Development and application of a multi- domain dynamic model for direct steam generation solar power plant

Master of Science Thesis

KTH School of Industrial Engineering and Management Energy Technology EGI_2017-0092-MSC EKV1214

Division of Heat & Power Technology SE-100 44 STOCKHOLM

Development and application of a multi- domain dynamic model for direct steam

generation solar power plant Anthony Rousset

Linear Fresnel solar field (Source: ©CEA-INES)

Abstract

Nowadays, one of the solutions considered in order to face the issue of global warming and to move towards a carbon neutral society relies on the use of solar energy as a renewable and bountiful primary source. And, if photovoltaic technologies account for a large part in the solar energy market, recent years have witnessed the growth of non-concentrated and concentrated solar thermal technologies. Among them, concentrated solar power technology (CSP) which uses the optical concentration of direct solar irradiation to generate high pressure and high temperature steam in the absorber tubes of the plant, has become a promising approach reaching 4.9 GWe of installed capacity by the end of 2015 [1].

However, one of the main challenges faced by CSP technology concerns the variability of solar energy related for example to sunrise, sunset, passing clouds… In addition to that, when it comes to direct steam generation, the presence of a two-phase flow regime inside the absorber tubes leads to a strong dynamic behavior of the steam generation. It is consequently necessary to be able to simulate this dynamic behavior in order to better handle the design and operation of CSP plants. Such simulation tools can then be used for the implementation and the test of reliable control systems aimed at maintaining desired operating conditions in spite of changes in solar irradiation.

In this context, the National Institute for Solar Energy (INES), part of the French Alternative Energies and Atomic Energy Commission (CEA) wishes to upgrade their dynamic simulation tool that would enable its teams to reproduce the behavior of a prototype based on the Fresnel solar field technology including direct steam generation which was built and commissioned at Cadarache, Aix-en-Provence.

This Master thesis work takes place within this framework and aims at developing a multi-domain dynamic model of the aforementioned prototype. To do so, three models respectively in the thermal- hydraulic, the optical and the control-command domains are built and combined using a co-simulation approach relying on an in-house simulation platform called PEGASE.

Master of Science Thesis EGI 2017:xxxx

Development and application of a multi- domain dynamic model for direct steam

generation solar power plant

Anthony Rousset

Approved Examiner

Andrew Martin - KTH/ITM/EGI

Supervisor Andrew Martin

andrew.martin@energy.kth.se Commissioner

INES - CEA

Contact persons Roland Bavière

roland.baviere@cea.fr Valéry Vuillerme

valery.vuillerme@cea.fr

More specifically the development of the following models has been addressed:

 a thermal-hydraulic model of the two-phase flow circulating inside the vaporizer field of the prototype and realized with the thermal-hydraulic code CATHARE [2] (Advanced Thermal- Hydraulic Code for Water Reactor Accidents) applied to solar thermal biphasic issues,

 an optical model of the receiver programmed using the Modelica language and the Dymola (Dynamic Modelling Laboratory) simulation software,

 control-command models (PID controller, control architecture…) adapted and built upon blocks taken from a modelling library included in the PEGASE platform.

Each model was first developed and tested on a standalone basis. These models were then coupled using the PEGASE co-simulation platform. A sunny day was simulated using the multi-domain model and the controllability of the plant was analyzed. At this stage, the study focused on the steam separator level regulation. A thermal-hydraulic study also focused on potential instabilities in the vaporizer that can occur under certain circumstances of water temperature at vaporizer inlet and solar heat flux. This analysis was carried out with a CATHARE standalone model.

Perspectives of the present work include a complete validation of the developed models from future experimental data and further developments should aim to extend the modelling scope of the numerical simulator towards a representation of all the hydraulic parts of the CSP prototype. Control schemes and regulation tools would have to be extended as well in order to move towards a more representative control architecture of the prototype. Particularly, the steam quality at vaporizer outlet is an important variable to regulate. Indeed, this parameter is usually kept between 60% and 80% [3]. It must be high enough to limit the power consumption of the recirculation pump but not too high in order to prevent absorber dry-out.

Keywords

Concentrated Solar Power (CSP), Direct Steam Generation (DSG), dynamic modelling, co-simulation, thermal-hydraulic calculation, two-phase flow, Ledinegg instabilities, control-command, regulation, PID controller.

SAMMANFATTNING

Solenergi, som är en förnybar och riklig primärkälla, är en av de lösningarna som anses kunna lösa problemet med global uppvärmning och bidrar i omvandlingen till ett kolneutralt samhälle. Andelen fotovoltaiska teknologier på energimarknaden är övervägande, men andelen koncentrerad och icke- koncentrerad solterminsteknik har ökat under de senaste åren. Bland solterminsteknikerna är koncentrerad solenergiteknik (CSP), som använder den optiska koncentrationen av direkt strålning för att generera högtrycks- och högtemperaturånga i anläggningens absorberarrör, ett lovande tillvägagångssätt som har nått 4.9 GWe installerad kapacitet i slutet av 2015 [1]. En av de största utmaningarna med CSP-tekniken är solenergins variation vid till exempel soluppgång, solnedgång och passerande moln, vilket beror på varierad tillgång av solljus. Det finns också utmaningar med direkt ånggenerering via tvåfasflödes regimer inuti absorberarrören eftersom det leder till ett starkt dynamiskt beteende vid ånggenereringen. Det är följaktligen nödvändigt att kunna simulera detta dynamiska beteende för att bättre hantera design och drift av CSP-anläggningar. Sådana simuleringsverktyg kan sedan användas för att genomföra tester för att erhålla tillförlitliga styrsystem som upprätthåller önskade driftsförhållanden trots förändringar i solstrålningen.

I detta sammanhang vill National Institute for Solar Energy (INES), som är en del av den franska alternativa energikommissionen och atomenergi kommissionen (CEA), förbättra dess dynamiskt simuleringsverktyg som skulle möjliggöra för sina team att reproducera beteendet hos en prototyp baserad på Fresnel solfältsteknik inklusive direkt ånggenerering som byggts och beställts vid Cadarache, Aix-en- Provence. Denna masteruppsats sker inom ramen för detta och syftar till att utveckla en dynamisk modell med flera domäner av den ovan nämnda prototypen. Tre modeller i termisk-hydraulisk, optisk och kontrollkommando domäner har byggts och kombinerats med hjälp av en co-simuleringsmetod som bygger på en intern simuleringsplattform som heter PEGASE. Mer specifikt om utvecklingen av modellerna enligt nedan:

 En termisk-hydraulisk modell av tvåfasflöde som cirkulerar inuti förångarens fält på prototypen har realiserats med termisk-hydraulisk kod CATHARE [2] (Advanced Thermal-Hydraulic Code for Water Reactor Accidents) som appliceras på soltermisk bifasiska frågeställningar.

 En optisk modell av mottagaren har programmerats med hjälp av Modelica-språket och simuleringsprogrammet Dymola (Dynamic Modeling Laboratory).

