When solar radiation hits a surface, the photons can be absorbed, reflected, or transmitted. In the case of opaque (not transparent) materials, none of the photons are transmitted. If the material is dark and dull (not reflective or shiny), very few of the photons are reflected. As such, the majority of photons incident on dark opaque surfaces will be absorbed. As s result of absorption, the photons are converted to thermal energy (or heat). At the same time, because of the temperature of the material, the surface emits radiation back to its surroundings at a rate that is dependent on the emissivity of the material. Heat can also be lost to the surroundings by conduction and convection, but that is not the focus of this lesson.
To learn the interaction of the solar radiation with opaque materials and parameters that characterize heat transfer, please read the following text:
Duffie, J.A., and Beckman, W.A., Solar Engineering of Thermal Processes, Wiley and Sons, 2013, Chapter 4, Sections 4.1-4.10.
While reading especially work through the examples that illustrate the use of the main equations.
Some additional note on materials selection:
When designing a solar thermal conversion system, the selection of materials is critical. Choosing collector materials that are good absorbers (i.e. carbon black) will help your system to perform well. Sometimes the best material can be too costly to justify. As such, it becomes a balance of priorities towards an optimal system. If a material of high absorptance (α=0.95) costs $10/lb and a material of higher absorptance (α=0.98) costs $20/lb, the best option to achieve a desired solar gain may be to use the cheaper material and increase the system aperture or total collector area.
In addition to the cost and physical radiation properties of materials, we must be careful to select materials that will hold up under extreme climatic and environmental conditions. For example, in sandy desert environments, the abrasive sand can have a negative impact on the reflective properties of concentrating trough collector systems over time. Thus full understanding the in situ performance of a material over a period of decades is important to the design and optimization of solar thermal energy conversion systems.
1. How would you define absorptance?
ANSWER: Absorptance is the ratio of the fraction of the incoming raditation that is absorbed by the material to the total incident radiation
2. How would you define emittance?
ANSWER: Emittance is the ratio of the radiation that is emitted by a material surface to the radiation that would be emitted by a blackbody at the same temperature
3. How would you define reflectance?
ANSWER: Reflectance of a surface is the ratio of the radiation that is reflected (i.e. not absorbed or transmitted) to the total incident radiation.
4. For best performance of flat-plate collectors, it is generally more important to maximize absorption of radiation rather than minimize emission of heat. If the highest temperature of the material surface is desired, which three options from the Table below would you pick?
|Aluminum, SiO2 coated||H||0.11|
|Carbon black in acrylic binder||H||0.94|
|Lampblack in epoxy||N||0.96|
|Paint - Parson's black||H||0.98|
|Paint - Acrylic white||H||0.26|
|Paint - White (ZnO)||H||0.12-0.18|
bThe numerator is the emittance at the temperature (K) of the denominator.
cNormal solar absorptance.
From quick look at the data, the three materials with the highest absorptance should provide the highest performance (see Section 4.10 D&B) :
- Carbon Black
- Parson’s Black
On the previous page of this lesson, we looked at how the wavelength of the incident radiation matters because the wavelength determines the amount of energy that is transmitted. Some specially selected or designed materials may absorb radiation in one range of wavelengths very efficiently while may be highly reflective in a longer wave length range. Such materials are referred to as selective surfaces. The concept of selectrive surface is discussed in Section 4.8. of D&B book, and Example 4.8.1. and 4.8.2. show how the radiation properties of such materials can be calculated. Please review those examples in detail. In this lesson assignment, you will be asked to perform a similar calculation.