Theoretical evaluation of thermal and energy performance of tropical green roofs

The thermal and energy efficiency of tropical green roofs is assessed by a theoretical model to clarify the contribution of underlying factors. The suitability of 1400 high-rise public housing blocks in Hong Kong for rooftop greening was assessed by remote sensing images. Weather and microclimatic-soil monitoring data of an experimental green roof provided the basis for computations. Roof greening prevented a huge amount of solar energy at 43.9 TJ in one summer from penetrating the buildings to bring significant energy saving. Thermal performance of humid-tropical green roofs, with greater latent heat dissipation, is twice more effective than the temperate ones. The energy balance model shows that solar energy absorption by bare and green roofs depends on shortwave rather than longwave radiation. Heat flux into a building indicates a one-day time lag after a sunshine day. With restricted evapotranspiration, bare roofs have more sensible heat and heat storage than green roofs. The bare roof albedo of 0.15, comparing with 0.30 of green roof, renders 75% higher heat storage. Small increase in convection coefficient from 12 to 16 could amplify 24% and 45% of latent heat dissipation respectively for bare and green roofs. Doubling the soil water availability could halve the heat storage of green roofs.