Statistical investigation of the optimal averaging time for solar irradiance on horizontal and vertical surfaces in the UK

As output from photovoltaic (PV) panels is closely related to sunlight leve ls, monitoring solar irradiance levels is crucial for system design and pre dictive purposes. With advances in PV technology, urban sites at northerly locations, where both horizontal and vertical solar irradiance make signifi cant contributions, are becoming increasingly important. The aim of this pa per is to compare solar irradiance for horizontal and vertical orientations and to assess the relative effectiveness of differing averages, from 10 mi n to 1 h, for use in short-term prediction of solar irradiance levels for a UK site. Vertical and horizontal solar irradiance observations were collec ted from a monitoring station on the roof of a five-storey building at an u rban site in Newcastle-upon-Tyne, UK (latitude 55 degrees N). 10-min data w as collected fur 13- or 15-day periods in two summers (1994, 1995) and two winters (1993, 1994). Although mean levels for horizontal and vertical irra diance were different, as expected, general patterns were very similar indi cating the possibility of predicting vertical irradiance from horizontal at the same location, 10-min, 20-min,30-min and I-h averaging times were comp ared utilising autocorrelation coefficients and ARIMA models to assess the information lost when using longer averaging intervals. For consideration o f short-term changes, 10-min averages were most informative whilst hourly a verages were substantially poorer. (C) 2000 Elsevier Science Ltd. All right s reserved.