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

Table 3

Summary of several current studies of RR materials applied for building envelopes.

Reference	Method to derive retroreflectance	Energy saving, urban albedo, UHI mitigation, and so forth
[16]	Model to calculate the hourly reflectance of directional reflective materials (DRMs) as a function of zenith and azimuth angles.	In summer, the selected metric estimates the mean energy saving for building energy models accurately and the peak heat absorption of surface by 8% (<40 W/m^2) error. In winter, the potential energy savings of buildings are overestimated by an error of less than 20%, and mean hourly and peak heat absorptions are estimated by the error of 22 kJ/m^2 and 9 W/m^2, respectively.
[18]	An ad hoc experimental setup was designed and used for assessing the angular reflectance of RR films.	The potential of RR materials for mitigating UHI was estimated in terms of cooling potential, defined as a decrease of circulating energy contributing to the canyon overheating, through a 2D algorithm. The decrease of the energy kept inside the canyon due to the use of RR materials has been evaluated for three values of latitude (30°, 40°, and 50°) with a south-exposed façade. In addition, the effect of RR materials on the energy circulating inside the canyon has been evaluated also for different façade exposures (south, east, and west).
[22]	Using two sorts of a 3-mirror type and a 4-mirror type of corner reflectors and geometrical optical analysis method to evaluate the reflection performance to the direct-beam solar radiation and RR properties of RR materials.	The RR effective area of 3-mirror or 4-mirror type of corner reflectors was defined, and the relationship between the incident angle and the RR effective area was also examined. However, there was no specific discussion on the energy saving of buildings and UHI mitigation.
[19, 20, 48]	A method of using spectrophotometer measurement and thermal measurement in the outdoor environment.	The urban albedo was carried out by using a simple 2D shape and Monte Carlo theory, with different reflective characteristics of building envelope materials. The results showed that the RR envelope of buildings is more effective to increase the albedo of urban canyons than the other diffusive or mirror reflective envelopes. Also the impact of RR envelope on thermal loads of buildings was examined by simulation building and New HASP/ACLD-β. The results indicated that RR building envelopes can decrease the annual cooling loads as well as the annual total thermal loads and increase the annual heating loads.
[23]	Using simulations and EnergyPlus to evaluate the impact of RR envelope on surface temperature and energy saving.	The average temperature reductions of the control building surface during daytime at typical seasonal days in Miami and Minneapolis cities were examined between RR testing and regular diffusive façade by simulation numerical analysis. A cross-regional energy analysis of eight cities in U.S. was also examined by simulation numerical analysis.