Preliminary Analysis of Direct and Indirect Heat Rejection Systems for a Small sCO2 Brayton Cycle Using an Existing Natural Draft Dry Cooling Tower

Abstract

The supercritical carbon dioxide (sCO2) Brayton cycle has been the focus of much research in recent years because of its high efficiency and compactness. One of the key issues is the heat rejection of about half heat addition to the sCO2 cycle, which needs a strong and reliable cooling system. In this paper, indirect and direct cooling systems using a 20-m natural draft dry cooling tower (NDDCT) were proposed and investigated. One-dimensional models for these two cooling systems were selected based on the experimental data of the cooling tower. The effects of the ambient temperature on the heat rejection rate and sCO2 outlet temperature for indirect and direct cooling systems were investigated, respectively. The results show that the optimal values of the water mass flow rate can be found in the indirect cooling system, and several optimal values are obtained under different ambient temperatures. Under the same operation temperature, the overall cooling performance of the direct cooling system is better than the indirect system, especially under low ambient temperature conditions. Under 15°C ambient temperature, the exergy efficiency of the direct cooling system is greater than that of the indirect cooling system (>200%). The results of this work illustrate that the direct cooling system is more appropriate for the Gatton cooling tower and also provide some references for the dry cooling system design for small-scale sCO2 cycles in the future.