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

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

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.

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.