 Modeller av kontrollkommandon (PID-kontroller, kontrollarkitektur ...) har byggts och anpassats i moduler som hämtats från modelleringsbibliotek som ingår i PEGASE-plattformen.

Varje modell utvecklades och testades på fristående basis. Modellerna kopplades sedan samman i PEGASE-co-simuleringsplattformen. En solig dag simulerades därefter med en flerdomänmodell och styrningsförmågan av anläggningen analyserades. Vid detta stadium fokuserade studien på att reglera nivån av ångseparerande. En termisk-hydraulisk studie fokuserade sedan på potentiella instabiliteter i förångaren som kan uppstå under vissa omständigheter av vatteninloppstemperatur och solvärmeflöde. Denna analys genomfördes med en CATHARE fristående modell.

Perspektiven för det aktuella arbetet omfattar en fullständig validering av de utvecklade modellerna med hjälp av framtida experimentella data. Vid en vidareutveckling bör inriktningen vara att utvidga modellernas omfattning av den numeriska simulatorn till att representera alla hydrauliska delar av CSP prototypen. Styrsystem och regleringsverktyg skulle också behöva förbättras för att få en mer representativ kontroll arkitektur av prototypen. I synnerhet är ångkvaliteten vid förångarens utlopp en viktig variabel att reglera. Faktum är att den här parametern vanligtvis hålls mellan 60% och 80% [3]. Det måste vara tillräckligt högt för att begränsa recirkulationspumpens elförbrukning men inte för hög för att förhindra att absorberen torkar ut.

Nyckelord

Koncentrerad solenergi (CSP), Direkt ånggenerering (DSG), dynamisk modellering, co-simulering, termisk hydraulisk beräkning, tvåfasflöde, Ledinegg instabiliteter, kontrollkommando, reglering, PID-kontroller..

-i- Contents

1 INTRODUCTION AND BACKGROUND ... 1 1.1 Host institution ... 1

CEA Tech: a research unit of CEA ... 1 1.1.1

Liten Institute ... 2 1.1.2

INES: the National Institute for Solar Energy ... 3 1.1.3

1.2 Context of the study ... 3 Solar energy share in the current panel of Renewable Energies ... 3 1.2.1

Solar thermal electricity ... 4 1.2.2

Potential instabilities in the vaporizer ... 9 1.2.3

1.3 Objectives and Methodology of the Master Thesis ... 13 CEA Linear Fresnel power plant prototype ... 13 1.3.1

Objectives and Methodology ... 13 1.3.2

Literature Review ... 14 1.3.3

2 MODELLING TOOLS AND NUMERICAL MODELS ... 20 2.1 Co-simulation principle and tools ... 20

CATHARE: a thermal-hydraulic code jointly developed by the consortium 2.1.1

CEA/AREVA/EDF/IRSN ... 20 DYMOLA & MODELICA ... 22 2.1.2

PEGASE: the numerical co-simulation platform ... 23 2.1.3

2.2 Thermal-hydraulic model ... 25 Description of the model elements ... 27 2.2.1

Head loss calculation... 31 2.2.2

Sensors and sub-modules ... 33 2.2.3

2.3 Optical model ... 33 Presentation of the model ... 33 2.3.1

Energy balances taken into account ... 34 2.3.2

2.4 Control-command models ... 35 Single feedback PI controller ... 35 2.4.1

Feedforward plus feedback controller ... 36 2.4.2

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3 APPLICATIONS OF THE NUMERICAL MODELS ... 39

3.1 Validation test ... 39

Test description ... 39

3.1.1 Interpretation and limitations ... 40

3.1.2 3.2 Investigation of the pressure drop vs mass flow rate internal characteristic of a vaporizer branch .. 41

Thermal-hydraulic model of a vaporizer branch ... 41

3.2.1 Scope of the study ... 43

3.2.2 S-shaped internal characteristics ... 44

3.2.3 Diagnostic Charts ... 46

3.2.4 Example of use ... 47

3.2.5 3.3 Co-simulation on a sunny day ... 49

Co-simulation chain ... 49

3.3.1 Results of the co-simulation on a sunny day ... 50

3.3.2 Discussion and limitations ... 52

3.3.3 3.4 Assessment of the controllability of the system ... 53

Steam separator level fluctuations ... 53

3.4.1 Implementation of a single feedback PI controller ... 54

3.4.2 Necessity of an anti-windup control ... 59

3.4.3 Implementation of the stationary mass flow rate calculation ... 60

3.4.4 Influence of a simulated cloud passage ... 63

3.4.5 Test of a control strategy based on the recirculated mass flow rate ... 65

3.4.6 4 DISCUSSION AND FURTHER DEVELOPMENTS ... 68

4.1 Strengths of the study ... 68

4.2 Further developments ... 68

Improvement of the thermal-hydraulic model ... 68

4.2.1 Validation of the co-simulated model ... 68

4.2.2 Development of further control strategies ... 69

4.2.3 5 GENERAL CONCLUSION ... 70

REFERENCES ... 72

APPENDIX 1: Concentration ratio ... 75

APPENDIX 2: Pressure variations balance in the recirculation loop ... 76

APPENDIX 3: Implementation of the stationary inlet mass flow rate calculation ... 77

APPENDIX 4: Diagnostic charts of S-shaped internal characteristics ... 79

-iii- List of Figures

Figure 1.1: Distribution of CEA centres over the French territory [4] ... 1

Figure 1.2: Branch of the organizational chart of the CEA referring to the Master Thesis environment ... 2

Figure 1.3: Augustin Mouchot's Solar concentrator at the Universal Exhibition in Paris, 1878 [13]... 5

Figure 1.4: Overview of the four forms of CSP technologies [15] ... 6

Figure 1.5: Layout of the four forms of CSP technologies [16] ... 7

Figure 1.6 : Different operation modes for direct steam generation CSP plants [18] ... 9

Figure 1.7: S-shaped pressure drop vs mass flow rate internal characteristic curve [23] ... 10

Figure 1.8: Various types of pressure drop vs mass flow rate internal characteristics ... 11

Figure 1.9: Different external characteristic curves drawn on a pressure-drop vs mass flow rate internal characteristic [20] ... 12

Figure 1.10: Simplified flowsheet of the Linear Fresnel power plant prototype in Cadarache [24] ... 13

Figure 1.11: Pressure drop vs mass flow rate internal characteristic for various orifice ratios ... 15

Figure 1.12: Layout of the nodalization scheme of the DISS thermal-hydraulic model (RELAP5) ... 16

Figure 1.13: Structure of a PI control loop with anti-windup [18] ... 18

Figure 1.14: Layout of the DISS facility configured in the recirculation mode [18] ... 19

Figure 2.1: View from GUITHARE related to the void fraction in a branch of the vaporizer prototype ... 21

Figure 2.2: Principle diagram of the Exchange Zone in PEGASE ... 23

Figure 2.3: View of the PEGASE post-processing interface ... 25

Figure 2.4: View of the thermal-hydraulic model from GUITHARE (schematic diagram not scaled) ... 26

