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Research Papers: Power Engineering

Thermal Modeling of a Solar Steam Turbine With a Focus on Start-Up Time Reduction

[+] Author and Article Information
James Spelling

Department of Energy Technology,  Royal Institute of Technology, SE-100 44 Stockholm, Swedenjames.spelling@energy.kth.se

Markus Jöcker

 Siemens Industrial Turbomachinery, SE-612 83 Finspång, Swedenmarkus.jocker@siemens.com

Andrew Martin

Department of Energy Technology,  Royal Institute of Technology, SE-100 44 Stockholm, Swedenandrew.martin@energy.kth.se

J. Eng. Gas Turbines Power 134(1), 013001 (Nov 04, 2011) (8 pages) doi:10.1115/1.4004148 History: Received April 13, 2011; Revised April 26, 2011; Published November 04, 2011; Online November 04, 2011

Steam turbines in solar thermal power plants experience a much greater number of starts than those operating in baseload plants. In order to preserve the lifetime of the turbine while still allowing fast starts, it is of great interest to find ways to maintain the turbine temperature during idle periods. A dynamic model of a solar steam turbine has been elaborated, simulating both the heat conduction within the body and the heat exchange with the gland steam, main steam and the environment, allowing prediction of the temperatures within the turbine during off-design operation and standby. The model has been validated against 96 h of measured data from the Andasol 1 power plant, giving an average error of 1.2% for key temperature measurements. The validated model was then used to evaluate a number of modifications that can be made to maintain the turbine temperature during idle periods. Heat blankets were shown to be the most effective measure for keeping the turbine casing warm, whereas increasing the gland steam temperature was most effective in maintaining the temperature of the rotor. By applying a combination of these measures the dispatchability of the turbine can be improved significantly: electrical output can be increased by up to 9.5% after a long cooldown and up to 9.8% after a short cooldown.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Turbine start-up time as a function of the minimum temperature measured in the unit before start-up

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Figure 2

High- and low-pressure units of the SST-700RH, including the location of the casing temperature measurement points (not to scale)

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Figure 3

High- and low-pressure units of the SST-700RH, showing the geometry blocks in which the different heat conduction equations apply

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Figure 4

High- and low-pressure sections of the finite-volume model, showing the different boundary conditions

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Figure 5

Input and boundaries of the steam turbine model, including segments for Stodola modeling

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Figure 6

Steam expansion h-s diagram, showing the different terms involved in the temperature calculations

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Figure 7

Flow passage heat transfer coefficients

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Figure 8

Steam turbine rotor showing gland steam injection

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Figure 9

Fraction of gland steam supplied by the external steam source

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Figure 10

Validation input data for the steam turbine models. Steam temperature and pressure given as percentage of nominal boiler values. Due to insufficient insulation, the plant was not operated on October 22nd.

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Figure 11

Validation output data for the steam turbine models. Temperature given as percentage of nominal boiler value.

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Figure 12

Evolutionary algorithm population after 600 evaluations. Position of the Pareto-optimal front shown in red.

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Figure 13

Comparison between simulated values and measured data for the optimized steam turbine models. Thick colored lines show simulated values, thin black lines show measurements. Temperatures given as percentage of nominal boiler temperature.

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Figure 14

Correction factor magnitudes and sensitivity of the measured temperatures to a ± 25% change

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Figure 15

Impact of applying each of the three improvements to the steam turbine. Influence on the casing and rotor temperatures shown separately for both the high- and low-pressure turbines.

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Figure 16

Increase in daily output resulting from the application of a combination of heat blankets and increased gland steam temperature on the day following a long cooldown period

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Figure 17

Increase in daily output resulting from the application of a combination of heat blankets and increased gland steam temperature on the day following a short cooldown period

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