Development of a Retro-Reflective Material as Building Coating

J. Yuan, C. Farnham, K. Emura

Osaka Metropolitan University, Japan

Energy and Buildings 103 (2015) 107–117

Introduction: Urban Heat Island Challenge

Buildings account for ~50% of urban heat emissions, exacerbating the urban heat island effect.

Key benefits of retro-reflective materials (RRM):

Energy and Buildings 103 (2015) 107–117

RRM vs Traditional Reflective Materials

Diffuse Reflection
Diffuse reflection scatters light in all directions

Issues:

  • Heat transfer to adjacent buildings
  • Reduced urban canyon albedo
  • Limited heat island mitigation
Retro-Reflection
Retro-reflection directs light back to source

Advantages:

  • Minimizes heat transfer to surroundings
  • Increases urban canyon albedo by 15-20%
  • More effective heat island mitigation
Energy and Buildings 103 (2015) 107–117

Innovative RRM Design

Developed novel RRM with enhanced performance:

  • High-performance glass covering:
    • Visible transmittance: >91.3%
    • UV transmittance: >92.5%
    • Reflectivity: 2%
  • Plastic micro-prisms with cubic corners
  • White appearance with high reflective luminance
RRM Structure
Multi-layer RRM structure with glass covering
Energy and Buildings 103 (2015) 107–117

Durability Testing Results

485-day outdoor exposure test (Jan 2014 - Apr 2015):

  • No surface degradation observed under 100× magnification
  • Solar reflectance (ρtmp): 0.81 → 0.79 (2% decrease)
  • Retro-reflectance (ρret): 0.44 → 0.42
  • Full reflectance recovery after cleaning
Microscope Image
RRM surface after 485 days exposure (100×)

Outperformed conventional materials by 50-300% in durability tests.

Energy and Buildings 103 (2015) 107–117

Urban Canyon Albedo Simulation

2D model comparing reflective materials in east-west urban canyon:

Case Wall Configuration Albedo Performance Heat Absorption
A Both walls: diffuse 3rd (Baseline) High
B North: diffuse
South: retro		 2nd (+12%) Medium
C Both walls: retro Best (+18%) Low
D North: diffuse
South: mirror		 Worst (-5%) Highest

RRM configuration showed maximum albedo improvement across all seasons.

Energy and Buildings 103 (2015) 107–117

Building Thermal Performance (Shanghai)

Annual energy impact of RRM coating:

Cooling Benefits

  • Summer cooling load reduction: 13-15%
  • Peak cooling demand reduction: 11-27%
  • Annual cooling load decrease: 157 MJ/m² (19%)

Heating Impact

  • Winter heating load increase: 5-7%
  • Annual heating load increase: 71 MJ/m² (6%)

Net annual results:

Energy and Buildings 103 (2015) 107–117

Key Findings & Conclusions

Developed highly durable RRM with glass covering maintains reflectance (ρtmp~0.81, ρret~0.44) for 485+ days outdoor exposure
RRM configuration increases urban canyon albedo by up to 18% compared to conventional materials
Significant cooling load reduction (19% annually) outweighs modest heating load increase (6%)
Net 4.3% reduction in annual thermal loads with 3.3 kg/m²-year CO2 reduction
Proven effective solution for urban heat island mitigation and building energy efficiency
Energy and Buildings 103 (2015) 107–117

Future Research Directions

Future Research
Potential applications and research directions for RRM technology
Energy and Buildings 103 (2015) 107–117