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Improving Concentrating Solar Power Plant Performance through Steam Turbine Flexibility
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology. (Concentrating Solar Power)
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The amount of incoming solar energy to earth is greater than any other source. Among existing technologies to harness solar energy there is concentrating solar power (CSP). One advantage of CSP is that is dispatchable, meaning that it can provide power even when the sun is not shining. However, CSP is undergoing challenges which hinder its development such as operating variabilities caused by the fluctuations of the sun or the fact that these systems are not yet cost competitive with respect to other technologies.  

One way of improving the performance of CSP plants (CSPPs) is by increasing their operational flexibility, specifically their capability for fast starts. In this way it is possible for the CSPP to harness the solar energy as soon as possible, thus producing more energy and increasing its profitability. Over 90% of CSPPs use a steam turbine to generate electricity. Steam turbines are not currently designed with the flexibility required by the CSP application. Steam turbine start-up is limited by thermal stress and differential expansion. If not carefully controlled, these phenomena either consume lifetime or even result in machine failure.

The aim of this work was to understand the improvement potential of steam turbine start-up and quantify this in terms of CSPP performance indicators. For this, a thermo-mechanical steam turbine model was developed and validated. The model was then used to analyze potential improvements and thermal constraints to steam turbine start-up operation. Furthermore, a CSP plant techno-economic model was developed including steam turbine details. This modeling approach including two levels of detail allowed for the particularities of the component to be included within the dynamics of the plant and thus be able to connect the perspectives of the equipment manufacturer with those of the plant operator. Reductions of up to 11.4% in the cost of electricity were found in the studies carried out.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017. , p. 75
Series
TRITA-KRV ; 17/04
Keywords [en]
Concentrating Solar Power, Steam Turbines, Transients, Start-up
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-211780ISBN: 978-91-7729-388-0 (print)OAI: oai:DiVA.org:kth-211780DiVA, id: diva2:1131039
Public defence
2017-09-08, M3, Brinellvägen 64, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency
Note

QC 20170814

Available from: 2017-08-14 Created: 2017-08-11 Last updated: 2017-08-15Bibliographically approved
List of papers
1. Geometric Modularity in the Thermal Modeling of Solar Steam Turbines
Open this publication in new window or tab >>Geometric Modularity in the Thermal Modeling of Solar Steam Turbines
2014 (English)In: Proceedings of the SolarPACES 2013 International Conference, Elsevier, 2014, Vol. 49, p. 1737-1746Conference paper, Published paper (Refereed)
Abstract [en]

To optimize the start-up schedules of steam turbines operating in concentrating solar power plants, accurate predictions of the temperatures within the turbine are required. In previous work by the authors, thermal models of steam turbines have been developed and validated for parabolic trough solar power plant applications. Building on these results, there is an interest to increase the adaptability of the models with respect to different turbine geometries due to the growing trend of having larger steam turbines in parabolic trough and solar tower power plants. In this work, a modular geometric approach has been developed and compared against both the previous modeling approach and 96h of measured data from an operational parabolic trough power plant. Results show a large degree of agreement with respect to the measured data in spite of the different detail levels. The new model allows for simple and fast prediction of the thermal behavior of different steam turbine sizes and geometries, which is expected to be of significant importance for future concentrating solar power plants.

Place, publisher, year, edition, pages
Elsevier, 2014
Keywords
steam turbine, thermal stresses, start-up, finite element method
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-126931 (URN)10.1016/j.egypro.2014.03.184 (DOI)000340733700178 ()2-s2.0-84902271058 (Scopus ID)
Conference
International Conference on Solar Power and Chemical Energy Systems, SolarPACES 2013, Las Vegas, NV, United States, 17 September 2013 through 20 September 2013
Funder
Swedish Energy Agency
Note

QC 20140922. QC 20160129

Available from: 2013-08-22 Created: 2013-08-22 Last updated: 2017-08-11Bibliographically approved
2. Operational Improvements for Startup Time Reduction in Solar Steam Turbines
Open this publication in new window or tab >>Operational Improvements for Startup Time Reduction in Solar Steam Turbines
Show others...
2015 (English)In: Journal of engineering for gas turbines and power, ISSN 0742-4795, E-ISSN 1528-8919, Vol. 137, no 4, article id 042604Article in journal (Refereed) Published
Abstract [en]

Solar steam turbines are subject to high thermal stresses as a result of temperature gradients during transient operation, which occurs more frequently due to the variability of the solar resource. In order to increase the flexibility of the turbines while preserving lifting requirements, several operational modifications for maintaining turbine temperatures during offline periods are proposed and investigated. The modifications were implemented in a dynamic thermal turbine model and the potential improvements were quantified. The modifications studied included: increasing the gland steam pressure injected to the end-seals, increasing the back pressure and increasing the barring speed. These last two take advantage of the ventilation and friction work. The effects of the modifications were studied both individually as well as in different combinations. The temperatures obtained when applying the combined modifications were compared to regular turbine cool-down (CD) temperatures and showed significant improvements on the startup times of the turbine.

