College of Natural and Applied Sciences

Permanent URI for this community

http://192.168.13.253:4000/handle/123456789/1249
.

Back to the main page

Browse

Browsing College of Natural and Applied Sciences by Subject "Absorption of light"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Selective Absorbing Coatings ForPhoto-Thermal Solar Energy Conversion and Some Useful Factors in the Design of Flat Solar Collectors
    (University of Dar es Salaam, 1976) Kivaisi, Rogath T.
    For efficient heat energy conversion of solar energy, the receiving surface should absorb strongly in the solar spectrum region but emit as little as possible in the infrared region. In this case, thin film coatings play an important role in producing selective absorbers. Such a surface can be obtained by coating a semiconductor film on a metal havinga high infrared reflectance.A number of works dealing with solar selective surfaces are reviewed. Various techniques of producing solar selective surfaces are described. These include vacuum evaporation, chemical vapour deposition (CVD), wave-guide techniques, electroplating, spraying and painting. The results of these coatings are given for each method of preparation. The performance of the coatings at elevated temperatures in vacuum is also given.The general theory of thin film optics is reviewed. The form of the electromagnetic fields is derived from the famous Maxwell’s equations. Fresnel 's laws are derived from the boundary and continuity conditions. A brief discussion is given on the interaction of electromagnetic waves with matter. The general application of this theory is given in the section of materials. Expressions for reflectance, transmittance and absorption are given in terms of matrix formulation. With the help of these expressions, and given the optical constants of materials, the reflectances (or absorptances) of solar absorber coatings are calculated. An IBM computer has been used to calculate these quantities.Heat losses by conduction, convection and radiation are discussed. Most attention is paid to a flat solar collector design although it can be extended to a circular collector employs solar concentratingsystems. Correspondingly, methods of reducing heat losses are covered. To understand the nature of heat-radiating surfaces, Planck's radiation law is derived. Materials which may be used for solar energy collection are dealt with in detail. In addition, the basic theory of these materials is covered. It includes semiconductors as well as metals. The dependence of optical properties of gold, copper, Aluminium, nickel, germanium, lead sulphide and silicon monoxide on wavelength is given.Vacuum evaporation is used for coating the metal substrates, absorbers and antireflection layers. These are evaporated in a convectio-nal vacuum system at a pressure less than 105 torr. The evaporant is heated using Sloan 5 kW electron beam gun fitted with electromagnetic poles. The film thickness is monitored optically by measuring the reflection of a beam of light using a photo-cell through an amplifier and a recorder. In addition, a quartz crystal monitoring system is used to control both the rate of deposition and film thickness. Initially the glass slides are cleaned ultrasonically.The spectral reflectances of the samples are measured in the range.3-2.7 um using Perkin Elmer 350 spectrophotometer and Beckman model IR 20 Infrared spectrophotometer in the range 2.5-40 urn.Theoretical designs consisting of a smooth metal substrate (Al, or Ni), a semiconductor film (Ge or PbS) and antireflection coating (SiO) are reported. The dependance of solar absorptance on absorber thickness is investigated. The theoretical and calculated experimental solar absorptance of the coatings are compared. The maximum solar absorptance obtained with the designs is .94. The emissivities of the coatings are calculated at 400 K and at 500 K. The aluminium coated samples show very low emissivities (N 1-2%) with a/c values ranging from 52 to 63 at 400 K and from 44 to 56 at 500 K.The samples are subjected to high humidity at room temperature and cycled between room temperature and the required operating temperatures (100-2400 C). The spectral reflectance of each sample is measured after each testing. A visual examination of the samples using a conventional microscope is studied. The results of optical and physical deterioration of the samples when subjected to the environmental testing are reported