Recall that there are three types of heat transfer: Conduction, which we covered two weeks ago, Convection, that we covered last week, and Radiation – the subject of this week’s blog.
Although, Radiation has a bad reputation, associated with nuclear power plants or worse with nuclear bombs. Radiation isn’t necessarily bad, we are all exposed to radiation every day of our lives, and this has been true since the dawn of humanity. Our sun’s gentle glow is just radiation. In fact, every object in the universe to include our own bodies are constantly emitting radiation. So, let’s explore this topic in greater detail.
What is Electromagnetic Radiation
Radiation is short for Electromagnetic (EM) radiation and comes in a variety of frequencies. At low frequencies, we have radio and television waves. At somewhat higher frequencies we have radar and microwaves (the same thing as in your microwave oven). At still higher frequencies, we get infrared (IR) light. As we go to still higher frequencies, we have ordinary visible light. Continuing up we get ultraviolet (UV) light (UV light can give us ordinary sunburns). At even higher frequencies, we get to X-Rays (think dentist office), and finally gamma rays. Both IR and visible light transfer heat and are collectively referred to as thermal radiation.
Radiation is different from other types of heat transfer
Radiation heat transfer is different from the other two types of heat transfer, Conduction and Convection. It’s different because unlike conduction or convection it does not require the presence of any material medium to take place. Recall for conduction heat is transferred through solid materials from hot to cold. For convection, we required the presence of a fluid (a liquid or gas) to move heat from a hot surface to a cold one. However, Radiation heat transfer can occur in a vacuum or through liquids or even solids. For example, heat from our sun warms the Earth through the mechanism of radiation heat transfer.
As mentioned above, every object is emitting radiation. For most objects that radiation is in the form of infrared (IR) radiation. Some objects, such as our sun or light bulbs also emit radiation as visible light in addition to the IR. Since every object is emitting IR radiation all the time, we must analyze the radiation heat transfer in all directions.
Size and temperature matter
How much heat is being transferred depends on the relative temperature and size of the objects being considered. For example, consider the sun, the sun is very hot (approximately 10,000 degrees Fahrenheit) and extremely large. Therefore, the amount of radiative heat transfer from the sun to your body is large. However, the skin temperature of your body is only about 95 degrees, plus we are relatively small. So, while you will radiate heat back to the sun, it’s such a small amount that we can effectively ignore it. But if we think about our body temperature relative to other objects in our homes, then the two-way flow of heat becomes more important.
So does color
An additional complication of radiation is that some materials and surfaces reflect radiation (light colored surfaces tend to reflect), others absorb the radiation (darker surfaces tend to absorb), and other simply allow the radiation to pass through, i.e., transmit. Most materials that we work with in home construction absorb and reflect a percentage of the radiation while windows transmit, absorb, and reflect the radiation all at the same time.
For home design in Colorado, we don’t worry about radiation heat transfer too much, except when it comes to windows. Windows can play a major role when it comes to keeping your home at a comfortable temperature. South-facing windows can help keep the house warm during the winter, but the same windows can overheat the home during the summer. We’ll do a deep dive into the science of windows in a future blog.
Delta Energy Solutions has you covered
The subject of radiation heat transfer is far too complex to properly cover in a simple blog. But our heat transfer specialist at Delta Energy Solutions will consider all three types (Conduction, Convection, and Radiation) of heat transfer when designing your dream home.
Next week we’ll wrap all three types of heat transfer up into a concrete example, stay tuned!