Figure 2.5: Structure of an axial element (1-D module) ... 27

Figure 2.6: Meshing of the ISO39CHP25 pipe ... 28

Figure 2.7: Spatial discretization of the standard 0-D module ... 29

Figure 2.8 : Layout of the Fresnel Trapezoidal Receiver modelled with Dymola [35] ... 34

Figure 2.9: View of the PI controller architecture from the FbsEditor ... 36

Figure 2.10: Feedforward plus feedback control diagram [36] ... 37

Figure 2.11: Layout of the modelled part of the prototype... 38

Figure 3.1: Evolution of the recirculated mass flow rate as a function of the ΔP in the pump ... 40

Figure 3.2: View of the thermal-hydraulic model of one branch of the vaporizer from GUITHARE ... 42

Figure 3.3: S-shaped internal characteristic occurring at high mass flow rates (a) and S-shaped internal characteristic occurring at lower mass flow rates (b) ... 45

Figure 3.4: Diagnostic chart related to the inlet mass flow rate gap ... 46

Figure 3.5: Diagnostic chart with outlines related to the gap of ΔP (in Pa) ... 48

Figure 3.6: Co-simulation principle diagram ... 49

Figure 3.7: DNI and heat flux densities applied to the vaporizer for a sunny day ... 50

Figure 3.8: Excerpt of the fluid thermal-hydraulic properties available from the co-simulation results ... 52

Figure 3.9: Instability of the steam separator level in the base case ... 54

Figure 3.10: Identification test of the steam separator level evolution process ... 55

Figure 3.11: Architecture of a parallel PID controller ... 56

Figure 3.12: Ziegler-Nichols closed-loop critical gain method for the tuning of PI corrective parameters ... 58

Figure 3.13: Steam separator level response to a change in set point according to the chosen set of PI parameters ... 59

Figure 3.14: Necessity of an anti-windup control ... 60

Figure 3.15: Dynamic error of the regulation process (hatched) [40] ... 61

Figure 3.16: Evolution of the steam separator level on a sunny day with clear sky according to the implemented control strategy ... 62

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Figure 3.17: Evolution of the steam separator level on a sunny day with a 15min-cloud passage according to the implemented control strategy (a) and corresponding DNI (b) ... 64 Figure 3.18: Evolution of the steam separator level during the simulated cloud passage according to the implemented control strategy ... 65 Figure 3.19: Steam separator level and recirculated mass flow rate responses to a step of pressure in the pump ... 66 Figure 3.20: Evolution of the steam quality at vaporizer outlet in the case of a recirculated mass flow rate increase ... 67

-v- List of Tables

Table 1.1: List of the solar DSG power plants currently in operation or under construction [14] ... 5

Table 1.2: Exceeding limits for turbine inlet temperature and pressure according to IEC Standard [30] ... 17

Table 1.3: Operating modes investigated in the DISS solar field by Valenzuela et al. [18] ... 19

Table 2.1: T22 Stainless Steel thermal properties at 20°C ... 29

Table 2.2: Boundary conditions related to the base case ... 30

Table 2.3: Values of A coefficient as a function of the bend angle ... 32

Table 2.4: Sensors and sub-modules positions in the system ... 33

Table 2.5: Heat fluxes exchanged in the optical model ... 34

Table 3.1: Test operating conditions... 39

Table 3.2: Validation test in a single-recirculation-loop configuration ... 39

Table 3.3: Boundary conditions of the model ... 42

Table 3.4: Binding values of the investigation ... 43

Table 3.5: Different values of the parameters tested ... 47

Table 3.6: Main features of the base case investigated ... 53

Table 3.7: Features of the operating point used to tune PI parameters ... 54

Table 3.8: Tuning of the corrective parameters according to Ziegler and Nichols closed-loop critical gain methodology [36] ... 57

Table 3.9: Critical values and suggested ones from Ziegler-Nichols methodology ... 57

Table 3.10: Fine-tuned PI corrective parameters ... 58

Table 3.11: Dynamic precision criteria for the simulated cases during a sunny day with clear sky ... 62

Table 3.12: Dynamic comparison of the implemented control strategies during a sunny day with a 15min cloud passage ... 63

Table 3.13: Dynamic comparison of the implemented control strategies focused on the cloud passage ... 63

-vi- Abbreviations

ADEME French Environment and Energy Management Agency

CATHARE Advanced Thermal-Hydraulic Code for Water Reactor Accidents

CEA Alternative Energies and Atomic Energy

Commission

CFD Computational Fluid Dynamics

CNRS National Centre for Scientific Research

CSP Concentrated Solar Power

CSTB Building Scientific and Technical Centre

DNI Direct Normal Irradiance

DSG Direct Steam Generation

DYMOLA Dynamic Modelling Laboratory

ECN École Centrale de Nantes

EDF Électricité de France

FMI Functional Mock-up Interface

FMU Functional Mock-up Unit

FTR Fresnel Trapezoidal Receiver

HEM Homogeneous Equilibrium Model

HTF Heat Transfer Fluid

ICT Information and Communication

Technologies

IDE Integrated Development Environment

IEA International Energy Agency

IEC International Electrotechnical Commission

INES French National Institute for Solar Energy IRSN Radio-protection and Nuclear Safety Institute

KTH Kungliga Tekniska Högskolan (Royal Institute

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of Technology)

LITEN Laboratory of Innovation in new

Technologies for Energy and Nanomaterials

LSHT Laboratory of High Temperature Solar

Systems

OFI Onset of Flow Instability

ONB Onset of Nucleate Boiling

PEGASE Advanced Management Platform for Energy Systems

PI Proportional - Integral controller

R&D Research and Development

SEE Sustainable Energy Engineering master’s

program

SEGS Solar Energy Generating Systems

-viii- Nomenclature

Latin symbols Units Description m² Collector area

m² Receiver area

- Geometrical concentration ratio

- Optical concentration ratio m² Valve flow coefficient

D_i m Bend internal diameter

kJ.kg^-1 k-phase enthalpy¹

, W.m² Incident beam irradiation upon the collector W.m² Irradiation over the receiver area

K - Singular head loss coefficient

K_cr - Ziegler-Nichols critical gain

- Derivative gain of a parallel PID controller

- Integral gain of a parallel PID controller

- Proportional gain of a parallel PID controller

- Proportional gain of a series PID controller

kg.s^-1 k-phase mass flow rate

kg.s^-1 Mass flow rate through the valve

P Pa Pressure

kW Thermal power coming from the preheater

kW Thermal power received by the vaporizer prototype m³.s^-1 k-phase volumetric flow rate

R_C m Radius of curvature of the bend

- k-phase Reynolds number

S_m m² External area of one mesh in the absorber model

1 liquid phase (k=l) or vapor phase (k=g)

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T_cr s Time period of the sustained oscillations in the

