Investigation of a Combined Inverted Brayton and Rankine Cycle

Ian Kennedy, Tomasz Duda, Zheng Liu, Bob Ceen, Andy Jones, Colin Copeland

Research output: Contribution to journalArticle

Abstract

Waste heat recovery is a vitally important technology to address increasingly stringent emissions legislation and environmental concerns over CO 2. One such means of recovering thermal energy is the inverted Brayton cycle (IBC). This paper presents an experimental study of a novel combination of the IBC with a Rankine cycle for the first time. The IBC requires cooling of the exhaust gases after expansion. If the gases contain water vapour, as is the case for hydrocarbon combustion, and cold enough coolant is available, the water can be condensed, pressurized and re-boiled for expansion in a Rankine cycle. The steam produced from the cycle can be utilized in a number of ways. In this study, steam is injected through a series of de Laval nozzles directed into the main turbine to produce additional shaft power in a compact arrangement. To minimize the size of the system, additive manufacturing was used for the heat exchangers, giving high performance per unit volume. The study demonstrates the feasibility of the cycle in producing power from waste heat using humid gas that already is present in most applications. The experimental results show that the system is able to generate power at very low exhaust temperatures where the standard IBC would cease to operate. With an IBC inlet temperature of 370 °C, approximately 5 kJ/kg of specific shaft work was produced with 5 g/s of steam flow rate. At higher exhaust temperatures, the IBC and the Rankine cycle started to work together to increase the shaft power resulting in much higher specific work. At 620 °C, a specific shaft work of 41 kJ/kg was generated at a steam flow of 9 g/s. For the present turbomachinery sizes, this corresponded to 1933 W of power at 47 g/s of main exhaust flow. A model of the thermodynamic system was created in order to study the sensitivity of the system to parameters such as the steam expander pressure ratio and efficiency. Higher steam pressure and higher steam expander efficiency both led to greater power generated for the same operating point, particularly at high IBC turbine inlet temperatures. The peak specific work for the range of parameters explored in the paper was 68 kJ/kg with a steam expander efficiency of 70% and exhaust conditions of 600 °C and 50 g/s. The plots produced in this study can be used as a guide for others considering this system to understand the expected power generated under a range of conditions.

Original languageEnglish
Article number121009
Pages (from-to)1-11
Number of pages11
JournalJournal of Engineering for Gas Turbines and Power
Volume141
Issue number12
Early online date1 Oct 2019
DOIs
Publication statusPublished - 1 Dec 2019

ASJC Scopus subject areas

  • Engineering(all)

Cite this

Investigation of a Combined Inverted Brayton and Rankine Cycle. / Kennedy, Ian; Duda, Tomasz; Liu, Zheng; Ceen, Bob; Jones, Andy; Copeland, Colin.

In: Journal of Engineering for Gas Turbines and Power, Vol. 141, No. 12, 121009, 01.12.2019, p. 1-11.

Research output: Contribution to journalArticle

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