Place, publisher, year, edition, pages
ASME Press, 2015
Keywords
Steam Turbines, Start-up time
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-148147 (URN)10.1115/1.4028661 (DOI)000350145500024 ()2-s2.0-84940473273 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20150408

Available from: 2014-07-30 Created: 2014-07-30 Last updated: 2017-08-11Bibliographically approved
3. Differential Expansion Sensitivity Studies during Steam Turbine Startup
Open this publication in new window or tab >>Differential Expansion Sensitivity Studies during Steam Turbine Startup
2015 (English)In: Journal of engineering for gas turbines and power, ISSN 0742-4795, E-ISSN 1528-8919, Vol. 138, no 6, article id GTP-15-1419Article in journal (Refereed) Published
Abstract [en]

In order to improve the startup flexibility of steam turbines, it becomes relevant to analyze their dynamic thermal behavior. In this work, the relative expansion between rotor and casing was studied during cold-start conditions. This is an important property to monitor during startup given that clearances between rotating and stationary components must be controlled in order to avoid rubbing. The investigation was performed using a turbine thermal simplified model from previous work by the authors. The first step during the investigation was to extend and refine the modeling tool in order to include thermomechanical properties. Then, the range of applicability of the model was validated by a twofold comparison with a higher order finite element (FE) numerical model and measured data of a cold start from an installed turbine. Finally, sensitivity studies were conducted with the aim of identifying the modeling assumptions that have the largest influence in capturing the correct thermal behavior of the turbine. It was found that the assumptions for the bearing oil and intercasing cavity temperatures have a large influence ranging between ±25% from the measured values. In addition, the sensitivity studies also involved increasing the initial temperature of the casing in order to reduce the peak of differential expansion. Improvements of up to 30% were accounted to this measure. The studies performed serve as a base toward further understanding the differential expansion during start and establishing future clearance control strategies during turbine transient operation.

Place, publisher, year, edition, pages
American Society of Mechanical Engineers (ASME), 2015
Keywords
Thermal expansion, Steam Turbines
National Category
Mechanical Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-156419 (URN)10.1115/1.4031643 (DOI)000374713500011 ()2-s2.0-84947466310 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20160122

Available from: 2014-11-28 Created: 2014-11-28 Last updated: 2017-08-14Bibliographically approved
4. Investigation into the Thermal Limitations of Steam Turbines During Start-up Operation
Open this publication in new window or tab >>Investigation into the Thermal Limitations of Steam Turbines During Start-up Operation
2017 (English)In: Journal of engineering for gas turbines and power, ISSN 0742-4795, E-ISSN 1528-8919Article in journal (Refereed) Accepted
Abstract [en]

Liberalized electricity market conditions and concentrating solar power technologies call for increased power plant operational flexibility. Concerning the steam turbine component, one key aspect of its flexibility is the capability for fast starts. In current practice, turbine start-up limitations are set by consideration of thermal stress and low cycle fatigue. However, the pursuit of faster starts raises the question whether other thermal phenomena can become a limiting factor to the start-up process. Differential expansion is one of such thermal properties, especially since the design of axial clearances is not included as part of start-up schedule design and because its measurement during operation is often limited or not a possibility at all.The aim of this work is to understand differential expansion behavior with respect to transient operation and to quantify the effect that such operation would have in the design and operation of axial clearances. This was accomplished through the use of a validated thermo-mechanical model that was used to compare differential expansion behavior for different operating conditions of the machine. These comparisons showed that faster starts do not necessarily imply that wider axial clearances are needed, which means that the thermal flexibility of the studied turbine is not limited by differential expansion. However, for particular locations it was also obtained that axial rubbing can indeed become a limiting factor in direct relation to start-up operation. The resulting approach presented in this work serves to avoid over-conservative limitations in both design and operation concerning axial clearances.

Keywords
Steam turbines, differential expansion
National Category
Energy Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-211778 (URN)10.1115/1.4037664 (DOI)000415792800018 ()2-s2.0-85029781596 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20170814

Available from: 2017-08-11 Created: 2017-08-11 Last updated: 2019-04-05Bibliographically approved
5. Improving Concentrating Solar Power Plant Performance by Increasing Steam Turbine Flexibility at Start-up
Open this publication in new window or tab >>Improving Concentrating Solar Power Plant Performance by Increasing Steam Turbine Flexibility at Start-up
2017 (English)In: Solar Energy, ISSN 0038-092X, E-ISSN 1471-1257Article in journal (Refereed) Submitted
Abstract [en]

Among concentrating solar power technologies, solar tower power plants currentlyrepresent one of the most promising ones. Direct steam generationsystems, in particular, eliminate the usage of heat transfer uids allowing forthe power block to be run at greater operating temperatures and thereforefurther increasing the thermal eciency of the power cycle. On the otherhand, the current state of the art of these systems does not comprise thermalenergy storage. The lack of storage adds to the already existing variability ofoperating conditions that all concentrating power plants endure due to theuctuating nature of the solar supply. One way of improving this situationis increasing the operating exibility of power block components to betteradapt to the varying levels of solar irradiance.In particular, it is desirable for the plant to achieve fast start-up times inorder to be available to harness as much solar energy as possible. However,the start-up speed of the whole plant is limited by the thermal inertia ofcertain key components, one of which is the steam turbine. This paperstudies the potential for power plant performance improvement through theincrease of steam turbine exibility at the time of start-up. This has beenquantied by carrying out power plant techno-economic studies in connectionwith steam turbine thermo-mechanic behavior analysis. Dierent turbineexibility investigations involving the use of retrotting measures to keep theturbine warmer during oine periods or changing the operating map of the turbine have been tested through multi-objective optimization consideringannual power performance and operating costs. Results show that reductionsof up to 11% on the levelized cost of electricity are possible through theimplementation of these measures, which in turn has a favorable impact onpower plant protability.

Place, publisher, year, edition, pages
Elsevier, 2017
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-211779 (URN)
Funder
Swedish Energy Agency
Note

QC 20170814

Available from: 2017-08-11 Created: 2017-08-11 Last updated: 2019-09-04Bibliographically approved

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