Ziegler-Nichols methodology

s PID derivative time constant

s PID integral time constant m.s^-1 k-phase velocity

x - Steam quality

Greek symbols Units Description

α - Volumetric void fraction

Volume of the mesh occupied by steam Total volume of the mesh

∆ Pa Singular head loss

∆ Pa Friction head loss

- Thermal efficiency of the FTR kg.m^-3 k-phase density²

kg.m^-3 Average density of the two-phase flow

s^-1 Time constant of the Laplace transfer function modelling the steam separator level evolution process W.m^-2 Heat flux density available for the absorber tube W.m^-2 Concentrated solar heat flux density on the outer glass

surface of the FTR

W.m^-2 Heat flux density on the upper insulator surface of the FTR

, , W.m^-2 Heat flux density applied to the i^th mesh of the j^th passage in the vaporizer branch

m Friction perimeter

2 liquid phase (k=l) or vapor phase (k=g)

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ACKNOWLEDGMENTS

First of all, I would like to start this Master Thesis by expressing my respectful thanks to my supervisors at CEA-INES for giving me the opportunity to write it under their dedicated and valuable management. Thanks, then, to Roland Bavière and Valéry Vuillerme for the trust and the confidence they have granted me during the six past months as well as for the instructive help and constant support. The hindsight of Valéry Vuillerme combined with the unfailing faith of Roland Bavière in the pursuit of our research have made this work possible.

My appreciation also goes to Pr. Andrew Martin who accepted to be the examiner of this project and who has allowed a genuine smooth and cooperative development of this Master Thesis. Thank you.

I wish to thank the several teams of the LSHT for their convivial and federative welcome as well as for the quality of their scientific advice. I especially thank Nicolas Lamaison for the interest he has shown on the technical and scientific issues raised during this period and for which he gave me wise and helpful advice. His assistance about the phenomena of static instabilities in a heated channel was precious. Many thanks to Jérôme Pouvreau from Grenoble CEA centre for his help on generating simultaneous computations with CATHARE code on the common cluster.

I would also like to include in these acknowledgments all the trainees, the PhD students and other members of App’INES association for their sympathy, their advice and the great atmosphere they contributed to bring to my Master Thesis through the several extra activities and the numerous

“fika³” we have shared. Many thanks to Julie, Nicolas, Nadine, Bertrand, Quentin, Marie, Gaëlle, Anne-Claire, Coralie, Philémon, Houssame, Lauren, Léa, Blaise, Gabriele, Aissata, Elise, Audrey, Antoine, Chaima, Sanae, Léo.

The following Master Thesis concluding my Double-Degree between KTH University and École Centrale de Nantes, I would also like to take this opportunity to thank all the people who contributed to make this journey a rewarding experience both in terms of personal and professional development. Thanks, thus, to École Centrale de Nantes for giving me the possibility and the support of completing my degree in such a fulfilling context and to KTH University for its warm and caring welcome and for allowing me to pursue my academic training in a great freedom with all the resources and the assistance needed. I am thinking in particular to my international coordinators in France: Sabine Vermillard, Claire Delhomme, Olivier Kermorgant and in Sweden: Elin Wiljergård, Vera Nemanova who have supported me with goodwill and great interest.

Many thanks as well to my student fellows and all my friends in France, in Sweden, and literally from all over the world, who have provided me with knowledge, openness, values, and unforgettable shared moments.

Finally, I cannot end these acknowledgments without express my very profound gratitude to my parents and my family for providing me with unfailing support and continuous encouragement throughout my years of study. Their contribution in my academic career is invaluable.

To all the people mentioned here, Thank you, Tack, Merci.

Anthony Rousset, 2017.08.16

3 From Swedish : « coffee break »

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FOREWORD

This Master Thesis has been conducted as the Degree Project of the Sustainable Energy Engineering (SEE) master’s program and concludes a Double Degree carried out between KTH University (Stockholm, Sweden) and École Centrale de Nantes (France). It has been performed from the 20^th of February 2017 to the 18^th of August 2017 within the Laboratory of High Temperature Solar Systems (LSHT) of the CEA (Alternative Energies and Atomic Energy Commission) based at the National Institute for Solar Energy (INES) in Le Bourget-du-Lac, France.

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1 INTRODUCTION AND BACKGROUND

1.1 Host institution

As mentioned previously in the Foreword, this Master thesis has been conducted in collaboration with the host institution which is presented in the following part.

CEA Tech: a research unit of CEA 1.1.1

Founded in 1945 by General de Gaulle in order to support the research in nuclear energy, the French Alternative Energies and Atomic Energy Commission (CEA) is nowadays a major institution in research, development and innovation in four main areas [4] :

 defense and security

 nuclear and renewable energies

 technological research for industry

 fundamental research in the physical sciences and life sciences

This EPIC (state owned industrial and commercial company) actively participates in collaborative projects with a large number of academic and industrial partners and is established in nine centres spread throughout France (Figure 1.1). It works in partnership with many other research bodies, local authorities and universities. Within this context, the CEA is a stakeholder in a series of national alliances set up to coordinate French research in energy, life sciences and health, digital sciences and technology, environmental sciences, human and social sciences.

Figure 1.1: Distribution of CEA centres over the French territory [4]

4 - GRENOBLE CEA and INES centres

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Its technology research unit, CEA Tech, develops a broad portfolio of technologies for ICTs, energy, and healthcare through its three labs: Leti (Laboratory of Electronics and Information Technology), Liten, and List (Laboratory of Systems and Technologies Integration).

Liten Institute 1.1.2

Liten institute (Laboratory of Innovation in new Technologies for Energy and Nanomaterials) is one of the three labs of CEA Tech and it is a major European research institute engaged in developing sustainable energy technologies. The institute is spearheading the EU's efforts to limit dependency on fossil fuels and reduce greenhouse gas emissions [5]. Indeed, Liten’s programs focus on three major areas: renewable energy and storage; energy efficiency and lowering CO₂ emissions; and materials implementation and recycling to promote raw-materials-efficient processes and, ultimately, support the circular economy. As outlined by the Head of Liten, Florence Lambert in [6], the mission of the institute is “to support France’s energy transition strategy through technology research and development”. This institute is then divided into several departments, themselves divided into several services and laboratories as it is shown in Figure 1.2.

