Technical note: An investigation of possible improvements in accuracy of regressions between diffuse and global solar irradiation

Abstract

Horizontal hourly and sub-hourly diffuse and beam irradiance are required for the estimation of global irradiance on a given tilted surface. This information is used by engineers and architects for various solar energy applications and also to calculate solar heat gain in buildings. Although horizontal global irradiance is a commonly measured parameter for many sites, horizontal diffuse irradiance is not so readily obtainable. For such sites that measure global irradiation alone a simple but reasonably accurate method is required to estimate diffuse irradiance from its global counterpart. The aim of this study is to evaluate the accuracies of estimation of diffuse irradiance by developing annual, seasonal and monthly regressions between sub-hourly diffuse-ratio and clearness-index. An error analysis has been undertaken to compare these regression models. Practicalapplications: Diffuse and beam components of global irradiation are required to accurately simulate solar heat gain in buildings. The UK Meteorological office keeps records of diffuse and global irradiances for 9 stations, but there are a further 84 stations that solely record global irradiance. Hence there is a need to estimate diffuse or beam irradiance for these sites. This paper presents an investigation into the uses of annual, seasonal and monthly regressions between diffuse and global horizontal sub-hourly solar irradiation. These regressions have proven to be a simple but reasonably accurate method of estimating diffuse irradiance from its global counterpart.

Extensive worldwide validation and climate sensitivity analysis of direct irradiance predictions from 1-min global irradiance

“…These errors can be only minimized if careful corrections and filtering are exerted on the raw data first. Such refinements have been attempted in various ways in recent studies, at least to some extent; e.g., Clarke et al, 2007;De Miguel et al, 2001;Muneer et al, 2007;Posadillo and Lopez Luque, 2009;Ruiz-Arias et al, 2010;Tapakis et al, 2015;Younes et al, 2005). It is however difficult to know whether the various levels of quality control in current use are sufficient, or conversely too stringent (thus eliminating too many good data points).…”

Evaluation of solar modelling techniques through experiment on a 627 kWp photo-voltaic solar power plant at Edinburgh College—Midlothian Campus, Scotland

“…As indicated in Figure 8, the principal input variables to determine the slope irradiation (once the position of the slope has been decided) are the horizontal irradiation and the time of day (and date), which allow the position of the sun in the sky to be calculated. As data readings were taken over 5 minute time intervals, not at an exact time, it was decided to follow the approach used by Clarke, et al (2007) and take the mid-point of the time period as the data point, ie. 2.5 minutes before the logged time.…”

“…Three different equations (5.1, 5.2, and 5.3) were selected to generate a range of results, one from Muneer et al (2000), the others from Clarke et al (2007). They are shown below with coefficients selected for Edinburgh: Once both the diffuse and global (total) horizontal irradiation are known, it becomes possible, for a given collector inclination and orientation, to calculate the global slope irradiation (generally referred to as the slope irradiation).…”

Signature of the North Atlantic Oscillation on British solar radiation availability and PV potential: The winter zonal seesaw

“…The UKMO recordings of the diffuse component were discontinued in 1997; the direct component, which could be determined by subtraction, is therefore equally unknown from then. In the cloudy British climate, the diffuse component constitutes a significant part of global radiation (Clarke et al, 2007) and can exceed 50% at the monthly timescale. We therefore extend the analysis of the radiation-WiNAOI relationship to the direct and diffuse components for the available data.…”