Urban heat island and Cool Roofs

COOLING BUILDINGS, CITIES AND THE PLANET WITH COOL ROOFS

COOL ROOFS

  • High solar reflectance and high thermal emittance (emissivity)
  • Reflects most incident solar energy and radiate absorbed heat to the sky, especially at night
  • This issue is of high importance in warm and temperate zones
  • Solar reflection and thermal emittance (emissivity) of materials used in buildings are important parameters for the energy efficiency in buildings

Source: https://heatisland.lbl.gov/coolscience/cool-roofs

URBAN HEAT ISLAND

  • Is a metropolitan area which is warmer than its surrounding rural areas
  • The main cause is the modification of the land surface by urban development using materials which retain heat, with the construction of buildings and pavements
  • Especially in warm and temperate climates, heat islands contribute to human discomfort (interior and exterior), higher energy bills, peak summer loads, more electricity infrastructure, summer blackouts, more global warming, an increase of air pollution, and health problems
  • Recurring heat waves increase these problems

Urban Heat Island profile Image from Lawrence Berkeley Lab

URBAN HEAT ISLAND AND COOL ROOFS

  • Positive correlation between white reflective roofs and cooler temperatures is shown
  • Mitigation of the urban heat island effect can be done using cool roofs which reflect more sunlight and absorb less heat

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Source: (City of Sydney, 2009)

ENERGY FLOWS. COOLING THE PLANET WITH COOL ROOFS

  • Short-wave and long-wave radiant flows to and from the earth’s surface and the atmosphere are shown
  • Cool roofs are able to cool the region and the planet increasing albedo (solar reflection), like the north and south poles which are covered by ice, inducing a negative radiative forcing equivalent to offsetting emitted CO2
  • Increasing global solar reflection of roofs and pavements by 0.25 and 0.15 respectively, will induce a negative radiative forcing on the earth equivalent to offsetting about 44 Gt of CO2 emissions, counteracting the effect of the growth in CO2 equivalent emission rates for 11 years (Akbari, H., Menon, S. & Rosenfeld, A. 2009, ‘Global cooling: increasing world-wide urban albedos to offset CO2.’, Climatic Change 94 (3): 275– 286)
  • We can cool faster with materials radiating mainly in the atmospheric window wavelength (8-14µm)

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Source: Kiehl, J.T. & Trenberth, K.E. 1997, ‘Earth’s annual global mean energy budget, Bull. Am. Meteorol. Soc. 78.’, American Meteorological Society. Pages 197–208

ENERGY SIMULATIONS WITH ENERGYPLUS

  • EnergyPlus is the most accurate building energy simulation software, managed by the Department of Energy in the United States
  • Performed hundreds of energy simulations with cool roofs using EnergyPlus to calculate heating and cooling loads to maintain the home and the office building between 20oC – 25oC during their occupation. Analyzed data to show cost-efficient solutions and the cost effectiveness of cool roofs
  • Reducing solar absorptance savings in the cooling system are greater than the penalty in the heating system (in Barcelona) using cool roofs
  • Roof solar absorptance (0.2, 0.4, 0.6, 0.8). 0.2 would be a white cool roof and 0.8 a standard brown roof
  • Roof thermal transmittance (U) (without insulation: 1.86 for home and 1.74 for office, and 0.67, 0.4, 0.33 W/m2·K). Technical building code (CTE) requirement in Barcelona is 0.4, more strict CTE requirement is 0.33
  • Scientific studies and experiments showed that the energy savings provided by cool roofs are greater than the results in energy simulations because the software does not consider the lowest temperatures around the building, the energy savings due to ventilation and air infiltration with lower temperatures, and that the cooling systems work at higher performance. In addition, the climate files are not usually updated
  • In the United States, colder cities also regulate with cool roofs to mitigate the urban heat island and avoid peak consumption in summer
  • Simulations performed for a home and office building with a low level of air tightness, and considering an optimal level of air quality in offices, for homes and offices with a better level of air tightness and offices with heat recovery the benefits of using cool roofs are greater

Single family home

  • Cool roof analysis in a 100 m2 single family home located in Barcelona:

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Solar absorbance and thermal transmittance (U) of the roof

Heating, cooling and total loads as a function of roof solar absorptance for four values of roof thermal transmittance (U). 0.2 would be a white cool roof and 0.8 a standard brown roof

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With solar absorptance 0.2, cooling loads increases when insulation on the roof is increased

“Heating (kW·h/m2·year)” penalties (positive number), “Cooling (kW·h/m2·year)” savings (negative number) and “Total (kW·h/m2·year)” (heating penalties minus cooling savings) difference and “Cooling/Heating” (cooling savings divided by heating penalties) reducing solar absorptance by 0.1 (e.g. from 0.8 to 0.7). A cool roof should be used if “Total (kW·h/m2·year)” is a negative number and “Cooling/Heating” is greater than 0

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  • Reducing solar absorptance savings in the cooling system are greater than the penalty in the heating system
  • Greater savings are acheived by reducing solar absorptance on roofs with less insulation, however the proportion between cooling savings and heating penalties is greater in roofs with more insulation
Solar absorptance, roof thermal transmittance (U) and home air tightness

“Heating (kW·h/m2·year)” penalties (positive number), “Cooling (kW·h/m2·year)” savings (negative number) and “Total (kW·h/m2·year)” (heating penalties minus cooling savings) difference and “Cooling/Heating” (cooling savings divided by heating penalties) reducing solar absorptance by 0.1 (e.g. from 0.8 to 0.7) for four levels of air tightness

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  • Reducing solar absorptance, greater savings and better ratio between cooling savings and heating penalties are achieved in homes with better level of air tightness. Heating loads are higher in a home with a low level of air tightness

Office building

  • Cool roof analysis in an office building with 2 floors (800 m2 per floor) located in Barcelona:

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Solar absorbance and roof thermal transmittance (U)

Heating, cooling and total loads as a function of roof solar absorptance for four values of roof thermal transmittance (U). 0.2 would be a white cool roof and 0.8 a standard brown roof

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With solar absorptance 0.2, total loads (heating plus cooling) does not decrease when insulation on the roof is increased

“Heating (kW·h/m2·year)” penalties (positive number), “Cooling (kW·h/m2·year)” savings (negative number) and “Total (kW·h/m2·year)” (heating penalties minus cooling savings) difference and “Cooling/Heating” (cooling savings divided by heating penalties) reducing solar absorptance by 0.1 (e.g. from 0.8 to 0.7). A cool roof should be used if “Total (kW·h/m2·year)” is a negative number and “Cooling/Heating” is greater than 0

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  • Reducing solar absorptance savings in the cooling system are greater than the penalty in the heating system

EXPERIMENTS IN SYDNEY WITH COOL ROOFS

  • Measurements were gathered in the two outdoor small structures to compare the differences on the roof temperatures and room air temperatures
  • Built with 9 mm wood, 60 mm rigid polystyrene insulation (R-value 1.4 m2·K/W) in floor, roof and walls
  • The terracotta coloured roof has solar reflectance 0.24 and thermal emittance 0.93
  • The cool roof has solar reflectance 0.72 and thermal emittance 0.9

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Roof surface temperature and room air temperatures were 25 oC and 5 oC lower respectively under the cool roof in the daytime

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ROOF ENERGY FLOWS

  • Energy flows into and through an insulated roof:

Source: Gentle, A.R., Aguilar, J.L.C. & Smith, G.B. 2011, ‘Optimized cool roofs: Integrating albedo and thermal emittance with R-value.’, Elsevier. Solar Energy Materials & Solar Cells 95 (2011) 3207–3215

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