Design and evaluation of compact heat exchangers for hybrid fuel cell and gas turbine systems

Abstract

Hybridized Carbonate and Solid Oxide fuel cell power plants are currently under investigation to fulfill demands for high efficiency and low emissions. Selection and design of high performance heat exchangers are essential for such applications. In this work, various compact heat exchanger (CHEX) technologies pertinent to gas-gas recuperative duties are presented. The CHEX types considered include brazed plate-fin, fin-tube, microchannel, primary surface and spiral. Based on a comparative rating procedure, two CHEX designs namely, plate-fin and microchannel were chosen for further review. Plain, strip, louver, wavy and semicircular surface geometries were then evaluated with a numerical CHEX sizing procedure. The brazed plate-fin CHEX having the louver fin geometry was determined the most conducive with hybrid fuel cell and gas turbine systems. Multiple numerical modeling efforts were carried out to develop plate-fin heat exchanger design recommendations. A model was created for the transient thermal simulation of counterflow heat exchanger partition plates. For this analysis, an alternating direction implicit finite difference scheme was written in the Java programming language to model temperature in the working fluids and partition plate.

Thermal stress was then calculated in various partition plate designs for steady state and transient modes of operation. Thermal stress was modeled in two heat exchanger materials, stainless steel 304 and Inconel 625. A primary creep law was developed for Inconel 625 to simulate creep behavior in high temperature (up to 1150 °K) heat exchanger partition plates. The results of the transient thermal simulation clearly show the effect of temperature ramping rate on the rate of heat transfer between the working fluids and partition plate. Thermal stress results confirm that additional stress produced in heat exchanger partition plates during transient operation is negligible for temperature ramping rates consistent with high temperature fuel cells. Based on this result it is suggested that employing slow temperature ramping permits the use of higher performance heat exchanger designs, given that damage generally accrued during transient operation is circumvented. Thermal stress results also show that heat exchanger partition plate aspect ratio (Width/Length) plays a major role on the amount of thermal stress produced within the plate. More importantly, this change in aspect ratio has an even larger effect on creep behavior.