Performance analysis of the evacuated tube counter-flow absorber and direct-flow absorber to optimize the heat extraction rate for high flow rate applications.

Abstract

The evacuated tube collector (ETC) has gained extensive use in low-temperature applications due to its cheapness and high efficiency. The ETC can be used with a concentrator for medium temperature applications, in the range of 140-200 °C . However, the heat extraction rate of the absorber tube is a limitation factor, particularly at higher heat flux and high flow rates. The energy gained is not directly proportional to the concentration factor used. This work thus proposes a counter-flow copper absorber for increasing the heat extraction rate and compares its performance to the conventional direct-flow absorber. The designs are both optimized by varying the absorber diameters, and a material property analysis is done. COMSOL Multiphysics is used for the simulations. The performance of the 2 systems is evaluated using a conjugate heat transfer model at flow rate ranges of 0.02-0.2 kg/s and uniform theoretical heat flux of 1000, 2000, and 3000 W/m². Analysis of the results indicates that the counter-flow with 0.01 and 0.02 m inner and outer diameter respectively has 4 times more energy gain than the direct-flow with a 0.01 m diameter. Increasing the heat flux by 2 at 0.02 and 0.2 kg/s flow rate increases the temperature by 1.5 and 1.1 for the counter-flow absorber and 1.2 and 1.04 for the direct-flow absorber. Tripling the heat flux at the same flow rate range increases the temperature by 2 and 1.4 for the counter-flow absorber and 1.5 and 1.07 for the direct-flow absorber. The counter-flow absorber is thus the best choice at higher heat flux and high flow rates which are typically required for industrial heating.