Thermal performance of climber greenwalls: Effects of solar irradiance and orientation

Thermal performance of greenwalls, a critical and common concern, is regulated by solar irradiance vis-a-vis orientation and shading. A field experiment was conducted in humid-tropical Hong Kong to address the research question under typical summer-weather scenarios: sunny, cloudy and rainy. On a large circular concrete tank, climber-greenwall experimental plots were established with duplication in four cardinal compass directions. Air and infrared-radiometer surface temperature sensors monitored at different greenwall positions: ambient-air (control), bare-concrete-surface (control), vegetation-surface, behind-mesh-airgap, and behind-mesh-concrete surface. Pyranometers were installed vertically at four orientations and horizontally at tank-top (control) to monitor solar-energy input. Habitat verticality induces notable variations in solar-energy capture at four orientations by daily total, peak level, intensity, duration and timing. On sunny day, solar fraction reaching east side was only 37.1% of tank-top. Early morning sunshine striking east side nearly perpendicularly brings maximum intensity. South side facing the sun but at tangential incident angle has only 23.3% reception. Strong irradiance drives high control-surface temperature, but also induces notable vegetation-surface and adjacent ambient-air cooling by transpiration. A threshold solar intensity of about 300 Wm(-2) is necessary to impart notable cooling-effect. Summer-sunny day and rainy-day sunshine-burst episodes could satisfy this condition; cloudy day and rainfall periods with attenuated-diffused sunlight could not. Cloudy day and rainfall periods suppress cooling differences by orientation. Behind-mesh concrete surface is consistently cooler than control concrete surface in the three summer-weather scenarios. Behind-mesh-air remains warmer than ambient-air but cooler than two adjoining surfaces (vegetation and behind-mesh-concrete), indicating air-barrier effect and restricted air exchange between ambience and airgap. It implies that greenwall can bring bidirectional cooling, but transpiration cooling of anterior (ambient) air is more effective than shading and thermal-insulation cooling of posterior (airgap) air and concrete-surface. The findings could inform greenwall design to enhance ecosystem services for climate-change adaption and urban heat island amelioration. (C) 2015 Elsevier Ltd. All rights reserved.