were within the expected variation range of each element. The relationship between air temperature, MRT, and SET* is shown in Figure 5. SET* is indicated by a contour line. Above-standard conditions were set for the other elements. If the evaluation point moved from a sunny to a shaded place, the MRT decreased by 13 °C and SET* decreased by 2.8 °C. To obtain the same decrease in SET* due to air temperature reduction by mist spraying, it must be lowered by 4.2 °C. Similarly, it is difficult to considerably reduce MRT using cool walls and pavements. Examples of the effects of adaptation measures obtained by demonstrative experiments are shown in Figure 6. As MRT was measured by a globe thermometer, the solar absorptance was set to 1.0, which was much larger than that of the human body. The measurements were taken at various places and times under typical summer weather condition, therefore, a simple mutual comparison was not appropriate. It was, however, possible to qualitatively recognize the characteristics of each adaptation measure [14]. Shielding of solar radiation to pedestrians was a more effective method of lowering MRT and SET*.

3.2.2. MRT and Surface Temperature Reduction Evaluation

The relationship between solar transmittance τ and MRT reduction by adaptation measures such as an awning, is shown in Figure 7. If the influence of long-wave radiation was ignored, complete shielding of solar radiation decreased the MRT by 15 °C.

The relationship between the surface temperature T_s of the adaptation measures and the solar absorptance a when the heat transfer coefficient h is 23 W/(m²K), emissivity ε is 0.97, and net infrared radiation q is −93 W/m² for different values of the evaporation rate E is shown in Figure 8. Although net infrared radiation q and the evaporation rate E varied depending on weather conditions such as surface temperature, air temperature, and wind velocity, they were set to specific values to allow simple evaluation. Even if the evaporation rate E was 0 L/(m²h), when the solar radiation absorptance a was 0.1, the surface temperature T_s was almost the same as the air temperature. The surface temperature T_s when the heat transfer coefficient h is 46 or 92 W/(m²K) is shown in Figure 9. A fractal-shaped sunshade was developed focusing on the utilization of the effect caused by increasing the heat transfer coefficient [17]. As the heat transfer coefficient h increased, the surface temperature T_s approached the air temperature value regardless of the solar absorptance a and evaporation rate E.

The relationship between the MRT reduction and the solar absorptance a when the evaporation rate E is 0 L/(m²h) for different values of the shape factor Φ of the human body is shown in Figure 10. When the shape factor Φ and solar absorptance a were large, the MRT increased due to the effect of long-wave radiation from the adaptation measures.

The relationship between the MRT reduction and the solar transmittance τ when the evaporation rate E is 0 L/(m²h) and the shape factor of the human body Φ is 0.3 for different values of the solar absorptance a is shown in Figure 11. When the targeted MRT reduction was 10 °C, the required solar transmittance τ plus solar absorptance a was 0.4 or less.

The relationship between the MRT reduction and the solar transmittance τ when the evaporation rate E is 1.0 L/(m²h) and the shape factor of the human body Φ is 0.3 for different values of the solar absorptance a is shown in Figure 12. If the evaporation rate E was 1.0 L/(m²h) or more, MRT decreased by 10 °C regardless of solar transmittance τ and solar absorptance a.

4. Discussion

In a typical summer day weather condition, if the evaluation point moved from a sunny to a shaded place, the MRT decreased by 13 °C and SET* decreased by 2.8 °C. Watanabe et al. have revealed that the globe temperature in sunlight was higher than that in the building shade by 16.7 °C and was 13.9 °C higher than that in the pergola shade in a clear day with global solar radiation of 800 W/m² [55]. Changes in the MRT in this study and the globe temperature by Watanabe et al. due to solar shielding corresponded relatively. It is difficult to reduce MRT to this level using cool walls and pavements. Through several examples of the effects of adaptation measures obtained by demonstrative experiments [14], it can be seen shielding of solar radiation to pedestrians is a more effective method of lowering MRT and SET*. If the influence of long-wave radiation was ignored, complete shielding of solar radiation decreased the MRT by 15 °C. Even if the evaporation rate E was 0 L/(m²h), when the solar radiation absorptance of the adaptation measures a was less than 0.1, the surface temperature T_s was almost the same as the air temperature. A fractal-shaped sunshade was developed focusing on the utilization of the effect caused by increasing the heat transfer coefficient [17]. As the heat transfer coefficient h increased, the surface temperature T_s approached the air temperature value regardless of the solar absorptance a and evaporation rate E. When the shape factor between the human body and the adaptation measures Φ and solar absorptance a were large, the MRT increased due to the effect of long-wave radiation from the adaptation measures. When the targeted MRT reduction was 10 °C, the required solar transmittance τ plus solar absorptance a was 0.4 or less. If the evaporation rate E was 1.0 L/(m²h) or more, MRT decreased by 10 °C regardless of solar transmittance τ and solar absorptance a.

5. Conclusions

Through several examples of the effects of adaptation measures obtained by demonstrative experiments, it can be seen that shielding of solar radiation to pedestrians is a more effective method of lowering MRT and SET*. The influence of the solar transmittance of adaptation measures such as shading, on MRT is approximately 1.5 °C per 0.10. The influence of the solar absorptance of adaptation measures such as an awning, on MRT is approximately 1.0 °C per 0.10, which also depends on the shape factor between the human body and adaptation measures. The influence of the evaporation rate on MRT is approximately 1.0 °C per 0.10 L/(m²h). If a shielding device that reflects a large amount of solar radiation and facilitates high levels of evaporation is developed, MRT and SET* will both decrease.