Thursday, August 24, 2017

Urban heat island effect and cool paints

A lot of people are asking about how the cool paint works. Looking at one photo, it appears fairly dark so the paint is absorbing some light energy. All components of sunlight, if they are absorbed, heat a surface. So untraviolet heats, visible light heats and infrared heats if they are absorbed. We cannot see the ultraviolet and we cannot see the infrared, so we do not know if they are being reflected instead of being absorbed by looking. If something is white we know it is reflecting the visible portion of sunlight (about 43% of the energy from the sun is visible energy). may help. Green vegetation does shade and also does reflect infrared, so trees are a good option. I am an advocate of using solar air heaters on poles to shade streets. By the conservation of energy, if air is heated by solar air heaters, other objects can not be heated with the same energy (can either be in air or in the other objects). My understanding of this paint is that it reflects infrared. With an urban heat island effect, the sunlight (includes infrared and ultraviolet) enters the city and gets reflected around onto walls and so on. They absorb the sunlight to some extent and heat up. The sunlight coming in has mainly short wavelength. After buildings heat up they emit radiation of a longer wavelength. Some of this longer wavelength radiation will probably be reflected by the paint and some will go out to space. The infrared of sunlight is high frequency (short wavelenth ) infrared. The heated buildings emit longer wavelength infrared radiation, mainly. With white cool roofs some sunlight is reflected onto other buildings causing heat problems. One of the big problems can be windows with sunlight entering. Window glass transmits radiation up to about 2.5 microns (the energy goes through the glass if its wavelength is less than about 2.5 microns) and about 97% of solar energy has wavelength less than 2.5 microns, so virtually all enters via a window.
Now how much of this energy escapes? Well if the walls heats up to 50 deg C, then radiation from them that is above 2.5 microns will not escape. The answer is that far less than 1% of this radiation can escape through the glass because more than 99% of the energy radiated by the 50 deg C walls is of wavelength greater than 2.5 microns (using a blackbody approximation). If a particular cool paint does reflect infrared, where is the infrared radiation going to go? Remember angle of incidence= angle of reflection. One may note that green vegetation reflects solar energy of wavelength between 0.75 to 2.4 microns significantly. Most of this could be reflected through your window into your house (glass lets in radiation of wavelength less than about 2.5 microns). This radiation would heat up objects and the radiation from the hot objects would not be able to get out through the window again (wavelength too long). About 42% of solar energy is energy of wavelength 0.75 to 2.4 microns.
Note that: 

1) Radiation of less than about 2.5 microns in wavelength can enter via ordinary glass windows. About 96.6% of sunlight (solar radiation) is energy of wavelength less than 2.5 microns.
2)  If you have black roads and they heat up in the sun, almost all of the radiation from the hot roads will NOT be able to enter through windows (wavelength too long) and so this helps cool houses regarding radiation, but hot roads do warm the air above them by conduction.
3) If you have trees, a significant amount of sunlight reflected from trees will be able to go through windows into your house (reflected radiation between 0.75 and 2.4 microns in wavelength by green vegetation is significant - see ). About 42.4% of solar radiation is radiation of wavelength 0.75 to 2.4 microns. But, because of evapotranspiration, trees also cool.
4) With the usual black roads, almost all solar energy is absorbed (this makes these roads and air immediately above it hot), but there is little reflection of solar energy into your house from the standard black roads.
5) With "cool surfaces paint" solar energy can be reflected into your house through windows.
6) Once radiation has entered your house via windows it heats objects, which then radiate energy of a wavelength that cannot exit your house via windows (except for a tiny amount).
7) We can usually make a rough approximation and treat bodies as blackbodies. Any blackbody radiates heat in mainly the infrared range, if it has a temperature of less than 100 deg C or so. Radiation from a 100 deg C blackbody "peaks" at about 7.8 microns. Light (visible) is in the 0.4 to 0.76 micron range. Infrared radiation has wavelength longer than about 0.76 microns.
8) If the "cool paint" reflects solar radiation of wavelength between 0.76 microns and 2.5 microns (which it almost certainly will, unless it is whitish and reflects the light portion), then that radiation can enter buildings through windows and the heat will generally remain trapped in the building (will not be able to come out via windows). About 41.8% of solar radiation is radiation of wavelength between 0.76 and 2.5 microns. 
9) If the cool paint is the old style white paint that reflects the light portion, then that radiation can enter buildings through glass and remain trapped as lower frequency radiation after heating objects in the house. About 43% of sunlight (solar radiation) is light energy.
10) Using a blackbody approximation, if a dark road heats up to 60 deg C, then about 39.1% of the radiation from this dark road is radiation having wavelength between 8 and 14 microns (could escape to space via the atmospheric window).
11) With reflection from smooth surfaces, angle of incidence=angle of reflection.
12 ) With dark roads that heat up a lot in the sun, the air above these roads is heated. This can result in higher than usual cloud formation and convectional rain. Cloud formation can drastically reduce the heating up of cities by shading them. The cloud itself radiates heat to space and low clouds in low latitudes cool Earth.

CONCLUSION: It there is a large area of buildings consisting of windows and the sun is not directly overhead, reflection of light and higher frequency infrared radiation into buildings could cause more problems than dark coloured surfaces. Using solar air heaters on poles to shade streets would get rid of heat. By the conservation of energy, if air is heated by solar air heaters, other objects can not be heated with the same energy (the heat can either be in the air or in other objects).

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