Use of sunshine number for solar irradiance time series generation

Abstract

It is a common observation that the amount of solar energy incident on the ground strongly depends on the state of the sky. Larger amounts of radiation are received when the sky is free of clouds. Moreover, when clouds are present, the incident radiation depends on the cloud types. Two quantities are commonly used to describe the state of the sky. First, there is the total cloud cover amount (sometimes called the cloudiness degree or point cloudiness), which represents the fractional total cloud amount observed by eye. It is expressed in tens (or, sometimes, in oktas) of the celestial vault. The total cloud cover amount is essentially an instantaneous quantity. A daily averaged total cloud cover amount may be computed. The days are sometimes classified according to this average value. This is justified by the observed persistence of cloud cover amount. For a given time interval S within the daytime, the bright sunshine duration s may be evaluated and the relative sunshine σ (sometimes call the bright sunshine fraction, or the sunshine fraction) may be defined by σ ≡ ?s/S. In many cases S is the interval between the sunrise and the sunset (i.e. the day-light duration) in a given day and s is the measured number of daily bright sunshine hours. Shorter S intervals are also used. Of course, a low σ value is an indication for a high cloud cover amount and the relative sunshine is the second common (indirect) indicator of the state of the sky. Any solar radiation computing model should take account of the state of the sky. This may be done through a variety of means, ranging from very complicated computer codes to empirical relations (for reviews and model classifications see e.g. May et al. (1984), Bener (1984), Davies et al. (1988), Festa and Ratto (1993) and the chapters of this book). Choosing among the existing models usually takes into account two features: (1) the availability of meteorological and other kind of data required as input by the model and (2) the model accuracy. For many practical purposes and users the first criterion renders the sophisticated programs based on the solution of the radiative transfer equation unusable. As a consequence, the other models (which we call here simple models) were widely tested. According to their complexity, these models may be classified as: very simple models for computing global solar irradiance (and its components, in some particular cases). Here a very simple model is defined as follows: (i) very simple clear sky models do not require any meteorological parameter as input and (ii) very simple cloudy sky models require as input a single meteorological parameter associated to the cloudiness degree (e.g. the total cloud cover amount or the relative sunshine). The very simple models are important because the majority of the people involved in practical solar energy applications have access (for various reasons) to this kind of models only. simple models to compute direct, diffuse and global solar irradiance. These models require more than one meteorological parameter as input. The accuracy of simple and very simple models for computing global solar irradiance was tested and reported in a large number of papers (see, for instance, Badescu (1997)). A new kind of simple solar radiation model is presented in this chapter. So far, most models use only one parameter describing the state of the sky, i.e. relative sunshine or total cloud amount. The original feature of the new model is the usage of two such parameters. First, it is the common total cloud cover amount. Second, a two-value parameter (the sunshine number) stating whether the sun is or is not covered by clouds is also defined and used. For given radiation component, the kind of data required depends on application and user. For example, average monthly or daily data are required to conduct feasibility studies for solar energy systems. Data for hourly (or shorter) periods are needed to simulate the performance of solar devices or during collector testing and other activities. The new category of models is developed for users in need of instantaneous global irradiance). Actinometric and meteorological data from Romania are used in this work. However, the proposed models are of general interest as they can be easily fitted to data from other countries. The main message of this chapter is the proof that models based on two parameters related to the state of the sky strongly increase the computation accuracy. © 2008 Springer-Verlag Berlin Heidelberg. All rights are reserved.