A comparative overview of various single-shaft and parallel-flow Brayton cycles developed from turbochargers

Abstract

Automotive turbochargers can be used to develop gas turbine cycles; however, turbochargers operate at low pressure ratios where cycle performance is sensitive to the addition of pressure-drop components. Parallel-flow Brayton cycles have been proposed to reduce the effect of pressure losses on cycle performance. This analytical study therefore compares various parallel-flow Brayton cycle configurations to their single-shaft counterparts, considering combustion, recuperation, as well as a concentrated solar power input via a solar dish and an open-cavity tubular receiver to identify where parallel-flow cycles are advantageous. Results show that the main shaft turbocharger choice greatly influences whether a single-shaft or a parallel-flow cycle is more beneficial. In recuperated solar cycles with a 6 % combustion chamber pressure loss, the parallel-flow low-temperature-turbine configuration with the solar receiver before the power turbine (in parallel with the main shaft) can achieve a peak thermal efficiency of 23.5 %, with 3 kW of power output, at a pressure ratio of 1.6. This can be compared with a peak thermal efficiency of 21.8 % at a pressure ratio of 1.75 for its single-shaft counterpart. In recuperated parallel-flow cycles and recuperated solar parallel-flow cycles, thermal efficiency performance improves under increased combustion chamber pressure losses, from 6 % up to 11 %, in contrast to the declining performance of single-shaft cycles. More specifically, at a pressure ratio of 1.8, results show that the parallel-flow low-temperature-turbine configuration can outperform its single-shaft counterpart when combustion chamber pressure losses exceed 8.7 %. The study highlights the potential of parallel-flow Brayton cycles for recuperation and concentrated solar power integration, particularly in low-pressure-ratio systems, offering practical guidance for turbocharger and cycle configuration selection.

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