Urban Rooftop Heat Reduction

Authors:

S. Sakaia,∗, M. Nakamuraa, K. Furuyaa, N. Amemuraa, M. Onishib, I. Iizawac, J. Nakataa, K. Yamajid, R. Asanod, K. Tamotsue

Larger dimensions=larger surface temps

Fractal dimension of a natural tree is approximately 2.

Fractal shades are not swollen with winds and do not suffer strong drag force.

Authors:

A.R. Gentle, J.L.C. Aguilar, G.B. Smith

Three parameters to consider, Solar albedo, Thermal Emittance E, and sub roof R-value.

Sensitivity to R and E drops as albedo rises.

Thermal mass, roof to wall ratio(area), roof slope, orientation, time dependance, magnitude, air exchange. All kept constant for this study.

Cool roofs reduce peak demand and ameliorate heat island effect.

Components of heat flows between roof and interiors solve between Troof and Tzone define each flux.

Local climate elements at sampling time, solar flux, ambient temp, humidity, its impact on sky temp, downwelling thermal radiation, wind speed. Spectral properties of outer roof surfaces, and thermal resistance to heat flow or R-value.

Roof related heat gains peak when solar flux is the highest and cooling power demands usually peak an hour or two later.

It is desirable to achieve maximum energy savings (or minimum discomfort days) per dollar invested in the overall roof structure, that is cost benefits for return on investment. Payback period from energy savings from cooling.

Roof albedo negatively affects heating requirements by approximately 10% where air exchange affects heating negatively by a factor of 2 or 4.

R>3.06 has litte benefit and may even reduce annual energy savings if albedo is high.

high t is important, R should not be too high as to interfere with nightime cooling.

High R in combo w/thermal mass can trap daytime heat gain over the following night unless cool air ventilation is available and utilized.

Doubling R beyond 1.63 benefit shrinks.

R 0.5, Asol=0.2, E=0.9 11.5 kwh/day cooling
R1.63, Asol=0.2, E=0.9 7.3 kwh/day cooling
R3.06, Asol=0.2, E=0.9 5.75kwh/day cooling

White surface should reflect IR radiation.

Hi Bino,

When you have a chance, could you please find a nice picture of avatar for our rooftop project? Also header as well. Thanks.

Regards,
Masa

http://www.greenroofs.com/projects/plist.php

http://www.sciencedirect.com/science/article/pii/S0360132308003028

http://www.sciencedirect.com/science/article/pii/S037877880700179X

http://www.sciencedirect.com/science/article/pii/S0960148103003987

http://www.sciencedirect.com/science/article/pii/S0378778807000965

http://www.sciencedirect.com/science/article/pii/S0378778811004129

http://www.sciencedirect.com/science/article/pii/S0378778812001491

http://www.sciencedirect.com/science/article/pii/S0196890411001348

http://www.sciencedirect.com/science/article/pii/S0378778810003191

http://www.sciencedirect.com/science/article/pii/S0378778808000352

http://www.nyc.gov/html/coolroofs/html/home/home.shtml

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