Enhancing heat transfer in air tubular absorbers for concentrated solar thermal applications

The development of pressurised air tubular absorbers has been a major challenge for solar thermal applications involving high temperatures. The low internal heat transfer coefficients that are expected when air is used as the heat transfer fluid can be possibly alleviated through the use of enhancement methods such as helically coil/wire and twisted tape inserts as well as dimpling. A parametric study on the heat transfer and pressure drop performance of solar tubular air absorbers with and without these heat transfer enhancements was conducted using a simplified steady-state heat transfer model with boundary conditions representing typical solar applications. CO2 and He as working gases were also considered. This study showed that tubes with deeper protrusions on the inner wall offer superior overall performance in terms of heat transfer and pressure drop compared to a plain tube and tube with different enhancement methods. The differential temperature between the tube wall and bulk air flow can be minimised using such modifications. Consequently, tube lengths that can meet practical solar receiver sizes are possible for gas temperatures of over 800 °C. The use of CO2 and He as working gases can further reduce the total pressure drop in a solar tubular gas receiver.