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Research Papers: Gas Turbines: Aircraft Engine

Inverted Brayton Cycle With Exhaust Gas Recirculation—A Numerical Investigation

[+] Author and Article Information
Martin Henke

e-mail: Martin.Henke@DLR.de

Manfred Aigner

German Aerospace Centre (DLR),
Institute of Combustion Technology,
Pfaffenwaldring 38-40,
Stuttgart 70569, Germany

Contributed by the Aircraft Engine Committee of ASME for publication in the Journal of Engineering for Gas Turbines and Power. Manuscript received June 27, 2013; final manuscript received July 4, 2013; published online August 19, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 135(9), 091203 (Aug 19, 2013) (7 pages) Paper No: GTP-13-1193; doi: 10.1115/1.4024954 History: Received June 27, 2013; Revised July 04, 2013

Microgas turbine (MGT) based combined heat and power (CHP) units provide a highly efficient, low-pollutant technology to supply heat and electrical power from fossil and renewable energy sources; however, pressurized MGT systems in an electrical power range from 1 to 5 kWel utilize very small turbocharger components. These components suffer from higher losses, like seal and tip leakages, resulting in a reduced electrical efficiency. This drawback is avoided by an inverted Brayton cycle (IBC) based system. In an IBC hot gas is produced in a combustion chamber at atmospheric pressure. Subsequently, the exhaust gas is expanded in a turbine from an atmospheric to a subatmospheric pressure level. In order to increase electrical efficiency, heat from the turbine exhaust gas is recuperated to the combustion air. After recuperation, the gas is compressed to atmospheric pressure and is discharged from the cycle. To decrease the power demand of the compressor, and thereby increasing the electrical cycle efficiency, it is crucial to further extract residual thermal power from the gas before compression. Coolant flows provided by heating applications can use this heat supply combined with heat from the discharged exhaust gas. The low pressure levels of the IBC result in high volumetric gas flows, enabling the use of large, highly efficient turbocharger components. Because of this efficiency benefit and the described cooling demand, micro-CHP applications provide an ideal field for utilization of the IBC. To further increase the total efficiency, discharged exhaust gas can be partially recirculated to the air inlet of the cycle. In the present paper a steady state analysis of an IBC with exhaust gas recirculation (EGR) is shown, and compared to the performance of a conventional Brayton cycle with equivalent component properties. Using EGR, it could be found that the sensitivity of the electrical cycle efficiency to the coolant temperature further increases. The sequent discussion focuses on the trade-off between total efficiency and electrical efficiency, depending on coolant temperature and EGR rate. The results show that EGR can increase the total efficiency by 10% to 15% points, while electrical efficiency decreases by 0.5% to 1% point. If the coolant temperature is below 35 °C, condensation of water vapor in the exhaust gas leads to a further increase of heat recovery efficiency. A validated in-house simulation tool based on turbocharger maps has been used for the calculations.

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Figures

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Fig. 1

Schematic representation of the heat transfer between gas and coolant in a WHX. (1) Condensation ignored. (2) Condensation considered.

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Fig. 2

WHX heat flux diagram with a higher coolant outlet temperature demand. (1) Condensation ignored. (2) Condensation leading to an acausal pinch decrease.

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Fig. 3

The coolant flow is decreased to remain a constant pinch

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Fig. 4

Schematic overview of the analyzed IBC with an optional second recuperator and EGR

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Fig. 5

IBC and BC electrical efficiency over power output curves at different shaft speeds and coolant temperatures

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Fig. 6

Electrical and total efficiency over coolant temperature of the IBC with and without a second recuperator

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Fig. 7

Total efficiency comparison of BC and IBC (with different EGR rates) at different coolant temperatures

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Fig. 8

Total efficiency gain of IBC without a second recuperator, induced by latent heat flux, depending on EGR rate and Tcoolant

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Fig. 9

Effect of EGR rate on inlet gas temperature (Tmix) and CIT, depending on coolant temperature

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Fig. 10

Electrical efficiency values depending on configuration, EGR rate, and coolant temperature

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Fig. 11

Dependencies of electrical efficiency on ambient temperature

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Fig. 12

Dependencies of total efficiency on ambient temperature

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