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Research Papers

Thermodynamic Analyses of Single Brayton and Combined Brayton–Rankine Cycles for Distributed Solar Thermal Power Generation

[+] Author and Article Information
W. Lipiński

e-mail: lipinski@umn.edu
Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the Journal of Solar Energy Engineering. Manuscript received August 21, 2011; final manuscript received December 23, 2012; published online April 29, 2013. Assoc. Editor: Manuel Romero Alvarez.

J. Sol. Energy Eng 135(3), 031008 (Apr 29, 2013) (8 pages) Paper No: SOL-11-1179; doi: 10.1115/1.4023591 History: Received August 21, 2011; Revised December 23, 2012

This paper reports theoretical efficiencies of single Brayton and combined Brayton–Rankine thermodynamic power cycles for distributed solar thermal power generation. Thermodynamic analyses are conducted with a nominal heat input to the cycle of 150 kW and component parameters for a 50 kWe gas microturbine for selected working fluids including air, Ar, CO2, He, H2, and N2 for the Brayton cycle and for the topping cycle of the combined system. Cycle parameters including maximum fluid temperature based on solar concentration ratio, pressure loss, and compressor/turbine efficiencies are then varied to examine their effect on cycle efficiency. C6-fluoroketone, cyclohexane, n-pentane, R-141b, R-245fa, and HFE-7000 are examined as working fluids in the bottoming segment of the combined cycle. A single Brayton cycle is found to reach a peak cycle efficiency of 15.31% with carbon dioxide at design point conditions. Each Brayton cycle fluid is examined as a topping cycle fluid in the combined cycle, being paired with six potential bottoming fluids, resulting in 36 working fluid configurations. The combination of the Brayton topping cycle using carbon dioxide and the Rankine bottoming cycle using R-245fa gives the highest combined cycle efficiency of 21.06%.

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References

Figures

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Fig. 1

Component schematic of the single Brayton cycle indicating the evaluated thermodynamic states

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Fig. 2

Example temperature-entropy plane diagram for single Brayton cycle indicating the evaluated thermodynamic states

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Fig. 3

Component schematic of the combined Brayton–Rankine cycle indicating the evaluated thermodynamic states

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Fig. 6

Brayton cycle efficiency versus cycle pressure ratio

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Fig. 5

Optimal receiver temperature versus solar concentration ratio

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Fig. 4

Efficiency of a receiver-Carnot heat engine system as a function of receiver temperature for C = 50, 70, and 100

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Fig. 8

Effect of turbine inlet temperature on Brayton cycle efficiency

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Fig. 7

Brayton cycle volumetric network versus cycle pressure ratio

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Fig. 9

Effect of compressor isentropic efficiency on Brayton cycle efficiency

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Fig. 10

Effect of turbine isentropic efficiency on Brayton cycle efficiency

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Fig. 11

Effect of pressure loss in heating stage on Brayton cycle efficiency. Cooling stage pressure loss is held constant.

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Fig. 12

Comparison of steam Rankine and single Brayton cycle efficiency plotted as a function of the cycle pressure ratio. Brayton cycle curves are identical to those of Fig. 7.

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Fig. 13

Cycle efficiency for each configuration of fluids in the combined cycle, grouped by topping cycle fluid

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Fig. 14

Net cycle power output for each configuration of fluids in the combined cycle, grouped by topping cycle fluid

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