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Publication Title | Extremum-seeking control of a supercritical carbon-dioxide closed Brayton cycle in a direct-heated solar thermal power plant

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Energy 60 (2013) 380e387

Contents lists available at ScienceDirect Energy

journal homepage: www.elsevier.com/locate/energy

Extremum-seeking control of a supercritical carbon-dioxide closed Brayton cycle in a direct-heated solar thermal power plant

Rajinesh Singh a, *, Michael P. Kearney a, Chris Manzie b

a School of Mechanical & Mining Engineering, The University of Queensland, St. Lucia, Queensland 4072, Australia b Department of Mechanical Engineering, The University of Melbourne, Parkville, Victoria 3010, Australia

articleinfo

Article history:

Received 21 March 2013

Received in revised form

4 July 2013

Accepted 1 August 2013

Available online 6 September 2013

Keywords:

Solar thermal

Supercritical carbon dioxide Closed Brayton cycle Adaptive control Extremum-seeking

Power maximisation

1. Introduction

Fossil fuels presently serve as the main source of electricity generation in Australia. In 2010e2011, approximately 90% of Australian electricity generated was from fossil fuel sources [1], leading to the nation having one of the highest emissions in- tensities in the world. Concerns over the effects of fossil fuel use on the environment and its projected future scarcity are fuelling the development of renewable energy technology. As a further incen- tive, legislation has also been introduced that commits Australia to reductions in CO2 (carbon-dioxide) emissions from the electricity grid to 20% of 2000 levels by 2050. One option for efficient gener- ation of electricity from solar energy is through CST (Concentrating Solar Thermal) power plants. Significant opportunities exist for electricity generation using CST power plants to replace or sup- plement local generation in remote communities and mining op- erations around Australia and achieve savings in fuel and transport costs [2].

CST power generation technology is still in its infancy, with current generation CST power plants having high capital costs and

* Corresponding author.

E-mail addresses: r.singh5@uq.edu.au (R. Singh), m.kearney@uq.edu.au

(M.P. Kearney), manziec@unimelb.edu.au (C. Manzie).

0360-5442/$ e see front matter ! 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.energy.2013.08.001

abstract

One promising avenue for the development of next generation CST (Concentrating Solar Thermal) technology focuses on the use of a direct-heated sCO2 (supercritical-CO2) CBC (closed Brayton cycle) as the generator power cycle. Initial investigations into such a CST plant, while promising, have found its power output and efficiency to be sensitive to fluctuations in solar heat input and ambient temperature over a day and between seasons. Given the difficulty in developing complete models across all operating conditions due to non-linearities in CO2 properties, an extremum-seeking controller is proposed to maximise the power output of the CBC as the solar heat input and cooling-air temperatures change. This controller achieves this effect by manipulating the CO2 mass inventory in the CBC. Slack variables are introduced into the extremum-seeking control performance metric to impose constraints on turbine inlet temperature and pressure to protect the CBC from damage. The performance of the proposed scheme is tested through simulations on representative summer and winter days. Simulations indicate that the performance of the CBC under ESC (extremum-seeking control) based inventory-control com- pares favourably to operation with a fixed-CO2 inventory in both summer and winter and does not require retuning between seasons.

! 2013 Elsevier Ltd. All rights reserved.

high unitized-electricity costs compared to conventional fossil-fuel based power plants. One approach to reduce both capital and unitized-electricity costs is to use a more efficient and compact power cycle, such as a cycle based on sCO2 (supercritical-CO2).

The use of sCO2 as a power cycle working-fluid has been growing in recent years due to associated benefits such as highly compact power plant and high cycle thermal efficiencies at turbine inlet temperatures achieved in solar thermal [3] and nuclear power plants [4]. The simplicity and potential ability of real-gas closed Brayton cycles such as one with supercritical-CO2 to be integrated with existing inexpensive solar collectors is also an attractive feature [5]. The supercritical-CO2 CBC is also being considered for power generation from other medium-grade heat-sources including waste heat [6] and for integration with post-combustion CO2 capture for efficiency improvements in coal-fired power sta- tions [7].

Experimental campaigns into the Supercritical-CO2 closed Brayton cycle with configurations similar to the one investigated in this work already exist and preliminary plant performance and stability has been demonstrated [8]. Investigations are also being conducted into cycle performance during the startup and shut- down phases for power plant peaking [9]. Other experimental demonstration campaigns are also being conducted for different

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