Experimental investigation on bimetallic tube compositions for the use in latent heat thermal energy storage units

Abstract

Based on the high energy density of phase change materials, latent heat thermal energy storage devices can play an important role in the future energy market. Therefore, the latent heat thermal energy storage technique is an interesting technology for industrial applications (e.g. batch processes) and power cycles. A key technology for such a storage device is the design of the heat exchanger tube, because the heat transfer rate by charging and discharging is the limiting factor based on the low thermal conductivity of the phase change material. The heat exchanger tube material used for such an application should have a high thermal conductivity and also a high mechanical resistance. Such a behavior can be found in a combination of different materials. The present paper deals with the design of such a heat exchanger tube composition consisting of a plain steel tube and an aluminum tube where fins can be attached. A novel bimetallic tube composition will be presented and compared with three common compositions. First, the mechanical stability of the bimetallic compositions was determined. Additionally a creep test of the used aluminum under operation conditions for a storage device using sodium nitrate as phase change material confirmed the utilizability for the operation in a latent heat thermal energy device.One of the main challenges for the compositions under investigation is based on the different thermal expansion coefficient for aluminum and steel, which results in different strain and creeping tendencies of the aluminum at operation temperature of the storage system, which is up to 340°C. A good heat transfer from the heat transfer fluid through the steel tube to the storage material around the fins can only be guaranteed through a close and stable connection between the two tubes. Compared to former solutions the fin circumference and the fin design are independent from the connection to the steel tube and allows individual arrangements of tubes and high packing densities.The experimental investigations have shown that the novel bimetallic tube composition is able to compensate these different strains and is capable to guarantee a stable connection between the steel and the aluminum tube. This high pressure and high temperature resistant bimetallic heat exchanger tube is easy to assemble and may play a key role for the development of thermal energy storages and other heat exchanging processes.