Messa a punto e analisi prestazionale di un collettore ibrido fotovoltaico termico a fluido termovettore acqua

Currently, commercial solar cells typically have an electrical efficiency ranging from 5% to 25%, meaning that a significant part of the incident solar energy can be harvested in the form of heat and used for low-temperature heating. For this reason, much research effort has been spent on the development of hybrid photovoltaic-thermal (PVT) collectors technology (water or air). Two are the main benefits related to PVT technology: first, the efficiency of PV cells can be increased by actively cooling the PV laminate and the removed heat can be subsequently used. Second, incorporating a PV and a thermal system into a single unit, the total area dedicated to solar energy devices can be reduced. For these reasons, many studies concur that a well-designed hybrid system can achieve better performances compared to two separated systems. In this sense, the PVT technology most investigated in recent times is based on systems which use water as heat transfer fluid, because they achieve higher overall efficiencies than air systems due to the greater heat conductivity, and hence greater heat capacity, of water. Aim of the thesis is the design of an innovative covered hybrid PVT collector with water as heat transfer fluid. First of all the heat exchanger of the experimental collector was analyzed and designed by means of computational fluid dynamics (CFD) calculations comparing two absorber pipe arrangement: serpentine and harp. The temperature distribution through the pipes is not uniform in both cases: temperature increases from left to right in first case, and from the bottom to the top in the second one. Furthermore, serpentine absorber reaches higher temperature gradient than the harp configuration. As a consequence, each PV cell placed on a specific portion of the absorber will have a different temperature and, thus, a different electrical performance. Electrical efficiency of PV cells is affected by their temperature and consequently by the temperature distribution on the underlying absorber plate. In fact when a temperature gradient exists, not all cells may be able to operate in the same manner (i. e. at their maximum power point) at the same time, causing parallel-series mismatches. For that reason it is foundamental theoretically investigate on electrical efficiency in function of the temperature distribution in a PVT collector, taking into account two different configurations of solar absorbers analyzed above. In order to analyze the different influence that the two temperature distributions in the absorber can have on PV conversion efficiency, electrical simulation were carried out considering series and parallel configurations of tipical PV cells’ electrical wirings,. Subsequently, each electrical layout was coupled with both the thermal distributions on absorber plates, obtaining the specific working temperature of each of the 60 PV cells. The results demonstrate that harp absorber, thanks to a more homogeneous thermal distribution and to lower average temperatures is more suitable for PVT application than serpentine absorber. Moreover, in order to overcome the described temperature effect, that is decisive in the characterization of the electrical performance of a PVT module, the experimental collector adopt a silicon thin film double junction technology characterized by lower temperature coefficient. This process combines an amorphous silicon top layer over a microcrystalline silicon layer. The top cells absorb and converts the visible solar spectrum, while the bottom one is sensitive to near infrared wavelengths. Morever, amorphous silicon undergoes a sort of regeneration process, which increases the electrical efficiency (initially lost due to the Staebler-Wronski effect). That effect, known as thermal annealing, can increase the electrical production of the PVT component. The PV sandwich was bonded mechanically with the two designed roll-bond aluminum absorber with harp channels arrangement. The whole components are enclosed into an aluminum frame and covered by a glass 4 mm thick. The air gap between the PV laminate and the cover glass is 200 mm depth. In order to reduce thermal losses and increase the output fluid temperature from the collector, thermal insulation of mineral fiber mat material, 500 mm thick, is applied in the rear side of the absorber. In order to evaluate the performance of the designed PVT collectors compared to a commercial collector, an experimental campaign data was performed from August to December 2013. A monitoring equipment has been connected to the PVT system in order to measure thermal and electrical performance. The comparison shown that the developed PVT collector produce 3% more specific energy than the commercial PVT. Finally the experimental PVT collector was compared with a simple PV sandwich characterized by the same electrical features of the experimental collector. The results shown that, although the two technology produces almost the same electrical energy, the PVT collector is able to convert the remaining part of solar radiation with a maximum efficiency of 50%. Further works can be focused on summer performance of PVT collectors in order to understand the regeneration effect of a-Si/μc-Si due to high temperature. Electrical efficiency of PV cells is affected by their temperature and consequently by the temperature distribution on the underlying absorber plate. In fact when a temperature gradient exists, not all cells may be able to operate in the same manner (i. e. at their maximum power point) at the same time, causing parallel-series mismatches. For that reason it is foundamental theoretically investigate on electrical efficiency in function of the temperature distribution in a PVT collector, taking into account two different configurations of solar absorbers analyzed above. In order to analyze the different influence that the two temperature distributions in the absorber can have on PV conversion efficiency, electrical simulation were carried out considering series and parallel configurations of tipical PV cells’ electrical wirings,. Subsequently, each electrical layout was coupled with both the thermal distributions on absorber plates, obtaining the specific working temperature of each of the 60 PV cells. The results demonstrate that harp absorber, thanks to a more homogeneous thermal distribution and to lower average temperatures is more suitable for PVT application than serpentine absorber. Moreover, in order to overcome the described temperature effect, that is decisive in the characterization of the electrical performance of a PVT module, the experimental collector adopt a silicon thin film double junction technology characterized by lower temperature coefficient. This process combines an amorphous silicon top layer over a microcrystalline silicon layer. The top cells absorb and converts the visible solar spectrum, while the bottom one is sensitive to near infrared wavelengths. Morever, amorphous silicon undergoes a sort of regeneration process, which increases the electrical efficiency (initially lost due to the Staebler-Wronski effect). That effect, known as thermal annealing, can increase the electrical production of the PVT component. The PV sandwich was bonded mechanically with the two designed roll-bond aluminum absorber with harp channels arrangement. Fig.2. Schematic of the PVT collector and realized product The whole components are enclosed into an aluminum frame and covered by a glass 4 mm thick. The air gap between the PV laminate and the cover glass is 200 mm depth. In order to reduce thermal losses and increase the output fluid temperature from the collector, thermal insulation of mineral fiber mat material, 500 mm thick, is applied in the rear side of the absorber. In order to evaluate the performance of the designed PVT collectors compared to a commercial collector, an experimental campaign data was performed from August to December 2013. A monitoring equipment has been connected to the PVT system in order to measure thermal and electrical performance. The comparison shown that the developed PVT collector produce 3% more specific energy than the commercial PVT. Finally the experimental PVT collector was compared with a simple PV sandwich characterized by the same electrical features of the experimental collector. The results shown that, although the two technology produces almost the same electrical energy, the PVT collector is able to convert the remaining part of solar radiation with a maximum efficiency of 50%. Further works can be focused on summer performance of PVT collectors in order to understand the regeneration effect of a-Si/μc-Si due to high temperature.

L’attività di ricerca proposta nel presente documento mira alla progettazione e all’ottimizzazione di un componente fotovoltaico innovativo volto al recupero della frazione di radiazione solare incidente sui moduli e non convertita in elettricità, sotto forma di energia termica atta ad essere impiegata all’interno dei fabbricati. Tale componente verrà quindi definito ibrido in quanto pensato per la cogenerazione di elettricità e calore da fonte solare.