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Photovoltaic-thermal (PVT) collectors, are useful systems to absorb solar energy and convert that to electrical and thermal energy. However, to make the best out of solar irradiation, it is vital to improve the thermal and energy efficiencies of such systems. Porous metal foams are suggested to be utilized in the present study for cooling of the PV cells and increase the thermal and electrical efficiencies. Moreover, the effects of various parameters, such as porous layer thickness, solar heat flux, and Reynolds number on these efficiencies are investigated numerically. Results mainly focus on the effects of the porous media on both thermodynamic and hydrodynamic characteristics of the system, i.e., velocity profiles and the pressure drop. The results indicated that the usage of porous media can improve both efficiencies (between 3 and 4% for the electrical and between 10 and 40% for the thermal efficiency) with a subsequent pressure loss. But for cases with a thickness of more than half of the channel height, it has a negative effect. In the best condition, Rp=0.5, the overall efficiency reaches up to 95%, however, in this case, the pressure drop rises considerably (up to 600 Pa), which causes destructive effects on the system and imposes additional costs.

In common parabolic trough solar collectors (PTC), solar irradiation is concentrated at the bottom of the absorber tube, causing high surface temperatures, thermal stresses, and deflections with subsequent damages and costs. Also, the performance improvement of PTCs is always of significant importance. The new idea of rotating the absorber tube is proposed, and its effects on the PTC performance are studied experimentally. The effects of the working fluid flow rate (Re number) and the rotational velocity of the absorber tube (Ta number) on the energy characteristics of the PTC are analyzed. Results indicate that the optimal selection of the rotational speed makes it possible to reduce and control the temperature of the absorber by about 60% reduction in the fluid-tube temperature difference, and 15% reduction in the maximum surface temperature. Furthermore, about 17% enhancement in the efficiency of the PTC is attained. The share of the natural and both the rotational and mass flow forced convection forces in the total heat transfer is analyzed through Gr, Re, Ta, and a convection evaluation parameter (CEP). Ta/Re ratio and CEP provide valuable information such that, CEP T a / R e ≈ 0.5 led to the highest efficiency (average and maximum efficiencies of 30% and 87% respectively).

Mixed convection inside a square cavity with internal rotating heater and cooler is analyzed numerically by simultaneous application of porous media and nanofluid as a heat transfer enhancement technique. Optimized multi-block porous foams are utilized to enhance the heat transfer. This type of medium could improve the heat transfer rate with manipulation and selection of porous regions’ pore size (or permeability) by amplifying the flow in critical regions and weakening it in non-effective areas. The whole cavity domain is assumed to be made of 25 distinct porous blocks. At first, the effects of the various rotation directions have been investigated and then the optimum distribution of pore size in the porous media is determined in a manner to maximize the heat transfer rate using the pattern search optimization algorithm. Finally, simultaneous effects of application of multi-block porous media and nanoparticle addition to the base fluid on the average Nusselt number are studied in various conditions. For this purpose, various volume fractions of the nanoparticles are implemented to investigate the effects of the different values of the volume fraction on Nu number. The optimization has done for different Ri and Ra numbers for achieving to the best distribution in each condition. In the best condition, 20.4% increase in the heat transfer is obtained.