Systematic testing of hybrid PV-thermal (PVT) solar collectors in steady-state and dynamic outdoor conditions

• Published in: Applied energy 2019-04, Vol.240, p.1014-1030
• Main Authors: Guarracino, Ilaria, Freeman, James, Ramos, Alba, Kalogirou, Soteris A., Ekins-Daukes, Nicholas J., Markides, Christos N.
• Format: Article
• Language: English
• Subjects: Captadors solars; Collector performance analysis; Dynamic modelling; Energia solar; Energies; Experimental characterisation of solar collectors; Hybrid PVT collectors; Solar collectors; Solar energy; Àrees temàtiques de la UPC
• ISSN: 0306-2619; 1872-9118
• DOI: 10.1016/j.apenergy.2018.12.049

•A methodology for the outdoor testing of hybrid PVT collectors is presented.•Glazed and unglazed collectors featuring c-Si and thin-film PV modules are compared.•Steady state and dynamic thermal performance of the PVT collectors are characterised.•Thermal and electrical energy yields over diurnal operating periods are assessed.•Experimental performance is compared to model simulations under intermittent cloud conditions. Hybrid photovoltaic-thermal (PVT) collectors have been proposed for the combined generation of electricity and heat from the same area. In order to predict accurately the electrical and thermal energy generation from hybrid PVT systems, it is necessary that both the steady-state and dynamic performance of the collectors is considered. This work focuses on the performance characterisation of non-concentrating PVT collectors under outdoor conditions. A novel aspect concerns the application of existing methods, adapted from relevant international standards for flat plate and evacuated tube solar-thermal collectors, to PVT collectors for which there is no formally established testing methodology at present. Three different types of PVT collector are tested, with a focus on the design parameters that affect their electrical and thermal performance during operation. Among other results, we show that a PVT collector suffers a 10% decrease in thermal efficiency when the electricity conversion is close to the maximum power point compared to open-circuit mode, and that a poor thermal contact between the PV laminate and the copper absorber can lead to a significant deterioration in thermal performance. The addition of a glass cover improves the thermal efficiency, but causes electrical performance losses that vary with the glass transmittance and the solar incidence angle. The reduction in electrical efficiency at large incidence angles is more significant than that due to elevated temperatures representative of water-heating applications. Dynamic performance is characterised by imposing a step change in irradiance in order to quantify the collector time constant and effective heat capacity. This paper demonstrates that PVT collectors are characterised by a slow thermal response in comparison to ordinary flat plate solar-thermal collectors, due to the additional thermal mass of the PV layer. A time constant of ∼8 min is measured for a commercial PVT module, compared to