Beam Irradiance Research Articles

The probability density and autocorrelation of short-term global and beam irradiance

For individual hours, a characteristic bimodal pattern of short-term global and beam irradiance is frequently observed, with modes at high and low irradiances and with low probabilities near the hourly averages. For such hours, averaging over the hour will imply smoothing of quite significant variations within the hour. Models for the probability density distributions of short-term (5 min or less) irradiances are presented in this paper. These distributions are not unique functions of the hourly averages, but depend heavily also on the irradiance variability within the hour. This intrahour variability is found to depend on the averaging time and also on the interhour variability among three hourly averages, namely, the hour in question, the preceding and the deceding hour. The distribution differences between 5 min averages and instantaneous values are, however, negligible for most practical purposes. The lag one autocorrelation is evaluated as a function of averaging time, and a first order autoregressive model is presented. With hourly averages as the only input the probability density and autoregressive model in combination produce time series of short-term intrahour averages having realistic distributions and autocorrelation structure.

Daylight measurement data: Methods of evaluation and representation

The International Daylight Measurement Programme will generate a large amount of data from distinct locations worldwide. To ensure comparability of results, data must be identified uniquely and standardisation of data analysis including the determination of derived quantities is necessary. A number of formulae are proposed for the normalisation and analysis of direct beam, global and diffuse irradiances and illuminances and sky luminance distributions. Graphical representation of results is discussed and a graphical approach, the P-G-D diagram, is put forward as a solution for irradiance and illuminance data presentation.

A probability density model for hourly total and beam irradiance on arbitrarily orientated planes

An analytical and adjustable model is proposed, by which the probability density of hourly global irradiance may be derived from long-term mean global irradiation. The model explicitly accounts for the bimodal character of the probability density of solar irradiance, and it is tuned to a developmental data sample and verified against an independent data base. In combination with previously published deterministic models this probability density model even forms a composite model which yields the probability density of hourly irradiance (total, beam or diffuse) on arbitrarily orientated planes. This composite model is also verified against the independent data base.

A model for the diffuse fraction of hourly global radiation

Hourly values of diffuse and beam irradiance are often required in cases where, at best, only global irradiance is available. For use in the evaluation of the climatological potential for solar energy utilization in Norway, an analytical model is proposed expressing the hourly diffuse fraction of global irradiance in terms of hourly solar elevation and clearness index. This model, developed for average snow-free close to sea level conditions in Norway, fairly well fits the average picture of an extensive independent data base.

Estimation of hourly beam and diffuse solar radiation

Abstract Measurements of beam and diffuse solar irradiance on a horizontal surface have been utilized to verify simple methods of prediction of solar radiation. Measurements of hourly beam and diffuse solar irradiance on a horizontal surface have been made at Roma, a town 35 km from Maseru, on a few selected clear cloudless days during 1984–1985 and the results are presented. The results are compared with those estimated theoretically using the models proposed by Hottel, H. C. Solar Energy18, 129 (1976) and by Liu, B. Y. H. and Jordan, R. C. Solar Energy4, 3 (1960). It is suggested that the theoretical models of Hottel and of Liu and Jordan are suitable for estimating hourly beam and diffuse irradiation for any location in Lesotho.