Conduction, Convection and Radiation Heat Transfer

You may copy whatever you want from these web pages for personal use or for academic purposes. You may not copy information from this web site for monetary gain.

Information on the general physics of heat transfer is readily available on the internet. This page does not make an effort to repeat that information or add anything to it. This page covers the basic principles necessary to understand solar home design and modelling.

Energy in the form of heat is typically transferred from a higher energy state to a lower energy state. In most cases this means transferring heat from a warmer location to a cooler one. There are 3 basic ways that this takes place.

Conduction.

Conduction always requires mass contact. That means something with mass in contact with something else with mass. An object or substance with mass can be solid, liquid or gas. Neighboring molecules are responsible for transferring energy from one object or substance to another or for moving energy deeper into a substance or emitting it at the surface. Heat energy increases the energy of the molecule and free electrons can transport that energy to neighboring molecules. The energy transfer is always from higher energy to lower energy or from a warmer area to a cooler one. If the outside temperature is cooler than the inside then energy transfer is to the outside. If the inside is cooler than the outside then energy transfer is to the inside. The same formula is used regardless of direction. Energy transfer due to conduction is typically computed using a simple formula.

Q = (delta T) / R * A

Where, Q is the heat transferred in Btu/sf-hr,

delta T is (T warmer - T cooler) in degrees F,

R is the R value of the material in sf-deg F-hr/Btu,

and A is the area of the surface in square feet.

The driving force is delta T. The greater delta T the faster energy moves. The resistance to that movement is in R. The R value is the reciprocal of the U-value. R values or U values for materials are given for a set of standardized conditions. That means if your conditions are not the same as the standardized conditions then you cannot assume the material has the R value or U value given.

Convection

Convection is like conduction. Instead of the absolute temperature for the energy transfer surfaces, delta T is a reflection of the temperature difference before and after the transfer took place. In a convective heat transfer, winds or some gas at a higher velocity than the subject sweeps by the subject surface and grabs all the high energy molecules and sweeps them away. Those molecules have a lot of potential energy. Eventually they smash into something else and that potential energy is converted into kinetic energy. The energy is imparted into the surface that they smashed into. Its much like a car hitting a brick wall at high speed. Its amazing we can't hear it. All we get to hear is this whistle when it tries to squeeze itself through a very small hole or crack at high velocity. The formula for computing convective heat loss is:

Q = (Delta T) / R * A, just like the conduction heat transfer equation. The difference is that Delta T in this case is the temperature difference before and after the conduction took place. It is a number between 1 and 10. When in doubt use 7.

Radiation

If you go to this website, http://jersey.uoregon.edu/vlab/elements/Elements.html and click on each element you should note that each element has a unique absorption spectrum in the visible light range. If you select the emission option you will see that each element emits energy at very particular wavelengths, the same wavelengths it absorbs energy at. It does not absorb or emit energy at any other wavelengths, just the ones given in the absorptance or emittance spectrum. It does not emit energy at a particular wavelength until it has reached the temperature associated with that wavelength. When an object is red hot, then it glows red and it is giving off energy specifically in the frequencies that correspond to that color or that energy level or the temperature associated with it. When its even hotter it will appear yellow. The higher energy or the hotter colors move to the left toward the blue.

Here is an example from http://astro.u-strasbg.fr/~koppen/discharge/ its the emission spectra of carbon

and this is the emission spectra of oxygen

The atoms absorb and emit energy at different frequencies depending on their own bonding and electron configurations.

This color bar showing the visible light range with the wavelengths came from http://mrsec.wisc.edu/Edetc/cineplex/movies/CConnect/BlueLight/moviepages/em_el.htm

Shorter wavelengths mean higher frequencies. Higher frequencies are related to higher temperatures. A hotter object is closer to the blue end of the spectrum and cooler objects are closer to the red. At Yellowstone, the hottest springs are blue.

At room temperature an ideal black body is emitting energy mostly in the long wave infrared region of the electromagnetic spectrum and at all wavelengths. Many objects in our surround act like this. They absorb heat from warm air, heated objects near or in touch with them or directly from the sun. They then heat up a little and start getting rid of that heat by emitting energy in the long wave portion of the IR spectrum. If you heat that black body up enough it will start emitting energy in the visible wavelengths.

