John A. Duffie

Estimation of the diffuse radiation fraction for hourly, daily and monthly-average global radiation

Hourly pyrheliometer and pyranometer data from four U.S. locations are used to establish a relationship between the hourly diffuse fraction and the hourly clearness index kT. This relationship is compared to the relationship established by Orgill and Hollands and to a set of data from Highett, Australia, and agreement is within a few percent in both cases. The transient simulation program TRNSYS is used to calculate the annual performance of solar energy systems using several correlations. For the systems investigated, the effect of simulating the random distribution of the hourly diffuse fraction is negligible. A seasonally dependent daily diffuse correlation is developed from the data, and this daily relationship is used to derive a correlation for the monthly-average diffuse fraction.

Diffuse fraction correlations

The influence of climatic and geometric variables on the hourly diffuse fraction has been studied, based on a data set with 22,000 hourly measurements from five European and North American locations. The goal is to determine if other predictor variables, in addition to the clearness index, will significantly educe the standard error of Liu- and Jordan-type correlations (IdI = f(k1)). Stepwise regression is used to reduce a set of 28 potential predictor variables down to four significant predictors: the clearness index, solar altitude, ambient temperature, and relative humidity. A piecewise correlation over three ranges of clearness indices is developed to predict the diffuse fraction as a function of these four variables. A second piecewise correlation is developed for predicting the diffuse fraction as a function of the clearness index and solar altitude, for use when temperature and relative humidity are not available. A third piecewise correlation of the Liu- and Jordan-type is developed from the same data set. Comparing this correlation with the new correlations provides a direct measure of the value of added predictor variables. The full diffuse fraction correlation reduced the residual sum squares by 14% when compared to the correlation that is a function of the clearness index only. The correlation including the clearness index and solar altitude diminished the residual sum squares by 9%. The correlations exhibited some degree of location dependence. This is expected, as the climates are quite different. The correlations also showed some seasonal dependence; the errors are higher in the fall and winter than on an annual basis.

A design procedure for solar heating systems

This paper is concerned with the design of solar space and water heating systems for residences. A simulation model capable of estimating the long-term thermal performance of solar heating systems is described. The amount of meteorological data required by the simulation in order to estimate long-term performance is investigated. The information gained from many simulations is used to develop a general design procedure for solar heating systems. The result is a simple graphical method requiring monthly average meteorological data which architects and heating engineers can use to design economical solar heating systems. A method of estimating the monthly average radiation on tilted surfaces is given in the Appendix.

World distribution of solar radiation

Abstract A series of 12 monthly world maps of the daily means of total solar radiation incident on a horizontal surface has been prepared. Isolines of constant radiation have been drawn at intervals of 50 langleys per day (cal/sq cm, day). Data for the computation of mean radiation levels were compiled from many sources, including weather reports and summaries, personal communications from weather and research organizations, and IGY and IGC reports. For areas in which few or no radiation data were available, such information was synthesized by use of sunshine-hours data and approximation formulae applicable to the particular climate type. Radiation data were obtained for 668 stations, whereas sunshine-hours values were used in estimating radiation from 233 additional locations. The maps are recommended for general use in appraising the collector area requirements for solar applications in broad regions. They do not, of course, reveal the influence of microclimatic factors on local solar-radiation levels. In addition to the maps, the paper contains an explanation of the various radiation instruments, the methods for data handling, the estimation of radiation values from sunshine-hours data, and the limitations and utility of the radiation maps.

Solar Heating and Cooling.

We have adequate theory and engineering capability to design, install, and use equipment for solar space and water heating. Energy can be delivered at costs that are competitive now with such high-cost energy sources as much fuel-generated, electrical resistance heating. The technology of heating is being improved through collector developments, improved materials, and studies of new ways to carry out the heating processes. Solar cooling is still in the experimental stage. Relatively few experiments have yielded information on solar operation of absorption coolers, on use of night sky radiation in locations with clear skies, on the combination of a solar-operated Rankine engine and a compression cooler, and on open cycle, humidification-dehumidification systems. Many more possibilities for exploration exist. Solar cooling may benefit from collector developments that permit energy delivery at higher temperatures and thus solar operation of additional kinds of cycles. Improved solar cooling capability can open up new applications of solar energy, particularly for larger buildings, and can result in markets for retrofitting existing buildings. Solar energy for buildings can, in the next decade, make a significant contribution to the national energy economy and to the pocketbooks of many individual users. very large-aggregate enterprises in manufacture, sale, and installation of solar energy equipment can result, which can involve a spectrum of large and small businesses. In our view, the technology is here or will soon be at hand; thus the basic decisions as to whether the United States uses this resource will be political in nature.

Transient Considerations of Flat-Plate Solar Collectors

A mathematical study showed that a quasi-steady-state model which assumes zero capacitance adequately represents the performance of a flat-plate solar collector if hourly meteorological data are the best available. The use of more complex models which account for capacitance is justified only if better data can be obtained.

A methodology for the synthesis of hourly weather data

Abstract The ability to generate weather data from limited inputs and location independent correlations would allow simulations of environmentally driven systems to be run at locations for which detailed weather records do not exist. A further improvement would be to generate just one year of data which would yield the same simulation results as those of long-term data, thereby minimizing the computation needed to assess long-term system performance. In this paper, a methodology to calculate such a year of data is described. It is a combinationand modification of several previously published generation methods. Statistics obtained from long-term (22 years), Typical Meteorological Year, and synthetically generated weather data are compared at three locations (Albuquerque, NM, Madison, WI, and New York, NY).

A method of simulation of solar processes and its application

Abstract TRNSYS, a program for simulating the dynamic thermal behavior of transient systems, is a general program for solving sets of differential and algebraic equations which describe solar energy systems. It is based on a modular approach which enables the user to readily simulate a wide variety of systems. The program consists of component models (for collectors, controls, storage tanks, heat exchangers, furnaces, building loads, integrators, recorders, etc.) and an executive routine. The designer selects both his components and the design parameters describing the components and specifies, in a simple fashion, the way in which the components are interconnected. The whole process is analogous to specifying an experimental system. The designer also selects the information he wants from the simulation, again in a manner analogous to a physical experiment, and includes the appropriate instrument components in his simulation. The use of the program is illustrated by a comparison of methods of operating a solar heating system.

A slit-and -expanded aluminum-foil matrix solar collector

Abstract This paper proposes the use of porous matrices as the heat-absorbing media in air-cooled solar collectors. Because heat-transfer and fluid-friction data have been obtained on slit-and-expanded aluminum-foil matrices, this type is particularly discussed. Porous matrices absorb solar radiation in depth, and when arranged in a parallel-flow system, their upper surface areas are subjected to cool air, so that the upper losses are minimized. They also have high heat-transfer area/volume ratios. A matrix collector is also inexpensive and simple. An analysis of heat-transfer and fluid-friction characteristics of such a collector indicates an efficiency higher than that of flat-plate collectors, and friction losses that are much lower.

Analysis of collector-storage building walls using phase-change materials

The use of thermal storage walls that serve both as solar collector and thermal storage is well known. The wall is usually composed of masonry or containers filled with water to provide sensible heat storage, i.e., storage resulting from the specific heat capacity of a material as it increases in temperature. An interesting alternative to the standard materials are phase-change materials (PCMs) which employ latent heat storage. Latent heat storage utilizes the energy associated with a change of state of a material such as the transition from a solid-to-liquid, or liquid-to-gas. The solid-to-liquid phase change is preferred for many applications because of the much smaller volume change resulting in this transition for a given amount of energy storage. This paper summarizes the results of a simulation study of the use of PCMs as a collector-storage wall.