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Research Papers

Full State Feedback Control of Steam Temperature in a Once-Through Direct Steam Generation Receiver Powered by a Paraboloidal Dish

[+] Author and Article Information
José I. Zapata

Solar Thermal Group,
Research School of Engineering
Australian National University,
Canberra 0200, Australia
e-mail: jose.zapata@anu.edu.au

The Hartman–Grobman theorem establishes that under certain conditions, the linear approximation has the same qualitative behavior as the nonlinear model near the operating point [15].

The subscript 0 indicates a specific value for a variable or sets of variables.

The system will converge to the equilibrium point x0,y0 when subject to u0,v0.

Other studies employing moving-boundary formulations of two-phase flow heat exchangers have obtained further model reductions that tend to preserve only the dominant behavior of the system [20,21].

Typical receiver heat losses in operation are approximately 45–60 kW [25].

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received July 13, 2014; final manuscript received October 14, 2014; published online November 17, 2014. Assoc. Editor: Markus Eck.

J. Sol. Energy Eng 137(2), 021017 (Apr 01, 2015) (10 pages) Paper No: SOL-14-1202; doi: 10.1115/1.4028919 History: Received July 13, 2014; Revised October 14, 2014; Online November 17, 2014

Once-through direct steam generation (DSG) plants convert water into superheated steam suitable for a steam turbine with a single pass of the fluid through the receiver. The control problem in such a plant is to set a feed-water mass flow that maintains a desired steam condition (e.g., temperature) while rejecting the disturbance effect of variable direct normal irradiance (DNI). A mass flow control strategy preserves the simplicity of the plant, but is challenging to implement from a control perspective, as the disturbance effect is nonlinear and difficult to measure, due to the complex physical nature of two-phase flow and the receiver geometry. A model of the receiver behavior can be incorporated into the controller design in the form of a state observer, to estimate the internal behavior of the receiver during operation. This paper presents the design, testing an experimental implementation of full state linear feedback controller for the steam temperature for a once-through DSG system. The system consists of a 500 m2 paraboloidal dish concentrator and a monotube cavity receiver at the Australian National University. The controller manipulates the feed-water mass flow at the receiver inlet to maintain a predetermined specific enthalpy at the receiver outlet, compensating for variations in DNI and other ambient conditions. The controller features three separate regulation mechanisms: a feedforward (FF) law to anticipate changes in DNI; a full state feedback (FSF) loop with a state observer for the receiver; and an additional integrator loop for robustness. Experiments on the Australian National University (ANU) system show that the linear controller maintains steam temperatures to within 3% of a set reference of 500 °C during clear sky conditions, subject to adequate controller tuning. These results show that it is possible to control the ANU system with an FSF loop and state estimator, opening the possibility to test more advanced state based controllers.

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Figures

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Fig. 1

SG4 steam generation system diagram

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Fig. 2

Signal diagram for the SG4 receiver outlet temperature controller. Thick arrows represent vector signals and thin arrows represent scalar signals.

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Fig. 3

Complex plane diagram with dominant pole original and desired locations. Inset shows location of zero b7 and the pole/zero cancellation effect of the controller

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Fig. 4

Simulated performance of the receiver outlet temperature controller, when transitioning from manual to automatic feed-water mass flow settings, and maintaining temperature during variations in DNI. Vertical dotted lines indicate the period where the temperature controller is active. (a) DNI. (b) Feed-water and outlet mass flow. (c) Receiver outlet temperature, with dotted lines to indicate 500 ± 15 °C. (d) Average receiver pressure. (e) Cumulative length of fluid regions, with respect to tube length (horizontal dotted line).

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Fig. 5

Simulated performance of the receiver outlet temperature controller, when rejecting a triple drop to zero in DNI, akin a cloud disturbance. Simulations include the controlled without the AW and FF terms to show their effect on the controller. Simulations also include a PI controller with kp = 8×10-5 and ki = 2.22×10-7 for comparison. (a) DNI. (b) Feed-water and outlet mass flow. (c) Receiver outlet temperature, with dotted lines to indicate 500 ± 15 °C. (d) Average receiver pressure. (e) Cumulative length of fluid regions, with respect to tube length (horizontal dotted line).

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Fig. 6

Unsuccessful attempts at experimental temperature control on 25 October 2013. Vertical dotted lines indicate periods when the temperature controller is engaged. (a) DNI. (b) Feed-water mass flow. (c) Receiver outlet temperature. Horizontal dotted lines indicate 500 ± 15 °C. (d) Inlet, outlet, and average receiver pressure. (e) Cumulative length of fluid regions, with respect to tube length (horizontal dotted line).

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Fig. 7

Successful experimental test of temperature control on 30 October 2013. A vertical dotted line indicates when the temperature controller was engaged. (a) DNI. (b) Feed-water mass flow. (c) Receiver outlet temperature. Horizontal dotted lines indicate 500 ± 15 °C. (d) Inlet, outlet, and average receiver pressure. (e) Cumulative length of fluid regions, with respect to tube length (horizontal dotted line).

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