Accurate estimation of the energy gain from solar radiation in buildings is necessary for building energy performance characterization, model predictive control (MPC), fault detection and diagnostics, etc. Solar radiation affects the buildings’ internal air temperature dynamics, either (directly) by penetrating the glazing or (indirectly) through the opaque building envelope. Nevertheless, no research has investigated the on-site data-driven modelling of the indirect effects of solar radiation i.e., additional solar gain through the opaque building envelope, marked as indirect solar gain. Therefore, this work aims to develop grey-box model-based techniques to characterize the dynamics of indirect solar gain. A test box, with overall dimensions of 120 x 120 x 120 cm3, that represents a simplified scale model of a building is examined, to provide an initial understanding of this matter. This test box is south-north orientated and has only one window of 60 x 60 cm2, positioned on its southern wall. On-site data associated with this text box was recorded during the summer (June-July) in Almeria, Spain. This simplified reduced-size text box satisfies the research goal very well to serve as a pilot case study, since the indirect solar gain was the dominant effect of solar radiation. Based on the in-situ data from this case, a three-dimensionally decomposed solar irradiance integrated grey-box modelling technique is proposed for characterizing the dynamics of indirect solar gain. Preliminary results from this study show that this technique can effectively reveal the key dynamics of indirect solar gain and outperform the classic grey-box model, based on limited low-frequency on-site measured data.
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