## 1 Introduction  
At present, pollution released from combustion in industries is one of main causes of environmental problems. To reduce the pollutant emission from the production of thermal energy, solar energy receives an increasing attention as a clean and abundant renewable source [1]. The evacuated-tube solar collector (ETSC) is a system used to absorb solar energy from radiation of the sun and convert into thermal energy. There have been several studies on the improvement of thermal performance of ETSC and the applications of the ETSC in industries. [PERSON] et al. [2] studied the thermal performance of evacuated-tube solar collector with U-tube by analyzing a network of thermal resistances. They found that the efficiency of evacuated-tube solar collector with U-tube and exit fluid temperature would increase by 10% and 16%, respectively, if the thermal conductivity increased from 5 to 40 W/m\(\cdot\)K. Some researchers tried to modify the inner structure of evacuated tube to reduce thermal resistance. [PERSON] et al. [3] improved performance of evacuated tube heat pipes by filling the gap between the heat pipe and inner wall of an evacuated tube with oil and foamed copper. The oil was added to store thermal energy supplied to the heat pipe after the sun set, and the foamed copper was used to enhance heat conduction. They found that the evacuated tube filled with oil and foamed copper could improve heat transfer rate causing the increase in its efficiency by 55.6%. [PERSON] et al. [4] studied the effect of CuO/water nanofluid and copper metal foam as a solar absorber on the performance of direct absorption parabolic trough solar collector (DAPTSC). concluded that the combination of 0.1 vol% of CuO/water nanofluid and copper metal foam in solar collector tube provided the maximum temperature difference of working fluid by 16.3 \({}^{\circ}\)C. The thermal efficiency increased by 42.48%. Using of the combination of nanofluid and foam copper was better than using each one alone. [PERSON] et al. [5] used carbon nanoparticles dispersed in acetone as heat transfer fluid inside the evacuated tube to increase thermal performance of solar absorption cooling system. The solar collector working with nanofluid achieved 91% of maximum thermal efficiency compared with conventional working fluid. [PERSON] et al. [6] reviewed the influence of nanoparticles and base fluid type on the thermal efficiency, entropy generation and pressure drop of parabolic trough collectors (PTC). They found that copper nanoparticle and MWCNT nanoparticle that were metallic nanofluid and non-metallic nanofluid, respectively, could provide the highest enhancement of thermal efficiency. The addition of nanoparticle in nanofluid could provide higher thermal efficiency and exergy efficiency. [PERSON] et al. [7] evaluated the heat transfer coefficient of the gravity-assisted heat pipe containing graphene nanoplatelets-pentane nanofluid inside. The graphene nanoplatelets-pentane nanofluid would promote the Brownian motion and thermophoresis effect which had a positive effect on heat transfer coefficient. The heat transfer coefficient of the system was improved to 5300 W/m\({}^{2}\):K at 0.3 Wt.% of nanoparticles in base fluid. Other studies about the improvement of evacuated-tube solar collector by using the nanofluid and metal foam for thermal absorber had shown similar associations [8; 9; 10]. In addition, [PERSON] et al. [11] represented the improvement of evacuated-tube solar collectors performance by using phase change materials (PCM). Paraffin, their chosen PCM, was added in the tubes for storing the thermal energy supplied to heat pipe. The efficiency of the improved solar collectors increased by 26% compared with the standard one. [PERSON] et al. [12] filled the thermiond D-12 oil as a heat transfer fluid in the evacuated-tube solar collector with parabolic trough. This design could increase hot water temperature from 40 \({}^{\circ}\)C to 68 \({}^{\circ}\)C under the low solar intensity condition and increase the thermal efficiency by 30%.