Comparative study of different honeycomb geometries for the suppression of convective heat transfer in flat plate solar collectors
Table Of Contents
<p> </p><p>Declaration ii<br>Certification iii<br>Dedication iv<br>Acknowledgements v<br>Abstract vii<br>Table of Contents viii<br>List of Figures xi<br>List of Plates xiii<br>List of Appendices xiv<br>List of Symbols xv<br>
Chapter ONE
<br>INTRODUCTION<br>1.1 Introduction 1<br>1.2 Statement of the Problem 3<br>1.3 Objective of the Study 4<br>1.4 Justification of the Study 4<br>Chapter TWO
<br>LITERATURE REVIEW<br>2.1 Flat Plate Solar Collector 5<br>2.2 Absorber Plate 6<br>2.3 Cover Sheets 8<br>9<br>2.4 Solar Water Heating 9<br>2.5 Honeycomb Cell 12<br>2.6 Review of Past Work 17<br>2.7 Conclusion of the Review of Literature 20<br>Chapter THREE
<br>DESIGN THEORY AND CALCULATIONS<br>3.1 Design Theory 21<br>3.2 Evaluation of the Overall Loss Coefficient (Ul) 21<br>3.3 Heat Removal Factor 24<br>3.4 Absorbed Solar Radiation(S) 24<br>3.5 Collector Performance 27<br>3.6 Calculation of the Hourly Radiation 27<br>3.7 Honeycomb Design 28<br>3.8 Design Calculations 28<br>Chapter FOUR
<br>CONSTRUCTION, TESTING AND RESULT<br>4.1 Introduction 43<br>4.2 Construction Processes 43<br>4.3 Assembly of component parts 47<br>4.4 Description of the Experimental Set-Up 49<br>4.5 Experimentation 53<br>4.6 Result and Discussion 54<br>10<br>Chapter FIVE
<br>SUMMARY, CONCLUSION AND RECOMMENDATION<br>5.1 Summary 71<br>5.2 Conclusion 72<br>5.3 Recommendation 73<br>References 74</p><p> </p> <br><p></p>Project 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%
Project Overview
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.
Convective heat loss through the air layer is due to the air circulation initiated within the layer
when the viscous forces cannot damp out disturbances to the air layer. This condition may be
brought about when the air layer is subjected to an adverse temperature gradient i.e. the
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temperature of the air next to the absorber plate is higher than that next to the glass cover. In
such case there will be density variation between air near the absorber plate and that near the
cover glass. Thus top-heavy situation becomes unstable when the buoyancy forces (tending to
cause motion) are greater than the viscous damping forces.
The Various heat losses from the absorber to the ambient are shown in figure 1.1.
Figure1.1: Various heat losses from the absorber to the ambient (Tiwari, 2006)
Sayigh (1978) established that in a flat plate solar collector, the convective loss is about
30%. Suppressing this loss will lead to a better efficiency. There are several ways in which this
can be achieved, which include the evacuated collector, the pressurized collector and the honey
comb collector. The least difficult method of suppressing convection is the use of a honey comb
structure.
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Honeycomb solar collector is used to describe an array of cells which are essentially individual
and having a small aspect ratio. Cells of honeycomb can be of various sizes and shapes such a
cylindrical, square, rectangular, hexagonal etc. A schematic diagram of a square cell honeycomb
is given in figure 1.2 below
Figure1.2: Schematic diagram of a square cell honeycomb (Meinel et.al. 1976)
1.2 Statement of the Problem
Water heating constitutes about 50% of domestic heating requirement. This is mostly
done by the use of electric heater or burning of fossil fuel. Due to erratic power supply in
towns and cities as well as non availability in most villages of Nigeria, Fossil fuel (i.e. fuel
wood, kerosene etc.) which is the most viable alternative is expensive and contribute to
global warming remains the main energy source.
In Nigeria with abundant sunshine, solar energy is the most available and accessible form
of energy. The acceptability of solar system has increased as an alternative to the use of fossil
fuel due to the low running cost and sustainable environmental development. In order to
further increase the acceptability of solar system there is need to look inward to improve on
the performance of the system.
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1.3 Objective of the Study
The objective of this work is to evaluate the performance of flat plate solar collectors with
different honey comb geometries by comparing the rise in water temperature and collector
efficiency in order to determine the most appropriate geometry for water heating.
1.4 Justification of the Study
1. Many studies on the convection suppression with the use of honeycomb have been
carried out but there has not been any concerted effort to compare the efficiency of
different geometries of honeycomb
2. There has been high demand of solar collectors running at high efficiency to
significantly save fuel
3. Solar energy is available in abundance in Nigeria and should be tapped and used
efficiently.
4. Solar water heaters are easy to operate, have long life span and are environmental
friendly (i.e. do not contribute to global warming).
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