19 oct. 2016

PhD position - Labex SOLSTICE 2016 - Optical method crosses for the characterization of a field of heliostats in a solar installation with concentration


 

Abstract

Solar tower power plants use thousands of heliostats and a fast and precise characterization of their optical defects constitutes an important technological obstacle. The objective of the thesis is to increase the capacities of an innovative method of characterization of the optical defects of heliostats. This patented backward-gazing method is developed at PROMES for two years. The thesis will consist in using a fluxmetry method on a white target, constituting the current state of the art, for the calibration of heliostats by combining it to the backward-gazing method with four cameras. The main operational parameters of the methods will be studied, discussed and optimized thanks to numerical simulation studies. The experimental backward-gazing and fluxmetry methods will be implemented at the solar tower facility THEMIS in Targassonne. The new optical hybrid method will be applied to some heliostats and validated by the check of the correction brought to the heliostat structure.

Detailed description

The conversion of the concentrated solar energy in domestic or industrial electricity is one of the most promising ways for the production of renewable energies of the XXIth century. The future solar tower power plants will possess a solar receiver (or a thermochemical reactor) installed at the top of a tower of hundred meters in height, and surrounded with tens of thousands of segmented mirrors placed at ground level (heliostats), assuring the tracking of the sun and the concentration of solar beams inside the receiver. Among the numerous technological challenges remaining to solve, one is the decrease of time and efforts dedicated to adjust and characterize the heliostats before the plant start-up, and to follow the heliostat opto-mechanical errors during the plant operation. The acquired experience on the existing solar facilities (for example in France the 1MW-CNRS Solar furnace of Odeillo or the THEMIS solar tower in Targassonne) leads to predict that these operations could require several months or even several years by applying the current techniques (review by Xiao and al., on 2012) in the industrial scale. The development and the operational implementation of more effective methods thus establishes one of the main keys for the development of this energy sector in the next decades. In this perspective, we intend to widen the capacities of the backward-gazing method. This method was patented (Henault and Caliot, in 2014) and is in a development phase at PROMES. Its principle consists in measuring, from several points of observation (in practice, the digital cameras) situated in the vicinity of the theoretical focal point, the visible radiative intensity the mirror surface reflects and originated from the sun. With these images, the real deformations of the segmented mirror are then estimated by means of specific algorithms. The method allows discriminating, in a quantitative way, between the canting errors of the heliostat, the pointing errors, and the defects of the segmented reflector surface (including waviness).

Ph.D. Objective

The objective of the thesis is to develop the current capacities of the backward-gazing method to characterize the defects of the optical surfaces of heliostats. The precision of this new method must be improved thanks to the use of experimental results obtained by a fluxmetry method on passive target. From the combination of these two methods will emerge a hybrid optical method enabling the comprehensive characterization and with a high accuracy the heliostats of the Themis solar tower. This method applied to a group of heliostats will be used to correct the mirror orientations by means of their actuators (screw, etc.). Finally, a characterization after the correction will validate the optical hybrid complete method.

Work Programme

The research work will begin with a literature review of the methods used to characterize the reflecting surfaces in concentrated solar tower power plants. Techniques such as the deflectometry, the colored targets, the photogrammetry, etc., will be reviewed and compared with the backward-gazing method. The studies on the prediction of the concentrated solar flux distribution (flux maps) on a target with Ray-Tracing or Convolution methods will be reviewed and discussed.

A training for the software and experimental devices developed at PROMES (Ray-Tracing, Backward-gazing setup, fluxmetry) will be realized through simulation campaigns and experiments at Themis (Targassonne, France).

A theoretical study using different software will be led. Simulations image capture by cameras (COSAC software) will allow optimizing theoretically the operational parameters of the backward-gazing method for an application to the solar tower power facility Themis: number of cameras, distance, relative position around the target point, etc. A wavefront reconstruction will be undertaken to identify various optical aberration sources such as defocus, astigmatism, etc. Then, simulations of flux maps on a target (COSAC software or SOLSTICE) will be realized by using real heliostats presenting structural defects or pointing errors, with the aim of quantifying the effects of the defects on the variation of the size and the position of the focal point during the day.

The objective being to develop a hybrid characterization method for the defects of heliostats thanks to the backward-gazing method the fluxmetry, both experimental methods will be implemented on one or several test heliostats to obtain raw data. Then, the post-treatment of the backward-gazing method will calculate the defects of the mirrors which will serve as initial information to feed an optimization algorithm. This algorithm will search iteratively the defects of surfaces minimizing the discrepancies between the measured and computed flux maps on the target. This combination will establish the heart of the development of the optical hybrid method. Finally, thanks to the identification of the defects, a correction of heliostats will be realized and an experimental campaign will validate the comprehensive hybrid method for heliostat characterization and correction.

References

- Xiao, J., Wei, X., Lu, Z., Yu, W., Wu, H. , A review of available methods for surface shape measurement of solar concentrator in thermal power applications, Renewable and Sustainable Energy Reviews, 16, pp. 2539-2544, 2012.
- Henault, F., Caliot, C., Installation concentratrice de rayonnement cosmique équipée d'un système de contrôle de surface optique réfléchissante, Demande de Brevet en France n° 14/52622, 2014.

Supervision and location of the thesis

Thesis joint supervision:
Cyril Caliot (Chargé de Recherche CNRS, Habilité à Diriger des Recherches)
+33 4 68 30 77 44,
Laboratoire Procédés, Matériaux et Energie Solaire (PROMES), 7 rue du Four Solaire, 66120 Font-Romeu-Odeillo-Via.
François Hénault (Ingénieur de Recherche CNRS, Habilité à Diriger des Recherches)
+33 4 76 63 57 78,
Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université J. Fourier – B.P. 53, 38041 Grenoble.

The Ph.D. Thesis will take place at the PROMES laboratory at the Odeillo site.

Chronogramme

Required skills

He/She candidate will have to justify for the following skills:
1. Optics and/or in Radiative Transfer,
2. Programming and/or scientific computation (e.g. image processing, Matlab, python, C, C ++, etc.),
3. Written and oral expression in French and in English.
He/She will be motivated by both experimental and numerical aspects of the thesis subject.

Funding

The thesis will be co-funded by the project Next-CSP (coordinator PROMES, G. Flamant) and the Labex SOLSTICE (person in charge PROMES, G. Flamant) for an amount of about 1750 € (monthly) gross salary.


Documents needed for the registration in thesis at the Doctoral School Energy Environment E²:
- Registration form
- Statements of the marks (evaluations) of the 1st and 2nd year of Master (or equivalent) (translated if needed, in English or in French)
- The present summary of the thesis subject
- The present chronogram of the progress of the thesis

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