Figure 1.2: Branch of the organizational chart of the CEA referring to the Master Thesis environment

CEA

DRT (Technological Research Division)

LIST LITEN

DEHT DTS

DTBH (Thermal, Bioresources and

Hydrogen Department)

SCSH SBRT (Thermal

Network and Bioresources Service)

LS2T LSHT (Laboratory of High Temperature

Solar Systems)

Master Thesis (INES)

LTCB LPB

SCTR DTNM LETI

DRF DEN DAM

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INES: the National Institute for Solar Energy 1.1.3

INES is a research institute based in Le Bourget-du-Lac, France. This is an independent institution connected to the Grenoble CEA centre. It is moreover a reference centre in France since its creation in 2005, and one of the first in Europe, dedicated to research, innovation and training on solar energy. Set up with the support of the Savoie Departmental Council and Rhône- Alpes Regional Council, it hosts teams from the CEA and the University of Savoie, and is supported by the CNRS (National Centre for Scientific Research) and the CSTB (Building Scientific and Technical Centre). The research at INES is mainly directed towards the following fields:

 Solar Photovoltaic

 Solar Thermal Energy

 Energy Efficiency in Buildings

The institute’s laboratories, clean rooms, pilot facilities and demonstrators allow its scientists and technicians to work on optimizing all aspects of solar photovoltaic energy, from cells to systems and from positive-energy buildings to solar mobility. In addition to the solar photovoltaic energy field, INES teams also dedicate their studies on solar thermal energy for cooling and heating, and investigate ways of optimizing passive energy through energy management in buildings and energy efficiency technologies [7].

Within the LSHT, the research is mainly focused on district heating networks (development and integration of large-scale networks), CSP technology and durability of the materials used in the CSP field (mirrors, solar absorbers…), heating and cooling valorization. A part of the research carried out in the laboratory also refers to solar gasification.

1.2 Context of the study

This paragraph aims to briefly remind the current technical-economic context in which this Master Thesis was written. In addition to that, a theoretical background about two-phase flow instabilities in heated channels is drawn in the last part.

Solar energy share in the current panel of Renewable 1.2.1

Energies

1.2.1.1 Renewable power generation in the past few years

According to [8], renewable power generating capacity saw its largest annual increase ever in 2016, with an estimated 161 gigawatts of capacity added. This report also highlights that the world now adds more renewable power capacity annually than it ads (net) capacity from all fossil fuels combined. Indeed, in 2016, renewable energies accounted for an estimated nearly 62% of net additions to global power generating capacity. Solar PV saw record additions and represented about 47% of newly installed renewable power.

In France, the past two years have also been thriving years in the field of renewable energy R&D.

Indeed, the French government unveiled its industrial renewal policy, which includes an environmentally-friendly mobility plan; the Energy Transition for Sustainable Growth Act was passed; and, on top of that, the United Nations COP21 conference on climate change was held in

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Paris in December 2015 and adopted by the majority of the countries attending the conference [6].

Furthermore, it can be noticed that progress on these initiatives continue as we move towards a new French ecological policy lead by the well-known environmental activist and new minister of the Energy Transition: Nicolas Hulot.

1.2.1.2 Position of the solar thermodynamic sector

In this favorable trend, the solar thermodynamic sector has also experienced an important growth from 355 MWe of installed capacity in 2005 to 4.7 GWe in 2016 [9] (4.9 GWe according to [1]). This growth was driven by Spain (2.3 GWe of installed capacity in 2016) and the United States (1.8 GWe of installed capacity in 2016). According to CSP Today, an English firm specialized in the sector, projections trends indicate that there is a potential growth of the solar thermodynamic sector between 10 and 22 GW installed by the end of 2025, reflecting pessimistic, conservative and optimistic scenarios. In the long run, the International Energy Agency (IEA) forecasts that the contribution of solar thermodynamic energy in the total world electricity production will reach 11% in 2050 [10].

With more than 1 000 GW of installed capacity in this ambitious scenario, the concentrated solar power plants, whether or not associated with energy storage, could ensure an annual production of 4 770 TWh, it is to say, the equivalent of the consumption of the United States. At the French level, ADEME (the French Environment and Energy Management Agency) has also developed a scenario called "an electric mix 100% renewable in 2050 "published last fall 2015, in which the projections of the solar thermodynamic capacity reaches 430 MW in 2050 [9].

Today, the main markets in the sector are South Africa, the Middle East, the Maghreb, India, China and Chile. Among them, one of the most ambitious country is Morocco, where the Noor program foresees the development of solar power plants with a total capacity of 2 000 MW by 2020.

Solar thermal electricity 1.2.2

1.2.2.1 A brief history

Since the dawn of time, solar energy has ensured the natural cycles of our planet through atmospheric circulation, water cycle, photosynthesis…

However, the first known machine using solar energy is the invention of the French engineer Augustin Mouchot in 1866 which was a machine made up of a dish concentrator and a small glass boiler enabling to power a small steam engine [11]. In 1878 he also presented at the Universal Exhibition in Paris its prototype of a solar concentrator for which he won the gold medal of the exhibition (Figure 1.3). His assistant, Abel Pifre, continued his work and demonstrated a solar powered printing press in “Jardin des Tuileries” in 1882. The American engineer John Ericsson built similar devices in the United States around 1884, based on parabolic troughs [12].

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Figure 1.3: Augustin Mouchot's Solar concentrator at the Universal Exhibition in Paris, 1878 [13]

The first solar thermodynamic plant came up almost thirty years later, in 1913, from the work of the American engineer Frank Shuman who built a parabolic trough power plant in Egypt capable of running an engine used for the irrigation of cultures. It is worthy to note that direct steam generation was used in this first plant. Despite these pioneer achievements, the first research centres dedicated to solar energy studies were only created in the 1980’s after the two oil price shocks in 1973 and 1979 decreased the influence of oil on the energy market. They were built particularly in Spain (“Plataforma Solar de Almeria”), in France (“THEMIS” platform in Targassonne) and in the United States.

Table 1.1 shows a list of the DSG power plants currently in operation or under construction.

Table 1.1: List of the solar DSG power plants currently in operation or under construction [14]

Plant Location Power Technology⁴ Starting

year Operator

PS10 Spain, Andalusia 11 MWe Tower 2007 Abengoa Solar PS20 Spain, Andalusia 20 MWe Tower 2009 Abengoa Solar Puerto Errado I Spain, Murcia 1.4 MWe Fresnel 2009 Novatec Solar

Puerto Errado II Spain, Murcia 30 MWe Fresnel 2012 Novatec Solar Lidell (coal

powerplant booster)

Australia, New South

Wales

18 MWth Fresnel 2012 Ausra &

Novatec Solar

TSE1 - PT Thailande,

Kanchanaburi 5 MWe Parabolic

Troughs 2012 Solarlite

Ivanpah USA,

Mojave desert 400 MWe Towers (3) 2013 Brightsource

Energy

Dhursar India,

Rajasthan 100 MWe Fresnel 2014 Areva/Rajasthan

Sun Technique Kogan Creek

(coal powerplant booster)

Australia,

Queensland 44 MWth Fresnel 2013 CS

Energy/Areva

4 See 1.2.2.2 Concentrated solar power technology

-6- Khi Solar One South Africa,

Northern Cape 50 MWe Tower 2014 Abengoa Solar

Alba Nova 1 France, Corse 12 MWe Fresnel 2015 Solar Euromed

1.2.2.2 Concentrated solar power technology

Concentrated solar power technologies aim to generate high-temperature steam by using mirrors to concentrate the sun thermal energy. The high-temperature steam can then be used to drive a turbine and produce electrical power but it can also serve as a storage medium or as industrial process heat. Concentrating solar power plants can integrate thermal energy storage systems to generate electricity during cloudy periods or even several hours after the sunset. They can also be combined with other non-solar fuel based power plants into hybrid power plants in order to provide dispatchable power.

