Solar Heating And Cooling
Author:
Frank Kreith
ISBN: 0070354863
Publisher: Taylor & Francis
Sample Page
Solar Heating And Cooling
INTRODUCTION: WHY SOLAR ENERGY?
CONVERSION OF SOLAR ENERGY TO HEAT
Solar energy is transmitted from the sun through space to the earth by
electromagnetic radiation. It must be converted to heat before it can be
used in practical heating or cooling systems. Since solar energy is
relatively dilute when it reaches the earth, a system used to convert it to
heat on a practical scale must be relatively large. Solar energy collectors,
the devices used to convert the sun's radiation to heat, usually have a
surface that efficiently absorbs radiation and converts this incident flux
to heat, which raises the temperature of the absorbing material. Part of
this energy is then removed from the absorbing surface by means of a heat
transfer fluid, which maybe either liquid or gaseous.
Since 1900 at least 50 solar collector designs have been demonstrated as
functional. These designs are separated into two generic classes:
concentrating and noconcentrating. Nonconcentrating, or flat-plate,
collectors intercept solar radiation on a metal or glass absorber plate from
which heat is transferred and used in the thermal application. Since the
temperature of the absorber plate is greater than that of the environment,
unrecoverable heat losses occur from the entire absorbing surface of the
collector to the environment. Consequently, 100 percent collector efficiency
cannot be realized in practice.
Concentrating collectors attempt to reduce heat loss by using an absorber are smaller than the area that intercepts the sun's rays - the aperture area. This performance improvement is accomplished by reflecting the sun's rays from the large aperture area to the small absorbing area by use of shaped mirrors or other reflecting surfaces. Since only the direct or collimated portion of solar radiation is amenable to effective concentration, most concentrators must move to track the sun and cannot collect as much diffuse radiation as flat-plate collectors. In Chap. 3, where several collector designs are analyzed in detail, it will be shown that these two negative aspects of concentrators can be offset by their greater efficiency, particularly in sunny regions of the world.
Since solar energy is available only during daylight hours and during periods when the sun is not significantly obscured by clouds, a means of providing continuous heat from this intermittent source is required,. In nearly all applications a form of thermal energy storage is used for this purpose. Three practical storage media have shown acceptable performance. Pebble or rock beds and water have been the two most widely used means of sensible heat storage (storage in a material by virtue of a temperature rise). The first is used with air-cooled solar conversion devices; the second, with liquid-cooled devices. The third storage method uses the latent heat of materials undergoing a phase change. Such materials can store energy per unit volume than sensible heat storage materials, but to date they have shown unacceptable reliability in repeated freeze-thaw cycles. Since the heating and cooling loads on a building do not occur in phase with useful solar collection, all practical solar building thermal systems require storage. The size and cost of storage are considered in detail in Chap. 4.
This book deals primarily with the design and economic analysis of heating and cooling systems for buildings. Solar energy can, of course, be applied for other end uses, and there are other solar technologies [2] besides those treated here. The following section is a brief overview of the most important solar technologies and their applications.
OVERVIEW OF SOLAR TECHNOLOGIES
Solar energy technologies can be divided into two broad classifications: natural
collection systems and technological conversion systems (see Fig. 1.5).
In natural collection technologies the total biosphere - earth, wind, and water
- provides free collectors. Since no collectors need to be built, the energy
costs for natural systems are determined by the converter, e.g., the wind
turbine. In technological conversion systems the amount of collectable energy is
determined by the amount of solar energy per unit area of collector at a given
geographic location.