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On the way out the door one summer day, my husband mentioned that since he moved to a top-floor office at work, he's not as cold in the summertime. Why? Most likely because the ceiling temperature is warmer under the roof. Human comfort is strongly affected by thermal radiation, as well as the local air speed. There are university groups that study this topic, and the conclusions aren't surprising. For example, a flat roof painted white keeps humans more comfortable in the summertime in Arizona -- at the same room temperature -- than a black one. So actually one could save energy by turning up the thermostat, and maintain the same comfort level. My own winter experience is that radiant heat is more comfortable than forced-air heating systems, even at a lower air temperature.
The conversation reminded me of the importance of thermal radiation when convection coefficients are low. In fact, if you figure out an effective radiative heat transfer coefficient for electronics applications, it can be the same magnitude as natural convection in air.
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Radiation Shielding
CoolingZone’s radiation heat flow calculator is for a single heat shield between parallel plates. It figures the heat flow between two plates at given temperatures (if only you knew that ahead of time!) with a heat shield between. The heat flux between the plates is halved by the installation of the heat shield, even when all surfaces are treated as perfect emitters. (Think car-exhaust systems; it's common practice.)
I used the calculator to look at the effect of low emissivity coating on the various surfaces. It doesn't matter which surface you choose -- any one surface with low emissivity (I used 0.15 as an estimate) cut the heat flux by almost 75% over having a blackbody shield in place. Two surfaces cut it even further, by 85%.
Compared with no heat shield between high-emissivity plates, a single shield with low emissivity on both sides reduced the heat flow by 93%! Think about that next time you're contemplating outdoor equipment that's going to sit in the sun.
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Solar Radiation Data
I recently had fun tinkering with solar radiation data. There are a few helpful resources available although they're usually kind of buried; also, the depth of the data varies by region. If it's third-world, tropical data you're looking for, you can find an estimate based on the latitude band on this Solar Energy World Distribution link. If you're interested in the reflectivity (albedo) of the ground and atmospheric absorptivity for tropical climates, it's at the Intensity of Solar Radiation link. If, on the other hand, you're interested in United States data for your own home solar project, the Renewable Resource Data Center at NREL has neat maps. You can get a map giving the average, maximum, and minimum solar radiation hitting a collector (stationary tilted south-facing or tracking, and others) by month.
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