In the case of power generation, the solar thermal energy collected is converted into electricity, usually by means of two fluids: a heat transfer fluid (HTF) and a thermodynamic fluid. The heat transfer fluid, also called intermediate fluid, is used to convey the heat. The thermodynamic fluid, also called working fluid, allows to operate and drive the working machines (turbines, steam engines...). In the case of DSG plants, water represents both the heat transfer fluid and the thermodynamic fluid. The steam is then directly generated in the receiver.

Concentration technologies of CSP systems usually exist in four forms comprising: parabolic trough technology, linear Fresnel technology, parabolic dish Stirling technology and solar tower technology. Figure 1.4 illustrates these four different types of concentrating technology and their main components are specified in Figure 1.5.

Figure 1.4: Overview of the four forms of CSP technologies [15]

-7- 1.2.2.2.1 Parabolic Trough Technology

Nowadays, this is the most widespread technology. Indeed, due to feedback from SEGS plants in California connected to the grid for more than 20 years, this is the most mature technology. In this type of plants, the sun's energy is concentrated by parabolic curved, trough-shaped collectors onto a receiver pipe running along the inside of the curved surface. This energy heats the HTF (oil or water) flowing through the pipe, and the heat energy is then used to generate electricity in a conventional steam generator. A collector field comprises many troughs in parallel rows aligned either on a north-south axis or on an east-west axis. North-South oriented solar fields show more seasonally variable optical performance, with maximum efficiency during the summer and lower efficiency in the winter.

The East-West orientation is characterized by fewer seasonal variations, but they cannot harvest as much energy as the North-South oriented fields annually. Thanks to a single-axis tracking system, the sun can continuously be focused on the receiver pipes.

1.2.2.2.2 Linear Fresnel Technology

A significant cost factor in the technology of cylindrical parabolic collectors relies on the shaping of the glass to obtain its parabolic shape. A possible alternative is to approximate the parabolic shape of the collector by a succession of planar mirrors. This is the principle of the Fresnel concentrator. Each of the mirrors can be rotated along the course of the sun to permanently redirect and concentrate the sun's rays towards a tube or a set of fixed linear receiver tubes. The prototype built by the CEA and which is considered in this report uses this technology.

Figure 1.5: Layout of the four forms of CSP technologies [16]

1.2.2.2.3 Solar Tower Technology

In contrast to the previous technologies which constitute line focusing CSP technologies, this technology is part of the point focusing CSP technologies and uses many large, sun-tracking mirrors (heliostats) to focus sunlight on a receiver at the top of a tower. According to [17], the concentration of the sunlight leads to temperatures between 200°c and 600°C for linear

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concentrators with concentration ratios⁵ between 10 to 100, and to temperatures up to well above 1000°C for the point focusing concentrators with their concentration ratios between 100 and 10 000. A heat transfer fluid heated in the receiver is used to generate steam, which, in turn, is used in a conventional power cycle to produce electricity. Early power towers used steam as heat transfer fluid whereas current American designs use molten nitrate salt because of its superior heat transfer and energy storage capabilities. Current European designs use air as heat transfer medium because of its high temperature and its good manageability.

1.2.2.2.4 Parabolic Dish Stirling Technology

Parabolic dish systems consist of a parabolic-shaped point focus concentrator in the form of a dish that reflects solar irradiation onto a receiver located at the focal point. These concentrators are mounted on a structure with a two-axis tracking system to follow the sun. The collected heat is typically used directly by a heat engine mounted on the receiver and moving with the dish structure. The dilatation of a gas inside the engine triggers the movement of a piston which makes an alternator to rotate. Stirling and Brayton cycle engines are currently favored for power conversion. The modules have generally a maximal generation capacity of 50 kWe and have achieved peak efficiencies up to 30% net [15].

1.2.2.3 Operation modes of a DSG power plant

One of the main constraint of a CSP plant is to supply steam to the turbine at the most stable thermodynamic conditions under nominal operating conditions despite transients due to the variability of the direct solar irradiation along the day (sunrise, sunset, clouds…). Moreover, vaporization and superheating of the fluid involve different physical phenomena. Indeed, vaporization is characterized by a two-phase flow regime whereas a monophasic heat transfer occurs in the superheater of the solar field. In order to overcome this issue, several solar field designs can be considered. They are illustrated in Figure 1.6.

In the “once through mode”, feedwater is preheated, evaporated and superheated all along the absorber as it circulates from the inlet to the outlet of the collector row. Thus, this design is the simplest one but its controllability remains difficult, particularly when it comes to regulate the superheated steam parameters at the collector field outlet [18].

As it concerns the “injection mode”, several injections of water are made along the absorber rows in order to regulate the steam temperature and pressure. In the same way as in the “once-through mode”, vaporization of the fluid is made as it circulates from the inlet to the outlet, leading again to some regulation issues. In the beginning of DSG age, this concept was thought promising but it is barely used today [14].

Finally, in the “recirculation mode”, the preheated water enters the first part of the solar field, called the vaporizer, where it is heated until saturation and evaporated. A steam separator is located at the end of the evaporating section and the water in excess is recirculated to the collector loop inlet where it is mixed with the preheated water. The excess water in the evaporating section guarantees good wetting of the absorber tubes and makes stratification impossible [18]. Saturated steam is led towards the inlet of the superheater section. Usually, the steam quality at vaporizer outlet is kept between 60% and 80% [3]. It must be high enough to

5 Ratio between the average irradiance integrated over the receiver area divided by the incident insolation upon the collector (See APPENDIX 1: Concentration ratio)

(Source: Advanced Renewable Energy Technologies lectures provided by Rafael Guédez on January 19^th, 2016 at KTH, Stockholm)

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avoid excessive consumption of the recirculation pump, and low enough to enable a margin with respect to steam saturation and to prevent absorber dry-out.

Even if this concept leads to additional technical facilities (recirculation pump, steam separator…) in comparison with the “once-through mode”, it remains the most controllable and is privileged in current DSG designs.

Figure 1.6 : Different operation modes for direct steam generation CSP plants [18]

Potential instabilities in the vaporizer 1.2.3

One application of the thermal-hydraulic model produced during the preparation of this Master Thesis is related to the investigation of potential instabilities in one branch of a vaporizer. Thus, this part aims to draw a brief theoretical background of two-phase flow instabilities in heated channels. The starting point of the analysis of two-phase flow instabilities can be determined with the publication of Ledinegg article in 1938 [19] but it became a well-documented thematic since 1960 and the development of industrial high-power-density boilers and boiling water reactors, due to the fact that heated channels are encountered in this kind of reactors.

