Fig. 1 Concentric collector with reflective solar film and vacuum in the annular gap (version A) Concentric collector with reflective solar film and vacuum in the annular gap (version A) More

Fig. 2 Concentric collector with reflective solar film and no vacuum in the annular gap (version B) Concentric collector with reflective solar film and no vacuum in the annular gap (version B) More

Fig. 3 Concentric collector without reflective solar film and with vacuum in the annular gap (version C) Concentric collector without reflective solar film and with vacuum in the annular gap (version C) More

Fig. 4 Concentric collector without reflective solar film and without vacuum in the annular gap (version D) Concentric collector without reflective solar film and without vacuum in the annular gap (version D) More

Fig. 5 Diagram of the numerical code Diagram of the numerical code More

Fig. 6 Experimental setup Experimental setup More

Fig. 7 Experimental bench Experimental bench More

Fig. 8 Comparison between numerical predictions and experimental results: ( a ) collector version D and ( b ) collector version C Comparison between numerical predictions and experimental results: (a) collector version D and (b) collector version C More

Fig. 9 Hourly variation of the temperature difference of water at entry and exit of the collector Hourly variation of the temperature difference of water at entry and exit of the collector More

Fig. 10 Hourly fluid outlet temperature and efficiency of collector version A Hourly fluid outlet temperature and efficiency of collector version A More

Fig. 11 Hourly variation of the ambient conditions, tested parameters, and efficiency of version B Hourly variation of the ambient conditions, tested parameters, and efficiency of version B More

Fig. 12 Hourly variation of fluid outlet temperature and efficiency of version C Hourly variation of fluid outlet temperature and efficiency of version C More

Fig. 13 Hourly variation of fluid outlet temperature and efficiency of version D Hourly variation of fluid outlet temperature and efficiency of version D More

Fig. 14 Efficiency of the four proposed versions Efficiency of the four proposed versions More

Fig. 15 Variation of the efficiency with the fluid flow velocity Variation of the efficiency with the fluid flow velocity More

Fig. 16 Variation of the efficiency with fluid flow velocity for versions A and B Variation of the efficiency with fluid flow velocity for versions A and B More

Fig. 17 Variation of the efficiency with fluid flow velocity for versions C and D Variation of the efficiency with fluid flow velocity for versions C and D More

Fig. 18 Variation of the temperature difference with solar radiation for the four versions Variation of the temperature difference with solar radiation for the four versions More