Comparative Study of Different Honeycomb Geometries for the Suppression of Convective Heat Transfer in Flat Plate Solar Collectors.
ABSTRACT
The performance of different honeycomb geometries have been compared experimentally in four identical solar water heaters equipped with circular, square and rectangular cell honeycomb structures while the fourth solar water heater was without honeycomb.
The collector with circular, square and rectangular honeycomb collected 19.32%, 9.78% and3.0% more energy respectively when compared with collector without honeycomb.
The overall analysis gave average exergetic efficiency of 48.8%, 44.9% and 42.1% for collector with circular, square and rectangular cell honeycomb respectively while the collector without honeycomb has an average exergetic efficiency of 40.9%.
INTRODUCTION
1.1 Introduction
Flat plate solar collectors are the most widely used solar collectors because of their simplicity and wide range of important potential applications. The flat plate solar collectors have been for many years the most popular device for heating water or other liquids to moderate temperatures (Rai, 2005).
Considerable effort has been made over the years to improve the efficiency and output temperatures of flat plate collectors. This effort has been made with several goals in mind.
One is to store heat more efficiently for use during nights and cloudy days. The other is to increase the temperature so that tasks other than simply providing hot water are possible.
The improvement in the performance of a flat plate solar collector requires the suppression of conductive, convective and radiative heat losses from the absorber plate. The use of top glass cover prevents the loss of energy via long wave radiation as glass is opaque to it.
The radiative losses from the absorber can further be reduced by the use of spectrally selective absorber coatings, Such coatings have a high absorbance of about 0.9 in the solar spectrum and a low emittance usually of the order of 0.1 in the infra-red spectrum in which the absorber radiates to the environment (Meyer et.al.1978).
The air layer between the absorber plate and glass cover is a good insulator against conductive heat loss as air has a low thermal conductivity. This remains so only if the air is stagnant.
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