Studies on methods of reducing heat losses from flat plate solar collectors. Annual progress report, August 1, 1974--July 31, 1975

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A method to generate synthetic hourly solar radiation globally

This paper develops a stochastic procedure for generating synthetic sets of hourly solar irradiation values, suitable for use in solar simulation design work. The daily atmospheric transmittance K t for the day in question is broken down into hourly irradiation events by a stochastic disaggregation procedure. The necessary disaggregation models were constructed using the quasi-universal hourly atmospheric transmittance k t , instead of the irradiation itself, as the random variable. Analyses of meteorological records revealed that both the marginal probability and stochastic features of sets of hourly events k t within an individual day can be closely predicted using the K t for the day. Results indicated that the parameters defining the disaggregation procedure were independent of geographical location. The values of k t are found to be closely modeled by a Beta distribution, and a mean correlation coefficient between successive hourly values of k t was found to be 0.54±0.14. An algorithm is described from which unlimited hourly solar irradiation data may, therefore, be generated using only the 12 monthly means of daily events, K t

Relationship between sunshine duration and solar radiation

This paper examines the relationship between sunshine duration and solar radiation received on the earth's surface. Sixty-nine thousand pairs of sunshine-radiation readings from 670 sites were analyzed. A generalization of the Ångström-Prescott equation of the form K=Kclear[β+(1-β)Sγ] was found to most efficiently fit the data and suggests the relationship between the average daily atmospheric transmittance K and the sunshine fraction S is non-linear. The suggested reason for this non-linearity is that a reduced sunshine fraction not only decreases the clear sky radiation duration, but also the radiation transmitted through clouds, i.e. clouds get optically thicker with decreasing S. This finding is supported on theoretical grounds and by analyzing instantaneous solar radiation measurements from Australia and Germany.Representing the sunshine fraction in terms of the proportion of beam radiation reaching the earth's surface S=Hb/Hb,clear leads to a fundamental connection between the monthly average diffuse fraction and the sunshine-radiation relationship. Moreover, it confirms the non-linearity of the latter relationship, which was previously questioned because of limited data and/or poor quality sunshine measurements

Methods for reducing heat losses from flat plate solar collectors: Phase II. Final report, February 1, 1976--August 31, 1977

Improvements to flat plate solar collectors for heating and cooling of buildings were investigated through two parallel studies. The first study, which deals with the free convective heat loss from V-corrugated absorber plate to a plane glass cover, has shown that, for the same average spacing, the free convective heat loss is greater for a V-corrugated absorber plate than for a plane absorber plate. However, provided the average spacing is large enough, the amount of increase is slight. The second study, which deals with the free convective heat loss in a honeycomb solar collector in which the honeycomb consists of a set of horizontal partitions, or slits, has shown that provided the solar collector is tilted to near vertical, such a honeycomb gives equivalent or superior free convective loss suppression than does a square-celled honeycomb having the same amount of material. Correlation equations for the free convective heat loss are given for both studies

A time series model for Kt with application to global synthetic weather generation

This article describes a general mathematical procedure for generating synthetic daily solar irradiation values. The procedure should be useful for simulating solar energy systems, requiring only the 12 monthly means ( K t ), as input. The procedure is based on a time series analysis of the daily K t values, described in the article. It incorporates the well-known probability distribution function for K t , by using a transformed variable, rather than K t , as the fundamental random quantity