During the past fifteen years the explosive growth of world population together with increasing mechanization and the growing aspirations of much of the world's population for higher standards of living have combined to make the scientific and engineering community acutely aware of our burgeoning energy needs. In the long view this is not a question of competition between fossil fuels, nuclear fuels, and solar energy, but rather one of maximum use of all of our available energy sources.

Although the solar energy impinging upon the earth and its atmosphere is some 32,000 times greater than the total amount of all energy which we use, its practical and efficient collection and its translation into energy thermodynamically available for practical use is a difficult problem. Much solar energy never reaches the earth's surface because of cloud cover. Since the energy is diffusely spread, large area solar collectors are needed and the cost of such collectors and their maintenance is high. The efficiency of collection is highest at low temperatures, but energy collected at low levels is not useable for many of our energy needs. Finally, because of the variable nature of the source, solar energy must be stored for many applications and this introduces storage losses and the additional cost of construction and maintenance of a storage system.

Despite these massive problems, there are many uses for which solar energy is now practical. In outer space beyond the earth's cloud cover, use of solar converters for the generation of power is well proven. Here the complete predictability of the source and the virtual lack of equipment cost limitations makes this feasible.

Terrestrial applications are more limited, Solar cells are often practical where very limited amounts of electrical energy converted directly from solar energy, are needed. However, where large quantities of energy need to be collected the applications are more practical if solar storage is a secondary factor or perhaps even inherent in the nature of the end product. For this reason the most widespread current application of solar energy is in relatively low-temperature water heating. Solar distillation and solar water pumping have the same inherent advantage since no energy storage problems involved are beyond the storage of the distilled or pumped water itself. Another area in which energy storage is partly negated is in solar energy-operated refrigeration systems, for here the increased demand for refrigeration is usually coupled with increased solar availability.

This book is devoted to a series of authoritative technical treatments needed in the engineering of low-temperature level solar energy applications particularly related to flat-plate collectors. The initial chapter concerns the prediction of the availability of solar energy and has been prepared by Benjamin Y. H. Liu and R. C. Jordan of the University of Minnesota. The second chapter proceeds to considerations of the measurement of solar radiation and has been prepared by John I. Yellott, Director of the Yellott Solar Energy Laboratories of Phoenix, Arizona. The third chapter concerns the design

factors influencing flat-plate solar collector performance and has been prepared by Austin Whillier, currently of the Mining Research Laboratory of South Africa and formerly Director of the Brace Research Solar Institute of McGill University, Barbados, West Indies. The fourth chapter by H. Tabor, Director of the National Physical Laboratory of Israel, concerns the very important subject of selective surfaces for solar collectors. Chapter five by N. K. D. Choudhury of the Building Research Institute, Roorkee, India resumes the status of the various applications of low-temperature solar devices, particularly as related to tropical regions of the world. Chapter six prepared by G. O. G. Löf, Research Associate, University of Wisconsin (and Consulting Engineer of Denver, Colorado) and D. J. Close of the Division of Mechanical Engineering of the Commonwealth Scientific and Industrial Research Organization, Melbourne, Australia, provides a detailed discussion of solar water heaters, by far the most widely accepted low-temperature application of solar energy.

The purposes of this publication are to provide (a) authoritative scientific and technical design information for the collection of solar energy through the use of flat-plate collectors, and (b) to provide engineering information on the design and performance of solar water heaters, currently the most widely used practical application of solar energy. Other applications such as water distillation, pumping, and house heating, are also practical but the engineering information is less fully developed and such applications are not as widespread at this time. However, in all cases the evaluation of available energy and the collector design are the starting point for such system designs.

The total publication is a contribution of the American Society of Heating, Refrigerating and AirConditioning Engineers and has been prepared by the 1964-66 Technical Committee on Solar Energy Utilization of which all of the chapter authors have been members. We sincerely hope that this material will be found useful in furthering the low temperature engineering applications of solar energy and that it may aid in stimulating further international interests in harnessing this vast and challenging energy source.

R. C. Jordan, Chairman ASHRAE, Technical Committee on Solar Energy Utilization

B. Y. H. Liu, Vice Chairman ASHRAE, Technical

Committee on Solar Energy Utilization

The availability of solar energy is an important factor for consideration in the design of flatplate solar heat collectors. Since collectors of the flat-plate type can utilize both the direct and diffuse components of solar radiation, and in many localities substantial quantities of solar energy can be collected on partly cloudy days, flat-plate collectors should not be operated on perfectly clear days only. Consequently, data on clear-day radiation alone, such as those presently available in the ASHRAE Handbook of Fundamentals for cooling load calculations, are inadequate for evaluating collector performance. Considerably more information than clear-day radiation is needed for this purpose.

In

No method is presently available by which one can calculate from theory alone the solar energy available during hazy or partly cloudy days. Theoretical methods developed for calculating solar radiation are primarily for cloudless skies only. this paper we shall summarize the various methods which have been developed for treating the availability of solar energy during all days, and to present the available data on solar radiation useful in preducting the performance of flat-plate collectors of any design and operated at any specified temperature levels. Theory of operation of the flat-plate collector and other aspects of the problem involved in the utilization of solar energy by flat-plate collectors will not be considered in this paper, since they are dealt with in the other papers in this series.

Solar Radiation Outside the Atmosphere of the Earth

Outside the atmosphere of the earth both the instantaneous intensity of solar radiation incident upon a surface of any orientation and the total radiant energy received by a surface over any specified interval of time can be readily calculated. Some of these calculations will be considered in this section. The results obtained are generally useful.

If Lo is the intensity of solar radiation incident upon a surface, expressed in Btu/hr-ft, then

(1.1)

= r I cose SC

In this equation Isc is the solar constant, i.e. the rate at which a surface of unit area, normal to sun's rays and at the mean distance of the earth from the sun, receives energy from the sun. The solar constant is 442 Btu/hr-ft or 2.00 cal/cm-min. The symbol r is the square of the ratio of the mean distance between earth and sun to the actual distance

1-3