Simulation and experimental performance analysis of a modified PV panel to a PVT collector Evangelos Sakellariou ⇑ , Petros Axaopoulos Τ.Ε.I. of Athens, Department of Energy Technology Engineering, Laboratory of Renewable Energy Sources, 17, Agiou Spyridona, GR12210 Egaleo, Greece article info Article history: Received 19 June 2017 Accepted 28 June 2017 Keywords: Retrofitted PV/T PV/T Hybrid PV panel Copper-made PV/T absorber PVT steady state model abstract A retrofitted PVT collector assembled and installed together with a conventional PV panel, on northern hemisphere, facing south-west and tilted to 40°. In order to evaluate the system’s performance during the autumn to winter period, outdoor experiments were conducted and a PC-based automatic data acqui- sition unit has been used. The results show that the PVT collector managed to slightly increase its average electrical energy yield by 0.32% during the experimentation period and obtained additional thermal energy with daily average efficiency of 20.33%. Moreover, PVT’s moderate thermal insulation revealed a detrimental factor regarding its thermal energy generation. Furthermore, a steady state model was cre- ated based on Hotel-Wilier equation modified for PVT collectors by Florschuetz. Two typical days were chosen and simulated for the model’s evaluation. The simulation outcomes revealed good agreement between the model and the measurements, with average figures of 3.12% for useful heat and for electric- ity between À4.46% and 10.72%. Finally, the PVT’s model, managed to predict the system’s operation on an average day’s values with À8.25% and 7.49% for useful thermal energy and electrical energy respec- tively, while the corresponding monthly figures of standard deviation were 2.23% for useful heat energy and 5.71% for electricity. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction The solar industry is largely comprised of two dominant tech- nologies, the Photovoltaics (PV) and the Solar Thermal (T) systems. Photovoltaics use solar energy to generate electricity, while solar thermal systems use the sun with the purpose to generate thermal energy. Nevertheless, PV efficiency deteriorates at high cell tem- peratures while high collector’s temperature is welcome for better efficiency in thermal systems. The above controversial operation of those technologies has driven into the PVT which is a combined collector. When solar irradiation incidents in a PV panel, only a small frac- tion of this energy about 5–15% becomes electrical energy, while another portion of 7–10% is reflected and the greatest portion becomes heat. The remaining heat at the PV panel increases the cells’ temperature and thus decreases the electrical efficiency. However, if a heat-absorber is installed on the rear side of the PV panel, the cells’ temperature will be decreased by removing the cells’ heat and thus the cells’ efficiency will be increased. Major review papers for PVT collectors have been carried out by Zondag (2008a), Abdul Hamid et al. (2014), Moradi et al. (2013), Charalambous et al. (2007) and Zhang et al. (2012). The PVTs are roughly divided into two categories according to the heat removal fluid, the liquid and the air based PVT collectors. There are also combined PVT collectors with air and liquid mean as illustrated in Abdul Hamid et al. (2014). Indicative thermal and electrical PVT efficiencies are illustrated in Table 1 by Zondag (2008a). In the table below, it is observed that the unglazed (no cover) sheet and tube PVT collector have 52% thermal efficiency and 9.7% elec- trical efficiency. Most pioneering studies on PVT collectors are carried out with retrofitted collectors due to lack of commercially available PVT col- lectors. Therefore, a vast amount of knowledge has been gained regarding the absorber configuration from retrofitted PVT collec- tors. Therefore, many researchers have investigated the impact of the collector configuration on its performance. Furthermore, many works have been carried out regarding the ratio of W/D which influences directly the collector’s efficiency, as well the cost and its weight (Zondag, 2008b; Charalambous et al., 2007). Many researchers did not optimize geometrically the PVT collector because the main aim of their work was to eval- uate the PVT collector behavior (Charalambous et al., 2011a,b) optimized the stripe and the serpentine plate tube collectors http://dx.doi.org/10.1016/j.solener.2017.06.067 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: evangelosenergy@gmail.com (E. Sakellariou), pax@teiath.gr (P. Axaopoulos). Solar Energy 155 (2017) 715–726 Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener