1. Characterisation and assessment of solar reflective
pavements for urban applications
Caracterización y evaluación de pavimentos de alta
reflectancia solar para aplicaciones urbanas
Michele Zinzi1, Emiliano Carnielo2, Ambra Angelini3, Gaetano Fasano1
1ENEA UTEE-ERT , 2Università of Roma Tre, 3University of Venezia
EESAP, San Sebastian, 1-3 July 2013
2. Contents of the presentation
1. Definitions
2. Introduction
3. Experimental charactersation
4. Numerical analisys
5. Conclusions and next works
3. Definitions
Solar reflective materials, also known as cool materials, are
characterised by:
• High solar reflectance, ability of the material to reject the incident solar radiation
• High thermal emissivity, ability of the material to radiate away the stored heat
Conventional construction materials are generally characterised by
• High emissivity
• Low solar reflectance (0.05 for new asphalts up to 0.25 for concrete or most tiles)
• Surface temperatures can be higher up to 30°C with respect to the air temperature
Cool materials minimise solar heat gain keeping the construction
surfaces cooler under the sun, reducing: the cooling load in buildings
and the heat released to the outdoor environment by convection.
4. Cool materials (and strategies) include:
• White products
• Light coloured products
• Cool coloured products
Solar reflectance of white coatings
0
10
20
30
40
50
60
70
80
90
100
300 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500
wavelength
A
B
C
D
Definitions
5. Introduction
• Climate changes global air
temperature increase
• More frequent and intense heat waves
• The 80% of the EU population will live
in urban area
• Additional hazard Urban heat island
(UHI) Increase of the air
temperature in urban area respect to
the surrounding country
6. Introduction
• Climate changes global air
temperature increase
• More frequent and intense heat waves
• It is expected the 80% of the EU
population will live in urban area by
2030
• Additional hazard Urban heat island
(UHI) Increase of the air
temperature in urban area respect
to the surrounding country
7. A number of problems arise:
• Energy end uses for cooling
– Buildings which accounts for approximately 40% of total final energy
consumption and 36% of CO2 emissions in Europe
– Dramatic increase of cooling demand in Italy in the last ten years
• The electricity bill during the peak hours
• The security of energy supply (planned black out!)
• Indoor and outdoor thermal comfort
• Health and safety for the weaker classes
– Aged people
– Children
– Low income people, forced to live in un-pleasant and un-safe houses
Introduction
Cool materials can be used for outdoor and building applications
8. Cool materials for urban environment can be:
• Coatings applied on an existing substrate
• Pavements constructed using a binder more
reflective than tar (high absorbing)
• The material tested in this study is a high
porous concrete used for roads and
pavements.
• It is characterised by a high water draining
power.
• The concrete binder is treated to obtain
different shades of grey.
• Three colours and two different sizes of the
grains were tested:
1. Off white - Grain Size 6 mm
2. Off white - Grain Size 12 mm
3. Light Grey - Grain Size 12 mm
4. Dark Grey - Grain Size 12 mm
Tested materials
9. • The optical characterization with the spectrophotometer Perkin Elmer
Lambda 950, equipped with a 15 centimetres diameter integrating sphere.
• The integrating sphere accessory is necessary when testing diffusing
materials.
• 5 mesurements were carried out on each samples because of the high
rugosity of the samples.
• Measurements were carried out between 300 and 2500 nanometres, with a
spectral resolution of 5 nm.
Reflectance values [%]
Experimental characterisation
ρe ρNIR ρv ρe - ρe new asphalt ρe - ρe aged asphalt
White - Grain Size 6 mm 45 46 46 41 29
White - Grain Size 12 mm 56 56 57 51 39
Light - Grey Grain Size 12 mm 30 31 31 26 14
Dark - Grey Grain Size 12 mm 22 23 23 18 6
New Asphalt 5 7 3 --- ---
Aged Asphalt 17 20 14 --- ---
11. Numerical analysis
• ENVI-met used to evaluate the impact of
pavements on outdoor temperature.
• The software uses the method S.V.A.T. (Soil,
Vegetation and Atmosphere Transfer)
• It operates at micro-scale to simulate the time
evolution of thermo-fluid dynamic parameters.
• Portions of an urban area with buildings, roads
and vegetation, including thermal and optical
properties of elements.
• The test was carried out in Prati, a densely
urbanised district in the centre of Rome.
12. Numerical analysis
• Simulations were performed in spring and in summer covering the central hours of
the day, from 10 to 14.
• Three different values of solar reflectance for pavements: 10% for a conventional
asphalt, 32% and 56% for grey and off-white concrete pavements.
• In spring the air temperature decreases by an average of 3.3(1.5)°C passing from
an urban texture made up of conventional asphalts to off-white (grey) concrete.
• In summer the air temperature decreases by an average of 4.4(2)°C passing from
an urban texture made up of conventional asphalts to off-white (grey) concrete.
13. Numerical analysis
• Vertical distribution of the air
temperatures in the two receptors
• Red lines receptor in the “hot spot”,
green lines in the square with greens
• The three curves refer to the three
different pavements solutions
• To be noted the cooling effect of the
trees (0.5°C less) at the building
height.
14. Numerical analysis
• Comfort analysis based on the Thom Index
• Function of dry and wet bulb temperatures
• DI [°C] = 0.4 · ( Ta + Tw ) + 4.8
• Calculation in the “hot spot” R2
15. Conclusions
• The tested products have higher solar reflectance than conventional
asphalts
• Significant air temperature reduction at urban level can be achieved (UHI
mitigation)
• Improvement of thermal comfort conditions, especially in extreme climatic
conditions, are calculated
Improvements are needed for the spectral selectivity – high NIR reflectance
Calculation with more reliable thermo-fluid dynamics codes and case studies
More accurate analysis with other reliable indexes for outdoor thermal comfort
Thanks for the attention!
Michele Zinzi
Tel. +390630486256
email: michele.zinzi@enea.it
skype: michele.zinzi