Emissivity

The emittance of a material refers to its ability to release absorbed heat energy. This is a number between 0 and 1 or 0 and 100%. A true black body has an emissivity of 1. It is capable of getting rid of any heat energy it absorbs because it emits energy in all wavelengths, ideally. That means regardless of temperature, it is capable of getting rid of the energy it gains. There is little such thing as an ideal black body. Most bodies are "grey bodies" which means they absorb and emit energy at certain wavelengths only. Nothing gives off radiant energy unless it is at the appropriate temperature to do so. The higher the temperature the more energy is given off per unit time when compared to something at a lower temperature. Many common non metal materials have an emissivity somewhere around 85% to 95%. That means under average habitable Earth conditions they are capable of getting rid of 85% to 95% of the heat energy they gain by re-emitting that energy as heat in the long wave portion of the IR spectrum.

Many metallic materials on the other hand may have emissivity values closer to 1 or less than 1 (1%). Metallic materials such as chrome, left out in the sun side by side with something painted black, will get much hotter than the black surface. It is true that the black surface is absorbing all wavelenghts of the visible spectrum (ideally), converting the energy from the photons it gains into higher temperature, then radiating that energy in the long wave portion of the IR spectrum. The key is that it is releasing everything it absorbs. It probably has the ability to release energy in multiple wavelengths simultaneously so it can keep up with the incident solar gain. The chrome on the other hand may be highly reflective to the incoming radiation but it cannot easily get rid of the energy it gains . It heats up more and more. Chrome has a very low emissivity, a piece of wood painted with latex or oil based black paint has a fairly high emissivity. Left out in the hot sun the chrome will feel much hotter than the black painted wood.

Emissivity is dependent on wavelength. When a material is capable of absorbing energy at a particular wavelength it is also typically capable of emitting energy at that same wavelength.The chrome may not be able to radiate the energy it absorbs until it reaches a signficantly high temperature. The black painted piece of wood will likely never get that hot when both are exposed to direct sunlight.

The ability of an object to absorb energy, or more specifically heat energy, or thermal radiation is called its absorption factor. The spectral absorption factor is equal to the emissivity of an object. This relation is known as Kirchholff's law of thermal radiation. The absorption and emissivity of a material are the same number and absorptance spectrums are frequently generated from emmisivity spectrums.

The reflectance of a material is the difference between the aborptance and 1. The absorptance and reflectance sum to 1 for any material or 100% at a given wavelength. Reflectivity at one set of wavelengths does not mean you can deduce emissivity at any other wavelengths as they do not sum to 1 or 100%. If a material such as a white painted piece of plywood has 95% reflectivity in the visible portion of the spectrum that does not mean it has 5% emissivity in any part of the IR portion of the spectrum. It means that if the material reflects 95% of the radiation at wavelength 800 nm, then at 800 nm it has an emissivity of 5%.

Radiant barrier materials like Aluminum have low emissivity in the mid and long infrared spectrum, or in the range 3-15 micrometers. That means they have low absorption in those frequencies, which means they have high reflectivity to those frequencies. They may absorb a lot of energy at other wavelengths.

Most objects at room temperature will emit radiation concentrated in the range of 8 - 25 micrometers.

1 nanometer (nm) = 0.001 micrometers.

This website has a list of common materials and gives the emissivity at various temperatures for those materials. This information is critical because emissivity or absorptance is only a valid parameter at a certain temperature which correlates to a specific frequency. http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=Emissivity.htmHYPERLINK "http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=Emissivity.htm&ID=254"&HYPERLINK "http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=Emissivity.htm&ID=254"ID=254

If you go to http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html there is a list of emissivity coefficients for many common materials. The emissivities given in the table are for approximatey 80 degrees F.

Infrared Services has 3 full pages of emissivity values at room temperature for an extensive list of materials. This information is for use with their infrared cameras. http://www.infrared-thermography.com/training.htm

Return to Main Page on Modelling Basic Solar Design Click Here