1. Introduction
In the last years particular attention has been paid to the development of economically viable solar thermal and electrical systems, with the twofold aim at improving power density and reducing system capital costs. As well known, for a long time the massive development of solar technologies has been limited by their poor economic profitability due to the high system capital costs.
Nevertheless, the recent remarkable decrease of capital costs of solar thermal collectors (SC) and photovoltaic (PV) panels and the simultaneous increase of sustainable energy policies, adopted by the majority of governments worldwide, are promoting more and more the uptake of small-scale solar systems. Among European Union countries, one of the goals of the established energy policies is to achieve a widespread adoption of small SC and PV systems, in order to enhance the utilization of renewable energy sources [
1,
2].
In this framework, photovoltaic/thermal (PVT) collectors are particularly promising due to their combination of conventional PV and SC collectors in a single component [
3,
4]. PVT collectors are typically manufactured by using a conventional thermal collector whose absorber is covered with a suitable PV layer. The thermal energy is distributed to a fluid, typically air [
5] or water [
6,
7,
8], whereas the PV layer produces electricity [
3,
4]. The overall result of this technology is a simultaneous production of electricity and heat [
9], with a reduction of the PV modules’ electrical efficiency losses.
In fact, the electrical efficiency of a PVT collector may be even higher than that of a conventional PV panel given moderate PVT operating temperature increases [
3,
4,
10]. In order to study the effect of the operating temperature on the electrical and thermal efficiencies, numerous numerical and experimental works on this topic were carried out in the last years [
11,
12,
13]. Kalogirou and Tripanagnostopoulos [
14] carried out a numerical simulation analysis of the energy performance of two PVT collector models (one made from polycrystalline silicon, pc-Si, and another with amorphous silicon, a-Si) for three locations at different latitudes (Nicosia, Athens and Madison). The obtained results (also tested experimentally) showed that although a PV system produces about 38% more electrical energy, the studied PVT system also covers, depending on the location, a large percentage of the hot water needs. Aste et al. [
6] designed an experimental innovative water glazed PVT component, consisting of a thin film PV technology and a roll-bond flat plate absorber, and developed a mathematical model for the prediction of the electrical and thermal outputs. Through such a model, validated by means of the obtained experimental data, the daily mean electrical efficiency of the PVT resulted to be about 6.0%, whereas the PV module showed a daily average efficiency of 6.2%. The PVT collector also produced thermal energy, with a daily efficiency of 25.8%. Touafek et al. [
15] modelled and simulated a novel design of a PVT collector including an absorber plate integrated with sheet galvanized steel. The advantages of this collector with respect to other configurations are better heat absorption and lower production cost. The simulation model allows assessing the temperatures levels of such layers and the effect of some parameters on the electrical and thermal performances. The authors also compared the prototype collector’s performance with that of existing configurations. Fan et al. [
16] reported the results of a 10 months experimental analysis of a 900 Wp liquid type glazed mc-Si PVT system, developed in Singapore. The analysis carried out on the operational data showed that the PVT system is capable of achieving 41.1% of the average monthly conversion efficiency. Dry and wet stagnation tests were also performed, showing that the maximum temperature of the water under dry and wet stagnation conditions was 64 °C and 65 °C, respectively. Herrando and Markides [
17] developed a numerical model of a water cooled PVT collector in order to estimate the year-long techno-economic performance of the system for a typical house in London, UK. The results showed that for the simulated low solar irradiance levels and low ambient temperatures, a higher coverage of total household energy demands and higher CO
2 emission savings can be achieved through the complete coverage of the solar collector with PV and a relatively low collector cooling flow-rate. They concluded that hybrid PVT systems offer a notably improved proposition over PV-only systems [
18]. Guo et al. [
19] reported the results obtained through a simulation model of a novel tri-functional PVT collector, validated by means of experimental data. The investigated collector operates in PV/water-heating and PV/air-heating modes, as a function of the energy demands. The authors highlighted the good consistency between simulation and experimental results, showing also that the tri-functional PVT collector is more efficient than a disjointed configuration of a PVT water and air collector. In addition, the authors also investigated the collector performance under different flow rates, wind speeds, inlet air temperatures, initial water temperatures, as well as the annual thermal efficiencies of such collector in three different climates in China (38.5%, 38.9%, and 40.1% in Hefei, Beijing, and Xining, respectively). Sardarabadi et al. [
20] experimentally investigated the effects of adding nanofluids (SiO
2/pure water, 1 wt % and 3 wt %) as a coolant flowing through the flat plate PVT collector from both energetic and exergetic points of view. Daily experiments were performed for the PVT system, tilted by 32° and under constant mass flow rates. The results showed that silica/water nanofluid suspension significantly enhanced the energetic and exergetic performances of the system, showing an increase of overall energy efficiency by up to 7.9% and of total exergy by up to 24.31%.
As mentioned before, a number of papers available in the literature have investigated several PVT collectors from both numerical and experimental points of view. However, only a few papers couple experimental analyses with 1-year simulation models of the system. In addition, none of the investigated papers present an integrated approach aiming at comparing PVT vs. PV technologies, from both numerical and experimental points of view.
In this framework, this paper aims to cover this lack of knowledge, reporting numerical and experimental studies where a PVT solar field is compared, from both energetic and economic points of views, with a PV field (identical PV modules and different PVT collectors are simulated/experimented). Note that simulations and experiments were carried out by taking into account the same PV types, whereas different PVT technologies were considered. Specifically, the experimented PVTs collectors are unglazed types, consisting of the same PV models in combination with water heat extraction units. Differently, in order to evaluate the performance of the novel generation of PVTs, represented by glazed devices, an additional comparison with the PVs was carried out by means of dynamic simulations. In fact, this paper presents the design of an experimental set-up, consisting of PVT collectors and PV panels, as well as the results obtained through the thermo-economic dynamic simulation model, developed for the analysis and comparison of energy performance, are discussed. In particular, the experimental set-up, installed at the company AV Project Ltd., located in Avellino (Italy), consists of four flat polycrystalline silicon PV panels and four flat polycrystalline silicon PVT collectors (named Janus). The designed experimental set-up allows gathering the systems electrical and thermal efficiencies as well as the temperatures reached by both solar technologies (PV and PVT), in order to determine: (i) the technology showing the higher performance; (ii) validate the dynamic simulation model by comparing the obtained results with experimental ones.