A Finite-Time Thermodynamic Framework for Optimizing Solar-Thermal Power Plants

[+] Author and Article Information
A. McMahan, S. A. Klein, D. T. Reindl

Solar Energy Laboratory, University of Wisconsin—Madison, Madison, WI 53706

J. Sol. Energy Eng 129(4), 355-362 (Jan 22, 2007) (8 pages) doi:10.1115/1.2769689 History: Received July 21, 2006; Revised January 22, 2007

Fundamental differences between the optimization strategies for power cycles used in “traditional” and solar-thermal power plants are identified using principles of finite-time thermodynamics. Optimal operating efficiencies for the power cycles in traditional and solar-thermal power plants are derived. In solar-thermal power plants, the added capital cost of a collector field shifts the optimum power cycle operating point to a higher-cycle efficiency when compared to a traditional plant. A model and method for optimizing the thermoeconomic performance of solar-thermal power plants based on the finite-time analysis is presented. The method is demonstrated by optimizing an existing organic Rankine cycle design for use with solar-thermal input. The net investment ratio (capital cost to net power) is improved by 17%, indicating the presence of opportunities for further optimization in some current solar-thermal designs.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

T-S diagram showing a Carnot cycle operating between two finite thermal resources

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Figure 2

Results of a finite-time analysis of a Carnot cycle operating between 20°C and 300°C showing a parabolic relation between first-law efficiency and power at constant UA. Each line is for constant UA with the dashed line representing infinite UA.

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Figure 3

T-S diagram describing an internally reversible Rankine cycle operating between two finite thermal resources

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Figure 4

Maximum power behavior of a single-stage Rankine cycle operating between finite thermal resources having inlet temperatures of 20°C and 300°C, respectively

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Figure 5

Finite-time analysis of a parabolic trough solar power plant with constant heat transfer fluid inlet conditions. Dashed lines indicate constant solar field size; solid lines are of constant power cycle size.

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Figure 6

Finite-time analysis of a parabolic trough solar power plant with constant power cycle thermal resource inlet (solar field exit) conditions. Lines of constant total system cost shown.

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Figure 7

Power cycle model schematic, shows all modeled heat exchange processes and fluid flows

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Figure 8

Flow diagram for sample parabolic trough organic Rankine cycle power plant

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Figure 11

Heating and cooling curves for the optimized PTORC

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Figure 12

Part-load performance of the reference and optimized cycles considered in Sec. 3

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Figure 9

Power-efficiency relationships analogous to Fig. 6 showing lines of total system cost corresponding to the reference and optimized cases. The specific power-efficiency operating point for each cycle is indicated.

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Figure 10

Heating and cooling curves for the reference PTORC



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