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Research Papers: Gas Turbines: Cycle Innovations

Design Considerations for Supercritical Carbon Dioxide Brayton Cycles With Recompression

[+] Author and Article Information
John Dyreby

Solar Energy Laboratory,
University of Wisconsin-Madison,
1500 Engineering Drive,
Madison, WI 53706
e-mail: jjdyreby@wisc.edu

Sanford Klein, Gregory Nellis, Douglas Reindl

Solar Energy Laboratory,
University of Wisconsin-Madison,
1500 Engineering Drive,
Madison, WI 53706

Contributed by the Cycle Innovations Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received May 15, 2014; final manuscript received June 25, 2014; published online July 22, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(10), 101701 (Jul 22, 2014) (9 pages) Paper No: GTP-14-1240; doi: 10.1115/1.4027936 History: Received May 15, 2014; Revised June 25, 2014

Supercritical carbon dioxide (SCO2) Brayton cycles have the potential to offer improved thermal-to-electric conversion efficiency for utility scale electricity production. These cycles have generated considerable interest in recent years because of this potential and are being considered for a range of applications, including nuclear and concentrating solar power (CSP). Two promising SCO2 power cycle variations are the simple Brayton cycle with recuperation and the recompression cycle. The models described in this paper are appropriate for the analysis and optimization of both cycle configurations under a range of design conditions. The recuperators in the cycle are modeled assuming a constant heat exchanger conductance value, which allows for computationally efficient optimization of the cycle's design parameters while accounting for the rapidly varying fluid properties of carbon dioxide near its critical point. Representing the recuperators using conductance, rather than effectiveness, allows for a more appropriate comparison among design-point conditions because a larger conductance typically corresponds more directly to a physically larger and higher capital cost heat exchanger. The model is used to explore the relationship between recuperator size and heat rejection temperature of the cycle, specifically in regard to maximizing thermal efficiency. The results presented in this paper are normalized by net power output and may be applied to cycles of any size. Under the design conditions considered for this analysis, results indicate that increasing the design high-side (compressor outlet) pressure does not always correspond to higher cycle thermal efficiency. Rather, there is an optimal compressor outlet pressure that is dependent on the recuperator size and operating temperatures of the cycle and is typically in the range of 30–35 MPa. Model results also indicate that the efficiency degradation associated with warmer heat rejection temperatures (e.g., in dry-cooled applications) are reduced by increasing the compressor inlet pressure. Because the optimal design of a cycle depends upon a number of application-specific variables, the model presented in this paper is available online and is envisioned as a building block for more complex and specific simulations.

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References

Figures

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Fig. 1

Diagram of a recompression Brayton cycle, with model inputs shown in bold

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Fig. 2

Iteration process for the cycle model. The results presented in this paper were generated using 20 subheat exchangers per recuperator and the relative tolerance used for convergence was 10−6.

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Fig. 3

Thermal efficiency as a function of recompression fraction for the “Basic” design with a fixed low-side pressure of 7.69 MPa

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Fig. 4

Thermal efficiency as a function of recompression fraction for the “High Performance” design with a fixed low-side pressure of 7.69 MPa

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Fig. 5

Thermal efficiency as a function of recompression fraction for the “Dry Cooled” design with a fixed low-side pressure of 7.69 MPa

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Fig. 6

Optimal thermal efficiency for a SCO2 cycle with a compressor inlet temperature of 32 °C. The dashed line corresponds to a recompression fraction of zero (i.e., a simple cycle). The open circles mark the normalized conductance corresponding to a minimum temperature difference of 10 °C in the recuperators while the filled circles correspond to a 2 °C minimum temperature difference.

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Fig. 7

Optimal thermal efficiency for a SCO2 cycle with a compressor inlet temperature of 55 °C. The dashed line corresponds to a recompression fraction of zero (i.e., a simple cycle). The open circles mark the normalized conductance corresponding to a minimum temperature difference of 10 °C in the recuperators while the filled circles correspond to a 2 °C minimum temperature difference.

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Fig. 8

Optimal compressor inlet and outlet pressures for the 32 °C case (left) and the 55 °C case (right). The polytropic efficiency of the turbomachinery is 0.9.

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Fig. 9

Thermal efficiency as a function of compressor inlet temperature for various designs

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Fig. 10

Optimal low-side pressure (top), recompression fraction (middle), and LT recuperator conductance fraction (bottom) as a function of low-side temperature with a high-side temperature of 700 °C

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