An exhaustive review related to two-phase flow instabilities is given by Ruspini et al., in [20]. A popular classification of two-phase flow instabilities is established in Bouré et al. [21] which divides these instabilities in two groups: static and dynamic instabilities. Static instabilities characterize the steady-state discontinuities of a system and can be analyzed using the steady-state system conservation equations. Flow excursions or Ledinegg instabilities, which are mentioned later on, are part of this category. Dynamic instabilities, on the other hand, most often lead to oscillations (flow oscillations, acoustic oscillations…) and can be analyzed by considering the transient regime of the system [22].

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1.2.3.1 Pressure drop vs mass flow rate internal characteristic curve

The investigation of static instabilities is based on the knowledge of the pressure drop (across the channel) vs mass flow rate internal characteristic curve of a heated channel (represented in this case by the absorber tube of the vaporizer subjected to the concentrated solar heat flux) which is set at [22]:

 a given pressure at any point in the channel (for example the outlet pressure),

 a given temperature at the inlet of the heated channel,

 a given heat flux density.

Figure 1.7 illustrates the typical S-shape (or N-shape) curve that can be obtained for a boiling system in a channel together with two possible scenarios of external characteristic curves (curves A and B).

Figure 1.7: S-shaped pressure drop vs mass flow rate internal characteristic curve [23]

For high mass flow rates, there is no vapor in the channel and the fluid is in a single-phase liquid state. For lower mass velocities, assuming that they can be achieved without secondary instabilities (See 3.2.3), the internal characteristic of the channel corresponds to the one of a steam flow and has a parabolic shape.

For intermediate flows, the trend of the curve is of type 1 or 2 and is connected to the characteristics of single-phase flow (Figure 1.8). Only case 2 is likely to lead to the development of Ledinegg instabilities and this is the case represented in Figure 1.7.

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Figure 1.8: Various types of pressure drop vs mass flow rate internal characteristics

1.2.3.2 Ledinegg instability

As mentioned in Bouré et al. [21] and Zhang et al. [23], a fluid heated in a channel is likely to represent an unstable situation for a thermodynamic system because it may be the source of a flow redistribution when its internal characteristic is of the second type shown in Figure 1.8.

This flow redistribution, also known as flow excursion or Ledinegg instability, corresponds to a sudden change in the mass flow rate generally appearing during a slow variation or during a small amplitude step of the value of one of the power plant control parameters [22]. Then, the plant moves from a steady state to another one. However, as mentioned in Zhang et al., [23], the term

"instability" may be misleading because such a characteristic does not necessarily lead to static instabilities. Indeed, the external characteristic of the channel (the one of the pump supplying the channel) must be taken into account in order to determine the stability or instability of an operating point.

More precisely, the operating point will be stable if the slope of the internal characteristic at this point is greater than the slope of the external characteristic (Ledinegg criterion):

∆ ∆

(1)

Let’s consider point (a) shown in Figure 1.7, for example, and the curve A as being representative of the external characteristic of the channel at this point. Let’s suppose that the flow rate increases slightly. Since the internal pressure loss in the channel ∆ is smaller than the pressure difference available to ensure the flow, ∆ (red curve), the flow rate will increase and stabilize only when the criterion of Ledinegg is respected again. This is followed by a flow excursion between point (a) and this new stable point. Similarly, if the flow rate is slightly reduced from point (a), the pressure loss inside the channel will be greater than the pressure difference available to ensure the flow and the flow rate will therefore decrease until the Ledinegg criterion is respected. Another flow excursion will appear.

On the contrary, if we suppose that curve B represents the external characteristic of the channel, the internal curve slope being larger than the external curve slope makes point (a) to be a stable operating point.

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To put in a nutshell, Ledinegg instability is a phenomenon associated with the pressure drop vs flow rate characteristic curve mentioned above. Due to the S-shape of this curve, the system may experiment a flow excursion when it turns from an unstable to a stable operating point. However, in most practical cases this situation is unlikely to happen, since in operating conditions it is barely possible to achieve a static unstable point [20]. However, the modification of the external characteristic curve can also change the stability of an operating point. This effect is shown in Figure 1.9. If the external characteristic of a system operating initially at a stable operating point (point 3 in case 3), is for some reason modified (to case 5), then the system will experiment a flow excursion (from point 3 to point 2).

This is one of the reasons that has motivated the development of some tools (See 3.2 Investigation of the pressure drop vs mass flow rate internal characteristic) enabling INES teams to identify the type of the internal characteristic of one branch of the vaporizer (type 1 or 2) under certain circumstances of solar heat flux density and water inlet temperature in the vaporizer.

Figure 1.9: Different external characteristic curves drawn on a pressure-drop vs mass flow rate internal characteristic [20]

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1.3 Objectives and Methodology of the Master Thesis

After having set up the frame of this Master Thesis, the following paragraph highlights its main objectives, the methodology followed in order to reach them and finally, a summary of the literature review carried out about the major topics underlined by the thesis.

CEA Linear Fresnel power plant prototype 1.3.1

In the framework of its research program related to CSP technologies, the LSHT built and commissioned a Linear Fresnel power plant prototype at Cadarache, Aix-en-Provence. The system is composed of a north-south oriented solar field separated in two parts by a steam separator: the vaporizer (made of two separated branches) and the superheater. The operation mode of this power plant is then a “recirculation mode”. Both parts are associated with sixteen rows of mirrors but are of different lengths: around 75m for the vaporizer and 25m for the superheater. A storage system with three temperature stages can provide steam during 6 hours after the sunset. In order to tackle the variability of the sun and the two-phase flow instabilities in the vaporizer, INES teams wish to upgrade their dynamic modelling tool of this prototype.

Figure 1.10: Simplified flowsheet of the Linear Fresnel power plant prototype in Cadarache [24]

Objectives and Methodology 1.3.2

In this context, the main objectives of this Master Thesis were:

 the development of numerical models enabling to set up in the long run a complete multi-domain dynamic model of the prototype,

 the implementation and the test of some control strategies in order to assess the controllability of the prototype.

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As mentioned in 1.2.3, this Master Thesis also includes a study based on the investigation of the internal characteristic type of one branch of the vaporizer according to a set of solar heat flux density and water inlet temperature values.

The methodology followed throughout the thesis progress has been the following:

 Literature review of the state-of-the-art in the fields of dynamic modelling applied to CSP technology, control-command strategies and two-phase flow instabilities

 Development of a thermal-hydraulic model related to the part of the water-steam cycle surrounding the vaporizer

 Development of a thermal-hydraulic model of one branch of the vaporizer and investigation of its pressure drop vs mass flow rate internal characteristic type

 Development, implementation and test of control-command models within the numerical platform PEGASE (See 2.1.3 PEGASE: the numerical co-simulation platform)

 Setting up of a co-simulation involving the several models used to describe the prototype (thermal-hydraulic model, optical model, control-command models)

 Analysis of the results and determination of the perspectives

These several steps were made possible by constant and constructive meetings with the supervisors of this Master Thesis during which the results obtained were discussed and the models improved.

