The Solar Reflectance Index as a Tool to Forecast the Heat Released to the Urban Environment: Potentiality and Assessment Issues
Abstract
:1. Introduction
2. The Solar Reflectance Index (SRI) and Its Calculation
2.1. Surface Radiative Properties and SRI
2.2. Potentiality and Limitations of SRI
3. Sensitivity of SRI Calculation to Input Parameters, and Measurement Issues
3.1. External Convection Coefficient
3.2. Assessment of Solar Reflectance and Ageing Issues
3.3. Assessment of Thermal Emittance
3.4. Combined Effects of Measurement Issues
4. Conclusive Remarks
- The SRI has raised significant interest in the construction sector thanks to its relative ease of calculation and, above all, its effective representation of the thermal behavior of a built surface subjected to solar radiation.
- A linear correlation exists between SRI and both the surface temperature and the fraction of incident solar heat that is transferred to the near ground air. Since a given SRI value can be the result of different pairs of solar reflectance and thermal emittance values, SRI seems a more effective indicator than solar reflectance alone for comparative evaluation of the capability of built surfaces to limit urban warming.
- SRI calculation is based on the hypothesis of an adiabatic irradiated surface and does not consider the insulation or the inertia of the materials below; nonetheless it can work well as an indicator of the heat transmitted to the external near ground air, and therefore of the contribution to urban warming, since the heat flow rate conducted through a roof or into the ground can be lower by one or two orders of magnitude than solar irradiance.
- Values of the convection heat transfer coefficient specified for SRI calculation are arbitrary; nonetheless they are in good agreement with the literature and are therefore reasonable from the perspective of product comparison.
- The standard solar spectra currently recommended in the U.S. by CRRC and allowed in Europe by ECRC to calculate the weighted average of surface reflectivity, considering air mass 1.5 and direct normal radiation, are probably improper for the prediction of annual peak solar heat gain; thus, spectra for air mass 1 global radiation on a horizontal surface have been recommended in recent studies. On the other hand, the adoption of such spectra was shown to lower the measured performance by a non-negligible amount and to potentially inhibit a fair comparison between newly rated and already rated products. The issue still needs to be properly addressed.
- Two test methods of practical relevance are available to measure thermal emittance, one (ASTM C1371) returning a hemispherical value and the other (EN 15976) a near-normal value. Unless properly corrected, near-normal emittance may represent a non-negligible overestimation of hemispherical emittance, the one relevant to infrared heat transfer between a built surface and the sky.
- The combined use of air mass 1.5 direct normal radiation spectra and near-normal emittance can lead to a significant overestimation of SRI with respect to using air mass 1 global horizontal radiation spectra and hemispherical emittance, especially for surfaces with high or intermediate solar reflectance. The freedom of choice apparently allowed by standard test methods and rating systems may induce such an unfair situation.
- The surface performance achieved after ageing is relevant to the long-term behavior of built surfaces and therefore this should be considered in regulations rather than that initially measured. Nonetheless at least three years are needed for natural ageing—a requisite that is not easily accepted by the industry—and randomly variable conditions are also possible in the weathering sites due to variations in meteorology and air pollution.
- In many countries different from the U.S. the regulations on solar reflective materials are still under development and only initial values of surface properties are considered, or can be provided by product manufacturers. On the other hand, a fast improvement of commercial products and regulations may be impeded by the long time required for natural ageing.
- A recently developed laboratory test method for accelerated ageing—already a standard test method—makes it possible to condense into three days the three-year long process of natural ageing required by CRRC. It proved to be easy and fast to perform, repeatable, and, above all, able to reproduce the reflectance obtained in a wide range of naturally exposed roofing products.
- In order to create a fair global market for solar reflective products, as well as to favor their use where this is still undeveloped, a general alignment is required on measurement methods. The use of a common solar spectrum is desirable for solar reflectance measurement, possibly representative of peak heat load conditions. The use of hemispherical thermal emittance should also be clearly specified. Under these conditions, the effects of both reflectance and emittance can be effectively summarized by the SRI. In order to consider aged values of SRI—that is, those relevant to the long term performance of built surfaces—worldwide use of an accelerated soiling method such as that specified by ASTM D7897 may greatly speed up the development and qualification of durable products, thus favoring the diffusion of specific performance limits intended to improve building energy efficiency and limit urban warming.
Acknowledgments
Conflicts of Interest
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AM1GH | EN410G | G173GT | G173DN | E891BN | |
---|---|---|---|---|---|
UV (300–400 nm) | 6.8 | 6.4 | 4.7 | 3.5 | 3.0 |
Vis (400–700 nm) | 44.7 | 44.1 | 43.3 | 42.0 | 39.0 |
NIR (700–2500 nm) | 48.5 | 49.5 | 52.0 | 54.5 | 58.0 |
Sample | Reference Solar Spectrum | ||||
---|---|---|---|---|---|
Lab. code | Description | EN410G | G173GT | G173DN | E891BN |
0217 | Elastomeric water based paint on aluminum | 0.816 | 0.826 | 0.836 | 0.839 |
- | 0.799 | 0.810 | 0.815 | ||
0218 | Elastomeric water based paint on aluminum | 0.612 | 0.625 | 0.637 | 0.650 |
- | 0.606 | 0.623 | 0.635 | ||
0219 | Elastomeric water based paint on aluminum | 0.359 | 0.372 | 0.385 | 0.401 |
- | 0.367 | 0.380 | 0.389 | ||
0220 | Elastomeric water based paint on aluminum | 0.357 | 0.367 | 0.377 | 0.388 |
- | 0.358 | 0.367 | 0.378 | ||
0221 | Elastomeric water based paint on aluminum | 0.835 | 0.845 | 0.853 | 0.854 |
0.817 | 0.826 | 0.828 | |||
0222 | Elastomeric water based paint on aluminum | 0.828 | 0.838 | 0.846 | 0.848 |
- | 0.815 | 0.824 | 0.830 | ||
0004 | Engobed ceramic til | 0.347 | 0.356 | 0.365 | 0.377 |
- | 0.348 | 0.354 | 0.364 | ||
0011 | Engobed ceramic til | 0.125 | 0.128 | 0.132 | 0.138 |
- | 0.125 | 0.128 | 0.133 | ||
0015 | Engobed ceramic til | 0.577 | 0.583 | 0.588 | 0.589 |
- | 0.564 | 0.570 | 0.570 |
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Muscio, A. The Solar Reflectance Index as a Tool to Forecast the Heat Released to the Urban Environment: Potentiality and Assessment Issues. Climate 2018, 6, 12. https://doi.org/10.3390/cli6010012
Muscio A. The Solar Reflectance Index as a Tool to Forecast the Heat Released to the Urban Environment: Potentiality and Assessment Issues. Climate. 2018; 6(1):12. https://doi.org/10.3390/cli6010012
Chicago/Turabian StyleMuscio, Alberto. 2018. "The Solar Reflectance Index as a Tool to Forecast the Heat Released to the Urban Environment: Potentiality and Assessment Issues" Climate 6, no. 1: 12. https://doi.org/10.3390/cli6010012
APA StyleMuscio, A. (2018). The Solar Reflectance Index as a Tool to Forecast the Heat Released to the Urban Environment: Potentiality and Assessment Issues. Climate, 6(1), 12. https://doi.org/10.3390/cli6010012