Solar Energy Conversion II

Selected Lectures from the 1980 International Symposium on Solar Energy Utilization, London, Ontario, Canada August 10-24, 1980

A. F. Janzen, R. K. Swartman, A. F. Janzen, R. K. Swartman

• Publisher: Pergamon
• Year: 2013
• Print ISBN: 9780080253886
• eBook ISBN: 9781483155906
• Topic: Technology & Engineering
• Subtopic: Physics
• Index: Physics

Solar Energy Conversion II presents the proceedings of the 1980 International Symposium on Solar Energy Utilization, held in Ontario, Canada on August 10-24, 1980. This book provides information on the utilization of solar energy and on the difficulties encountered in its implementation. Organized into 42 chapters, this compilation of papers begins with an overview of the important parameter in solar radiation measurement. This text then examines the use of solar radiation measurement, the solar radiation scales, the solar radiation units, and the types of solar radiation. Other chapters consider the general problems linked with building up data banks of observed solar radiation data. This book discusses as well the fundamental modes of heat transfer. The final chapter deals with the necessity to incorporate energy education into other disciplines like space geometry. This book is a valuable resource for politicians, government officials, engineers, scientists, and research workers. Technologists working on solar energy will also find this book useful.

ACTIVE SYSTEMS AND STORAGE

4

LIQUID HEATING SOLAR FLAT PLATE COLLECTORS

A.A.M. Sayigh, College of Engineering, University of Riyadh, Riyadh, Saudi Arabia

ABSTRACT

The paper describes the basic modes of heat transfer, then shows their application to flat plate solar collectors used for liquid heating. The paper then examines the factors affecting the performance of a flat plate solar collector and the para meters that are used to describe performance.

KEYWORDS

Conduction

convection

radiation

heat losses from flat plate collectors

collector performance

efficiency

collector efficiency factor

heat removal factor

flow factor

HEAT TRANSFER BY CONDUCTION

This mode of heat transfer takes place mostly in a solid material and occurs due to temperature difference between different parts of the material. Conduction also takes place in liquids and gases, but it is associated with convection. Conduction in solids is a transfer of internal energy which is the energy due to molecular collisions of the material and leads to energy transfer to regions of lower kinetic energy. Under steady state a molecule will pass on the same amount of energy that it receives. In unsteady conditions the flow of energy is governed by the changing energy levels.

The theory of conduction heat transfer was established by Joseph Fourier and published in Paris in 1822, but earlier work was carried out by J.B. Biot in 1804 and 1816.

The Fourier equation is

(1)

Q_X = rate of heat transfer

A = cross sectional area of the slab

k = thermal conductivity

t = temperature

x = thickness of the slab

as shown in Fig. 1.

Fig. 1 Conduction in a plane slab.

The negative sign results from the convention of defining a positive heat flow in the direction of a negative temperature gradient

There are three and two-dimensional conduction, but for the sake of simplicity, only one-dimensional steady state conduction will be discussed here.

Using a rectangular coordinate system, the differential equation of conduction in the x-direction only is

(2)

HEAT CONVECTION

It is the gross motion of the fluid itself, so that fresh fluid is continually available for heating or cooling. This is subdivided into two different kinds, natural and forced. Heat transfer by natural convection occurs between a solid and a fluid undisturbed by other effects when there is a temperature difference between the two, as in a kettle of water. It is not often that a fluid can be regarded as entirely at rest, so frequently there is a small amount of forced convection as well. Forced convection requires a major applied motion of the fluid in relation to the source or sink of heat, so that natural convection is negligible. Within both modes, there are subdivisions of laminar and turbulent flow convection. In forced convection there are special cases where flow may be separated from solid surface, as in flow across the outside of a pipe, and also convection with phase change occurring in the fluid. This is, of course, encountered in a steam raising and condensing plant. Therefore, many factors are involved in heat convection.

Newton, in 1701, proposed a general equation to describe convection heat transfer, as shown in Fig. 2.

Fig. 2 Heat convection.

(3)

where h is the film coefficient expressed in J/m².s.°C or Btu/ft². hr.°F

Typical values are given in Table 1.

TABLE 1

Properties Of Some Surface Coating Used In Flat Plate Collectors.

^*α = absorptance for solar energy; ε = emittance for long-wave radiation at at temperatures typical of flat plate solar collectors.

^**Commercial processes.

TABLE 1

Convection System	Range of h, J/m^2,°C
Natural convection	3.5 − 50
Forced convection (air)	10 − 550
Forced convection (liquids)	100 − 5500
Boiling heat transfer (water)	1000 − 110,000
Condensation (steam, filmwise)	550 − 25,000
Forced convection (liquid metals)	3000 − 110,000

Dimensional analysis of natural convection provides a correlation for the film coefficient, h, in the Nusselt number, Nu, where Nu = hd/k as a function of Grashof and Prandtl numbers as follows:

(4)

where a and b are constants, and the Grashof and Prandtl numbers can be expressed as the Rayleigh number:

(5)

Varying Gr at some fixed Pr, laminar and turbulent flow regions have been observed in natural convection, and transition generally occurs in the range

Mc Adams recommends for vertical surfaces

Fishenden and Saunders recommend for square or rectangular horizontal surfaces up to a mean length of side of 2 ft.

RADIATION

Energy absorbed by any surface due to radiation behaves according to:

where

For thick or opaque bodies τ = 0,

In engineering applications of radiation, there will generally be a gas separating solid bodies; often this gas is air which can be a...

Table of contents

• Cover image
• Title page
• Table of Contents
• Pergamon Books of Related Interest
• Copyright
• FREQUENTLY USED CONVERSION CONSTANTS
• PREFACE
• ACKNOWLEDGEMENTS
• INTERNATIONAL ORGANIZING COMMITTEE
• SPEAKERS
• PANELISTS
• PARTICIPANTS
• SOLAR RADIATION
• ACTIVE SYSTEMS AND STORAGE
• ARCHITECTURE AND COMMUNITY PLANNING
• DIRECT AND BIO-CONVERSION
• CONTROL SYSTEMS
• GENERAL TOPICS
• SOLAR ECONOMICS AND COMMERCIALIZATION
• SOLAR ENERGY AND SOCIETY
• INDEX