Comparative study of different honeycomb geometries for the suppression of convective heat transfer in flat plate solar collectors

Thesis Overview

<p> INTRODUCTION<br>1.1 Introduction<br>Flat plate solar collectors are the most widely used solar collectors because of their<br>simplicity and wide range of important potential applications. The flat plate solar collectors have<br>been for many years the most popular device for heating water or other liquids to moderate<br>temperatures (Rai, 2005).<br>Considerable effort has been made over the years to improve the efficiency and output<br>temperatures of flat plate collectors. This effort has been made with several goals in mind. One<br>is to store heat more efficiently for use during nights and cloudy days. The other is to increase<br>the temperature so that tasks other than simply providing hot water are possible.<br>The improvement in the performance of a flat plate solar collector requires the suppression of<br>conductive, convective and radiative heat losses from the absorber plate. The use of top glass<br>cover prevents the loss of energy via long wave radiation as glass is opaque to it. The radiative<br>losses from the absorber can further be reduced by the use of spectrally selective absorber<br>coatings, Such coatings have a high absorbance of about 0.9 in the solar spectrum and a low<br>emittance usually of the order of 0.1 in the infra-red spectrum in which the absorber radiates to<br>the environment (Meyer et.al.1978). The air layer between the absorber plate and glass cover is a<br>good insulator against conductive heat loss as air has a low thermal conductivity. This remains<br>so only if the air is stagnant.<br>Convective heat loss through the air layer is due to the air circulation initiated within the layer<br>when the viscous forces cannot damp out disturbances to the air layer. This condition may be<br>brought about when the air layer is subjected to an adverse temperature gradient i.e. the<br>20<br>temperature of the air next to the absorber plate is higher than that next to the glass cover. In<br>such case there will be density variation between air near the absorber plate and that near the<br>cover glass. Thus top-heavy situation becomes unstable when the buoyancy forces (tending to<br>cause motion) are greater than the viscous damping forces.<br>The Various heat losses from the absorber to the ambient are shown in figure 1.1.<br>Figure1.1: Various heat losses from the absorber to the ambient (Tiwari, 2006)<br>Sayigh (1978) established that in a flat plate solar collector, the convective loss is about<br>30%. Suppressing this loss will lead to a better efficiency. There are several ways in which this<br>can be achieved, which include the evacuated collector, the pressurized collector and the honey<br>comb collector. The least difficult method of suppressing convection is the use of a honey comb<br>structure.<br>21<br>Honeycomb solar collector is used to describe an array of cells which are essentially individual<br>and having a small aspect ratio. Cells of honeycomb can be of various sizes and shapes such a<br>cylindrical, square, rectangular, hexagonal etc. A schematic diagram of a square cell honeycomb<br>is given in figure 1.2 below<br>Figure1.2: Schematic diagram of a square cell honeycomb (Meinel et.al. 1976)<br>1.2 Statement of the Problem<br>Water heating constitutes about 50% of domestic heating requirement. This is mostly<br>done by the use of electric heater or burning of fossil fuel. Due to erratic power supply in<br>towns and cities as well as non availability in most villages of Nigeria, Fossil fuel (i.e. fuel<br>wood, kerosene etc.) which is the most viable alternative is expensive and contribute to<br>global warming remains the main energy source.<br>In Nigeria with abundant sunshine, solar energy is the most available and accessible form<br>of energy. The acceptability of solar system has increased as an alternative to the use of fossil<br>fuel due to the low running cost and sustainable environmental development. In order to<br>further increase the acceptability of solar system there is need to look inward to improve on<br>the performance of the system.<br>22<br>1.3 Objective of the Study<br>The objective of this work is to evaluate the performance of flat plate solar collectors with<br>different honey comb geometries by comparing the rise in water temperature and collector<br>efficiency in order to determine the most appropriate geometry for water heating.<br>1.4 Justification of the Study<br>1. Many studies on the convection suppression with the use of honeycomb have been<br>carried out but there has not been any concerted effort to compare the efficiency of<br>different geometries of honeycomb<br>2. There has been high demand of solar collectors running at high efficiency to<br>significantly save fuel<br>3. Solar energy is available in abundance in Nigeria and should be tapped and used<br>efficiently.<br>4. Solar water heaters are easy to operate, have long life span and are environmental<br>friendly (i.e. do not contribute to global warming). <br></p>