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Enhancement of heat extraction from solar ponds by using twisted coil‐tubes

Generally, coil‐tubes are utilized to extract heat from solar ponds. There are different coil‐tubes in which their thermal and hydraulic characteristics depend on configuration and geometrical parameters. In this study, the performance of three well‐known coil‐tubes, namely helical, colubrine, and spiral, as the core of solar ponds is investigated at both smooth and twisted forms. Moreover, different levels of coil‐pitches and twist‐lengths are examined to explore the effects of geometrical parameters. First, the validity of the results is checked by the previous correlations proposed for the coil‐tubes. A good agreement is found, so that the average absolute deviation is about 6%. The results show that the heat extraction is affected by the configuration of coil‐tubes so that at the same geometrical parameters and operating conditions, the highest values are obtained for the spiral coil‐tube and the helical and colubrine coil‐tubes come in the second and third, respectively. It is found that, in all cases, twisting walls lead to higher rates of heat extraction. The enhancement range for the helical, colubrine, and spiral coil‐tubes is 0.3–19.8%, 0.3–46.1%, and 0.6–27.0%, respectively. At the same time, the pumping power augments in the range of 1.0–31.7%, 0.6–70.1%, and 0.4–43.5%. Furthermore, the models with lower level of coil‐pitch and twist‐length provide higher values of heat extraction and pumping power. At the considered ranges, the greatest overall performance is obtained for the colubrine coil‐tube with high coil‐pitch and twist‐length, which presents the performance index of 1.14.