Performance improvement of a double pipe heat exchanger proposed in a small-scale CAES system: An innovative design

Abstract

Compressed air energy storage (CAES) is a hopeful technology to overcome the intermittency of renewable energy systems and meet the high peak load demand. The objective of this study is to propose a double pipe heat exchanger (DPHX) working with CuO/water nanofluid in order to cool the compressed air before cavern in a small-scale CAES system. A new design of DPHX by considering different internal tube geometry (nine configurations) is proposed. To achieve these targets, a transient model for simulating the technical demeanor of the CAES system is developed. After simulating the behavior of the CAES system, DPHX is modeled by computational fluid dynamics (CFD) to evaluate the outcome of nanofluid as well as geometry design on the DPHX performance. The pressure drop is unchanged for all finned tube at higher Reynolds numbers. The numerical analysis through mathematical modeling of the charging process of the cavern denotes the effect of length and mass flow rate of the secondary fluid in the DPHX. The results illustrate that by enhancing the mass flow of the secondary fluid, the cavern temperature declines. The pressure inside the cavern has a small dependence on its temperature. The cavern pressure is invariant by increasing the secondary fluid flow. For proposed DPHX, the convective heat transfer coefficient increased up to 22% for cold fluid considering tube with four fins (air/nanofluid + finned tube (w = 3.5 mm and H = 1.0 mm)) and compared to the smooth tube. In addition, around 17% enhancement in convective heat transfer coefficient was achieved using tube with eight fins and with air/nanofluid as the working fluid (case with w = 3.5 mm and H = 1.0 mm), compared to tube with four fins. This shows the capability of the proposed finned tube along with the utilization of the nanofluid to increase the heat exchanger performance.