Experimental investigation of thermal and electrical performances of a nanofluid-cooled photovoltaic/thermal system equipped with a sheet-and-grooved serpentine tube collector

In the present experimental investigation, the thermal and electrical performances of a photovoltaic/thermal system equipped with a sheet-and-grooved serpentine tube collector are investigated. The water-magnetite nanofluid is used as the heat transfer fluid. The effect of nanoparticle volume concentration (0-1%), nanofluid mass flow rate (10-40 kg/h) and groove pitch (0, 0.54 and 8 mm) on the temperature of photovoltaic panel, thermal efficiency, electrical efficiency and overall efficiency is examined. A review of the literature reveals that the present study is the first experimental study on the performance of photovoltaic/thermal systems with a sheet-and-grooved serpentine tube collector. 

In order to evaluate and compare the performance of the PVT systems studied in the present study, weather conditions such as ambient temperature and solar radiation intensity should be similar in different experiments. Therefore, it was decided to perform the experiments using a solar simulator capable of producing a uniform heat flux (1000 W/m2) at a constant ambient temperature (22 °C). The collector is connected to the bottom of the photovoltaic panel and the serpentine tube is welded to the bottom of the collector. In this way, the heat of the photovoltaic panel is transferred to the collector and from the collector to the serpentine tube, and finally, the nanofluid flowing in the tube receives heat from the tube wall and heated. After leaving the serpentine tube, the heated nanofluid enters a heat exchanger and transfers its heat to a coolant and, as a result, cools down. The grooves create secondary flow in the nanofluid that disrupts the thermal boundary layer and increases heat transfer from tube wall to the nanofluid. On the other hand, the grooves increase the pressure drop of the nanofluid, and thus increase the pumping power required to make the nanofluid flow in the serpentine tube, which is not desirable at all. 

The results showed that the thermal, electrical and overall efficiencies of the system with a sheet-and-plain serpentine tube collector are in the range of 30.89-41.09%, 11.89-11.99% and 62.19-72.63%, respectively. These values for the system equipped with a sheet-and-grooved serpentine collector having groove pitch of 8 mm are 34.57-46.81%, 12.06-12.15% and 66.30-78.78%, respectively, and for the system equipped with a sheet-and-grooved serpentine collector having groove pitch of 5.4 mm are 37.03-50.89%, 12.29-12.38% and 69.37-83.47%, respectively. Among the systems studied, the best thermal, electrical and overall performance belongs to the system having groove pitch of 5.4 mm, while the worst performance belongs to the system equipped with a sheet-and-plain serpentine tube collector. In addition, the results showed that increasing the nanoparticle concentration and nanofluid mass flow rate leads to improved thermal and electrical performance of all three systems studied in the present study.

Using a grooved serpentine tube instead of a plain serpentine tube causes the hot fluid near the tube wall to mix with the colder fluid passing through the central areas of the tube, and thus, the temperature distribution of the fluid becomes more uniform, which improves its cooling capability. This effect is enhanced by reducing the groove pitch. On the other hand, the larger the number of grooves, the greater the pressure drop across the nanofluid current in the serpentine tube, which in turn increases the power required to pump the nanofluid into the collector and, as a result, reduces the net electrical output of the system. Fortunately, the share of pumping power in the electrical power generated by the system is about 1%, which makes the positive effect of using a grooved serpentine tube more than its negative effect, and as a result, the performance of the photovoltaic/thermal system with a sheet-and-grooved serpentine tube having groove pitch of 5.4 mm is better than other systems. Also, because the nanofluid has a higher thermal conductivity than the pure water, it can receive more heat from the tube wall, which better cools the panel and, thus, improves its electrical efficiency. On the other hand, at the same mass flow rate, the velocity of the nanofluid in the tube is lower than that of water, which leads to a lower pressure drop of the nanofluid compared to water. Overall, the results showed that nanofluid is a better coolant than water for use in the photovoltaic/thermal systems studied in the present study. Increasing the mass flow rate also leads to an increase in the coolant velocity, which leads to a simultaneous increase in heat transfer and pressure drop, the former of which is desirable and the latter of which is undesirable. The results showed that the positive effect of increasing the mass flow rate outweighs the negative effect, and as a result, increasing the mass flow rate leads to improved thermal, electrical and overall performance of the studied photovoltaic/thermal systems.