ass3.docx - SOLAR THERMAL COLLECTOR INTRODUCTION This is equipment in which solar energy is collected by absorbing radiation in an absorber and

ass3.docx - SOLAR THERMAL COLLECTOR INTRODUCTION This is...

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SOLAR THERMAL COLLECTOR INTRODUCTION This is equipment in which solar energy is collected by absorbing radiation in an absorber and transferring it to a fluid. FLAT PLATE COLLECTOR It is a collector with no optical concentrator. The collector area and the absorber area are numerically the same, efficiency is low, and temperatures of the working fluid can be raised up to only 100 degrees centigrade. It has five major parts: A metallic flat absorber plate; of high thermal conductivity made of copper, steel, or aluminium and having black Tubes or channels; these are soldered to the absorber plate. Water flowing through these tubes takes away the heat from the absorber plate. Transparent toughened glass sheet; it reduces convection losses through a stagnant air layer between the absorber plate and the glass. Radiation losses are also reduced as the spectral transmissivity of glass is such that it is transparent to short wave radiation and nearly opaque to long wave thermal radiation emitted by interior collector walls and absorbing plate. Fibre glass insulation; provided at the bottom and on the sides in order to minimize heat loss. EFFECT OF DESIGN PARAMETERS ON PERFORMANCE OF FLAT PLATE COLLECTOR
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Heat Transport System Heat from the absorber plate is removed by continuous flow of cold water through the metal tubes that are welded to the absorber plate for effective heat transfer. Cold water enters the bottom header, flows upwards and gets warmed by the absorber. Hot water flows out of the top header. When air is used, the air stream flows at the rear side of the collector plate. Fins welded onto the plate increase the air contact surface area. Selective surfaces These are surfaces which provide high absorptivity for incoming solar radiation and low emissivity for outgoing radiation. Solar radiation lies in short wavelength band up to 4 micrometers, while the absorber plate emits long wave radiation with a maximum wavelength of 8.3 micrometers. There is no natural surface with selective radiation characteristics. A selective surface is composed of a thin black metallic oxide on a bright metal base. E.g. black chrome coating (Cr 2 O 3 ) Number of covers A solar collector is provided with a transparent glass sheet over the absorber plate to minimize convection and radiation loss. Glass sheet absorbs heat radiation emitted by the absorber plate thus the glass sheet reduces the heat loss radiation by 10W/m 2 .K If you use two glass sheet covers, the heat loss co-efficient reduces further to 4W/m 2 .K Spacing The operating performance of a flat plate collector varies with the spacing between the absorber plate and cover as well as with tilt and service conditions. Spacing of 4 to 8cm is required for improved performance.
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LAWS OF THERMAL RADIATION Solar energy reaches the earth by radiation. The solar radiation is electromagnetic energy propagating through space at the speed of light. Emission of energy with regards to wavelength is not uniform but depends on temperature.
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