The main deliverables of the Master Thesis are (for the host institution):

 A thermal-hydraulic model of the part of the water-steam cycle surrounding the vaporizer

 A specific thermal-hydraulic model related to one branch of the vaporizer

 Numerical control-command models

 A set of documents enabling to perform co-simulations on the numerical platform PEGASE

 A technical note referring to the development of diagnostic charts enabling to investigate the pressure drop vs mass flow rate internal characteristic type of one vaporizer branch

 The present report

Regarding KTH University, the main deliverable is the present report, summarizing the main objectives and results obtained over the period of the Master Thesis preparation. A public seminar will also be held at KTH, Stockholm in order to present the content of this work and its results.

Literature Review 1.3.3

1.3.3.1 Two-phase flow instabilities

Concerning this field, the major part of the information gathered are summarized in 1.2.3.

Among the several types of two-phase flow instabilities, the analysis focused particularly on the Ledinegg ones. However, as it has been already mentioned, an exhaustive review of two-phase flow instabilities is available in [20]. [21] gives also an older review of this phenomenon in which the classification between static and dynamic instabilities is highlighted. In his PhD Thesis, R.Disenmeyer offers a way to tackle Ledinegg instabilities in a heated tube by increasing the pressure loss at the tube inlet, by means of a grid, or a section reduction [25]. Indeed, by reducing

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the orifice ratio at the tube inlet, it transforms the internal characteristic of the heated tube, conferring it a monotonic form and erasing its S-shape (Figure 1.11).

Figure 1.11: Pressure drop vs mass flow rate internal characteristic for various orifice ratios

(D_orifice/D_pipe) at the heated tube inlet [26]

1.3.3.2 Dynamic modelling of CSP plants

In order to promote the use of Direct Steam Generation in the CSP industry, one important factor is the availability of modelling resources enabling to predict the dynamic behavior of the solar power plants due to transients (starting, cloud passages…). Dynamic simulations also bring the opportunity to test regulation schemes without jeopardizing the real system. Thus, in recent years, several research projects have focused on the modelling of CSP plants. And, if CFD codes can be a suitable option in the case of local modelling, the need for fast numerical codes enabling to design a full scale DSG plant model, makes the 1-D modelling approach a more realistic alternative [27].

Currently, a wide range of 1-D tools (one-dimensional discretization along the axial coordinate) can be found but they can be classified in two very distinct categories. The first category gathers commercial software packages or in-house codes which use the Homogeneous Equilibrium Model (HEM) approach. In this approach, both phases are treated as an equivalent single fluid and the relative velocity between water liquid and steam phase is not taken into account. This, can lead to inaccurate results in the phase change section of the system [27]. The work of Zapata et al. [28] and Rodat et al. [24] enter this category. Indeed, in [28], a dynamic model of a solar steam receiver is implemented in TRNSYS 16 and validated with experimental data from the Australian National University 500m² dish system. In [24], two solar power plants (one using oil as heat transfer fluid and the other one using water/steam) are simulated with DYMOLA software based on the Modelica modelling language. In this work, the Thermosyspro library is used and extended to fit the requirements of the solar power plant model. In his PhD Thesis, A.

Aurousseau also relies on DYMOLA software in order to develop dynamic models of Linear Fresnel solar power plants [14].

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The second category of 1-D tools that have been used to develop dynamic CSP models concerns the thermal-hydraulic (TH) codes like the CEA in-house CATHARE code. Since the 1950s, nuclear industry has developed, improved and validated such thermal-hydraulic codes [27]. Some of them are RELAP5 [27], CATHARE [2], ATHLET [29] and TRACE.

In these codes, a two-phase fluid approach is adopted where thermodynamic quantities as well as momentum quantities are considered for each phase. Thus, interface interactions are taken into account. This approach is then more realistic than the one adopted in HEM models but its computational cost is higher. However, increasing computing capabilities make these type of codes perfectly usable nowadays to design and test future DSG plants. The work of Serrano- Aguilera et al. [27] is an example of the use of RELAP5 code to simulate a CSP single-loop system including transients (Figure 1.12). This model is validated with experimental data from the DISS (Direct Solar Steam) facility located at Plataforma Solar de Almería. The model produced considers, in addition, connection pipes, change of heights and thermal insulation in the passive sections. The same facility is considered in the model set up by Hoffmann et al. in [29] but using the thermal-hydraulic code ATHLET.

Figure 1.12: Layout of the nodalization scheme of the DISS thermal-hydraulic model (RELAP5)

1.3.3.3 Regulation and control-command of CSP plants

The main objective of steam generation in CSP plants is to supply a steam turbine in order to rotate an alternator which will produce electricity. However, in 1991, the International Electrotechnical Commission (IEC) established a quality standard for steam turbines, which has to be matched by every commercially operated turbine. In this standard, limits regarding the steam quality have to be achieved. Table 1.2 shows the limits for the main steam temperature and pressure, which are allowed to be exceeded for a certain duration. The absolute limits for temperature and pressure, which should not be exceeded, are respectively 28 K above the rated condition and 120% of the rated condition. This is one of the reasons motivating the implementation of regulation processes, among the ones related to the safety of the plant components.

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In his PhD Thesis, A. Aurousseau summarizes some regulation strategies according to the operation mode of the CSP plant [14] (See 1.2.2.3). Concerning the recirculation architecture, it solves the problem of the predictability of the end of the vaporization zone, which is the main problem encountered with the “once-through” mode. Moreover, this operation mode adds a parameter to the regulation: the recirculated flow rate.

Table 1.2: Exceeding limits for turbine inlet temperature and pressure according to IEC Standard [30]

Parameter Limits Duration Turbine inlet temperature Rated temperature Annual average must be the rated

temperature or below

< +8K exceeding rated temperature Annual average must be maintained

< +14K exceeding rated temperature Annual average must be maintained and accumulated duration <400h per year

< +28K exceeding rated temperature Annual average must be maintained, accumulated duration <80h per year,

max.duration <15min +28 K exceeding rated temperature Not allowed to exceed

Turbine inlet pressure Rated pressure Annual average must be the rated pressure or below

<105 % of rated pressure Annual average must be maintained

<120 % of rated pressure Annual average must be maintained and accumulated duration <12h per year

In [18], Valenzuela et al. investigate the controllability of this recirculation mode on the DISS facility based on PI control loops (Figure 1.13). In addition, PI controllers (or regulators) include a so-called “anti-windup” control in order to avoid that the integral term increase or decrease too much because of the use of saturators. Indeed, regulators use saturators because the actuators are operating parameters of the solar field (valve opening, pump rotation speed…) which have physical limits. If the signal from the controller exceeds a saturation limit, it no longer has any effect on the actuator, but the integral term of the error continues to increase, leading further to a wrong control action of the PI controller on the actuator. The numerical addition of an “anti- windup” control enables to avoid such a runaway of the calculation by binding the value of the integral term.