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This paper dealt with a series of numerical investigations on a new porous cooling channel applied to PV/T systems in order to improve the insufficient heat transfer in the conventional channel. The proposed porous cooling channel based on field synergy theory had a higher overall heat transfer coefficient, which enhanced the total efficiency of the PV/T system. The numerical model was validated with experimental data. The results showed that holes distributed non-uniformly near the outlet of the cooling water led to a better cooling effect, and a hole diameter of 0.005 m led to an optimal performance. The total efficiency of the PV module with the new cooling channel was 4.17% higher than the conventional one at a solar irradiance of 1000 W/m2 and an inlet mass flow rate of 0.006 kg/s. In addition, as the solar irradiance increased from 300 to 1200 W/m2, the total efficiency of the new PV/T system dropped by 5.07%, which included reductions in both the electrical and thermal efficiency. The total efficiency was improved by 18.04% as the inlet mass flow rate of cooling water increased from 0.002 to 0.02 kg/s.

The amount and shape of intermetallic compounds (IMCs) may influence the properties of the Al–Mg dissimilar welded joints. In the present study, friction stir welding (FSW) assisted with Zn interlayer was designed to weld Mg to Al alloys. The effects of the Zn interlayer on the microstructure and mechanical properties of the joints were studied in detail. The results indicated that Zn interlayer could improve performance of the joints and inhibit the formation of Mg/Al IMCs, which were replaced by Mg–Zn and Al–Zn solid solutions. The maximum tensile strength of the joint with Zn interlayer was 102 MPa, which was elevated about 27% compared with the joint without Zn interlayer.

With the increase of welding speed from 75 to 150 mm/min, the grain size of the nugget zone first decreased and then increased, and the smallest grain size was obtained at 100 mm/min. The maximum tensile strength of 445 MPa was achieved at the welding speed of 100 mm/min, equivalent to 70% of the base metal. The hardness value of the heat-affected zone increased with the rising welding speed. Joint hardness was affected by precipitated phases and it decreased overtly from thermo-mechanically affected zone to heat-affected zone. Owing to the fact that a number of big precipitates clustered in the heat-affected zone, the strengthening effect in spray formed ultra-high strength 7055 aluminum alloy was weakened which caused the hardness of the joint to reduce remarkably. Because of the recrystallization, most of the phases in nugget zone were dissolved and the rest were similar to those in the base metal, while AlCuMg phase and Al2Cu phase precipitated in heat-affected zone. The phase precipitated in the nugget zone and heat-affected zone decreased gradually with the increase of the welding speed.

High cell temperature of PV modules would reduce electrical efficiency and hinder the development of photovoltaic application. Many potential cooling channels have been proposed for PV modules cooling but perform inefficiently, due to the insufficient heat transfer between the cooling water and PV modules. In current research, a PV/T system with a special cooling channel based on field synergy theory, was designed to optimize the performance of PV/T systems. The performance of the new PV/T system was studied numerically, and mathematical models were validated with the experimental data. Results indicated that electrical efficiency of PV modules increased by 0.5% (from 11.4% to 11.9%) when the solar radiation decreased from 1200 to 300 W/m2. The electrical efficiency increased 1.11% (from 10.8% to 11.9%) as the inlet mass flowrate of cooling water increased from 0.0018 to 0.018 kg/s at the inlet temperature of cooling water of 20 °C. Additionally, the special channel with shark dorsal fin type sawtooth or regular type sawtooth, had the optimal parameter values, with the channel depth of 0.05 m, the sawtooth peak of 0.035 m and the sawtooth width of 0.01 m. The average cell temperature with the optimized channel was 6.05 °C lower than the conventional smooth channel, and the new cooling channel performed better with a high solar radiation.

The influence of wire feeding speed during laser welding with a filling wire was investigated by high-speed imaging and mechanical analysis of the droplet. A force relationship was found to analyze the droplet transition behavior at the end of the welding wire in the narrow-gap groove. Welding defects, mainly including nonfusion and porosity, were studied to reveal the formation mechanism and propose affective solutions. In this paper, it was found that the droplet transition was determined by the position of the melting end of the wire in the narrow-gap groove under the same parameters. The sidewall transition, the liquid bridge transition, and the spreading transition were found at different wire feeding speeds. As the wire feeding speed increases from 3 to 6 m/min, the weld depth increased 40%, the maximum weld width decreased 16%, and the minimum weld width increased 8%. More nonfusion and porosity defects and poor weld formation would appear at excessive wire feeding speed.