Jihui Yuan

A method to measure retro-reflectance and durability of retro-reflective materials for building outer walls

A method of determining the retro-reflectance of retro-reflective materials used for building coatings is proposed in this article. In addition, the durability of retro-reflective materials over long-term outdoor exposure is also estimated. Retro-reflective materials are currently limited to use in specific purposes, such as road traffic signs in Japan. To consider their application as a building coating, it is also necessary to examine the thermal performance and durability of retro-reflective materials. We proposed a method of determining the retro-reflectance of retro-reflective materials (capsule retro-reflective material and prism retro-reflective material were used in this experiment) by experiment. To explore the durability of retro-reflective materials over long-term outdoor exposure, we measured the changes in solar reflectance and retro-reflectance of retro-reflective materials exposed to the outdoors over about 25 months. The solar reflectance of capsule retro-reflective material decreased from 0.69 to 0.51 and that of prism retro-reflective material decreased from 0.83 to 0.81. The retro-reflectance of capsule retro-reflective material decreased from 0.18 to near 0 (0.0072) and that of prism retro-reflective material decreased from 0.44 to 0.42. At the end of the test period, we cleaned the surface of the retro-reflective materials. Both of the retro-reflective materials recovered about 50% of the lost solar reflectance by cleaning. The retro-reflectance of capsule retro-reflective material increased by about 0.08 (46% recovered) by cleaning and that of prism retro-reflective material increased by only 0.01 (with 50% recovered) by cleaning. We concluded that the durability of prism retro-reflective material is better than that of capsule retro-reflective material for use on building coatings.

Evaluation of the solar reflectance of highly reflective roofing sheets installed on building roofs

There is a strong need to prevent the heat island effect. One of the important countermeasures to the heat island effect is to reduce thermal storage in buildings. The highly reflective material on outer walls and roofs is a significant way to prevent heat from penetrating indoors; such materials have been installed on the surface of buildings in Osaka, Japan. To evaluate the solar reflectance of such highly reflective material, we measured the solar reflectance of reflective roofing sheets installed on the building roofs in Osaka, Japan, over a span of time from February 2010 until September 2011, and examined the change in solar reflectance during this time.

Measurement of the evaporative cooling effect: oscillating misting fan

ABSTRACT To determine the thermal effects of an oscillating mist fan (spraying 86 L/h of droplets with 25 µm mean diameter) on worker comfort, its effects on the thermal environment were measured in a large indoor space (37,500 m2/525,000 m3). It was found that the temperature dropped by 0.2–2.5 K, with local humidity increasing by 5%. Ventilation air-exchange calculations indicate that in hot summer conditions, an 8-h shift could be continuously cooled without creating high humidity, recovering to initial values after 16 h of ventilation at 0.3 ach. The cooling effect of the mist and fan was measured and compared with that of the fan. The mist and fan-cooling effect exceeded 100 W/m2 in all cases, while the mist and fan cooling exceeded the fan alone by 18 W/m2 on average, 24 W/m2 at the peak values and 11 W/m2 average during oscillation. The ASHRAE 55-2013 model is modified to include this additional cooling. Standard effective temperatures (SETs) are calculated with and without the mist fan-cooling effect. Linear approximations for reduction in SET were developed as a function of air temperature and mist-cooling effect. The deployment of this technology would improve thermal comfort for factory workers on hot summer days.

Potential for Application of Retroreflective Materials instead of Highly Reflective Materials for Urban Heat Island Mitigation

Research on urban heat island (UHI) mitigation has been carried out globally. Several strategies have been proposed or developed to mitigate UHI, including highly reflective (HR) envelopes of buildings, green roofs, urban vegetation, shading, heat sinks, and air-conditioning efficiency. Among these techniques, HR envelopes have been extensively studied as an effective method to mitigate the UHI effect by reducing energy consumption. However, because most of HR materials are diffusive, HR envelopes applied to vertical surfaces can reflect both onto roads and nearby buildings. Additionally, HR roofs cannot reflect all incoming solar radiation to the sky if there are high buildings around it. Thus, HR materials applied as building envelopes have a limited effect against the solar contribution to the UHI. In order to solve this problem, retroreflective (RR) materials, which reflect the solar radiation back towards the source, have been studied and developed to be applied as building envelopes instead of HR materials. This paper summarizes several previous researches on HR envelopes and cool roofs and summarizes several current researches on RR materials. The potential for application of RR envelopes in cities is proposed with consideration of economic and environmental factors.