KXGM 6102 Applied Heat Transfer

Critical review of thermal and energy performance of tropical

green roof Lecturer: Assc.Prof. Ir. Dr. YauYHProject Member: 1.WongKH (Leader)2.Mohd.Hafiz3.Mohd.Shazwan4.Krishan5.SimSY

Content 1 Introduction2 Thermal Conductivity3 Mathematical modelling of an green

roof4 Thermal performance5 Energy efficiency and saving 

Green roof

Advantages Disadvantages

Additional insulation to the roof structure

Increase building cost & maintenance cost

Reduce the building temperature

Provide habitat and biodiversity  

Reduce building noise  

Minimize the energy consumption  

Reduce the long wave radiation emitted by the building surface

Green roof structure and thermal conductivity

Structure & Layer

Green roof & other Bare roof

Green roof

White roof

Green roof performance & other

Roof type Conductance Thermal

Capacity Solar

Absorptance

  (W/m2 oC) kJ/oC  Common flat roof (BFR) 0.59 479 0.8

White roof (BWR) 0.59 479 0.4

Green roof(BGR) 0.42 519 0.37

Table 2 : Thermal and optical properties of the roof materials under dry conditions

Net RadiationI = Ki + Kr + Li + Lr

Earth’s Solar Energy Balance

Ki = incident shortwave radiationKr= reflected shortwave radiationLi = incident longwave radiationLr= reflected longwave radiation

I net

Ki &Li

Mathematical modeling of green roofs

Total Solar Radiation

Re-emitted Energy

Convection

Evapotranspiration

Reflected Radiation

ShadingThermal Mass (Varies with Material Properties)

Ki+Kr+Li+Lr = QE+QH+QS

QE, latent heat loss due to soil surface and vegetation evapotranspiration.Influencing factors are soil water content, LAI and relative humidity,

QH, sensible heat loss due energy difference btw the soil surface and atmosphere.(excess heat loss by convection)

QH= hc (Ts –Ta)hc = 12Wm-2 K-1

QS,heat absorbed in building materals, likely to be reliesed at night (contribute to urban island effect).

QS= ρIρ ratio of heat storage to the net radiation

Thermal performanceComparison of green roof to bare roof (Heat

Flux)

Comparison of green roof to bare roof (Temperature)

Insulated and un-insulated green roof

Energy efficiency and saving

Green Roof

Green Roof

Conventional Roof

Conventional Roof

ConclusionGreen roof shows a significant effect on the

building’s temperature It show a small temperature fluctuations of indoor

and outdoor temperature differentIt plays role as insulation and cool building by

direct shading , evapo-transpiration and photosynthesis of plant.

Higher leaf area index provide better reduction of temperature fluctuation.

High humidity of the soil will lower surface temperature of the green roof

In tropical region, solar radiation and building heat gain are high ,therefore green roof is highly recommended

Thank you

Q & A