Study on the effect of timestep and thermography method for pavement watering technology

Highlights

• For pavement watering experiments, a timestep of 10 min or less is suggested

• The optimal measurement frequency must be determined by an experiment beforehand.

• When using thermography method, the appropriate water film, emissivity and shading effect should be considered.

Abstract

In this study, effect of the timestep and thermography method when measuring the surface temperature of the road and surround temperature on pavement watering was verified. The study result showed that the temperature of the surface and surround air were dropped after watering according to the cooling effect, and the temperature change tends to occur considerably within 10 min in this experiment. Therefore the short timestep should be applied according to the site condition to assume the cooling effect of pavement watering. In addition, thermography method showed to be inaccurate right after the watering and when the solar radiation is strong. Therefore, it is necessary to consider the above when using thermography method.

Introduction

With the development of road systems, there has been a rapid increase in impermeability in urban areas with (Habete and Ferreira, 2016), and this is becoming a major cause of the urban heat island phenomenon (Mathew et al., 2016). This heat island phenomenon has an impact on the energy consumption, indoor comfort, vulnerability, and heat-related mortality and morbidity (Santamouris et al., 2020). Pavements are being developed that use increased porous/water-retention or by applying coating, PCM, etc. (Santamouris, 2013; Qin, 2015; Chen et al., 2018; Anupam et al., 2020). In addition, studies are conducted on the method of testing the developed pavement in controlled environments (Shin et al., 2019; Parison et al., 2020a; Parison et al., 2020b).

In addition to the development of pavement as a product, there is ongoing research on pavement-watering as a means of lowering ground temperature by actively spraying water on the pavement, mainly in Japan and France. (Hendel et al., 2014; Hendel and Royon, 2015; Hendel et al., 2015b; Hendel et al., 2015c; Hendel et al., 2016; Hendel, 2016; Kinouchi and Kanda, 1997; Nakayama and Fujita, 2010; Takahashi et al., 2010; Yamagata et al., 2008) Pavement watering is a system that is installed as part of the elements that make up the road, such as a median divider or cat's eye, and sprays water onto the road. By reducing the radiant heat emitted by lowering the temperature of the road surface through spraying water, it is possible to lower the temperature of the city center and to reduce fine dust. In addition, when the temperature of the road surface is lowered, the near-surface temperature, which is directly related to humans, eventually decreases (Qin, 2015; Chui et al., 2018). Unit products and large-scale experiments for this have also been proposed. (Japan Society for Testing Materials, 2015; Ko et al., 2018; Wang et al., 2019; Shin et al., 2019).

In Korea, pavement watering is referred to as Clean Road or Cooling & Clean Road (Kim et al., 2014; Kim et al., 2015a; Lee et al., 2019), and this term is more focused on the effect rather than on its method. Representative cases include Seoul, Chuncheon, and Daegu; and this system is gradually being expanded. In Seoul, the Clean Road project was promoted beginning in 2006 and was implemented starting on May 2007. Clean Road is operated according to the seasonal and weather conditions, such as yellow dust, fine dust, and excessive heat warnings. In Chuncheon, Clean Road is installed along 8.8 km of the main road in the downtown area. Like in Seoul, it is used in accordance with conditions such as yellow dust, fine dust, and excessive heat warnings. Additionally, it is useful during all 4 seasons and used in combination with the snow removal system during the winter season to enhance the environmental improvement effect (Lee et al., 2019). Daegu is the most representative case, in which the effectiveness was increased by using underground water discharged from the subway station. Along an approximately 9.1 km section, the underground water discharged from 10 subway stations along Daegu Subway Line No. 2 was sprayed on the road using the installed watering nozzles to deal with the heat island phenomenon. Before the Clean Road was implemented, a sprinkler truck was used, but the operation of the sprinkler truck caused traffic congestion and pedestrian damage, and hence the installation of a Clean Road was promoted. Existing studies on the Daegu case showed that the surface temperature of the road rapidly decreased up to about 15 °C after spraying water on the road from the nozzle (Kim et al., 2015a); and it was reported that about 5–15% of fine dust on the road was reduced compared to other areas where Clean Road was not installed (Kim et al., 2014).

Most of the previous studies were on verifying the trend through long-term monitoring with sensors such as the thermocouple and snapshots through infrared cameras. In addition, because it is long-term monitoring, the analysis was performed by utilizing Timesteps per hour, or 10 min at minimum. There was a study that performed the measurement in 10 s intervals, but this was for environmental analysis on the asphalt before the implementation of Clean Road (Kim et al., 2015b). Pavement has large thermal mass, so long-term measurement is considered reasonable, but if you measure the value when the temperature drops momentarily due to watering, the effect can be exaggerated and it cannot be assessed properly when the temperature is measured at the point of recovery. In other words, an appropriate timestep must be chosen to ensure measurements capture thermal equilibrium of the pavement and water film. Also, infrared cameras are widely used to evaluate the heat island reduction effect. But when we can identify the period and scale of difference between the measurement value and sensor measurement value (Takebayashi and Moriyama, 2007; Kimijima et al., 2009; Takebayashi and Masakazu, 2009; Chui et al., 2018), and also on the reason for such differences, each measuring instrument can be used appropriately to judge the validity of the value. Moreover, it is necessary to measure in relatively short timesteps to determine the impact of the road surface temperature treated with watering to the surrounding air with low thermal capacity compared to the pavement.

In this study, sensors are used to analyze the road surface temperature before and after watering with a relatively short timestep, and an analysis is performed on the difference compared to the timestep applied/used in existing studies. In addition, the surface temperature measured by an infrared camera is compared to the temperature measured by the sensor, and the issues requiring consideration when using the infrared camera were identified. Lastly, the aspect of temperature changes on the pavement surface and the air is analyzed with a short timestep to identify the impact of surface temperature on the surrounding air.