Research Papers: Energy Systems Analysis

Exergy-Based Study of a Binary Rankine Cycle

[+] Author and Article Information
Mathias Hofmann

Institute for Energy Engineering,
Technische Universität Berlin,
Berlin 10587, Germany
e-mail: hofmann@iet.tu-berlin.de

George Tsatsaronis

Institute for Energy Engineering,
Technische Universität Berlin,
Berlin 10587, Germany
e-mail: tsatsaronis@iet.tu-berlin.de

1Corresponding author.

2Or also economiser, superheater, etc.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received October 30, 2015; final manuscript received March 16, 2016; published online June 14, 2016. Assoc. Editor: Alojz Poredos.

J. Energy Resour. Technol 138(6), 062003 (Jun 14, 2016) (7 pages) Paper No: JERT-15-1414; doi: 10.1115/1.4033303 History: Received October 30, 2015; Revised March 16, 2016

The aim of this work is to study a binary Rankine process with a significantly higher efficiency compared to a conventional coal-fired power plant. This paper focuses on the design of the process and especially on an efficient combination of flue gas, potassium, and water streams in the components of the steam generator, such as economizers, evaporators, and superheaters, to decrease the overall exergy destruction. Based on a literature review, a base case for a coal-fired binary Rankine cycle with potassium and water as working fluids was developed and, in order to evaluate the thermodynamic quality of several variants, comparative exergy analyses were conducted. A simulation of the process and calculation of the values for the streams were carried out by using the flow-sheeting program CycleTempo, which simultaneously solves the mass and energy balances and contains property functions for the specific enthalpy and entropy of all the substances used. Necessary assumptions are predominantly based on literature data or they are discussed in the paper. We present the exergy analysis of the overall process that includes the flue gas streams as well as the potassium and water cycles. A design analysis and sensitivity studies show the effects of stream combinations and key parameters on the net efficiency, which is higher than 50%.

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Fig. 1

Principle of a binary Rankine cycle with liquid metal as working fluid for the topping cycle, see, e.g., Ref. [1, p. 357]

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Fig. 2

T-s diagram of a binary Rankine cycle. Topping cycle, 1-2-3-4, working fluid, e.g., potassium. Bottoming cycle, 5-6-7-8, working fluid water/steam.

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Fig. 3

Simplified flow diagram of the simulated coal-fired binary Rankine cycle with potassium and water as working fluids

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Fig. 4

Pressure profile of air and flue gas streams through steam generator for the system shown in Fig. 3, cp. Ref. [24, pp. 6–36]

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Fig. 5

T–ΔH˙ diagram for selected heat exchangers, reference

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Fig. 6

T–ΔH˙ diagram for selected heat exchangers, case 2




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