Comfort and energy savings with active green roofs

Published Date
Energy and Buildings
October 2014, Vol.82:492–504, doi:10.1016/j.enbuild.2014.07.055

Author 

• Pablo La Roche ^a
• Umberto Berardi ^b,,

• aCalifornia State Polytechnic University Pomona, Pomona, CA, USA
• bRyerson University, Toronto, ON, Canada

Received 30 April 2014. Revised 14 June 2014. Accepted 24 July 2014. Available online 2 August 2014. 

Highlights

• •
  Cooling and heating potential of green roofs strongly depends on the climate and building characteristics.
• •
  This paper presents and investigates a green roof adopting a variable insulation strategy thanks to a sensor-operated fan.
• •
  Test cell results have been monitored in a hot and dry climate with mild winters over several years.
• •
  Measurements are also compared with simulations for different weather conditions.
• •
  Results show particularly promising for energy saving and comfort improvements.
  
  
  Abstract
  Green roofs have been proposed for energy saving purposes in many countries with different climatic conditions. However, their cooling and heating potential strongly depends on the climate and building characteristics. In particular, the increase of the thermal capacity of green roofs compared to traditional roofs, if not controlled with insulation, may lead to higher cooling and heating loads. This paper discusses the energy saving potential of green roofs adopting a variable insulation strategy. A system consisting of a plenum located between a green roof and the room underneath and a sensor-operated fan that couples (or decouples) the green roof mass with the indoor environment was developed. The fan is activated and stopped using temperature based rules; the plenum is ventilated only when the fan works, creating a variable insulation system. Four cells with an insulated traditional roof, a non-insulated green roof, an insulated green roof, and a green roof with the variable insulation system have been tested in a hot and dry climate with mild winters over several years. This paper compares and discusses different plenum control algorithms. Results are particularly promising because the variable insulating system proved to adjust the thermal capacity of the roof effectively. In summer, the non-insulated green roof and the green roof with variable insulation system achieved the lowest indoor temperature; in winter, the insulated traditional roof and the variable insulation green roof system achieved the highest indoor temperatures. Measurements are hence compared with simulations. Finally, the energy saving potential of the new green roof system is evaluated.

Keywords

• Energy saving
• Green roof
• Smart control
• Cooling systems
• Sustainable design, Passive cooling

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Legend

C_e,g

latent heat flux bulk transfer coefficient at ground layer

C_f

bulk heat transfer coefficient

C_hg

sensible heat flux bulk transfer coefficient at ground layer

c_p,air

specific heat of air at constant pressure

F_f

net heat flux to foliage layer (W/m²)

F_g

net heat flux to ground surface (W/m²)

H_f

foliage sensible heat flux (W/m²)

H_g

ground sensible heat flux (W/m²)

I_s^↓

total incoming short-wave radiation (W/m²)

I_ir^↓

total incoming long-wave radiation (W/m²)

l_f

latent heat of vaporization at foliage temperature (J/kg)

l_g

latent heat of vaporization at ground temperature (J/kg)

K

soil thermal conductivity, water dependent

L_f

foliage latent heat flux (W/m²)

L_g

ground latent heat flux (W/m²)

LAI

leaf area index (m²/m²)

q_af

mixing ratio for air within foliage canopy

q_f,sat

saturation mixing ratio at foliage temperature

q_g,sat

saturation mixing ratio at ground temperature

Q_cond

conduction heat

Q_irr

radiation heat

Q_conv

convection heat

Q_evap

evapotranspiration heat

r″

surface wetness factor

T_af

air temperature with in the canopy

T_f

foliage temperature

T_g

ground surface temperature

T_∞

ambient temperature

V_∞

air velocity

w_af

wind speed within the canopy

α_f

albedo (short-wave reflectivity) of the canopy

α_g

albedo (short-wave reflectivity) of ground surface

ɛ_f

emissivity of canopy

ɛ_g

emissivity of the ground surface

ɛ₁

ɛ_g + ɛ_f − ɛ_g × ɛ_f

ρ_af

density of air at foliage temperature

ρ_ag

density of air at ground surface temperature

σ

Stefan–Boltzmann constant

σ_f

fractional vegetation coverage

 Table 1

Table 1.

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Published by Elsevier B.V.

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